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Fracture surface topography analysis (FRASTA)—Development, accomplishments, and future applications
Abstract
In the late 1970s, Dr. George Irwin suggested to his colleagues at the University of Maryland that valuable information about a fracture event was recorded in fracture surface topography. Under his urging, Takao Kobayashi, an associate professor at the time, began quantifying and interpreting topographical features. Over the subsequent 30 years the procedures for quantifying and interpreting fracture surface topography grew continuously into an established technology that allows a fracture event to be reconstructed in microscopic detail. FRASTA (fracture surface topography analysis) has now been applied to achieve solutions to a wide variety of failure problems. This paper chronicles the historical development of FRASTA, recounts several notable achievements, and presents the plan for further development and future applications.
... Despite its infinitesimal size, pre-existing microcracks and their evolution significantly determine the behaviour of concrete subjected to external loadings. In this context, surface tomography analysis [4,5] can be used to provide additional information on the load induced microcracking behavior in concrete. Moreover, microcracking in concrete initiates already at a load levels much lower than the ultimate load [6][7][8][9][10][11]. Thus, from a structural health monitoring point of view, detection of microcracking, which is a precursor to complete failure of the structure, can help take suitable precautionary measures in advance [12,13]. ...
... are determined by varying the volume fraction of aggregate accordingly in the Mori-Tanaka based homogenisation procedure, while keeping other parameters unchanged (group Model parameters). The following sources are used to determine the parameters: [ 4,5 ] aspect ratio is taken within the range measured in [69] , 6 -[24,70]. ...
... Update loading strain: E n+1 or Σ n+1 3 for k = 1 → max_iter do 4 Solve for mesoscopic stress and strain fields 5 Solving BVP for stress and crack growth at each mesoscopic material point: 6 for all voxels of mortar material do 7 Update the applied strain:˜ n+1 8 Check crack growth criteria (Equation (1) ...
Damage in concrete structures initiates as the growth of diffuse microcracks that is followed by damage localisation and eventually leads to structural failure. Weak changes such as diffuse microcracking processes are failure precursors. Identification and characterisation of these failure precursors at an early stage of concrete degradation and application of suitable precautionary measures will considerably reduce the costs of repair and maintenance. To this end, a reduced order multiscale model for simulating microcracking-induced damage in concrete at the mesoscale levelis proposed. The model simulates the propagation of microcracks in concrete using a two-scale computational methodology. First, a realistic concrete specimen that explicitly resolves the coarse aggregates in a mortar matrix was generated at the mesoscale. Microcrack growth in the mortar matrix is modelled using a synthesis of continuum micromechanics and fracture mechanics. Model order reduction of the two-scale model is achieved using a clustering technique. Model predictions are calibrated and validated using uniaxial compression tests performed in the laboratory.
... This kind of analyses was carried out by Macek et al. [6][7][8], where the relationships between bending-to-torsion ratio or strain sequence to surface topography parameters were demonstrated. This short communication presents the results of a fracture reconstruction method called FRActure Surface Topography Analysis (FRASTA) [9][10][11] to elucidate the fracture events in thin-walled tubular stainless steel specimens failed under monotonic tensile and fatigue loads. This method has not been applied for thin-walled tubular specimens so far, and it will be applied to the axial-torsion fatigue load case later. ...
... A FAPP represents a snapshot of the fracture progression at a given separation distance in which the black and white areas correspond to "no separation" and "separated or cracked" states, respectively. The analysis procedure is described elsewhere [9,10]. ...
The main objective of this short communication is to show the fracture progression in each loading case and complement knowledge about fracture mechanisms underpinning the tensile and fatigue performance of thin-walled tubes. For this purpose, the fracture surface topography analysis (FRASTA) method was used in the thin-walled tubular austenitic stainless-steel specimens. Two cases were analyzed: monotonic tension, and uniaxial fully-reversed fatigue. Furthermore, the fractures topographies were quantified through the profiles over their entire surfaces with the support of an optical confocal measurement system. The results showed the usefulness of the FRASTA method in identifying characteristic zones in the cracking process for the analyzed cases and motivates its development for other materials and complex loading cases.
... Fracture mechanics and fractography are two branches of modern engineering currently used to investigate the mechanical behaviour of structural elements during and after fatigue failure, respectively (Zeravcic et al. 2006;Macek et al. 2017Macek et al. , 2020bBranco et al. 2020;Kowal and Szala 2020). One of the methods of fracture-surface topography analysis is FRASTA, developed by SRI International (Kobayashi and Shockey 2010) and further improved by (Cao et al. 2014;Martelo et al. 2019). Another approach, called total fracture area method, was used by Macek to analyse the fracture topographies for various engineering materials and loading cases, including single bending, single torsion and their combinations (see, for example, the reference (Macek et al. 2020a(Macek et al. , 2021). ...
... This has been investigated for axisymmetric complex surfaces (Aono and Noguchi 2005), for short cracks under mixed mode loading (Zhang and Fatemi 2010), or for fatigue crack growth at the subgrain scale (McDowell 1989). A smaller number of papers has tried to investigate the relationships between fatigue and fracture parameters and fracture surface roughness (Arakawa and Takahashi 1991;Slámečka et al. 2010;Kobayashi and Shockey 2010). Nevertheless, the correlation between fracture roughness parameters and fatigue properties for notched samples made of aluminium alloy has not been systematically investigated yet. ...
The effect of cyclic loading on facture surface topology in notched components made by aluminium alloys is not completely clear. Fractogra-phy and fracture mechanics can help to understand this interdependency. This paper aims to study the distribution of the fracture surface roughness of notched 2017A-T4 aluminium alloy after bending fatigue using an optical focus-variation surface measurement technique by applying the fracture zone concept. The effects of stress level at the notch root and the load ratio on fatigue crack growth and fracture surfaces are analysed. Profile and areal surface parameters of four fracture surface regions were investigated at incre-mental crack lengths of the specimens. Studies have confirmed that the surface areas associated with the main stages of fatigue phenomenon (i.e. crack initiation , crack propagation, and final rupture) have significant differences in roughness which can be explained by the different loading scenarios. Overall, fatigue fracture surfaces have smallest roughness values at the crack initiation stage and a gradual increase during the fatigue crack growth stage.
... In case of work presented by Sampath [11] investigated determination of crack initiation stress -by correlating fracture cases. The observation of the damaged surface of a structural member has two aspects: (i) the orientation of the fracture surface [12][13][14] and (ii) its shape and roughness [15][16][17]. In open literature [12][13][14], the authors described multiaxial fatigue behavior of materials in the context of critical planes, according to damage accumulation methods. ...
... The authors showed a better correlation of the fatigue life estimation results using the damage accumulation method. Regarding aspect (ii), Kobayashi and Shockey [15] presented their own method for quantifying and interpreting fracture surface topography, Molent et al. [16] used the concatenation between fractal geometry and crack growth, Szala and Kot [17] studied the effect of electrostatic sprayed coatings on their microstructure, roughness and mechanical properties. The FRASTA (fracture surface topography analysis) system Kobayashi developed is based on a scanning laser confocal microscope for failure mechanisms characterization. ...
This paper presents the influence of multiaxial loading and fatigue behavior on a fracture surface in the context of its topography. For analysis were taken fracture surfaces obtained in experimental tests of S355J2 steel subjected to non-proportional bending with torsion. A high-resolution optical 3D profilometer was used in the examination and surface measurements. For a quantitative description of fracture mean surface roughness parameter Sa and a fractal dimension Df were employed. Surface topography studies were carried out on the entire fracture surface. Shear stress contribution expressed through ratio of maximum stresses λ, loading conditions represented by variance of normal and shear stress µ, and fatigue life influence on the fracture surface parameters were found. Such a method of applying advanced way of topography application is useful not only as a measurement methodology but also, as innovation method of surface evaluation.
... The results verified that the fatigue life values using the core material volume were within the scatter band and on the whole fracture surface. Based on reconstructing crack propagation in microstructural details by examination of fracture surface topographies, Kobayashi and Shockey [51] introduced a novel approach for fracture surface analysis called fracture surface topography analysis (FRASTA). ...
The use of a composite welded joint consisting of titanium and austenitic stainless steel metals is evidently a favourable selection for industrial applications employing the resistance spot welding
(RSW) operation. Nevertheless, achieving a high-quality welded joint proved challenging owing to the properties of the diverse range of materials’ used. To improve the quality of dissimilar
welded joints, the welding parameters should be selected precisely. With that in mind, the current research endeavoured to figure out the ideal RSW parameters for a dissimilar resistance spot welded joint between grade 2 titanium alloy (Ti) and AISI 304 austenitic stainless steel (ASS) with equal and unequal thicknesses of 0.5 and 1 mm. The RSW cases based on the selected thickness were referred to as the following: similar thickness of 1 mm for Ti and ASS as case E, dissimilar thickness of 0.5 mm for Ti and 1 mm for ASS as case F, dissimilar thickness of 0.5 mm for ASS and 1 mm for Ti as case I, and similar thickness of 0.5 mm as case J. Tensile shear force, failure mode, and micro-hardness were the metrics used to assess the dissimilar joint’s soundness. The RSW variables utilized in the scope of the present investigation were “welding current, pressure, welding time, squeeze time, holding time, and pulse welding”. Models from gradient boosting, CatBoost, and random forest machine learning (ML) algorithms were used to guarantee an accurate analysis, along with the artificial neural network regressions. Therefore, the ML and artificial neural network (ANN) models were trained using real data collected from 100 experimental RSW samples conducted under different RSW process parameters. Various transfer and training functions were applied with the multilayer perceptron employing the feed-forward-back propagation approach when building the ANN models. Also, for the first time in the RSW field, an estimation of the relative importance of the RSW variables regarding the predicted shear force is presented in the current study. Evaluating the experimental findings demonstrated that the highest shear force was 2.183, 2.589, 1.708, and 1.851 kN for cases E, F, I, and J. The microhardness data indicated that the nugget zone had a much higher hardness than the heat-affected zone (HAZ) and base metal (BM) zones, wherein case F showed the peak nugget hardness in comparison to the other cases. The best prediction model was found to be the ANN model when training the conjugate gradient with the Polak-Ribiere updates (Traincgp) training function with the hyperbolic tangent sigmoid transfer function (Tansig) with the mean squared error (MSE) and correlation coefficient (R2) values recorded as 0.01886 and 0.94973, respectively. However, the random forest algorithm gave the second best prediction of the MSE while the CatBoost and gradient boosting algorithms were third and fourth, respectively. The automotive and aerospace sectors are anticipated to benefit from the present research in applications associated with body component production that result in a superior strength-to-weight ratio.
... Steel adhesive joints after fatigue shear testing [11] or life assessment of notched austenitic steel specimens [12] were also better explained by studying the surface topography. Historically, fracture surface topography analysis was initiated by quantifying and interpreting topographical features [13]. Nevertheless, its application is becoming increasingly widespread. ...
In this paper, the topography of 6082 aluminium alloy specimens after fatigue bending tests was studied with a comprehensive evaluation of measurement noise caused by vibration. Roughness results were acquired by contactless Focus Variation Microscopy (FVM). Studied data were pre-processed, removing the non-measured points and outliers with regular methods, respectively, and high-frequency noise was considered. The variations in ISO 25178 roughness parameters were studied. Based on the previous studies, it was found that surfaces after fatigue bending tests can be difficult to consider when analyzing the measurement noise in a selected bandwidth. Some advantages of profile data extraction in selected directions, like horizontal, vertical or crack, were found deficient, even in studies by various functions, like autocorrelation, power spectral density, or texture direction ratio. When noise suppression methods depend on the details studied, boundary areas were extracted to compare and highlight the presence of high-frequency data characteristics. The proposed method was validated when contrasting standardised Gaussian or median filtering techniques with the spline filtering approach. A proper filter for the reduction of vibrational noise from the results of FVM topography measurements was suggested based on the proposed procedure. Finally, it was proposed how use the new method for reducing errors caused by high-frequency measurement noise in the surface topography of specimens after fatigue bending tests.
... These approaches involve a detailed study of the fracture surface of a material after failure to understand the causes and mechanisms that led to the rupture. These methods rely on observing and characterizing the morphological features of the fracture surface, including identifying origin marks, crack propagation lines, and other features that can indicate the type of failure [22,23]. ...
Fatigue crack growth behavior of AlSi10Mg aluminum alloy processed by laser powder bed fusion (L-PBF) was studied for three material conditions: as-built, stress relieved and hot isostatic pressed. An improvement in the fatigue crack growth resistance was found for the heat-treated conditions which was attributed to the residual stress relief induced by the heat treatments. The stress ratio did not affect the fatigue crack growth in regime II which was explained by the absence of crack closure. Fatigue crack growth rates were predicted using a model based on the cyclic plasticity of the tested aluminum alloy. The proposed model agreed well with the experimental results.
... Some scholars have observed the surface morphology of the specimen surface or fracture area more intuitively by using a threedimensional optical profilometer to determine surface topography parameters (Macek et al., 2023;Macoretta et al., 2024). In addition, different methods have been introduced for quantitative analysis of fracture surfaces, such as Macek's "entire fracture surface" method (Macek et al., 2023) and Kobayashi's "fracture surface topography analysis (FRASTA)" method (Kobayashi and Shockey, 2010). In this paper, ultrasonic fatigue tests under constant amplitude and two-step variable amplitude were carried out on SLM AlSi7Mg specimens to mainly study the VHCF behavior and ultra-slow crack growth behavior of the materials, respectively. ...
Purpose
The purpose of this paper is to determine the ultra-high cycle fatigue behavior and ultra-slow crack propagation behavior of selective laser melting (SLM) AlSi7Mg alloy under as-built conditions.
Design/methodology/approach
Constant amplitude and two-step variable amplitude fatigue tests were carried out using ultrasonic fatigue equipment. The fracture surface of the failure specimen was quantitatively analyzed by scanning electron microscope (SEM).
Findings
The results show that the competition of surface and interior crack initiation modes leads to a duplex S–N curve. Both manufacturing defects (such as the lack of fusion) and inclusions can act as initially fatal fatigue microcracks, and the fatigue sensitivity level decreases with the location, size and type of the maximum defects.
Originality/value
The research results play a certain role in understanding the ultra-high cycle fatigue behavior of additive manufacturing aluminum alloys. It can provide reference for improving the process parameters of SLM technology.
... To verify the reasons of fatigue failure in welded joints, many studies have investigated the fatigue failure mechanisms through the observation methods of fractural locations and fractural surface morphologies [23][24][25]. Takao Kobayashi et al. [26,27] used the fracture surface topography analysis method to quantify and analyze fracture surface topography, which could help researchers reconstruct a fracture event induced by fatigue loadings. Furthermore, Wojciech Macek et al. [28][29][30][31] applied the entire fracture Nomenclature C Master S-N curve parameter D 1 Distance from the applied load to the weld root at the base plate (see Fig. 4) D 2 Distance from the applied load to the weld toe at the attached plate (see Fig. 4) F i (i = 1, 2, 3⋯) Resultant forces along the weld line with respect to the natural axis F Applied Applied load on the base plate during experiment f i (i = 1, 2, 3⋯) Line forces along the weld line h Master S-N curve parameter [32][33][34] studied the fatigue failure mechanism of a dissimilar welded joint under various loading levels and found that the transition of failure locations from heat-affected zone (HAZ) at the high loading level to WM at the low loading level was induced by the competition between the un-uniform distribution of material's strengths and the random distribution of micro-defects. ...
... For example, grain size has a distribution of frequencies across the spectrum rather than one specific frequency. Similarly, other microscopic fracture features have a range of spectral distributions [67][68][69] . For a pair of fractured surfaces, the population of these features contains relevant information about the physical fracture processes present at each length scale (e.g. ...
The complex jagged trajectory of fractured surfaces of two pieces of forensic evidence is used to recognize a “match” by using comparative microscopy and tactile pattern analysis. The material intrinsic properties and microstructures, as well as the exposure history of external forces on a fragment of forensic evidence have the premise of uniqueness at a relevant microscopic length scale (about 2–3 grains for cleavage fracture), wherein the statistics of the fracture surface become non-self-affine. We utilize these unique features to quantitatively describe the microscopic aspects of fracture surfaces for forensic comparisons, employing spectral analysis of the topography mapped by three-dimensional microscopy. Multivariate statistical learning tools are used to classify articles and result in near-perfect identification of a “match” and “non-match” among candidate forensic specimens. The framework has the potential for forensic application across a broad range of fractured materials and toolmarks, of diverse texture and mechanical properties.
... Topography analysis of fracture surfaces caused by fatigue loading is very common in literature because it can provide important clues on the failure mechanisms. Kobayashi and Shockey [19] correlated fatigue behaviour under multiaxial loading with the topography characteristics of fracture surfaces. Molent et al. [20] effectively connected the fractal geometry with crack growth. ...
In this paper, the methods of compensation of differences in the results of entire bending-fatigued fracture surface topographies were presented. The roughness evaluation was performed with a focus variation microscope and confocal surface topography measurement techniques. The differences in the ISO 25178 roughness parameters were investigated and procedures for their compensation were studied. It was found that various types of optical measurements can cause differences in the errors occurring in the measurement process, such as outliers, and noise. The reduction of differences in the various optical roughness measurements can be attained when measurement errors are compensated. For this study, the applications of general procedures available in commercial software can be suitable for improvements of the roughness measurement results, such as raw data thresholding technique, digital filtering (S-filter), power spectral density, and autocorrelation function analyses. The validation of measurement techniques was proposed for areal and profile studies, including analysis of differences in the calculation areal ISO 25178 roughness parameters.
... Fracture surface investigation is conducted in SEM; the entire fracture surface is viewed initially to identify the origin of crack initiation and thereafter, specific zones from crack initiation and propagation regions are magnified further to obtain more detailed information. Different methodologies are adopted now-a-days [52][53] to carry out quantitative fractographic investigation viz. evaluation of fracture surface topologies through aerial, volume and fractal dimension parameters or by use of FRASTRA (Fracture surface topography analysis). ...
Recycling of Al-alloys is the need of today's world since the energy consumption is only 5 % of that needed for production of primary Al. However, there are several challenges of using recycled Al in load-bearing structures and components particularly in connection to fatigue performance since durability is an important criterion for their design. The present paper aims to catalogue and analyse the existing literature on fatigue behaviour of recycled Al-alloys, focusing on the process parameters, microstructure, tensile properties, presence of defects, etc. presented in a comprehensive overview on this important subject. An attempt has been made to identify the main knowledge gaps that must be addressed in future investigations to establish standards for robust design of recycled Al-alloy products. This paper is aimed to serve as useful input to chart the roadmap of investigation of fatigue of recycled Al-alloys in the academic and industrial community.
... The above literature studies have revealed that little research has been carried out on the fatigue behaviour of composites made up of A356-hematite particles. Takao Kobayashi and Donald [16] studied the fracture surface topography analysis (FASTA) to analyze the failure initiation time and load conditions, average crack growth rate and details such as accelerations, decelerations, hesitations, and arrests. The details of micro failure initiation and how a crack front interacts with microstructural features and fracture toughness and other continuum fracture mechanics parameters are got directly from a fracture surface. ...
This study intends to investigate how copper chill affects the fatigue behaviour of composites made of aluminium alloy A356 and hematite. It was cast by altering the weight fraction particles of hematite (0 to 12%wt in increments of 3%wt) by sand casting method with and without copper chills at its end to get isotropic and homogenous significant characteristics under liquid metallurgical way. The test specimens were prepared in accordance with ASTM specifications. Ducom-type fatigue testing equipment (rotating bending-low cycle fatigue) is used in experiments to examine fatigue behaviour. The micrographic images were taken with a scanning electron microscope (SEM) and interpreted uniform reinforcement of hematite particles, and X-ray diffraction (XRD) patterns were used to reveal microscopic details. The existence of the hematite particles and their phases was revealed by the X-ray diffraction analysis. The results show that the composites cast with copper chills have significantly greater fatigue strength than the casting obtained without copper chills. It was also observed that at 9%wt, copper chilled composite shows improve in fatigue strength about 10.2% as compared without chilled composites.
... Some investigations on the fracture surface topography analysis of fatigue damage have been conducted. Macek et al. [14] studied the effects of fracture on the monotonic tensile properties of thin-walled tubular austenitic steel specimens through the fracture surface topography analysis method, which was proposed by Kobayashi and Shockey [15]. A tensile load is a common load in load-bearing members. ...
... Advanced measurement methods, such as digital image correlation [22,23] and broadly understood microscopy [24][25][26] are particularly useful for quantitative fractography. In addition, the combination of modern research methods [27,28] with optical surface observation techniques [29,30] allow the development of alternative approaches to characterise the fracture surface, such as FRASTA (FRActure Surface Topography Analysis) method [31][32][33]. The aforementioned measurement tools also enable the development of classical computational models and the search for non-standard methods, for instance the fractal dimension [34]. ...
In this study, the fractal dimension of fatigue fracture surfaces is investigated in order to find an alternative failure loading indicator. Some of many metrological factors are generalized by reducing the fracture surface structure to one factor and develop an entire fracture surface procedure by analyzing the impact of surface slope and calculation resolution. Three notched geometries are studied under cyclic bending-torsion: 34CrNiMo6 high-strength steel bars with transverse blind holes; (ii) 34CrNiMo6 high-strength steel bars with lateral U-shaped notches; and (iii) 18Ni300 maraging steel hollow bars with transverse holes produced by selective laser melting. The surface topographies of fatigue fractures were measured with an optical profilometer. The bending-torsion ratio, maximum local von Mises equivalent stress and the number of cycles to crack initiation are examined using the fractal dimension. Moreover, a comparison was also made for conventional surface topography parameters such as root mean square height and void volume. A fatigue crack initiation model based on surface topography and loading conditions is proposed. The model relies on the product of the maximum local von Mises equivalent stress and the fractal dimension divided by the root mean square height to void volume ratio. The results show that the fracture plane geometry, expressed by the fractal dimension Df, can facilitate the estimation of post-failure loading history. In particular, the analysis based on the enclosing boxes method (EBM) is more accurate when it is used as an extra-fine resolution without any plane leveling.
... In the present study, the fatigue properties were evaluated by a plane bending fatigue test [40] , as the maximum tensile stress was applied at the surface. Note that fracture surface topography analysis (FRASTA) [41] and entire fracture surface method [ 42 , 43 ] were proposed to investigate the fractured surface for bending-torsion fatigue using such as fatigue test stand MZGS-100 PL [44] . ...
To investigate the improvement in the fatigue strength of magnesium alloy by peening methods, magnesium alloy AZ31 was treated by submerged laser peening (SLP), cavitation peening (CP), and shot peening (SP), and the fatigue properties were evaluated by a plane bending fatigue test. In the case of SLP, both the impact induced by laser ablation (LA) and that caused by laser cavitation (LC), which developed after LA, were used. In the present study, the fatigue life at a constant bending stress was examined to determine the suitable coverage. It was found that the fatigue strengths at N = 107 for the SLP, CP, and SP specimens treated by each optimum condition were 56%, 18%, and 16% higher, respectively, than that of the non-peened (NP) specimen, which was 97 MPa. The key factors in the improvement of fatigue strength by peening methods were work hardening and the introduction of compressive residual stress.
... In the literature, one can find studies that evaluate the topography of cracks using, for example, a confocal measurement system [42]. Fractal analysis [43] of fractures can be another step to better understand the genesis of the fracture in flat notched specimens. In the present discussion, a basic assessment of the crack development surface was carried out. ...
open access: https://doi.org/10.1016/j.engfailanal.2023.107199
... In the scientific literature on fractography, there are many attempts to find a way to quantify fatigue fractures and make them dependent on damage-generating factors (de Freitas et al., 2017;Deng et al., 2022;Merson et al., 2017;Westermann et al., 2016). One of these ways is the FRASTA method, but it focuses on regions from initiation to final fracture (Kobayashi and Shockey, 2010;Sampath et al., 2018). On the contrary, the method introduced by the authors is based on the entire fracture area and has been successfully tested for different materials and specimens geometries (Wojciech Macek et al., 2021c;Macek et al., 2021a). ...
In this study, different fracture surfaces caused by fatigue failure were generated from 18Ni300 steel produced by selective laser melting (SLM). Hollow round bars with a transverse hole were tested under bending-torsion to investigate the crack initiation mechanisms and fatigue life. Next, the post-failure fracture surfaces were examined by optical profilometer and scanning electron microscope. The focus is placed on the relationship between the fatigue features (e.g., bending-torsion ratio, fatigue crack initiation angles, and fatigue life) and the fracture surface topography parameters (e.g., height parameter Sx, volume parameters Vx, maximal pit and valley angles). The analysis was carried out using the entire fracture surface of the tested specimens. It was found that the decrease of the shear stress level significantly reduces the value of the fracture surface parameters. A fatigue life prediction model based on both the surface topography values and the applied load was proposed. Fatigue life predictions for different loading ratios agreed well with the experimental results and were slightly better than those of other existing models. The proposed model can be helpful for post-mortem analysis of engineering components sub-jected to multiaxial fatigue.
... Kobayashi (1984) suggested a method to explain the topography in quantitatively manner by using an instrument called parallax bar. The innovative concept in this case was to take into account the conjugate fracture surfaces, which entails profiling the roughness along the same line on opposing crack faces, comparing the profile mismatches, and interpreting the overlap between the conjugate surfaces as denoting the amount of inelastic deformation that is still to take place prior to fracture [4,5]. ...
The present work deals with a critical fractographic analysis into low carbon (0.18%-C) steel samples which were used for three different mechanical tests: tensile test; shear test; and toughness test. These mechanical tests were performed in standard sized specimens as recommended by ASTM. In each category of test, there were two different specimens with different physical states according to heat treated conditions. First specimen was in ‘as received’ condition and another was annealed. For annealing, sample was first heated up to austenitic temperature and inserted inside the sand for slow rate of cooling. As these two categories of samples were undergone through destructive tests, the variation in fracture behaviour of the samples was analysed by FESEM, XRD. A significant variation in fractographic images could be observed in different heat-treated samples. Micro-pores, dimples, cleavage facet, peaks, valleys, and cave formation were observed in the samples.
... FRASTA is a method for computationally reconstructing a fracture process such as crack initiation and propagation behaviors by comparing the topographic features of the fracture surfaces. The detailed procedure for FRASTA was described in previous studies [23][24][25], and consists of three steps: (a) obtaining topographic maps of the identical areas in two opposing fracture surfaces; (b) computationally superimposing the maps until there was no gap between them; and (c) virtually separating them in the loading direction. Because the gaps that appear correspond to the fractured areas, the fracture propagation process can be reconstructed through the gap formation sequence by increasing the relative distance between the two topographic maps. ...
This study investigated the hydrogen-related fracture behavior in as-quenched low-carbon martensitic steel under a constant loading tensile test with various applied stresses. We found that the fracture time in the constant loading tensile test decreased as the applied stress and hydrogen content increased. The fracture surface topography analysis revealed that when the applied stress was low, the intergranular fracture was initiated around the side surface of the specimen and gradually propagated into the inner part of the specimen. In contrast, several intergranular fractures were separately initiated inside the specimen when the applied stress was high. The mode of hydrogen-related fracture was controlled by the fracture stress and not by the global hydrogen content inside the specimen. Increasing the global hydrogen content caused a decrease in the duration required for the accumulation of critical local hydrogen concentration at the fracture initiation site (prior austenite grain boundary). Accordingly, we propose that the local state at the crack initiation site is constant under a given applied stress, even when the global hydrogen content is different.
... In some works, scientists have tried to combine all fractographic methods and link them to fracture mechanisms or fatigue life, especially using surface roughness parameters evaluated from the fracture surfaces [36][37][38]. Examples of quantitative fractography cases are presented in the works by Goldsmith et al. [39] and by Kobayashi et al. [40]. However, as far as the authors know, there are no studies in the literature connecting multiaxial loadings and surface fracture parameters in round bars with blind holes subjected to multiaxial loading. ...
The present study evaluates the fracture surface response of fatigued 34CrNiMo6 steel bars with transverse blind holes subjected to bending with torsion loading. The analysis of the geometric product specification was performed by means of height parameters Sx, functional volume pa-rameters Vx, and fractal dimension Df. Surface topography measurements were carried out using an optical profilometer with focus variation technology. The experimental results show that the doubling the bending to torsion moment ratio B/T from B/T = 1 to B/T = 2, maintaining the same normal stress amplitude, greatly reduces both Sa, Vv as well as the fractal dimension Df of the an-alyzed specimen fractures by 32.1%, 29.8%, and 16.0%, respectively. However, as expected, a two-fold increase in the B/T ratio, maintaining the same normal stress amplitude, resulted in a larger number of cycles to fatigue crack initiation, Ni, which can be explained by the lower shear stress level. These experiments prove that parameters Sx, Vx, Df are smaller for larger Ni values, which is an important finding. In addition, it was found a high consistency of surface topography measurements for the two sides of the broken specimens. The proposed methodology is both relia-ble and applicable for other engineering applications involving different geometries and loading conditions.
... The surfaces prepared in this way, together with the measurement results, can also be used for further comparative analyses focused on the cracking mechanisms. Such analyses may be carried by using, for example, the FRASTA method (Kobayashi and Shockey, 2010;Sampath et al., 2018) but this approach is not the subject of this paper. In the present paper, for further fracture surface topography analyses, the entire fracture method based on the reduced area will be used, due to the reduction of the unmeasured points near the fracture edge. ...
The paper studies the effect of strain-loading sequence on fatigue lifetime and fracture surface topographies in 7075-T651 aluminum alloy specimens. Fatigue tests were performed in two ways: (i) constant-amplitude loading and (ii) two series of variable amplitude loading with non-zero mean strain values. The topography of the fatigue fractures was measured over their entire surfaces with the help of an optical confocal measurement system. The results of fatigue tests in the form of equivalent strains, εaeq, such as the weighted mean of strain components, εa1, εa2, and fatigue life, Nf, were used as the sum of the partial number of cycles N1, and N2. This study indicates, inter alia, that the values of the fracture surface parameter core height Sk, found in the two-step loading program, are linearly dependent on the equivalent strain, and logarithmically dependent on the fatigue life.
... For example, grain size has a distribution of frequencies across the spectrum rather than one specific frequency. Similarly, other microscopic fracture features would have a range of spectral distributions [45], [46]. For a pair of fractured surfaces, the population of these features contains relevant information about the physical processes present at each length-scale. ...
Fractured metal fragments with rough and irregular surfaces are often found at crime scenes. Current forensic practice visually inspects the complex jagged trajectory of fractured surfaces to recognize a ``match'' using comparative microscopy and physical pattern analysis. We developed a novel computational framework, utilizing the basic concepts of fracture mechanics and statistical analysis to provide quantitative match analysis for match probability and error rates. The framework employs the statistics of fracture surfaces to become non-self-affine with unique roughness characteristics at relevant microscopic length scale, dictated by the intrinsic material resistance to fracture and its microstructure. At such a scale, which was found to be greater than two grain-size or micro-feature-size, we establish that the material intrinsic properties, microstructure, and exposure history to external forces on an evidence fragment have the premise of uniqueness, which quantitatively describes the microscopic features on the fracture surface for forensic comparisons. The methodology utilizes 3D spectral analysis of overlapping topological images of the fracture surface and classifies specimens with very high accuracy using statistical learning. Cross correlations of image-pairs in two frequency ranges are used to develop matrix variate statistical models for the distributions among matching and non-matching pairs of images, and provides a decision rule for identifying matches and determining error rates. A set of thirty eight different fracture surfaces of steel articles were correctly classified. The framework lays the foundations for forensic applications with quantitative statistical comparison across a broad range of fractured materials with diverse textures and mechanical properties.
... Wang et al. [18] applying the Scanning White-light Interferometric (SWLI) microscope for the study of surface roughness evolution during fatigue. The articles [19][20][21], described and developed a method for obtaining information on fatigue load from the fracture surfaces roughness characteristics. As for the information about the features of the fracture surface for the load during bending analyzed by, among others, Macek et al. [22,23] are rather rare. ...
This work deals with the post-failure analysis of the fatigued fracture in the context of surface topography parameters. The influence of material isotropy on fatigue fractures using metrological methods is presented. The study was carried out on fracture surfaces of different maximum stress at the notch root σmax and stress ratio R. Surface topography and fracture measurements were made using the Infinite Focus technique with the use of the changing the focus, which allowed the collection of data sets. The analysis of the geometric product specification was performed by means of height parameters according to ISO 25178, fractal dimensions as well as surface isotropy. Paper presents that higher values of skewness were received for the stress ratios R = -1, -0.5 than for R = 0, while lower kurtosis values for the stress ratios R = -1 and -0.5 than for R = 0 were observed.
... The crack propagation mechanism was investigated by the FRActure-Surface Topography Analysis (FRASTA) technique [25,26] . This method consists of confronting the spatial configurations of both fracture surfaces of the same zone in order to compare the profile mismatch. ...
High-entropy alloys (HEAs) have attracted a great deal of interest and high expectations not only from the scientific community, but also from the industrial point of view, mainly because of a remarkable combination of promising properties. However, the characterization of HEAs under fatigue loading is still very scarce, especially in the case of body centred cubic (BCC) HEAs. As an attempt to fill this gap, we report here the first study on the fatigue behavior of a refractory BCC HEA in four-point bending loading conditions. Equimolar HfNbTaTiZr HEA was subjected to load-control fatigue tests in air and at room temperature to assess its fatigue properties in the high-cycle regime. Experimental results show that the maximum stress level reached within one fatigue cycle exceeds the uniaxial yield stress of the material. This particularity is observed within the whole range of fatigue life investigated, i.e., from approximately 10⁴ to 10⁷ cycles, showing no macroscopic plastic deflection of the beam in the process. A fatigue endurance of approximately 430 MPa (at 10⁷ cycles) is found, for a stress ratio of approximately 0.1. A fatigue crack initiation mechanism occurs at the specimen surface through an intergranular phenomenon and propagates in a transgranular crystallographic manner. The fatigued samples underlined the low local lattice rotation, implying a restrained cyclic strain hardening. Finally, the fatigue study of the HfNbTaTiZr HEA highlights a fatigue endurance over an ultimate tensile strength ratio of 0.43, which is significantly larger than the actual conventional structural metallic materials under similar conditions.
... Related theoretical analyses in the former case are in [63,64] and, for glass material, to essentially static hardness testing by Rouxel [55] and applied to hierarchical structural aspects of tooth enamel by Yilmaz et al. [65]. Other benefits of structural considerations dealing with experimental fractographic analyses, building onto Hull's attribution to Hooke [3], are provided in [66,67], relating also to an exposition of ductile versus brittle fracturing considerations in the articles by Tekoglu et al. [68], Armstrong [48], Knott [69] and Matic et al. [70]. ...
Everyone has to deal with fracturing of materials at one level or another, beginning from normal household chores and extending to the largest scale of observations reported for catastrophic events occurring on a geological level or even expanded to events in outer space. Such wide perspective is introduced in the current introduction of this theme issue. The follow-on organization of technical articles provides a flavour of the range in size scales at which fracturing occurs in a wide diversity of materials-from 'fracking' oil extraction and earth moving to laboratory testing of rock material and extending to the cracking of tooth enamel. Of important scientific interest are observations made and analysed at the smallest dimensions corresponding to the mechanisms by which fracture is either enhanced or hindered by permanent deformation or other processes. Such events are irrevocably linked to the atomic structure in all engineering materials, a sampling of which is presented, including results for crystalline and amorphous materials. Hooray for the broad subject description that is hoped to be appealing to the interested reader.
© 2015 The Author(s) Published by the Royal Society. All rights reserved.
This paper presents a novel high‐resolution direct analysis of mating AA7050‐T7451 fracture surfaces at crack lengths from 0.1 to 1.3 mm ( K max of 4.51 to 16.39 MPa√m) for the purpose of analyzing the fatigue growth mechanisms. The various fractographic features observed at these crack lengths were then compared to the known accelerative effects of an applied loading sequence that contained underloads at regular intervals. A correlation was found between some of the observed features and the local fatigue crack growth rate. This study provides new insight into the prominent mechanisms that dictate crack growth rate and ultimately should assist with developing crack growth prediction methods that more accurately represent the physics of the fatigue crack growth mechanisms.
The experimental measurements of 2D micro/nanolayered dimples' dimensions (both geometry and pattern) on the published tensile fractographs of a series of austempered high-strength low-carbon bainitic steel are carried out and interpreted with the corresponding initial microstructural features and tensile mechanical responses as a function of austempering temperature.
Fracture is the ultimate step of any kind of continued deformation process undergone in a material. If this viewpoint is correct then it will be plausible to relate the geometric fracture surface appearances with the entire deformation path/history subjected to the material previously. The two-dimensional dimple geometry quantified from the open published tensile fractographs are investigated to predict the nature of variation in corresponding deformation and fracture properties of a Zircaloy-4 with systematic swaging and swaging + annealing treatments. The changes in microstructure and texture that led to non-identical two-dimensional ductile fracture characteristics/patterns at different initial feed rate during swaging and swaging + annealing treatments have been (quantitatively) assessed, compared and correlated with the respective mechanical responses of the material. Image texture analysis has also been carried out to understand the clear variation in fracture morphologies as a function of initial feed rate for both treatments.
The simulation of crack growth and estimation of fatigue life in engineering structures containing cracks and notches is very complex. The complexity can be simplified by making suitable assumptions based on the experimental observations. The present investigation is aimed to address these aspects. The effect of notch length (a1) and notch offset distance (H) on fatigue crack growth of a crack having length a0 and Paris crack growth parameters under three-point bending constant amplitude loading was investigated. Significant shielding effects on the crack growth rate have been observed for a1 ≤ 0.6a0 and H ≤ 6 mm. It was found that the shielding effects gradually reduced to 18.67 % for a1 > 0.6a0 and H > 6 mm. The effect of offset secondary machined notch was found negligible for a1 = a0 and H ≥ 10 mm. About 50% reduction in fatigue life as compared to a single crack of length a0 = 10 mm was seen for a1 ≤ 6 mm and H ≤ 8 mm. The shielding effect due to the presence of a notch was discussed on the basis of stress intensity factors and von Mises stresses computed by the finite element method.
As a help to steel design, microstructural and compositional effects on hydrogen embrittlement are reviewed for typical steel types. Control of a specific microstructural factor occasionally affects other factors, and deformation microstructures and cracking processes associated with observed degradation are remarked to characterize hydrogen effects.
Background
Metal surface roughness detection is an essential step of quality control in the metal processing industry. Due to the high manual involvement and poor efficiency of traditional roughness testing, rapid automated vision detection has received increasing attention in product quality control. Many methods have focused on extracting features related to roughness from images by means of mathematical statistics. However, these methods often rely on extensive experiments and complex calculations, while being sensitive to external environmental disturbances.
Method
In this paper, a convolution neural network-based approach for metal surface roughness evaluation has been proposed. The convolutional neural network was initialized using a transfer learning strategy, and the data augmentation technique was applied to the benchmark dataset for sample expansion.
Results
To evaluate this approach, samples of 4 types of roughness classes were prepared. The samples were divided into a training set, validation set, and test set in the ratio of 7:2:1. The accuracy of the neural network on the test set was found to be above 86%.
Conclusion
The effectiveness of the proposed approach and its superiority over manual detection have been demonstrated in the experiments.
Quantitative fractography has been hindered by the lack of tools capable of accurately characterizing fracture modes and crack initiation locations. An easily implementable, non-destructive image analysis-based tool utilizing fractal and topological techniques was developed to overcome these traditional shortcomings. With this tool, feature complexity, heterogeneity, and connectivity are quantified through the calculation of fractal dimensions and lacunarity and topology measurements, respectively. Validation of this tool was performed on complex fatigue fractures in additively manufactured Ti-6Al-4V, with fracture initiation sites at near surface, sub-surface, and internal defects and fatigue fracture modes being easily differentiated.
In this study, fracture surface topography parameters were measured to investigate the effects of multiaxial loading. In order to assess the metrological aspects of fracture for notched specimens made of high-strength steels processed by both conventional and additively manufacturing (AM) techniques, an optical surface profilometer was used. Three bending moment to torsion moment ratios (B/T) were studied, i.e. 2, 1 and 2/3. The geometries were solid round bars with lateral notches made of conventional steel and hollow round bars with transversal holes for AM specimens. The investigations indicate that arithmetical mean height, Sa, decreased and fractal dimension, Df, increased with higher B/T ratios and higher fatigue lives.KeywordsMultiaxial FatigueBending-TorsionFatigue FractureSurface Topography
This study investigated the hydrogen-related fatigue fracture in 2Mn-0.1C steel having a lath martensite microstructure. The presence of hydrogen significantly reduced the fatigue life. The transgranular surface was a main component in each stress range of the uncharged specimen, while the intergranular surface was frequently observed in the hydrogen-charged specimen. The crystallographic orientation analysis by electron backscattering diffraction revealed that the cracks mainly propagated along {011} planes regardless of the presence of hydrogen. Compared with the uncharged specimen, however, plastic deformation was localized near the fatigue crack in the hydrogen-charged specimen. According to the reconstructed fracture process by fracture surface topography analysis, the hydrogen-related fatigue fracture was discontinuous and composed of isolated nucleation of intergranular cracks and quasi-cleavage crack propagation initiated at the pre-existing intergranular cracks.
The paper describes the analysis of crack surface morphology of S355J2 steel specimens after bending-torsion fatigue. These experimental investigations of the surface topography were carried out using the focus variation microscope, an optical 3D measurement device. Selected results of measured fracture surfaces for S355J2 steel were analysed according to the surface texture ISO 25178 standard. Differences in roughness values for different loadings were demonstrated. For profile Rx and areal Sx parameters, characteristic relationships of fracture zones have been demonstrated. It has been shown that roughness profile Ra for the rupture area is higher than for the propagation area, as well as increases after both the LCF and HCF tests and increases with the torsional loading level. However, Sa in the propagation area increases and the rupture area decreases.
Hydrogen-related Crack initiation and propagation in tempered Martensitic steel were investigated by Fracture surface topography analysis (FRASTA) and crystallographic orientation analysis. Hydrogen-related Fracture morphologies of tempered Martensitic steel were characterized by intergranular and quasi-cleavage transgranular fractures. Fracture surface topography analysis (FRASTA) results suggested that hydrogen-related Crack initiation sites were inclusions and cracks propagated from Quasi-cleavage fracture to Intergranular fracture near the Crack initiation site. Crystallographic orientation analysis results suggested that Intergranular fracture propagated on prior austenite grain boundaries, whereas Quasi-cleavage fracture propagated along {011} planes near the Crack initiation site. However, Quasi-cleavage fracture consisted of not only {011} planes but also various other planes. In a previous study, hydrogen-related Fracture morphologies of tempered Martensitic steel tended to change from quasi-cleavage to intergranular with an increase in strength or an increase in hydrogen content, and quasi-cleavage fractures propagated along {011} planes. However, the results of the present study indicate that the Fracture propagation path changed from Quasi-cleavage fracture along {011} planes and other various planes within the prior austenite grains to Intergranular fracture on the prior austenite grain boundaries caused by the influence of an Inclusion.
The variation in mechanical properties of Ni-alloys 945X (UNS N09946) and 718 (UNS N07718) during slow strain rate tests (SSRTs) under two different test protocols, both involving hydrogen (H) charging, was examined through fracture surface analyses. The test protocols used were: H pre-charging of the specimen prior to in-situ H charging during SSRT (P1) and in-situ H charging during SSRT without pre-charging (P2). The ductility (elongation to failure) of Ni-alloy 945X was observed to be inferior to the ductility of Ni-alloy 718 under given SSRT conditions. FRActure Surface Topography Analysis (FRASTA) was applied to evaluate the fracture events during SSRT from crack initiation to final fracture. The fracture events were then correlated with the stress-elongation response to define the stress at which crack initiation occurs. Crack initiation occurs below the material's yield stress under P1 and above its yield stress under P2 for alloy 945X. On the other hand, crack initiation stress for alloy 718 was found to be higher than the material's yield stress under both test protocols. The local fracture toughness was estimated by measuring the Crack Tip Opening Displacment (CTOD) obtained from line profile analysis, based on fracture surface topography data, over the H induced fracture surface area. The average local fracture toughness of alloys 945X and 718 under H decreases significantly when compared with their in-air fracture toughness values and the reduction is higher for alloy 945X than that for alloy 718. Under a H environment, the inferior ductility of Ni-alloy 945X, compared to that of Ni-alloy 718, may be due to significant reduction in the fracture toughness of alloy 945X. The operative mechanism of H assisted cracking in these Ni-alloys is explained based on the effect of microstructural condition on H enhanced - slip localisation and decohesion.
The hydrogen-related fracture propagation process in martensitic steel was investigated through crystallographic orientation and fracture surface topography analyses. The hydrogen-related fracture surface consisted of three typical surfaces, namely smooth surfaces, surfaces with serrated markings, and surfaces with dimples. Crystallographic orientation analysis suggested that the smooth surface was generated by intergranular fracture at prior austenite grain boundaries, and the surface with serrated markings originated from quasi-cleavage fracture propagated along planes. According to the reconstructed fracture propagation process by fracture surface topography analysis, the intergranular fracture at prior austenite grain boundaries initiated and propagated suddenly at the early stages of fracture. The quasi-cleavage fracture along planes then gradually propagated within the prior austenite grains. At the final stages of fracture, ductile fracture accompanied by dimples occurred around the edge of the specimen. The results clearly indicated that the fracture propagation path changed with the proceeding fracture from the prior austenite grain boundaries to along planes within the prior austenite grains.
Fracture surface topography analysis (FRASTA) technique reconstructs fracture process in microscopic details by matching conjugate fracture surface topographs. It produces three different results: fractured area projection plot (FAPP), cross-sectional plot (XSP), and fracture progression curve (FPC). FAPP shows graphically where the crack initiation site(s) is and how it grew. By superimposing a series of FAPPs over a scanning electron microscope image of fracture surface, we can establish relationships between fracture process and microstructures. XSP is used to characterize the crack tip opening displacement or crack opening angle. Use of these two results is seen in published papers where the crack initiation mechanisms could not be determined by regular fractographic examination. However, these two results do not indicate the kinematics of crack growth. FPC shows the relationship between fractured area and the spacing between conjugate surface topographs. This relatively unexplored relationship shows the crack growth history from the onset of microcrack initiation to the rupture of the component, and by analyzing this relationship such information as crack initiation time, crack growth rate, changes in loading conditions or environment can be learned. This paper presents some examples of use of FPC in failure analysis.
FRASTA (fracture surface topography analysis), a newly developed fractographic technique that shows how a crack front interacts with microstructural features, is described and applied to a continuous fiber-reinforcd intermetallic matrix composite. The fracture surfaces of a tensile specimen were examined and their topographies quantified. The fracture process was reconstructed in microscopic detail by comparing the conjugate topographic features at various relative displacements using FRASTA. The results show the sequence of microfailure events occurring in the matrix, in the fiber, at the fiber/matrix interface, and in the fiber core, and hence they provide information useful for developing composite materials that promise to be more resistant to failure.
An understanding of the early stages of corrosion fatigue could lead to procedures and inhibitors for extending the economic life of commercial and military aircraft. Unresolved issues include the mechanism by which a fatigue crack emerges from a corrosion pit, the time at which the pit-to-crack transition occurs, and the growth rate of the microcrack to a length where it can be treated by fracture mechanics principles. Insight into these issues was sought through application of an advanced fractographic technique, FRASTA. The inelastic deformation on the fracture surfaces of aircraft skin produced under corrosion fatigue conditions was quantified and analyzed to reconstruct the sequential microfailure processes. This paper describes the technique and presents the results of its application.
Fracture surfaces contain detailed information about the microprocesses and history of cracks, and the FRASTA technique allows the extraction of this information by reconstructing the fracture event in microscopic detail. A unique advantage of the technique is that it produces a graphic depiction of events occurring inside a material, showing where and how microfractures occur and how they grow, as well as the location and rate of movement of macrocrack fronts. Thus, the sequence of microfailure events and the history of evolving macrofracture events can be observed. (from paper)
SRI International has developed a new fractographic-analysis technique, FRASTA, or fracture surface topography analysis, that overcomes the limitations of conventional techniques by quantifying and analyzing the topographies of conjugate fracture surfaces simultaneously to reconstruct a fracture event (via computer) in microscopic detail. The method involves the use of a FRASTAscope, which combines a confocal-optics-based scanning laser microscope and computer software to match three-dimensional features of conjugate fracture surfaces and reconstruct detailed fracture processes that occurred inside the material. FRASTA often can provide fracture toughness, crack nucleation times, and growth rates in the absence of instrumentation; i.e., without the need for a fracture-toughness test or instrumented impact test.
Studies of the fracture behaviors of strain-rule sensitive materials suggest sharply notched impact-type toughness evaluations are fast enough for adiabatic conditions at the crack border and are largely controlled by a minimum level of the crack toughness. With the possible exception of very high strength steels, when a fatigue-cracked steel specimen is loaded in a time longer than 10−3 seconds, the crack border is stationary prior to instability and the KIc value is well above the minimum level. Using KIc data and estimates of the isothermal-adiabatic boundary, fracture behaviors of strain-rate sensitive metals can be outlined over a wide range of crack speeds and stress rates.
This exploratory effort demonstrates the feasibility of determining crack initiation times and growth rates by posttest analysis of the fracture surfaces of stress corrosion cracking (SCC) test specimens. The fracture surface topography analysis (FRASTA) technique was applied to constant extension rate test (CERT) specimens of type 304 (UNSS30400) stainless steel fractured in several aqueous environments. Initiation times and propagation rates were determined for intergranular stress corrosion cracks in specimens tested at 288°C in clean oxygenated water containing a small amount of H2SO4. Cracking appeared to begin predominantly at the specimen's surface in clean water, but in the specimen's interior in the H2SO4-contaminated water.
One hundred and three cases with sellar-parasellar tumors were treated using microsurgery and gamma knife radiosurgery, and followed up for more than 5 years. The sellar-parasellar area is the most critical eloquent region not only in microsurgery but also in radiosurgery. The tumor volume and the distance from the tumor to the optic nerve are the most important dose-planning factors for these tumors, followed by the radiosensitivity of the tumor. Although the distance to critical structures is short and the marginal dose is limited in cases with suprasellar germinoma, optic glioma or craniopharyngioma, excellent results can be obtained due to the relatively high radiosensitivity of these tumors. When the tumor volume is large and radiosensitivity is low in cases with non-functioning pituitary adenoma or meningioma, surgical debulking of the tumor and decompression of and detachment from the optic pathways (3D strategy) are necessary prior to radiosurgery. As to functioning adenomas, when the tumor is small and the distance is large, a higher radiation dose can be given and favorable results obtained. There is no deterioration of neuroendocrinological functions despite the low radiosensitivity. Copyright (C) 2004 S. Karger AG, Basel.
The possibility of deducing load spectrum parameters from fatigue failure surfaces is explored by applying innovative, three-dimensional topographic characterization and analysis techniques to failure surfaces in aluminum sheet. Precise, high-resolution elevation maps of fracture surfaces were obtained using confocal optics scanning laser microscopy. Elevation power spectral density curves resulting from a fast Fourier transform of the elevation data appear sensitive to stress intensity range and environment. A conjugate fracture surface matching procedure, FRASTA, can detect and may provide a way to quantify overloads.
A new fractographic technique, FRActure Surface Topography Analysis (FRASTA), was applied to a crack in a boiler tube of a fossil-fired power plant in an attempt to determine when in its 22-year service history the crack initiated and the rate at which it grew. By comparing the topographies of the conjugate crack surfaces, we estimated that the crack nucleated about 68 000 h after onset of service and experienced several periods of accelerated and decelerated growth with rates ranging from 5 × 10 to 9.5 × 10 mm/h. These findings correlated roughly with chemical cleanings and startup/shutdown cycles. A similar analysis of a corrosion fatigue specimen tested under controlled conditions in the laboratory provided results consistent with measured values.
These results suggest that fractography can be used to extract valuable information on cracking kinetics in aged systems and thereby, lead to new ways to predict, monitor, and extend component life.
Advanced diagnostic instruments and analyses applied to failure surfaces and cross sections of bulk metallic glasses (BMGs) can provide insight into the deformation and failure of these materials and assist in prototyping new materials with improved failure resistance. Confocal- optics scanning laser microscopic analysis of conjugate fracture surface topographs suggests that the formation and stretching of ligaments are likely keys to the high impact toughness of Vitreloy.
The mechanisms of toughening in an intermetallic matrix composite were determined by FRASTA, a technique that reconstructs
microfailure details of a fracture event by comparing the topographies of conjugate fracture surfaces. Graphic portrayals
of the interaction of a crack front with microstructure features are presented for a forged gamma titanium aluminide reinforced
with 20 vol pct, 300-µm-diameter, 30-µm-thick TiCb platelets. The threefold toughness increase relative to unreinforced material
is attributed to (1) drag on the crack front by the TiCb platelets, (2) bridging of the crack faces by the TiCb platelets,
and (3) deflection of the crack front by microcracks in the process zone that nucleate and grow out-of-plane at TiCb platelets.
The fractographic results explain discrepancies between measured and predicted toughnesses, provide the mechanistic understanding
and microfailure data needed to develop more reliable models, and suggest microstructural modifications for developing more
fracture-resistant materials.
The fracture surface topography analysis (FRASTA) technique under development at SRI was applied to seek an explanation for
severe thermal embrittlement observed in cast duplex stainless steel. By comparing topographic features of conjugate fracture
surfaces, FRASTA showed that fracture in thermally embrittled cast duplex stainless steel occurs by microcrack initiation
at delta-phase grain boundaries and at alpha-phase/gamma-phase interfaces, and by microcrack growth along these boundaries
and interfaces. The critical crack tip opening displacement (CTOD) as measured from cross-sectional views generated by the
FRASTA technique indicated a microcrack initiation toughness,J
Ic, of 287 KJ/m2, in excellent agreement with measurements using conventional fracture mechanics procedures, and significantly less than the
toughness of unaged material.
This paper presents a method for predicting fatigue failure using a virtual prototyping software tool that allows the simulation of real material behavior. Computer models are used to simulate the three dimensional microstructure in which fatigue evolves. Because grain structure properties are randomly distributed through any macro-sized structure, Monte Carlo simulation is used to give a probabilistic distribution of fatigue failure outcomes over the operating life of the structure. The reliability of structural elements with complex stress distributions is predicted by integrating the fatigue simulation model within traditional structural finite element models (FEM). The reliability analyses results for all the individual elements are combined using system reliability modeling techniques to determine the fatigue reliability of the entire component. This results in prediction of fatigue reliability as cycles to failure probability or the probability of exceeding a fatigue threshold. In making the fatigue reliability predictions, only the material micrographs, stress-strain curves and a single fatigue test data point (at a single stress) were provided. The results show excellent correlation between the predicted fatigue life and experimental fatigue life
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Dynamic crack behavior in a field of rising stress intensity and increasing toughness
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Correlation of fracture surface topography with fatigue load history
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Identifying acoustic emission sources in aging bridge steel
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Advanced materials & processes. American Society for Metals
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