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Fatigue strength prediction of 410NiMo stainless steel with surrogate weld discontinuities
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Fractures nucleated from defects and subjected to cyclic loading can experience propagation for a range of stress intensity factor ΔK well below the so‐called long crack threshold. This phenomenon is attributed to the development of crack closure mechanisms which may differ from those observed in laboratory tests conducted in accordance with current standards. Cracks originating from material defects require a specific degree of extension to develop the plastic wake, thus achieving a stabilized condition called the long crack threshold. However, in certain materials, this stabilization length can extend up to several millimeters, effectively encompassing a significant portion of the component's fatigue life. Therefore, understanding and quantifying the development of ΔKth with crack extension is important for implementing a reliable assessment procedure based on the fracture mechanics theory. Herein, R‐curve measurements are presented for five distinct structural metallic alloys widely used in various industrial applications. Additionally, the impact of load ratio is investigated, providing a comprehensive analysis of fatigue crack resistance in terms of the R‐curve concept. This study underscores the necessity of ruling new experimental techniques to measure and implement the long crack threshold, thereby ensuring the development of a reliable and robust framework for fatigue assessment.
Shrinkage porosities and non-metallic inclusions are common manufacturing process based defects that are present within cast materials. Conventional fatigue design recommendations, such as the FKM guideline (“Forschungskuratorium Maschinenbau”), therefore propose general safety factors for the fatigue assessment of cast structures. In fact, these factors mostly lead to oversized components and do not facilitate a lightweight design process. In this work, the effect of shrinkage porosities on the fatigue strength of defect-afflicted large-scale specimens manufactured from the cast steel G21Mn5 is studied by means of a notch stress intensity factor-based (NSIF-based) generalized Kitagawa diagram. Additionally, the mean stress sensitivity of the material is taken into account and establishes a load stress ratio enhanced diagram. Thereby, the fatigue assessment approach is performed by utilizing the defects sizes taken either from the fracture surface of the tested specimens or from non-destructive X-ray investigations. Additionally, a numerical algorithm invoking cellular automata, which enables the generation of artificial defects, is presented. Conclusively, a comparison to the results of the experimental investigations reveals a sound agreement to the generated spatial pore geometries. To sum up, the generalized Kitagawa diagram, as well as a concept utilizing artificially generated defects, is capable of assessing the local fatigue limit of cast steel G21Mn5 components and features the mapping of imperfection grades to their corresponding fatigue strength limit.
The welded joints of hydraulic turbine runners have a higher failure probability than other runner regions due to the inevitable presence of welding flaws and dynamic stress concentrations. Welding processes are always associated with flaws caused by technical or metallurgical factors. In order to lower the failure probability, critical welding flaws should be detected and removed before the runner commissioning; otherwise, they will propagate under dynamic stresses and endanger the structure before it reaches its expected life. In such a case, the electricity production process should be halted and a mandatory unplanned maintenance should be applied. The production halt and the maintenance process (or runner replacement) impose high costs to the utility owner. A remedy to this is to have prior knowledge of potential welding flaws so as to select proper nondestructive testing (NDT) methods to detect and characterize them. In addition, better characterization of the potential flaws and their dimensions leads to more precise life estimations. In this study, different NDT methods are used on a series of T-joints designed to be representative in scale of real turbine runner joints. The outcomes of these methods are compared and used to characterize some of the detected indications. In order to confirm the NDT results, part of the joint containing a targeted indication has been extracted from the T-joints to undergo detailed evaluations. The objective is to provide detailed information on some common welding flaws in hydraulic turbine runners and to discuss the capability of different NDT methods used to characterize such flaws.
The failure of hydraulic turbine runners is a rare event. So in order to assess the reliability of these components one cannot rely solely on the number of observed failures in a given population. However, as there is a limited number of degradation mechanisms involved, it is possible to use physically-based reliability models. Such models are often more complicated but are able to account for physical parameters in the degradation process. They can therefore help provide solutions to improve reliability. With such models, the effect of materials properties on runner reliability can be highlighted. This paper presents a brief review of the Kitagawa-Takahashi diagram which links the damage tolerance approach, based on fracture mechanics, to the stress or strain-life approaches. Using simplified response spectra based on runner stress measurements, we will show how fatigue reliability is sensitive to materials fatigue properties, namely fatigue crack propagation behaviour and fatigue limit obtained on S-N curves. Furthermore, we will review the influence of the main microstructural features observed in 13%Cr-4%Ni stainless steels commonly used for runner manufacturing. The goal is ultimately to identify the most influential microstructural features and to quantify their effect on fatigue reliability of runners.
The reliability assessment of large rotating structures like hydroelectric Francis runners is often limited by our capacity to define a proper limit state combined with a relevant degradation model. In this paper, we propose that the proper limit state for fatigue reliability of such structures is the onset of high cycle fatigue (HCF). Based on this premise, a prior interval for our limit state based on available literature is presented. The prior assumptions are believed to be the first step toward validation of the applicability and suitability of the proposed model. The paper includes an overview of the theoretical background for reliability assessment of Francis turbine runners, the methodology used, and the results obtained from the information gathered from the available literature. (C) 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of CETIM
In the recent past, Atzori and Lazzarin proposed a diagram able to include the geometrical effects on the fatigue limit of defects, short cracks, long cracks and U-shaped notches of whatever severity. The diagram shows that defect sensitivity and notch sensitivity can be considered as two sides of the same medal, thus creating a bridging between Linear Elastic Fracture Mechanics and Notch Mechanics. The fundamental material parameter, dependent on the load ratio, necessary to define the diagram is the El Haddad-Smith-Topper length parameter a0 defined by the plain material fatigue limit and the threshold value of the mode I stress intensity factor for long cracks. It is worth noting that a0 does not contain any geometry-related parameter. The diagram has been validated in the past against a number of experimental results and has been subsequently extended to sharp and blunt V-shaped notches, dependent on their absolute dimensions. After summarising the theory supporting the approach, some application examples relevant to both defect and notch sensitivity will be given
The chemical composition and solubility of metals in welding fumes is thought to be related to the welder health but is not well characterized. We quantified and compared the total (insoluble + soluble) and soluble metal contents in fumes from flux-cored arc welding using non-stainless steel (FCAW/NSS) and stainless steel (FCAW/SS) wire. Welding was performed in an American Welding Society standard fume collection chamber. The total content of 13 analyzed metals was significantly higher in FCAW/NSS fumes than in FCAW/SS fumes (51.6 ± 5.7 vs. 40.2 ± 5.2%, p < 0.001). Soluble metal content was significantly higher in FCAW/SS fumes than in FCAW/NSS fumes (15.5 ± 5.8 vs. 6.49 ± 2.4%, p < 0.001) due to the presence of potassium and sodium. Different proportions of each element were observed between welding types. Iron, magnesium, and aluminum were significantly higher in FCAW/NSS fumes, whereas chromium, nickel, and potassium were more common in FCAW/SS fumes. The metal composition of FCAW fumes was more similar to that of shielded metal arc welding fumes than that of gas metal arc welding fumes. It seems unnecessary to measure soluble iron, nickel, aluminum, and barium to compare with their soluble ACGIH-TLVs at the FCAW welding process. But chromium should be specified in terms of its valance and solubility.
The method explained in this paper for quantitative evaluation of fatigue limit for materials containing defects is based on the experimental evidences that inhomogeneities and micro-notches can be treated like cracks. First, the basic concept of the area parameter model is explained introducing the various data obtained by the first author's group for over last 15 years. Evidences are shown that small cracks, defects and nonmetallic inclusions having the same value of the square root of projection area, area, have the identical influence on the fatigue limit regardless of different stress concentration factors. Various applications of these concepts to various defect types and microstructural inhomogeneities are shown. Since the estimation of fatigue strength is related to the estimation of the size of maximum defects occurring in a piece, the methods for searching the defects and the quality control of materials with respect to inclusion or defect rating as well as their statistical implications are discussed.
An explanation for non propagating fatigue cracks is presented based on the criterion that once the value of a particular strain intensity factor reduces to the threshold value for the material the crack should stop. Predicted lengths of these cracks based on solutions for the intensity factor are in good agreement with the experimental data. Intensity factor trends for cracks in notches are shown to vary from an initial decrease to a minimum value followed by an increase and eventual convergence with the trend for the equivalent long crack for sharp notches to the blunt notch curves that continuously increased during their approach to the long crack trend. The type of trend exhibited by a given notch depends both on notch geometry and notch size. In blunt notches the maximum value of the threshold stress for crack propagation is at initiation. However, for sharp notches the peak value of the threshold stress vs crack length curves shifts to a finite length. Stresses above the initiation level but below this peak stress level result in fatigue cracks which start but do not propagate to failure. Predicted values of the fatigue limit stresses for a variety of sizes in a circular and an elliptical notch are in good agreement with experimental results.
Scientific digital imaging in three dimensions such as when using X-ray computed tomography offers a variety of ways to obtain, filter, and quantify data that can produce vastly different results. These opportunities, performed during image acquisition or during the data processing, can include filtering, cropping, and setting thresholds. Quantifying features in these images can be greatly affected by how the above operations are performed. For example, during binarization, setting the threshold too low or too high can change the number of objects as well as their measured diameter. Here, two facets of three-dimensional quantification are explored. The first will focus on investigating the question of how many voxels are needed within an object to have accurate geometric statistics that are due to the properties of the object and not an artifact of too few voxels. These statistics include but are not limited to percent of total volume, volume of the individual object, Feret shape, and surface area. Using simple cylinders as a starting point, various techniques for smoothing, filtering, and other processing steps can be investigated to aid in determining if they are appropriate for a specific desired statistic for a real dataset. The second area of investigation is the influence of post-processing, particularly segmentation, on measuring the damage statistics in high purity Cu. The most important parts of the pathways of processing are highlighted.
Characterization of Shrinkage Porosity and Fatigue Properties of Cast CA-6NM Low Carbon Martensitic Stainless Steels ABSTRACT Cast CA-6NM low carbon martensitic stainless steel is used for hydraulic turbine runner manufacturing since the sixties of the last century owing to its high strength, toughness, cavitation-erosion resistance and corrosion to withstand high cycle loads in long lifetime operation. The ideal microstructure consists of a martensitic matrix with some dispersed austenite after appropriate tempering which gives these steels their unique properties. However, these castings contain several types of defects which may have detrimental effects on the performance of the produced parts. Several aspects of the casting defect characterization in these steels have not been extensively studied in particular. The main objectives of this investigation include the characterization of the casting defects described in the following three sections. In the first article, the relation between radiography mapping and actual macro-defect morphologies in several severity levels was studied destructively using the “Salami” cross-sectioning to document the real shape and morphology of defects. Results of these experiments represented defects as macro-shrinkage porosities with extremely sharp endings. The distribution of porosity size was measured via Murakami’s geometrical parameter (√area) and Feret’s diameter. Higher severity levels of macro-shrinkage porosities demonstrated defects with sharper endings and larger Murakami’s parameter. Metallographic characterization of cast microstructure was also performed to study both chemical composition and microstructure around macro-defects. In the second part, uniaxial tension-tension low cycle fatigue testing (LCF) was conducted on the cast CA-6NM fatigue samples. The impact of defects position and sizing parameter (i.e., Murakami’s geometrical parameter (√area) on the fatigue life was studied. To this end, a two-dimensional (2D) SEM fractographic examination was performed on the fracture surfaces. The fatigue life generally diminished when the defect size increased. However, the position of the defect was also of critical importance. Moreover, the evolution of the shrinkage porosities during fatigue crack propagation and variation of geometrical factor Y(a) (considering linear elastic fracture mechanics (LEFM)) were investigated. In the third part, non-destructive three-dimensional (3D) X-ray tomographic evaluation was performed before (i.e., pristine sample) and after (i.e., broken sample) fatigue failure. The results were compared with the two-dimensional (2D) fractographic observations. The 3D analysis provided the distribution of shrinkage porosities precisely and thoroughly. The defects possessed a tortuous and intricate morphology. Results after fatigue testing displayed that the position of shrinkage porosity was more effective than the size in initiating the fatigue crack ......
Keywords: Hydraulic-turbine, Macro-shrinkage porosities, Cast martensitic stainless steel, Characterization, “Salami” cross-sectioning, Fractography, X-ray computed tomography, Fatigue.
This paper presents experimental data from three-point bending fatigue tests of load-carrying cruciform joints with partial penetration fillet welds that are representative of hydraulic turbine runners. The cruciform joints were made by multi-pass FCAW of martensitic stainless steel plates (AISI 415), followed by weld toe grinding and tempering. The S–N approach is used to compare the fatigue performances for three different weld penetration depths covering both weld toe and weld root failures. In case of weld root failure, the variable fatigue lives at the weld roots in a S–N plot are consolidated by introducing an equivalent notch stress intensity factor under mixed-mode. The results demonstrate that the origin of fatigue failure mode transition is size dependent, and it also depends on the nominal stress range and weld toe surface finish. E316L austenitic weld metal yields lower fatigue performance than E410NiMo martensitic weld metal but does not require preheating.
This paper investigates the accuracy and reliability of nominal stresses, hot-spot stresses, effective notch stresses, notch-stress intensity factors (N-SIFs) and material length scale parameters in estimating fatigue lifetime of aluminium welded joints. This comparative assessment was based on a large number of experimental data taken from the literature and generated by testing, under either cyclic axial loading or cyclic bending, a variety of aluminium welded structural details. Whenever it was required, stress analyses were performed by solving bi-dimensional linear-elastic finite element models. The obtained results demonstrate that the effective notch stress method, the N-SIF approach, and the Theory of Critical Distances (TCD) provide a more accurate fatigue life estimation in comparison with the other methodologies. In this context, the TCD was seen to be easier to adopt, requiring less computational effort than the effective notch stress method and the N-SIF approach. Finally, based on the experimental results being re-analysed, a unifying value of 0.5 mm is proposed for the TCD critical distance, with this value allowing aluminium welded connections to be designed accurately irrespective of joint geometry’s complexity.
Multipass welding procedures are common methods for 13Cr4Ni steels' fabrication and repairs. Compared to a single-pass weld procedure, the weld
microstructure in a multipass weld is more heterogeneous due to the complex local thermal cycles imposed by adjacent weld passes. Furthermore, the final microstructure and mechanical properties of these steels are very sensitive to their thermal history which increases the microstructure heterogeneities. Thus, post-weld heat treatments are performed to reduce heterogeneities and produce a relatively homogenous weld. It has been found that the best option to improve mechanical properties of 13Cr4Ni steels is forming a “room temperature stable austenite” phase by heat treatments. This study focuses on the effects of these post-weld heat treatments on the austenite phase and carbide formations and the related evolutions of microhardness distribution. The study shows that nanometer-size carbides form at martensite lath interfaces and sub-block boundaries, and then at higher temperatures austenite lamellae appear at these locations. Results also show that the highest percentage of stable austenite achievable by a single-stage tempering was obtained at 610 °C. When the heat treatment temperature is lower than 610 °C, longer holding time produces softer steel while longer heat treatments at temperatures higher than 610 °C, produces harder steel. Still, double-stage heat treatments are more effective and produce the highest percentage of austenite and the lowest hardness of all heat treatments.
Based on an investigation performed using a set of five experimental FCAW electrodes, an improved version of the IIW basicity index formula is developed. This new methodology is described in two papers, titled Part 1 : Solidified Slag Composition of a FCAW Consumable as a Basicity Indicator and Part 2: Verification of the Flux/Slag Analysis Methodology for Weld Metal Oxygen Control To accomplish this purpose, the partition of the various elements contained in the formulation of one FCAW electrode is studied and modeled in Part 1. Correspondingly, the composition of the solidified slag is predicted for this particular electrode. To verify the model, the prediction of the slag chemical composition is compared with experimental measurements. Good accordance is found, which shows the model is applicable. Also, a new way of defining the basicity of a FCAW consumable based on the chemical composition of the slag is derived. In Part 2, comparison of this innovative methodology with the IIW formula is achieved, as well as with other means reported in the literature for expressing the flux/slag basicity. The newly defined basicity index is found to offer superior correlation with the weld metal oxygen content, demonstrating the validity of the assumptions made in the present investigation.
Nonmetallic inclusions in two weld metals were characterized with respect to variations in weld aluminum concentration. Two self-shielded flux cored arc welding (FCAW-S) electrodes were used to produce welds for optical, scanning and transmission electron microscopy. The inclusions in the weld with high-aluminum concentration were predominantly aluminum nitride. In contrast, the inclusions in welds with low-aluminum and high-titanium concentrations were mostly aluminum oxide and titanium carbonitrides. The measurements were compared with predictions from multiphase, multicomponent thermodynamic equilibrium calculations. The calculations agreed with the experimental measurements and predicted the formation of aluminum nitride in high-aluminum welds and also simultaneous formation of aluminum oxide and titanium carbonitrides. However, the predicted volume fractions were lower than experimental values.
A rapid test method is described for generating data points for a Haigh diagram for Ti-6Al-4V at a constant life of 107 cycles at room temperature. It involves subjecting specimens to loading blocks of 107 fatigue cycles and progressively increasing the load until failure occurs. An equivalent stress is obtained for each test specimen for plotting on the Haigh diagram. The method is applied to tests conducted at stress ratios (ratio of minimum to maximum stress) from R=-1 to R=0.9. The validity of the method is confirmed by comparing data with those obtained using the conventional S-N interpolation approach at values of R=0.1, 0.5, and 0.8. The rapid testing technique is then extended to the generation of a Haigh diagram for the same material subjected to prior low cycle fatigue (LCF) for ten percent of its LCF life. No degradation of the fatigue limit is observed from subsequent HCF testing using the rapid testing technique.
The fatigue crack growth behavior of a CA6NM weld heat affected zone (HAZ) was investigated. Fatigue crack growth tests in river water environment were conducted on as-welded and post-weld heat treated specimens at load ratios R = 0.1 and R = 0.7. For a fully open crack, i.e. at R = 0.7, the HAZ fatigue behavior was similar to that of the base metal. When crack closure occurred, i.e. at R = 0.1, the HAZ showed a lower near threshold crack growth resistance. The post-weld heat treatment was beneficial at R = 0.1 by relieving tensile residual stresses, while its effect was negligible at R = 0.7. The crack trajectory was influenced by the weld’s yield strength mismatch which promoted deviation towards the soft material (base metal). The main conclusion of this study is that the HAZ does not represent a weak link when its high load ratio fatigue behavior is considered. This confirms the validity of currently used fatigue assessment methods of hydraulic turbine runners.
ABSTRACT Intentional weld defect or flaw specimens can be required for training purposes, developing new non-destructive testing techniques, qualifying non-destructive testing procedures, obtaining mechanical property data and in support of safety cases. The single most important criterion in producing defects or imperfections is that they must accurately simulate flaws which can occur in welded components and structures. For this reason, in certain applications, saw cuts or machined slots which are more easily detected may not be considered acceptable as planar imperfections/defects for the purpose of NDT training or validation. Therefore, TWI has developed techniques for producing realistic imperfections/defects and, in the case of cracks, the desired morphology, including roughness, angles of tilt and skew to the surface. TWI can reliably produce weld specimens with imperfections/defects such as: lack of root fusion, lack of penetration, lack of sidewall or interrun fusion, joint misalignment, porosity, solidification cracking, cluster cracking, heat affected zone (hydrogen) cracking, undercut, brittle fracture or fatigue cracks, under or overfill of weld metal, inclusions (slag or metallic). Some of these are essentially produced by using bad welding practice (lack of root fusion, porosity, solidification cracks), by welding with techniques such as TIG bridging to obtain lack of side wall fusion or by welding under crack promoting conditions.
As an analog to the bending case published in an earlier paper, the stress singularities in plates subjected to extension in their plane are discussed. Three sets of boundary conditions on the radial edges are investigated: free-free, clamped-clamped, and clamped-free. Providing the vertex angle is less than 180 degrees, it is found that unbounded stresses occur at the vertex only in the case of the mixed boundary condition with the strength of the singularity being somewhat stronger than for the similar bending case. For vertex angles between 180 and 360 degrees, all the cases considered may have stress singularities.
In amplification of some work of Southwell, it is shown that there are certain analogies between the characteristic equations governing the stresses in extension and bending, respectively, if ν, Poisson’s ratio, is replaced by −ν. Finally, the free-free extensional plate behaves locally at the origin exactly the same as a clamped-clamped plate in bending, independent of Poisson’s ratio.
In conclusion, it is noted that the free-free case analysis may be applied to stress concentrations in V-shaped notches.
This paper addresses the problem of high cycle fatigue at notches and the role of short crack propagation in the fatigue notch factor kf. Ahead of a V-notched feature, the stress field is characterized by two parameters, i.e. the stress concentration factor kt and the normalized notch stress intensity factor kn. Whether fatigue strength at a given life is controlled by crack initiation (kf = kt) or by short crack propagation (kf < kt) depends on kt, kn and the material resistances to crack initiation and to short crack propagation. The analysis accounts for the effects of notch acuity, notch size, material and fatigue life on the fatigue notch factor kf. It opens the door to a new method for predicting fatigue life using two S-N curves for a given material; one being measured from a smooth specimen, the other from a severe V-notch.
In this work the well known local approach to predict the fatigue strength of sharply notched components, based on the analytic expressions of the local stress field as proposed in literature, is applied to welded joints in aluminium alloys in a simplified form oriented to practical applications. A particular value of the general expression of the local stress field parameter is taken into account, which the fatigue strength depends on. Then a simple model is proposed by the authors in order to estimate such a parameter, based on the calculation of a geometric (or structural) contribution to the local stress field, depending on the overall joint geometry, and a local contribution evaluated by considering a specimen with lateral V-notches characterised by the same weld toe profile and a depth proportional to the weld throat thickness. Doing so, the estimation of the fatigue strength of a welded joint can be reduced to the estimation of the fatigue strength of the equivalent V-notch subjected to a remote stress equal to the structural stress (that can be regarded as a ‘hot spot’ stress). Finally a simple fatigue strength diagram, in the form recently proposed by Atzori and Lazzarin and calibrated on experimental fatigue test results, is proposed, so that one can estimate the fatigue strength of a welded joint, in terms of structural stress at a given number of cycles, as a function of the equivalent V-notch depth. By considering this diagram, the scale effect and the effectiveness of the methods to improve the fatigue strength by smoothing the weld toe radius are also taken into account.
A substantial fraction of the mysteries associated with crack extension might be eliminated if the description of fracture experiments could include some reasonable estimate of the stress conditions near the leading edge of a crack particularly at points of onset of rapid fracture and at points of fracture arrest. It is pointed out that for somewhat brittle tensile fractures in situations such that a generalized plane-stress or a plane-strain analysis is appropriate, the influence of the test configuration, loads, and crack length upon the stresses near an end of the crack may be expressed in terms of two parameters. One of these is an adjustable uniform stress parallel to the direction of a crack extension. It is shown that the other parameter, called the stress-intensity factor, is proportional to the square root of the force tending to cause crack extension. Both factors have a clear interpretation and field of usefulness in investigations of brittle-fracture mechanics.
The paper addresses the estimation of the fatigue limit of components weakened either by U- and V-shaped notches or by defects, all under mode I stress distributions. When the influence of the opening angle is absent, a single formula is able to summarise both the notch sensitivity and the sensitivity to defects. Fatigue limit assessments need two material parameters, namely the plain fatigue limit and the threshold value of the long crack stress intensity factor range. The formula is compared with about 90 fatigue limits taken from the literature. Material properties and specimen geometries are given in detail. Afterwards, in the case of V-notches with large opening angles, the formula is modified, but without involving additional material parameters. A generalised Kitagawa diagram is obtained, that encompasses fatigue behaviour of stress raisers of different size, opening angle and notch tip radius.
In the welded joints, the conventional welding procedures result in a small value of the weld toe and the weld root radius. By assuming such a radius equal to zero, the paper demonstrates the effectiveness of the Notch-stress intensity factor approach in summarising a number of experimental data from failures occurring at the weld toe. Then it is shown that fatigue data from failures originated from both weld roots and weld toes can be summarised in a single scatter band by using the mean value of the strain energy density in a well defined volume (area) surrounding the critical points. Finally, a simplified application of the NSIF approach based on finite element analyses carried out with coarse meshes is presented.