Low Stress Amplitude Research Articles

Postponing Crack Nucleation in 2024 Aluminium Alloy by Dynamic Precipitation from the Supersaturated State

Recently a novel strategy to improve the fatigue resistance of precipitation hardened aluminium alloys has been proposed, which is based on dynamic precipitation in partially aged material. In this paper, the effect of mean stress and alternative temperature treatments that can enhance the high cycle fatigue resistance through the mentioned mechanism were investigated. The material used is an under-aged 2024 aluminium alloy, which showed superior fatigue life compared to the peak aged condition. The recorded behaviour was observed to be more effective at lower stress amplitudes and lower R values. As the dynamic precipitation may not be able to keep up with the damage evolution, dedicated experiments were conducted to insert periods of controlled healing in a stress free state.

Effect of Stress Ratio and Loading Mode on High Cycle Fatigue Properties of Extruded Magnesium Alloys

The aim of this study is to discuss an effect of stress ratio and loading mode on high cycle fatigue performances of extruded magnesium alloys. Axial loading fatigue tests under three conditions of stress ratio, R, of 0, -1 and-1.5, and also rotating bending fatigue tests have been performed in laboratory air at room temperature using hourglass shaped specimens of AZ31, AZ61, AZ80 and T5-treated AZ80 alloy. From the experimental results, some materials showed a specific stepwise S-N curve on which two knees appear. The shape of S-N diagram depended on a kind of tested materials, applied stress ratio and loading mode. It was suggested from the detail observation of fracture surface that fatigue crack initiation mechanism changed from a twin-induced failure mode at high stress amplitude level to a slip-induced one at low stress amplitude level. This transition was determined with the relation between the minimum stress during a fatigue cycle and the compressive yield stress at which deformation twin occurs.

OS2140 Ti-6Al-4V合金の環状切欠き丸棒のねじり疲労特性評価

Torsion fatigue tests were conducted for smooth bars and circumferentially notched bars of Ti-6Al-4V alloy. The purpose of this work is to evaluate fatigue strength and notch sensitivity under stress concentration factors of Kt=2.0 and 4.0. Fatigue life and fatigue strength decrease with increasing stress concentration. To investigate the crack initiation and propagation, the crack length were calculated with Hooke's law. When compared at the same fatigue life, the crack initiation life for Kt=4.0 was much shorter than that for Kt=2.0. Difference of the fatigue fracture mechanisms between blunt and sharp notch is considered to cause this phenomenon. Under low stress amplitude, the crack propagates under tensile mode toward a 45° to the axial direction. The crack propagated all over the blunt notch, and the crack propagated in the cross section of the sharp notch root under low stress amplitude. As a result, the fracture surface of blunt notch specimen under low stress amplitude was flat, and that of sharp notch specimen showed factory-roof pattern. The formation of factory-roof increases contact areas in the fractured surface and lengthens crack propagation time.

S-N Property and Fatigue Damage of Ultrafine Grained Cu Smooth Specimens

Fatigue tests were conducted using copper processed by equal channel angular pressing (ECAP) through 4, 8 and 12 pass numbers. At high and medium stress amplitudes, the largest fatigue life was attained in the specimens processed by 8 passes, whereas the smallest one was given in 12 passes. At low stress amplitudes, S-N curves tended to coincide with a specific value of stress amplitude regardless of pass numbers. The physical background of the effect of ECAP pass numbers on S-N property was discussed from the viewpoints of microstructure and surface damage formation.

High-strength bolt corrosion fatigue life model and application.

The corrosion fatigue performance of high-strength bolt was studied. Based on the fracture mechanics theory and the Gerberich-Chen formula, the high-strength bolt corrosion fracture crack model and the fatigue life model were established. The high-strength bolt crack depth and the fatigue life under corrosion environment were quantitatively analyzed. The factors affecting high-strength bolt corrosion fatigue life were discussed. The result showed that the high-strength bolt corrosion fracture biggest crack depth reduces along with the material yield strength and the applied stress increases. The material yield strength was the major factor. And the high-strength bolt corrosion fatigue life reduced along with the increase of material strength, the applied stress or stress amplitude. The stress amplitude influenced the most, and the material yield strength influenced the least. Low bolt strength and a low stress amplitude level could extend high-strength bolt corrosion fatigue life.

A fatigue assessment technique for modular and pre-stressed orthopaedic implants

Orthopaedic implants experience large cyclic loads, and pre-clinical analysis is conducted to ensure they can withstand millions of loading cycles. Acetabular cup developments aim to reduce wall thickness to conserve bone, and this produces high pre-stress in modular implants. As part of an implant development process, we propose a technique for preclinical fatigue strength assessment of modular implants which accounts for this mean stress, stress concentrating features and material processing.A modular cup's stress distributions were predicted computationally, under assembly and in vivo loads, and its cyclic residual stress and stress amplitude were calculated. For verification against damage initiation in low-cycle-fatigue (LCF), the peak stress was compared to the material's yield strength. For verification against failure in high-cycle-fatigue (HCF) each element's reserve factor was calculated using the conservative Soderberg infinite life criterion.Results demonstrated the importance of accounting for mean stress. The cup was predicted to experience high cyclic mean stress with low magnitude stress amplitude: a low cyclic load ratio (Rl=0.1) produced a high cyclic stress ratio (Rs=0.80). Furthermore the locations of highest cyclic mean stress and stress amplitude did not coincide. The minimum predicted reserve factor Nf was 1.96 (HCF) and 2.08 (LCF). If mean stress were neglected or if the stress ratio were assumed to equal the load ratio, the reserve factor would be considerably lower, potentially leading to over-engineering, reducing bone conservation.Fatigue strength evaluation is only one step in a broader development process, which should involve a series of verifications with the full range of normal and traumatic physiological loading scenarios, with representative boundary conditions and a representative environment. This study presents and justifies a fatigue analysis methodology which could be applied in early stage development to a variety of modular and pre-stressed prosthesis concepts, and is particularly relevant as implant development aims to maximise modularity and bone conservation.

Low Amplitude Characterization Tests Conducted at Regular Intervals Can Affect Tendon Mechanobiological Response

In bioreactor studies of tissue mechanobiology, characterizing changes in tissue quality is essential for understanding and predicting the response to mechanical stimuli. Unfortunately, current methods are often destructive and cannot be used at regular intervals on the same sample to characterize progression over time. Non-destructive methods such as low amplitude stress relaxation tests could be used, but then, the following dilemma comes into play: how can we accurately measure live tissue progression over time if the tissue is reacting to our measurement methods? In this study, we investigated the hypothesis that stress relaxation tests at physiological amplitudes conducted at regular intervals between stimulation periods do not modify tissue progression over time. Live, healthy tendons were subjected to mechanical stimuli inside a bioreactor for 3 days. The tendons were grouped based on the daily characterization protocol (24 or 0 stress relaxation tests) and their progression over time were compared. Stress relaxation tests at physiological amplitudes modified the tendon response to mechanical stimulation as observed through mechanical and histologic analyses. Possible solutions to eliminate or minimize the effect of stress relaxation tests are to use the mechanical stimuli to characterize tissue progression or to limit the number of stress relaxation tests.

Giga-Cycle Property of a New Age-Hardened Aluminium Alloy Containing Excess Solute Magnesium

The giga-cycle property of a newly developed Al alloy, which contains 0.5wt.% excess Mg solute compared to a standard age-hardened 6061 alloy (6061-T6), was investigated by using smooth specimens subjected to ultrasonic fatigue. The fatigue strength of the new alloy was higher than that of a normal 6061 alloy particularly at relatively low stress amplitude level. Several analyses (surface crack observation, fractography, FIB cross-sectioning, etc.) were also conducted to reveal the micro-mechanism of the observed strength properties. The following results were obtained: i) No fatigue limit was confirmed for both 6061 and new alloy. ii) Total life (Nf) of 6061 and new alloys was determined by a single fatigue crack initiated from a surface PSB crack. iii) Crack initiation resistance defined by N25 (number of cycles to reach ρ = 25 mm-2, where ρ is the PSB crack number density) for new alloy was higher than that of 6061. iv) The higher fatigue strength of new alloy was explained by the effect of excess Mg solute which increased the resistance against the formation of PSB cracks.

Effect of microstructural stability on fatigue crack growth behaviour of nanostructured Cu

Constant amplitude fatigue crack growth tests were carried out on commercial and high purity nanostructured copper processed by High Pressure Torsion (HPT). Due to strong grain refinement the HPT processed materials show higher tensile strength but also faster crack growth rates when compared to coarse grained material. Crack growth curves of nanostructured copper determined at different stress levels, however, showed that the occurrence of grain coarsening at low stress amplitudes leads to a retardation of crack growth in commercial and high purity HPT Cu. This effect was not observed for high purity HPT Cu with a bimodal microstructure. Crack propagation rates depend significantly on the coarsening phenomenon which on the other hand depends on the applied stress amplitude. A comparison of these results with cyclic deformation tests in the high cycle fatigue regime suggests that grain coarsening during crack growth depends more on the stored energy of the materials while a similar coarsening during cyclic deformation depends more on the activation enthalpy for annealing of defects.

Structural Analysis for EHF Hypervapotron Twin Fingers and Beryllium Tile Size Study

The impact of Be tile size on the stress exerted on the CuCrZr heat sink for the ITER EHF finger was examined. The study especially focused on the areas beneath the tiles that are exposed to the high convective heat flux. For reference, in a Be tile size of 50x50x8 mm3, the calculated equivalent strain range using elastic analysis for the path of interest through the side wall of the CuCrZr heat sink resulted in a peak value at the inner wall of ~0.492%. The corresponding fatigue lifetime of the heat sink locally is unacceptably low, 1400 cyclic operations. By using smaller tiles, lower stress amplitudes are observed due to a smaller deformation. In this paper, the total strain range under ITER projected pulsed operating conditions is analyzed for a range of Be tile sizes. The analysis model uses a complete pair of twin fingers as opposed to a sub-model of two tiles. The paper documents the calculated cyclic lifetime of the ITER EHF CuCrZr heat sink with respect to Be tile size and peak heat loads by evaluating the total strain range both from elastic and time independent elasto-plastic analyses for repeated cycle.

Residual stresses and high cycle fatigue properties of friction stir welded SiCp/AA2009 composites

3mm thick rolled SiCp/AA2009 sheet in T351 condition was subjected to friction stir welding (FSW). FSW generated high residual stresses with the peak value occurring in the weld center. While the FSW joint showed a shorter fatigue life than the base material (BM) at stress amplitudes higher than 150MPa, it had a fatigue life equivalent to the BM at lower stress amplitudes with a fatigue limit of about 100MPa. In the BM the fatigue crack initiated at the SiCp agglomeration or inclusions; however, in the joint the initiation zone was mainly characterized by the formation of dimples.

Evaluation of distribution of fatigue lives of the extruded magnesium alloy AZ61

In this study, fatigue tests were carried out to study the distribution of fatigue lives of the extruded Mg alloy AZ61 under constant stress amplitudes. The scattering in the distribution of fatigue lives becomes larger at lower stress amplitudes compared to higher stress amplitudes. During the fatigue process of the alloy, it was observed that cracks initiated from inclusions existing on the specimen surface, then propagated, and finally led to the final failure of the specimen. Statistical distributions of both densities and areas (sizes) of the inclusions were experimentally investigated in detail and were approximated by Weibull distributions. In addition, the distribution of \({{A}_{\rm max}^{1/2}}\), the square root of the maximum inclusion areas within a division of 0.75 mm2, was also investigated. The experimental data were approximated well by the extreme-value distribution. The distributions of the fatigue lives of the extruded Mg alloy were evaluated using both Monte-Carlo simulation and surface fatigue crack propagation behaviour. In the simulation, the random numbers corresponding to the above statistical distributions of the inclusions, i.e., densities, areas, and square root of the maximum inclusion areas were utilized. The evaluated distribution of the fatigue lives corresponded well with the experimental one. It was concluded that for the evaluation of the distribution of fatigue lives, employing the extreme-value distribution of the inclusions is recommended because of convenience and accuracy of the method.

The Effects of Laser Marking and Symbol Etching on the Fatigue Life of Medical Devices.

This paper examines the question;“ does permanent laser marking affect the mechanical performance of a metallic medical component?” The literature review revealed the surprising fact that very little has been presented or studied even though intuition suggests that its effect could be detrimental to a component's fatigue life. A brief investigation of laser marking suggests that defects greater than 25 μm are possible. A theoretical investigation further suggests that this is unlikely to cause issues with relation to fast fracture but is highly likely to cause fatigue life issues. An experimental investigation confirmed that laser marking reduced the fatigue life of a component. This combination of lines of evidence suggests, strongly, that positioning of laser marking is highly critical and should not be left to chance. It is further suggested that medical device designers, especially those related to orthopaedic implants, should consider the position of laser marking in the design process. They should ensure that it is in an area of low stress amplitude. They should also ensure that they investigate worst-case scenarios when considering the stress environment; this, however, may not be straightforward.

Stress-based fatigue life prediction models for AZ31B magnesium alloy under single-step and multi-step asymmetric stress-controlled cyclic loadings

The uniaxial ratcheting and low-cycle fatigue failure behaviors of the hot-rolled AZ31B magnesium alloy are studied by the asymmetric cyclic stress-controlled experiments at room temperature. The effects of loading histories with the variable mean stress, stress amplitude and stress rate on the ratcheting respond and fatigue life of the studied magnesium alloy are discussed. Considering the combined effects of ratcheting damage and fatigue damage on the material failure, the stress-based fatigue life prediction models are developed to evaluate the low-cycle fatigue life under the single-step and multi-step cyclic loadings. Results show that (1) the prior stress cycling with high mean stress or stress amplitude restrains the ratcheting strain in the subsequent cycling with low mean stress or stress amplitude. (2) Due to the rate-independent property of the studied material at room temperature, the effects of the loading history with variable stress rate on the ratcheting behavior are not obvious. (3) The comparisons between the measured and predicted results confirm that the developed model can give an accurate estimate of the low-cycle fatigue life for the hot-rolled AZ31B magnesium alloy under the single-step and multi-step asymmetric stress-controlled cyclic loadings.

Very high cycle fatigue behavior of shot-peened 3Cr13 high strength spring steel

The effect of shot peening (SP) on the very high cycle fatigue behavior of 3Cr13 high strength spring steel was investigated. The samples that underwent electro-polishing (EP) and SP treatments were analyzed at 20kHz by ultrasonic fatigue tests. The results showed that subsurface crack initiation occurred in both types of specimens at low stress amplitude. A fine granular area (FGA) was also observed in the vicinity of the inclusion. The stress intensity factor range at the front of the FGA was regarded as the threshold value that controls the stable internal crack propagation. The S–N curve decreased in very high cycle regime for both types of specimens, and the fatigue life for the subsurface initiation of the SP specimens was reduced compared with that of the EP specimens. Furthermore, the fatigue life of the internal crack in the FGA was estimated based on the crack propagation rate assuming Paris’ law. Considering the tensile residual stress in the interior of the SP specimens, the prediction of the S–N curves on the mean size of the non-metallic inclusions were consistent with the experimental results.

Fatigue-induced grain coarsening and crack growth behavior in ultrafine-grained copper under different loading histories

The crack growth direction in ultrafine-grained copper under constant stressing depends on the applied stress amplitude. The growth direction is perpendicular to the loading axis at low stresses and inclines at 45° to the loading axis at high stresses. To clarify physical causes of such different behaviors at high and low stress amplitudes, two-step fatigue stress tests are conducted. The formation mechanisms of crack paths at high and low stress amplitudes and effect of pre-stressing on subsequent crack growth direction are discussed considering the morphological features of surface-damage formed under different cyclic-stress histories.

OS1513 水素環境下における繰返し応力がTINI形状記憶合金の寿命に及ぼす影響

TiNi shape memory alloys (SMA) receive the great attention because of their unique shape memory properties and commercial potential. However, TiNi alloys are apt to absorb hydrogen and deteriorate in various hydrogen environments, and the behavior of TiNi alloy fatigue life under the hydrogen environment has not been elucidated yet. By comparing fatigue test in air and fatigue test in hydrogen environment, the fatigue life of later clearly decreased than the other. At this time, a brittle layer was not formed on the fracture surface of the specimen for fatigue test in hydrogen environment However, macroscopically fracture surface was showing signs of brittle fracture. Furthermore, by comparing fatigue test in hydrogen environment and constant stress test and SSRT, the relationship between maximum stress and charging time showed good agreement with each other in low stress area about 300 MPa. On the other hand, in high stress area about 1000 MPa, large difference can be seen in the life of each. Therefore, the destruction under high stress amplitude is controlled by the damage due to stress fluctuation. On the other hand, destruction under low stress amplitude is controlled by the hydrogen embrittlement and the maximum stress.

Effect of Hybrid Surface Treatment Composed of Plasma Nitriding and Fine Particle Bombarding on Fatigue Strength of Ti–6Al–4V Alloy

This study investigated the effect of hybrid surface treatment on the fatigue strength of Ti­6Al­4Valloy. The hybrid surface treatment was composed of plasma nitriding and fine particle bombarding (FPB). Plasma nitriding generated a hardened layer with no marked growth of the microstructure of the substrate, and subsequent FPB eliminated a compound layer formed outermost through plasma nitriding. Tensile strength was maintained at the initial level, although ductility was somewhat reduced by fracture of the hardened layer. Plasma nitriding decreased the fatigue strength because the brittle compound layer fractured at relatively low stress amplitude and hastened the propagation of cracks. On the other hand, the effect of the hardened layer appeared through the elimination of the compound layer, so that hybrid surface treatment greatly improved the fatigue strength. Correspondingly, a crack initiation site was found beneath the hardened layer and suggested that the formed layer strongly suppressed crack initiation from the surface. [doi:10.2320/matertrans.M2012262]

Tracing causes for the stress sensitivity of elastic wave velocities in dry Castlegate sandstone

SUMMARY The stress sensitivity of elastic wave velocities in dry rock is a resultant of two types of processes—elastic and non-elastic processes. Both processes are activated under stress and both are associated with stress-induced changes in the rock structure. Although they are of the same origin, their stress-dependency may differ. Inthisstudy,asetofteststhatseparatetheelasticandnon-elasticprocessesisusedtoevaluate the impact of each process on the stress sensitivity of the elastic wave velocities. The tests rely on comparing the stress sensitivity of wave velocities during uniform loading/unloading over a long stress interval (involving both elastic and non-elastic processes) with the stress sensitivity seen in low-amplitude stress oscillations (assumed to be affected mainly by elastic processes). Our study on dry, weak sandstone shows that the impact of elastic and non-elastic processes on the stress sensitivity of the elastic wave velocities is significantly different. This implies that the processes ought to be described separately in order to provide a better foundation for predictive rock physics models. Observationsonartificiallyfracturedsamplesindicatethatlarge,horizontalfracturesreduce the axial wave velocities, whereas they have no notable impact on the stress sensitivity of the velocities. This suggests that the closed macro-fractures contain damaged areas with reduced stiffness which are apparently insensitive to stress changes. A few basic processes—elastic opening and closure of cracks, friction-controlled shear sliding of closed cracks associated with opening or closure of wing cracks, and crushing of asperities in fractures or grain contacts—may be used to intuitively explain the observations.

Analysis on Fatigue Crack Propagation and Fractography of A7075 Aluminum Alloy

The fatigue fractography and surface of A7075 aluminum alloy was investigated by means of optical microscope, scanning electron microscope and technology of metallurgic replica. The results show that the fatigue crack generally initiated in the surface or near the surface of the sample area, such as hole and inclusion. The crack propagation regions under high stress amplitude was characterized by furrow and hollow, and obviously presented fine ripple vein. While lots of fatigue striations and sidesteps with the holes and secondary cracks generated in the crack propagation regions of low stress amplitude. Fatigue transient breaking area is a mixed fracture of brittle and ductile fracture. In the fatigue crack propagation stage, with the increasing of cycle times, secondary cracks gradually merged into primary crack on the path of crack propagation. The dislocation of crack tip,grain boundaries and different grain orientations have great influence on the path of crack growth, which can make cracks bending.