Although the role of fatigue failure in aseptic loosening of cemented total hip replacements has been extensively studied in femoral components, studies of fatigue failure in cement mantle of acetabular replacements have yet to be reported, despite that the long-term failure rate in the latter is about three times that of femoral components. Part of the reason may be that a complex pelvic bone structure does not land itself readily for a 2D representation as that of a femur.
In this work, a simple multilayer model has been developed to reproduce the stress distributions in the cement mantle of an acetabular replacement from a plane strain finite element pelvic bone model. The experimental multilayer model was subjected to cyclic loading up to peak hip contact force during normal walking. Radial fatigue cracks were observed in the vicinity of the maximum tangential and compressive stresses, as predicted by the FE models. Typical fatigue striations were also observed on the fracture surfaces post cyclic testing. The results were examined in the context of retrieval studies, 3D FE analysis and in vitro experimental results using full-sized hemi-pelvic bone models.
The long-term stability of cemented total hip replacements critically depends on the lasting integrity of the bond between the cement and the bone, often referred to as fixation. In vitro assessment of fatigue behaviour of cemented acetabular, as opposed to femoral, replacements is of particular interest due to the more aggressive nature of late "loosening" found in acetabular replacements, reported to be three times that in femoral cases. Quantitative assessment of fatigue behaviour of cement fixation on acetabular side has been difficult due to the complexity of the pelvic bone geometry and the associated loading conditions.The purpose of this work was to develop a framework for in vitro assessment of fatigue integrity of cement fixation in acetabular replacements. To this end, a newly developed hip simulator was utilised, where the direction and the magnitude of the hip contact force (Bergmann et al., 2001) under typical physiological loading conditions including normal walking and stair climbing were simulated for the first time. Preliminary hip simulator experimental results seem to be consistent with those from constant amplitude fatigue tests, in that debonding at the bone-cement interface is identified as the main failure mechanism, although the numbers of cycles to failure are significantly reduced in samples tested in the hip simulator. Finite element analysis of implanted bone samples was carried out, where the effects of loading mode on the stress distribution in the cement mantle and at the bone-cement interface were evaluated. The effects of model geometry on the stress state and failure modes were also examined and discussed based on the results of the present and previously published work.
The fatigue and fracture behavior of hard tissues are topics of considerable interest today. This special group of organic materials comprises the highly mineralized and load-bearing tissues of the human body, and includes bone, cementum, dentin and enamel. An understanding of their fatigue behavior and the influence of loading conditions and physiological factors (e.g. aging and disease) on the mechanisms of degradation are essential for achieving lifelong health. But there is much more to this topic than the immediate medical issues. There are many challenges to characterizing the fatigue behavior of hard tissues, much of which is attributed to size constraints and the complexity of their microstructure. The relative importance of the constituents on the type and distribution of defects, rate of coalescence, and their contributions to the initiation and growth of cracks, are formidable topics that have not reached maturity. Hard tissues also provide a medium for learning and a source of inspiration in the design of new microstructures for engineering materials. This article briefly reviews fatigue of hard tissues with shared emphasis on current understanding, the challenges and the unanswered questions.
Microdamage formation is a critical determinant of bone fracture and the nature and type of damage formed in bone depends on the interaction of its extracellular matrix (ECM) with the applied loading. More importantly, because bone is a hierarchical composite with multiple length scales linked to each other, the nature and type of damage in bone could also be hierarchical. In this review article, based on new unpublished data and a reanalysis of literature reports on in vivo and in vitro observations of microdamage, three length scales including mineralized collagen fibrils, lamellar and osteonal levels have been identified as the key contributors to microdamage hierarchy and energy dissipation in bone. Inherent hierarchy in bone's ECM therefore has specific microstructural features and energy dissipation mechanisms at different length scales that allow the bone to effectively resist the different components of the applied physiological loading. Furthermore, because human bones experience multiaxial cyclic loading and its ECM is subjected to variation with aging and disease, additional emphasis is placed on investigating how the nature of applied loading and the quality of ECM affect the hierarchy of microdamage formation with age.
In this paper, a new mechanical driving force parameter for long- and short-crack growth rate correlation is proposed. This new parameter, (ΔK+Kmax)0.5, does not utilize disputable crack closure data, instead it is calculated as a geometric mean of the positive part of the applied stress intensity factor (SIF) range, ΔK+, and the corresponding maximum value of the SIF, Kmax. The proposed parameter correlates fairly well the R-ratio effects on the threshold condition and fatigue crack growth rate at the low and intermediate stress intensities for six aluminum alloys investigated.
In this paper, low cycle fatigue studies on 99.3Sn–0.7Cu lead free solder was evaluated over a range of test temperatures (298, 348 and 398 K) and frequencies (10−3–1 Hz), tested at four values of total strain range (2, 3.5, 5 and 7.5%). The fatigue performance of lead free 99.3Sn–0.7Cu solder was compared to 63Sn–37Pb solder. The effects of temperature and frequency on the low cycle fatigue life were discussed and were presented in Coffin–Manson and Morrow model relationships. Frequency modified Coffin–Manson and Morrow models were proposed for predicting the effect of frequency on low cycle fatigue life of 99.3Sn–0.7Cu solder alloy.
High-temperature stress controlled tests for 1.25Cr0.5Mo steel were carried out at different loading conditions to investigate the fatigue–creep interaction behavior. Four fatigue–creep facture character maps have been established. It is found when the stress amplitude is less than the mean stress after a sort, drastic interactions between fatigue and creep will occur, the fracture life will decrease rapidly and the fracture ductility will reach its minimum. As the complex relationships between fracture life and its influencing factors can be well explained by the mean strain rate at half-life, which is considered as the main factor associated with the fracture life. Moreover, based on the ductility exhaustion theory and the effective stress concept, a new model for fatigue–creep life prediction under stress control is proposed. Most of the test data are predicted within a factor of ±×1.5, which is better than the frequency separation method (FS) and the strain energy frequency modified approach (SEFS).
Variable amplitude bending-torsion fatigue experiments were conducted on axle-shafts to determine the effects of overloads on the fatigue life of normalized SAE 1045 steel. Either periodic bending overloads or static bending loads were applied to these shafts to determine their effect on torsional fatigue. It was determined that these yield stress level bending excursions both decrease the torsional fatigue limit and shorten the torsional fatigue life at medium and long lifetimes. The magnitude of this effect is very similar to that of uniaxial overloads, and a multiaxial fatigue parameter was found which causes the uniaxial and multiaxial datasets to fall on a single parameter–life curve. It is postulated that the fatigue strength reduction is due to a reduction in crack face interaction which results in an increase in effective strain intensity.
A crack growth analysis based on a fracture mechanics approach was used to model the fatigue behavior of as-received 1045 steel specimens for three load spectra scaled to various maximum stress range levels. The crack growth analysis was based on an effective strain-based intensity factor, a crack growth rate curve obtained during closure-free loading cycles, and a local notch strain calculation based on Neuber’s rule.The crack-opening stresses were modeled assuming that the crack-opening stress when it is not at the constant amplitude steady-state level for a given stress cycle builds up as an exponential function of the difference between the current crack-opening stress and the steady-state crack-opening stress of the given cycle unless this cycle is below the intrinsic stress range for crack growth or the maximum stress in the cycle is below zero in which cases the crack-opening stress does not change.The crack-opening stress model was implemented in a fatigue notch model and the fatigue lives of the notched annealed 1045 steel specimens under the three different load spectra scaled to several maximum stress levels were estimated. The average measured crack-opening stresses for the various histories and levels were within between 8 and 13% of the average calculated crack-opening stresses. The fatigue life predictions based on the modeled crack-opening stresses were in good agreement with the experimentally obtained fatigue data.The averages of the measured crack-opening stresses and those calculated using the model were nearly the same for all the histories examined. When these average crack-opening stresses were used in the life prediction model they gave predictions as good as those obtained by modeling crack-opening stress on a cycle by cycle basis.The use of a crack-opening stress level corresponding to the cycle causing a reduction to a crack-opening stress reached for 1/200 of the cycles in the history gave a conservative estimate of average crack-opening stress for all the histories.
Several researchers have demonstrated that, under both uniaxial and in-phase biaxial loading, periodic overloads of yield stress level magnitude can be used to eliminate the effects of crack-face interference. In the current research a series of experiments using either axial or torsional overloads were conducted in order to determine their effects on the torsional fatigue of normalized SAE1045 steel tubes. It was determined that, in the high cycle regime, both types of overload had the same impact on fatigue life. The effect on the torsional high cycle fatigue of axial/torsional mean stress following an axial/torsional overload was found to be minimal for both the zero mean and peak strain cases examined. Lastly, the results of these experiments were compared with similar fatigue data for this material from both axle-shafts and in-phase overload tests. This larger data set was then used to evaluate various multiaxial parameters, and it was found that the Fatemi–Socie–Kurath parameter yielded the best data correlation.
The advantages of the Friction Stir Welding process compared to conventional fusion welding technologies have been clearly demonstrated in recent years. In this study, the metal matrix composite under investigation was a 7005 aluminium alloy reinforced with 10% of alumina particles Friction Stir Welded by employing a tool rotating speed of 600 rpm and a welding speed of 250 mm/min. The optical and scanning electron microscopy observations performed on the different zones of FSW joints cross-section revealed the different structures of the nugget, the thermo-mechanical affected zone and the heat affected zones thanks to the difference due to the strong grain refinement produced by the dynamic recrystallization acting during the severe plastic deformation to which the material is subjected during welding. Only a few data are available in the literature on the fatigue behavior of these materials. Thermoelastic stress analysis (TSA) was applied to the study of the damage behavior in Friction Stir Welded MMC sheets, during fatigue tests. Fatigue tests were carried out under pulsating (R = σmin/σmax = 0–0.1), tension loading using a resonant electro-mechanical testing machine (TESTRONICTM 50 ± 25 KN by RUMUL (SUI)). All the mechanical tests were performed up to failure which occurred at the interface with the welded area. The TSA measurement system allowed crack evolution to be observed in real-time during fatigue cycles and stress fields to be derived on the specimens from the temperature variation measured.
Low-cycle fatigue behavior of the HAYNES HR-120 alloy in the temperature range from 24°C to 982°C and total strain range from 0.4% to 2.3% was investigated under an axial total strain control mode in laboratory air. It was noted that increasing temperature generally led to a substantial decrease in the fatigue life of the alloy. It was found that the alloy could exhibit cyclic strain hardening or softening, which was closely related to the imposed total strain range and testing temperature. The microstructural evolution due to low-cycle fatigue deformation was characterized using optical, scanning-electron, and transmission-electron microscopy. It was observed that the precipitation of second-phase particles would occur at or above 761°C. The presence of these precipitates could be taken into account the greater the cyclic-hardening behavior and the reduction of fatigue life, with increasing temperature. In addition, strain fatigue parameters were determined at different temperatures, based on Coffin–Manson and Holloman equations.
The effect of frequency on fatigue behavior of an oxide–oxide continuous fiber ceramic composite (CFCC) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite–alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. Tension–tension fatigue tests were performed at frequencies of 0.1 and 10 Hz for fatigue stresses ranging from 75 to 170 MPa. Fatigue run-out was defined as 105 cycles at the frequency of 0.1 Hz and as 106 cycles at the frequency of 10 Hz. The CFCC exhibited excellent fatigue resistance in laboratory air. The fatigue limit was 170 MPa (88% UTS at 1200 °C). The material retained 100% of its tensile strength. Presence of steam significantly degraded the fatigue performance, with the degradation being most pronounced at 0.1 Hz. Composite microstructure, as well as damage and failure mechanisms were investigated. Examination of fracture surfaces revealed higher degrees of fiber pull-out in specimens tested at 10 Hz, indicating weakening of the fiber/matrix interface. A qualitative spectral analysis showed evidence of silicon species migration from the fiber to the matrix.
Among the recently developed titanium-based alloys for medical applications, Ti–13Nb–13Zr is distinguished by its biocompatibility, corrosion resistance and low Young’s modulus in the non-aged condition. However, fatigue data referring to this alloy are not available. In order to conduct this study, Ti–13Nb–13Zr was produced by arc melting followed by homogenization in vacuum, rotary swaging and solution heat treatment, resulting in bars with martensitic α′ microstructure. The low cycle fatigue properties of the material were obtained by means of total strain controlled tests. The fatigue behavior of the alloy in the range of 104–106 cycles was assessed through load controlled tests of smooth and notched samples, conducted both in laboratory air and in 0.9% NaCl solution.
The fatigue life and fatigue crack growth rate of AlLi 1460-T8 alloy under cyclic tension (asymmetry coefficient R = 0.1) are studied on flat samples at T = 293 K (air), 77 K (liquid nitrogen), 20 K (liquid hydrogen and hydrogen vapour), and 4 K (liquid helium). The fractured samples are examined macrofractographically and electron-fractographically. It is found that at temperature lowering from 293 K to 4 K the fatigue life increases and the fatigue crack growth slows down. The monotonicity of these temperature dependence is disturbed at 20 K in liquid hydrogen: in this medium the fatigue life either decreases or coincides with that in liquid nitrogen (17 K) depending on the stress amplitude; a similar decrease under these conditions is also observed for the crack growth rate. Proceeding from macro- and microfractographic data, the correlation is found between fatigue life, crack growth rate and fracture mechanisms in different cryogenic media. Possible mechanisms of temperature and liquid hydrogen medium effects upon the fatigue characteristics of the 1460 alloy are considered.
The recent development of multi-step aging treatments and thermo-mechanical processes to improve fracture toughness of optimum strength Ti-15V-3Al-3Sn-3Cr has increased the range of aerospace applications for this cold-formable alloy. This investigation describes the impact on fatigue performance of the microstructural changes induced by these new processes. Beta solution treated material exhibited a transition in fatigue crack growth rates, dramatically slowing when a critical plasticity dimension produced by the applied stress intensity was exceeded. In material exhibiting transgranular fracture, crack propagation was not significantly altered by changes in alpha morphology (including precipitate size, aspect ratio and distribution) resulting from multi-step heat treatment. Changes in grain and precipitate orientation due to warm or cold rolling prior to aging increased fatigue propagation rate when tested in L-T and T-L orientations.
Load history effects on room temperature fatigue-crack-growth threshold measurement are evaluated for titanium alloys Ti–6Al–4V and Ti-17. Baseline thresholds are determined by the use of a conventional load-shedding technique. Load history is synthesized by precracking at ΔK levels with Kmax greater than the subsequently measured threshold. Threshold for the latter is defined as the first notice of crack propagation under increasing ΔK, constant R, step loading.An empirical overload model is developed to account for precracking history on the threshold. Stress relief annealing after precracking and prior to threshold measurement is demonstrated to eliminate load history effects on rising ΔK, constant R, step loading threshold measurement, and to be comparable to that made using load shedding, but with considerable time saving.
Fatigue crack initiation life is predicted for notched bend specimens with two different notch acuities and two stress ratios using the equivalent strain energy density method. The material used was 17Mn4 steel, widely applied in pressure vessel construction. The monotonic and cyclic mechanical properties and the strain-life constants were experimental determined in this work. A comparison was made between predicted and experimental lives obtained for two different optically measured crack lengths. Good agreement was obtained for stress ratios, R, of 0 and −1, and for two notch stress concentration factor values. The observed mean stress effect on crack initiation lives is well modelled in the prediction by the method used with calculation of both strain range and mean stress at the root of the notch.
The fatigue crack growth behaviour of a ferritic stainless steel has been investigated as a function of test temperature, thermal exposure and frequency at intermediate growth rates. In general, fatigue crack growth rates increased with increasing temperature and in the temperature range 500–700 °C growth rates were described by a kinetic process with an activation energy of 48 kJ/mole. Higher than normal growth rates at 475 °C were observed and attributed to an embrittlement process which is known to occur in this temperature regime in high-chromium ferritic stainless steels. The influence of frequency on fatigue crack growth rates was examined at 500 and 655 °C for a load ratio of 0.1 and over four decades of frequency. A transition from time-independent to time-dependent behaviour was observed at each temperature as frequency was lowered. The frequency at which this transition occurred was dependent on temperature. For all temperatures investigated, near threshold crack propagation occurred by a crystallographic or faceted propagation mechanism. At high crack growth rates, crack-tip plasticity was significant and propagation proceeded by a ductile striation formation process. At intermediate growth rates a mixed-mode fatigue crack growth mechanism was observed where some intergranular fracture occurred.
Over recent years Australia has been involved in a number of full-scale fatigue testing programs in support of the through-life structural integrity of the Royal Australian Air Force’s (RAAF) F/A-18 fleet. It was recognised early in the acquisition cycle that the certification testing conducted by the manufacturer failed to considered damage tolerance requirements and would be unlikely to cover the typically more severe and diverse RAAF operations. Given similar aircraft structural integrity management philosophies, major benefits were to be realised through collaboration with the Canadian Forces (CF). In particular, as fatigue testing under representative CF/RAAF loading was the basis for both countries’ structural integrity management, the International Follow-On Structural Test Project (IFOSTP) was successfully concluded.This paper emphasises the Australian components of IFOSTP, including the damage tolerance testing and demonstration for the aft fuselage that incorporated the simultaneous application of both manoeuvre and dynamic buffet loads. Many of the innovations and consequences of this work program are highlighted, and may be applicable to future fighter aircraft structural integrity programs.
Fully reversed axial fatigue tests have been performed using smooth specimens of 18Cr–2Mo ferritic stainless steel (type 444) at ambient temperature, 673 K and 773 K in laboratory air, in order to understand the effect of temperature on high cycle fatigue behaviour. Fatigue strength significantly decreased with increasing temperature. When characterized in terms of fatigue ratio, fatigue strength still decreased at elevated temperatures compared with at ambient temperature. At all temperatures studied, cracks were generated at the specimen surface due to cyclic slip deformation, but crack initiation occurred much earlier at elevated temperatures than at ambient temperature. Subsequent small crack growth was considerably faster at elevated temperatures even though difference in elastic modulus was taken into account, indicating the decrease in the intrinsic crack growth resistance. Fractographic analysis revealed some brittle features in fracture surface near the crack initiation site at elevated temperatures, which was more pronounced and extensive at 773 K, indicating that embrittlement has occurred at the temperature range studied. It was believed that such embrittlement was responsible for the observed decrease in both crack initiation resistance and small crack growth resistance.
A non-destructive, magnetic Barkhausen emission (MBE) technique has been used to assess various stages of low cycle fatigue (LCF) damage in 9Cr–1Mo ferritic steel. The initial decrease in the MBE peak height in the early stage of LCF cycling indicates the cyclic hardening stage, in which the formation of dislocation tangles reduces the mean free path of the domain wall movement. The increase in the MBE level again on further cycling indicates the progressive cyclic softening stage where the rearrangement of dislocation tangles into cells enhances the domain wall movement. The unaltered behaviour of MBE on continued cycling shows the saturation stage where the stabilization of dislocation substructure maintains the MBE level. Finally, a sharp increase in the MBE peak value identifies surface crack initiation and propagation, which is ascribed to the movement of additional reverse domains produced at the crack surfaces. This study establishes that the MBE technique can be used to assess the progressive degradation in the fatigue life of the ferritic steel components.
A model devoted to the prediction of the high temperature creep–fatigue lifetime of modified 9Cr–1Mo martensitic steels is proposed. This model is built on the basis of the physical mechanisms responsible for damage due to the interaction of creep, fatigue and oxidation. These mechanisms were identified thanks to detailed observations previously reported in part I and part II of this study. These observations led to the distinction of two main domains, corresponding to two distinct types of interaction between creep, fatigue and oxidation. As no intergranular creep damage can be observed in the tested loading range, the proposed modelling consists in the prediction of the number of cycles necessary for the initiation and the propagation of transgranular fatigue cracks. Propagation rate measurements under high stress low-cycle fatigue conditions were carried out to calibrate the Tomkins model used to predict the life spent in crack propagation, whereas the initiation stage is predicted using the model proposed by Tanaka and Mura. The predictions obtained compare very favorably with the experimental creep–fatigue lifetimes. Finally the extrapolations and limits of the model are discussed.
Cyclic tests with or without tensile holding periods were conducted in air at 823 K on a modified 9Cr–1Mo martensitic steel. In addition to stress–relaxation fatigue (RF) tests with a hold time at maximum load, creep–fatigue (CF) experiments were carried out. These CF tests were strain-controlled during the cyclic part of the stress–strain hysteresis loop and then load controlled when the stress was maintained at its maximum value to produce a prescribed value of the creep strain before cyclic deformation was returned under strain-controlled conditions. This unusual testing procedure enabled larger viscoplastic strains to be reached during the holding period than during usual relaxation–fatigue (RF) tests. The relationship between the number of cycles to failure of pure fatigue tests and the cyclic strain range is established for pure fatigue tests. The lifetime reduction due to holding periods is highlighted and quantified. The fatigue lifetime reduction due to holding periods is all the more pronounced as the cyclic strain amplitude is low. No creep cavitation is visible by microscopic observations, while the environment is found to play a key role in damage accumulation and interaction. Two main failure mechanisms are observed depending both on the fatigue strain range and on the duration of the holding period. An attempt is made to explain the existence of these two domains in relation with oxidation effect.
Cyclic tests with compressive holding periods were carried out in air at 823 K on a modified 9Cr–1Mo martensitic steel. In accordance with other results published in the literature, compressive holding periods were found more deleterious than tensile ones, i.e. fatigue lifetimes were more severely reduced under compressive holding periods than under tensile holds. For all tested samples a strong influence of oxidation was observed, and a tensile mean stress was measured due to compressive holding periods. To understand the difference between compressive and tensile holding periods, a detailed study of the oxide layers mechanical behavior was carried out. Based on an extensive literature survey, focussed on the fracture behavior of oxide layers, and on Finite Element (FE) calculations, a general scheme for modelling creep-fatigue-oxidation interactions is proposed and discussed.
In this paper the influence of discontinuous ceramic particulate reinforcements on cyclic stress response, cyclic stress versus strain response, cyclic strain resistance, deformation and fracture behavior of 2009 aluminum alloy discontinuously reinforced with silicon carbide particulates are presented and discussed. The cyclic strain amplitude–controlled fatigue properties and fracture characteristics of the 2009/SiC composite specimens are discussed for a range of cyclic strain amplitudes and at two different temperatures. The conjoint influence of test temperature and strain amplitude on cyclic stress response, cyclic stress versus strain response, and cyclic strain resistance is highlighted. The intrinsic mechanisms governing stress response, cyclic deformation and fatigue fracture characteristics are presented and discussed in light of the competing and mutually interactive influences of intrinsic composite microstructural effects, deformation characteristics of the composite constituents, cyclic strain amplitude and concomitant response stress, cyclic ductility, and test temperature.
A study has been made to understand the low-cycle fatigue properties and cyclic fracture characteristics of 2014 aluminium alloy discontinuously reinforced with varying amounts of Al2O3 particulates. The 2014/A12O3 composite specimens were cyclically deformed over a range of cyclic strain amplitudes, using tension-compression loading under total strain-amplitude control. The 2014/AI2O3 composites exhibited softening at all cyclic plastic strain amplitudes and for different volume fractions of the discontinuous particulate reinforcement in the ductile metal matrix. The softening effect was greater at the higher cyclic strain amplitudes, and increased with test temperature. The intrinsic micromechanisms controlling the stress response characteristics during fully reversed cyclic straining are highlighted and the rationale for the observed softening behaviour is discussed. The cyclic strain resistance and resultant low-cycle fatigue life of the composites improved with increase in test temperature. The improvement was noteworthy at low cyclic plastic strain amplitudes and resultant long fatigue life. The kinetics governing the cyclic fracture process are discussed in the light of composite microstructural effects, cyclic plastic strain amplitude, concomitant response stress and test temperature.
The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.
The work examines the microstructural and fatigue properties of friction stir welds made of 2024-T3 aluminium alloy and provides extensive information towards their cyclic stress–strain behaviour, residual stress distribution and crack initiation sites. To eliminate the cost associated with the removal of the flow arm by milling and other costs associated with the quality control of the welding process (residual stress distribution, micro-hardness profile, welding scar, etc.), controlled shot peening is introduced. Tensile residual stresses introduced in the thermomechanical affected zone during welding are found to become compressive after peening. The effect can be held responsible for increasing the fatigue resistance of the weld beyond the values of the bare (parent) material.
The role of longitudinal residual stress on propagation of fatigue cracks was examined in friction stir welds produced in 2024-T351 aluminum alloy. Fatigue crack growth rate was obtained through constant ΔKIapp tests for notches at different distances from the weld centerline. Subsequently, crack growth was correlated to weld residual stress measured by the cut-compliance method. It was found that residual stresses correspond to low crack growth rates outside the weld zone during fatigue loading. Once in the weld zone, the crack growth was affected by microstructural and hardness changes. Furthermore, weld residual stresses were mechanically relieved and effects on crack propagation behavior were observed. A comparative analysis between relieved and unrelieved joints indicated that fatigue crack growth behavior is dominated by residual stress outside the weld zone.
The concept of the Fatigue Damage Map (FDM) is implemented to quantify the effects of exfoliation corrosion damage on the fatigue behaviour of the 2024-T351 aluminium alloy. This is achieved by using extensive experimental data involving tensile, fracture toughness, fatigue as well as fatigue crack growth tests on pre-exfoliated 2024-T351 specimens. The analysis suggests that exfoliation exposure leads to an increase in accelerated crack growth stage (Stage III) at the expense of Stage II (long crack growth). It also suggests that the transition from Stage I crack growth (short crack) to Stage II for the same stress level occurs at smaller crack lengths for the exfoliated material. Furthermore, stress intensity range values for onset of crack growth rate acceleration (Stage III) obtained from the experimental results for the pre-corroded specimens are in accordance with the results of the FDM analysis. The outcome of the analysis, demonstrates that the a priori negligence of the corrosion effect in structural integrity analyses of aged aircraft components may lead to significant overestimation of the damage tolerance ability of the structure.
The effects of weld residual stress and heat affected zone on the fatigue propagation of cracks parallel and orthogonal to the weld direction in friction stir welded (FSW) 2024-T351 joints were investigated. Crack propagation behaviour was sensitive to both weld orientation and the distance of the crack from the weld line. Growth rates both faster and slower than in the parent material were observed, depending on the crack orientation and distance from the weld. Weld residual stress was mechanically relieved and the effects on crack propagation observed. A comparative analysis of the results indicated that crack growth behaviour in the FSW joints was generally dominated by the weld residual stress and that microstructure and hardness changes in FSWs had a minor influence.
Ultrasonic equipment is described which is used to perform cyclic torsion and cyclic tension–compression fatigue experiments with the aluminium alloy 2024-T351 in the high and very high cycle fatigue regime. Displacement amplitude of one specimen’s end and cyclic resonance frequency are measured and controlled in closed-loop electronic circuits, and the specimens are loaded in pulse-pause sequences to prevent heating. Theoretical considerations and the practical realisation of a load train to perform ultrasonic torsion fatigue tests are presented. Cyclic torsion and cyclic tension–compression endurance data of 2024-T351 are compared using Mises equivalent stresses. At same equivalent stresses, numbers of cycles necessary to create cracks with minimum length 300 μm in cyclic torsion were 2–10 times higher than lifetimes in tension–compression tests. Crack initiation and propagation in cyclic torsion is in the directions of maximum shear stresses whereas fracture surface at crack initiation location produced by cyclic tension–compression is perpendicular to the principal stress. One direction of maximum shear (specimen’s length direction) is preferred, and circumferential fatigue cracks develop and propagate at significantly greater numbers of torsion load cycles.
The purpose of this research was to compare the resistance to the onset of multi-site damage in thin 2024-T3 and 2524-T3 sheet. Aluminum alloy 2524-T3 is a relatively new fuselage skin sheet material that may offer improved performance with respect to the onset of multi-site damage. The alloys were compared on the basis of crack size frequency distributions measured from multi-hole specimens with and without prior corrosion subjected to cyclic loading. The crack size distributions for the pristine bare and alclad sheet showed no significant difference between 2024-T3 and 2524-T3. When prior corrosion is present, however, thin, bare 2524-T3 offers improved resistance to the onset of multi-site damage.
Previous studies of fatigue crack growth in corroded aluminum have revealed that multiple crack-nucleating corrosion features often lead to the failure of individual test specimens. In the present work, this phenomenon was explored by performing quantitative fractography on forty 2024-T3 sheet aluminum fatigue specimens. Slightly over half of the specimens were found to have two or more crack-nucleating pits. The number of nucleating pits per specimen was found to be positively correlated with stress level, and an interactive effect with corrosion exposure duration was observed. A fracture mechanics-based model was developed to simulate the observed multiple crack growth process. Flaw interaction effects were investigated and the importance of modeling multiple crack growth at high stress levels was seen.
Much research has examined the viability of total fatigue life prediction methodologies based on the initial condition of the material. Further, corrosion attack is also of interest in terms of its damaging effects on structural durability. As a natural next step in understanding the effects of corrosion damage, this investigation was aimed at assessing the viability of a total fatigue life prediction methodology for material with pre-existing corrosion damage. The work covered here was mainly performed to further the breadth and scope of research and development, provide insights into several factors influencing fatigue resistance of corroded material with a basis in analytical and experimental data, and push closer to a fully conclusive analysis tool for truly predicting fatigue life in corroded materials. While the qualitative effects of corrosion and its effects on fatigue resistance are acknowledged by many researchers, a rigorous analytical technique for fully capturing the quantitative consequences of corrosion does not exist in closed form. Most current strategies call for overly conservative and costly repairs or make use of arbitrary factors of safety.
Experimental and analytical evidence indicates that ‘closure’ or interference of crack faces does not entirely isolate the crack tip from damaging strains due to cyclic loading. Therefore, an estimation of the effective stress intensity range at the crack tip, ΔKeff, should take into account the additional cyclic crack tip strain below the opening load. In this investigation, the estimation of ΔKeff is based on the traditional opening load method and also on numerous new methods that show improved correlation of fatigue crack growth rate stress ratio effects in general but mostly at near-threshold growth rates. The experimental results of this study also indicate that the fatigue crack growth rate is not determined solely by ΔKeff but also depends on Kmax. A procedure for determining an intrinsic crack growth rate curve is described.
The variation in damage accumulation rates which occur when periodic tension-compression and compression-tension overloads are introduced into an otherwise constant-amplitude loading history are examined in this investigation. Block loading histories consisting of either compression-tension or tension-compression overloads followed by smaller constant-amplitude fully reversed cycles were applied to smooth, axial, 2024-T351 aluminium specimens to determine the increase in damage accumulation rates for the fully reversed cycles following such overloads. The results show that the interactive damage per cycle remains constant for approximately 450 cycles following an overload and then it decays as a power law function of the number of cycles. The periodic application of a compression-tension or a tension-compression overload contributes significantly to damage accumulation during subsequent small fully reversed cycles, either above or below the constant-amplitude fatigue limit. Consequently, cycles with stress amplitudes below the constant-amplitude fatigue limit, which are usually omitted damage from calculations, should be accounted for in fatigue life prediction analyses for variable-amplitude histories.
The aim of this study is to identify domains where the interactions between mechanical, environmental and microstructural parameters may occur during corrosion fatigue crack growth in the aluminum alloy 2024. The scope considered encompasses the influence of frequency and of alternate immersion in saline solution. Corrosion fatigue crack propagation tests have been carried out under sinusoidal and saw-tooth waveforms, and at different frequencies, load ratios, grain orientations and tempers, in air, distilled water and 3.5% NaCl in permanent and alternate immersion. The stress corrosion cracking behavior of the alloy 2024 has also been considered in order to evaluate the possible contribution of this type of damage during corrosion fatigue crack growth. In 3.5% NaCl, growth rates were found to decrease with decreasing frequency. In alternate immersion, growth rates were increased by up to an order of magnitude for the ΔK values considered compared to permanent immersion and air. The possible mechanisms that govern the corrosion fatigue behavior of the 2024 alloy are discussed in terms of a competition between passivation and anodic dissolution and/or hydrogen embrittlement. Finally, it is proposed that the fatigue crack growth enhancement observed during permanent immersion is related to a crack-tip hydrogen embrittlement mechanism. Hydrogen would be produced by anodic dissolution in relation with film rupture periodicity and then be dragged into the process zone. In alternate immersion, precipitate-free zone dissolution would govern crack advance, as during stress corrosion cracking.
This paper examines the crack growth properties of a popular aircraft alloy, 2024-T351, whilst immersed in Corrosion Preventative Compound (CPC). Comparison tests were conducted in laboratory air and in distilled water. The results were very startling. The effect of the CPC was to increase the crack propagation rate to a value higher than that seen in distilled water. The loading regime maintained stress intensity within a narrow range, minimizing non-linear effects—resulting in very consistent results with minimal scatter. The accurate measurement of crack lengths in the different environments was facilitated by the use of the dynamic specimen compliance technique. Analysis conducted included a fractographic survey of the fracture surfaces in an attempt to discover the fatigue mechanism responsible for the results. Several possible mechanisms are proposed. It is vital that the mechanism responsible is identified so that the possible detrimental effect of CPC on fatigue life can be assessed vis-a-vis its beneficial effect in controlling corrosion.
In the present work, FSW of 2024-T351 Al alloy is characterised in terms of weld residual stress and cyclic properties. A fatigue endurance of the FSW joint was also investigated and discussed. Critical areas for natural fatigue crack initiation in FSW are pinpointed. The fatigue mechanism in FSW is identified to follow a multiple crack coalescence nature. The numbers of cracks participate in coalescence and the resulting crack growth rate is governed by the distance between the crack tips from crack initiation to coalescence. The above represents a complex condition for modelling. During fatigue bending tests, surface crack initiation and growth were monitored by means of a plastic replication technique. Detailed analysis revealed that under that the FSW specimen failures in fatigue bending tests are mainly a process of crack growth with initiation from defects and oxide inclusions, causing subsurface crack formation. Multiple crack initiation sites were observed from different microstructural regimes in the non-uniform residual stress distribution across the weld. This indicates that failure is dominated by fatigue crack propagation from defects. Therefore mechanisms that include features such as defect size and residual stress were considered when applying crack growth analyses to lifetime predictions. Based on crack growth and characterisation of FSW joints, a modified version of the Hobson–Brown is adopted. The good correlation achieved between the experimental data and the model predictions is presented in this paper. Satisfactory predictions of FSW lifetimes are derived from the model.
A variable-amplitude block-loading history consisting of high, near-yield-stress, underloads or compression-tension overloads followed by constant-amplitude small cycles was used to examine underload and compression-tension overload induced reductions in crack closure and the subsequent build-up of crack-opening stress to its steady-state level in a 2024-T351 aluminium alloy. Special attention was given to the near-threshold region, where the crack growth rate was less than 10−9 m cycle−1 The crack-opening stress level and the crack growth rate were measured for four different R-ratios of the small cycles following underloads and compression-tension overloads using a 900 power short focal length optical microscope. The crack-opening stress levels were measured at frequent intervals after the underload and compression-tension overload applications until the crack-opening stress returned to a steady-state level. The crack-opening stress build-up was then described by an empirical formula in terms of the ratio of the difference between the instantaneous crack-opening stress of the small cycles (Sop) and the post-underload and compression-tension overload crack-opening stress levels (Sopol), and the difference between the steady-state crack-opening stress of the small cycles (Sopss) and the post-underload and compression-tension overload crack-opening levels, (). Effective stresses calculated using this formula were used to predict crack growth rates. The formula's predictions showed good agreement with experimentally measured crack growth rates. For simplicity, both underloads and compression-tension overloads will be referred to as overloads in the remainder of this paper.
The fatigue failure process exploits the weakest links (discontinuities) within the test material, which act as nucleation sites for crack origins. This paper summarizes the results of a study on the 2024-T3 aluminum alloys in different forms (clad and unclad), loading directions, thickness and environment. Microstructural features such as particles, grain size, and clad layer, which dominate fatigue performance have been identified, classified, and statistically characterized. Two distinct mechanisms were found to be responsible for crack nucleation. Constituent particles were found to be the crack origins in unclad sheets. There was almost no evidence of multiple nucleation sites in the unclad material. The fatigue crack origins in clad sheets, however, were located at the surface of the clad. No constituent particles were found to be associated with crack nucleation. Multi-nucleation sites were observed in the clad material.
This work aims to provide extensive experimental evidence that reinforce the fact that block overloading can produce phenomena of either crack growth acceleration or retardation depending on its magnitude, duration and the fatigue damage stage characterising its onset. Compared to the majority of similar experimental works in the field, the investigation is based on the classification of identical overloading conditions applying to four distinct crack lengths corresponding to microstructurally short, physically short, long and very long crack conditions. The above is done in order to deliver a better understanding towards the capacity of the crack length to affect the post-overloading response, which in many cases has been hidden from the use of ΔK oriented experiments. The work, which is based on the popular 2024-T351 aluminium alloy, concludes that the effect of the aforementioned parameters on the material’s response to block overloading is complex and is represented by a mixture of acceleration or retardation mechanisms. Based on the experimental data, a sensitivity analysis is performed on the presence of load interaction mechanisms occurred or triggered by the onset and duration of overloading blocks. The boundary conditions indicating the onset of specific load interaction mechanisms have been established and incorporated into the fatigue damage map. The above methodology allows the mapping and therefore the discrimination of the overloading conditions in terms of fatigue life increase or decrease. The methodology is found to predict satisfactory the experimental results.
A methodology dedicated to the optimisation of the fatigue properties of aluminium alloys by controlled shot peening is presented. Selection of the peening conditions is made out of the use of the Design of Experiment and the Effects Neutralisation Model. Both techniques allowed the optimisation both in terms of life and crack growth rates. Experimental determination and further analysis of the residual stress relaxation patterns revealed that at high stress levels, low cycle fatigue, life improvement is predominantly due to slow crack growth rates, while in high cycle fatigue the extension of life is attributed to a prolonged period of crack arrest.
The ‘unified’ approach presented by Vasudevan and Sadananda is used to build an empirical crack growth model for aluminum 2024-T351, based on experimental results published in the literature. This model depicts the existence of two threshold parameters, and , and provides a method to describe crack growth without any reference to crack closure theories. The crack growth process (crack length as a function of load cycles) is solved numerically. A lower limit of initial crack length a* is found, below which cracks do not propagate. Results were compared to a model based on Walker–Kujawski (ΔK+·Kmax)0.5 parameter, and a very good agreement was found. Some contradiction was found in some of the published experimental results. The results used for curve fitting in the NASGRO program exhibit a slope (of the da/dn vs. ΔK curves) smaller than those described in Kujawski works, and therefore, the NASGRO data yields higher number of cycles to failure. The method described can be used to build empirical crack growth models for other materials, for which experimental data exists, thus providing a practical tool for structures' designers. The model described here will be used in the future to also describe the probabilistic behavior of the crack growth.
In a previous paper, a deterministic experimental crack growth model for aluminum 2024-T351 alloy was formulated based on published experimental results, using the ‘unified’ approach. In the present paper, random equivalent initial flaw size (EIFS) values obtained from other experimental results are introduced to the model, and the crack growth probabilistic behavior is demonstrated. The EIFS values computed from published experimental results of 3-D cracked fastener holes are well-approximated by a Weibull distribution. This approximated EIFS distribution is used as a distribution of the initial cracks in a large number of computations for a 2-D case which is very similar to the 3-D one, and serves for demonstration purposes. A computation program was written to numerically solve the behavior of crack lengths vs. the number of load cycles. It is shown that part of the initial cracks do not propagate, as their initial value is smaller than the required threshold criteria set by the ‘unified’ approach. A dispersion in the number of load cycles required to obtain a given crack length is shown. Although the coefficient of variation (COV) of this dispersion is smaller than the COV of the initial random cracks, it may be of concern to designers, as the life period of the design may change by a factor of more than 3.
A study has been made to understand the role of composite microstructure on failure through mechanisms governing the quasi-static and cyclic fracture behavior of aluminum alloy X2080 discontinuously-reinforced with silicon carbide (SiC) particulates. Two different volume fractions of the carbide particulate reinforcement phase, in the aluminum alloy matrix, are considered. Quasi-static fracture of the composite comprised cracking of the individual and clusters of particulates present in the microstructure. Particulate cracking increased with reinforcement content in the aluminum alloy matrix. Final fracture occurred as a direct result of crack propagation through the matrix between particulate clusters. The composite specimens were cyclically deformed under fully-reversed, total strain-amplitude-controlled cyclic straining, giving lives of less than 104 cycles to failure. The plastic strain-fatigue life response was found to degrade with an increase in carbide particulate content in the metal matrix. The cyclic fracture behavior of the composite is discussed in light of concurrent and mutually interactive influences of composite microstructural effects, matrix deformation characteristics, cyclic plastic strain amplitude and resultant response stress.
The phenomenological approach to fatigue crack growth through the Paris-Erdogan model suggests a linear relationship between the two parameters m and log C. Besides any physical interpretation, this relationship allows a statistical analysis of a set of 37 fatigue tests carried out on a 2091 AlCuLi alloy in the same experimental conditions. Fatigue crack growth rate data show a normal distribution for both m and log C. These statistical results allow a probabilistic prediction of fatigue lifetimes between two stated values of crack length.
In this paper, the cyclic strain resistance and cyclic fracture behaviour of aluminium alloy 2124 are examined. Test specimens of the alloy were cycled using tension-compression loading, under total strain control, over a range of plastic strains giving less than 104 cycles to failure. The alloy displayed hardening in both the longitudinal and transverse directions of the wrought plate. The observed hardening behaviour is ascribed to contributions from synergistic influences of dislocation multiplication and dislocation-dislocation interactions. The alloy followed the Coffin-Manson relation and exhibited a single slope for the variation of cyclic plastic strain amplitude with reversals to failure. Fracture of the alloy samples was predominantly transgranular for both orientations, with microscopic crack propagation along the grain boundaries. The low-cycle fatigue characteristics and fracture behaviour of the alloy are discussed in terms of competing and synergistic influences of cyclic plastic strain amplitude, response stress, intrinsic microstructural effects and dislocation-microstructure interactions during cyclic straining.