Prior Loading History Research Articles

Assessing fatigue crack growth thresholds for a Ti-6Al-4V (STOA) alloy using two experimental methods

Fatigue crack growth rate behavior in the threshold and near-threshold regimes for a Ti-6Al-4V solution-treated and over-aged (STOA) alloy was investigated using two test procedures: i) accelerated load reduction (ALR) utilizing faster load-shed rates than recommended in the ASTM standard E647 load-reduction (LR) test procedure, and ii) compression pre-cracking constant-amplitude (CPCA) or load-increasing (CPLI) and load-reduction (CPLR). Fatigue crack growth tests were conducted on compact, C(T), specimens (B = 6.35 mm) with two different widths (W = 51 and 76 mm) at two stress ratios (R = 0.1 and 0.7). The crack-growth-rate test data were compared to previous test data produced from the same batch of material using the ASTM LR and CPCA test procedures that were tested on C(T) specimens (W = 25, 51 and 76 mm) over a wide range in stress ratios (R = 0.1, 0.4, and 0.7).Although none of the load-reduction test procedures provided a good representation of the threshold value (ΔKth) at low R, the CPCA test procedure was the most promising. The LR, ALR, and CPLR test procedures were influenced by prior loading history, leading to higher ΔKth values and slower crack growth rates in the threshold regime due to a rise in the crack-opening loads. The LR and ALR tests at R = 0.1 and shed rates of −0.08, −0.32, and −0.95 mm−1 yielded ΔKth values that were 15 to 38 % higher than the estimated threshold stress-intensity factor range (ΔK*th)R=0.1 based on the crack-closure model (FASTRAN). The compression pre–cracking test procedures are more accurate alternatives for developing threshold and near-threshold fatigue crack growth rates. Specifically, the CPLR test procedure produced a ΔKth value that was 8–18 % higher than (ΔK*th)R=0.1. Additionally, LR and ALR tests produced different ΔKth values as a function of the specimen width for a given load ratio, which violates classical fracture mechanics principles. In addition, the spread in the ΔK-rate data in the low-rate regime was larger than the spread in the mid-rate regime as a function of R, a term referred to as “fanning”. In contrast, the CPCA test procedure produced consistent crack growth rates over the same range of ΔK values examined, independent of the specimen width and these data did not exhibit “fanning” with R. Further research is necessary to develop standard test procedures capable of providing a more definitive representation of the ΔKth value in the threshold regime. However, FASTRAN can serve as a crucial tool in aligning the actual ΔKth values with those obtained through CP testing, thereby bridging the gap and enhancing the reliability of the results.

Hysteresis behavior of EQ56 high strength steel: Experimental tests and FE simulation

The probability of offshore platforms suffering to extreme cyclic loads, such as storm surges and typhoons, increases highly at deep sea. It is essential to achieve the hysteresis behavior of steel material for evaluating the dynamic response of offshore platform under cyclic load. A total of 18 coupons are fabricated from the EQ56 high strength steel (HSS) plate to investigate the monotonic and cyclic behaviors. Loading protocols include monotonic tension, constant amplitude and changing amplitude strain-controlled cyclic loadings. It is revealed that the visco-plastic behavior of EQ56 HSS is insignificant under the monotonic tension at room temperature. The EQ56 HSS presents obvious Bauschinger effect, non-masing rule, stress hardening/softening behavior and strain history memory effect under strain-controlled cyclic loading. The prior loading history at low strain-amplitude levels has little effect on the saturation stress of EQ56 HSS while stress hardening can be reactivated to a new saturation level when the applied strain-amplitude is increased. The monotonic and the cyclic hysteresis curves of EQ56 HSS were compared, and hysteretic skeleton curves are fitted using the Ramberg-Osgood model. The essential parameters of the Chaboche combined hardening model are calibrated to describe the cyclic behavior of EQ56 HSS. The comparison between FE simulation and test results demonstrates that this model cannot characterize effectively the hysteretic behavior of EQ56 HSS because the strain history memory effect and coupled behavior of cyclic stress softening and hardening are ignored in its constitutive equation.

The dependence of acoustic emission behavior on prior fatigue loading history

Fatigue cracks are a major concern for aging metallic structures, as they can cause unexpected catastrophic failure. They often occur on hidden surfaces, e.g., under paint and rust or on the interface between two structural layers, which makes visual detection difficult. Acoustic Emission (AE) is a passive nondestructive evaluation (NDE) method that measures the initiation and progression of defects when a structure is continuously monitored. While using this method, which has been successfully applied to monolithic and composite structures, the fatigue life is related to the cumulative AE count. However, if the method is applied after the fatigue crack is initiated, the relationship between the fatigue life and the cumulative AE count may not be applicable. In this paper, the dependence of AE behavior on the prior fatigue loading history is studied using modified compact tension specimens. The cumulative AE count, AE correlation plots, and waveform characteristics are shown to be dependent on loading conditions in addition to the coupling state. Relative behaviors between AE sensors and trends in the cumulative AE count are illustrated by two AE characteristics that are shown as being independent of prior loading history and coupling discontinuities.

Effects of in vivo fatigue-induced subchondral bone microdamage on the mechanical response of cartilage-bone under a single impact compression

Subchondral bone (SCB) microdamage is prevalent in the joints of human athletes and animals subjected to high rate and magnitude cyclic loading of the articular surface. Quantifying the effect of such focal in vivo fatigue-induced microdamage on the mechanical response of the tissue is critical for the understanding of joint surface injury and the development of osteoarthritis. Thus, we aimed to quantify the mechanical properties of cartilage-bone from equine third metacarpal (MC3) condyles, which is a common area of accumulated microdamage due to repetitive impact loading. We chose a non-destructive technique, i.e. high-resolution microcomputed tomography (µCT) imaging, to identify various degrees of in vivo microdamage in SCB prior to mechanical testing; because µCT imaging can only identify a proportion of accumulated microdamage, we aimed to identify racing and training history variables that provide additional information on the prior loading history of the samples. We then performed unconfined high-rate compression of approximately 2% strain at 45%/s strain rate to simulate a cycle of gallop and used real-time strain measurements using digital image correlation (DIC) techniques to find the stiffness and shock absorbing ability (relative energy loss) of the cartilage-bone unit, and those associated with cartilage and SCB. Results indicated that stiffness of cartilage-bone and those associated with the SCB decreased with increasing grade of damage. Whole specimen stiffness also increased, and relative energy loss decreased with higher TMD, whereas bone volume fraction of the SCB was only associated negatively with the stiffness of the bone. Overall, the degree of subchondral bone damage observed with µCT was the main predictor of stiffness and relative energy loss of the articular surface of the third metacarpal bone of Thoroughbred racehorses under impact loading.

Effect of ratcheting fatigue damage on the evolution of tensile properties of Al–Mg alloy

This work aims to investigate the effect of damage accumulation on the evolution of tensile properties due to prior ratcheting fatigue in as-received and artificially aged conditions of Al–Mg alloy (AA 5754). Uniaxial asymmetric stress controlled tests for various combinations of mean stress and stress amplitude are performed at room temperature. Progressive degradation in the mechanical properties of structures and machines subjected to cyclic loading during service period can affect the structural integrity. In view of this, ratcheting fatigue tests are interrupted at various life fractions for particular mean stress and stress amplitude followed by tensile tests to understand the damage evolution during ratcheting for both conditions of alloy. Also, the effect of mean stress and stress amplitude on the post ratcheting tensile behaviour is analysed by conducting the ratcheting tests up to 150 cycles. Cyclic hardening is exhibited in the initial cycles followed by saturation for both as-received and artificially aged alloy. Ratcheting strain increases with the increase in both mean stress and stress amplitude. Drastic changes in tensile properties are noticed with ratcheting fatigue damage evolution for both conditions of alloy. The evolution of tensile properties due to prior loading history such as ratcheting fatigue can provide fundamental understanding in the design of structural components.

Multiaxial ratcheting behavior of zirconium alloy tubes under combined cyclic axial load and internal pressure

In this study, a series of uniaxial and multiaxial ratcheting tests were conducted at room temperature on zirconium alloy tubes. The experimental results showed that for uniaxial symmetrical cyclic test, the axial ratcheting strain ɛx did not accumulate obviously in initial stage, but gradually increased up to 1% with increasing stress amplitude σxa. For multiaxial ratcheting tests, the zirconium alloy tube was highly sensitive to both the axial stress amplitude σxa and the internal pressure pi. The hoop ratcheting strain ɛθ increased continuously with the increase of axial stress amplitude, whereas the evolution of axial ratcheting strain ɛx was related to the axial stress amplitude. The internal pressure restricted the ratcheting accumulation in the axial direction, but promoted the hoop ratcheting strain on the contrary. The prior loading history greatly restrained the ratcheting behavior of subsequent cycling with a small internal pressure.

Post-Cyclic Behaviour of Clayey Soil

The post-cyclic behaviour of a clayey soil after subjecting to cyclic loading is presented in this paper. Tests were carried out on the samples both by allowing the cyclic pore pressure developed during cyclic loading to dissipate and without dissipation. While the samples subjected to post-cyclic monotonic loading without dissipation of cyclic pore pressure exhibited a highly overconsolidated behaviour, the samples after dissipation of cyclic pore pressure exhibited a lightly overconsolidated behaviour. All the samples subjected to cyclic loading exhibited a higher undrained strength during post-cyclic monotonic tests, when compared to the samples without prior cyclic loading history. Post cyclic consolidation tests depicts that the samples show signs of apparent over-consolidation.

Using fracture mechanics for determining residual stress fields in diverse geometries

Component deterioration due to a crack is of the highest importance for the engineering community. Fracture mechanics have mainly been used for studying and evaluating crack or defect nucleation and propagation. This article presents a methodology based on inducing a crack (cut) into mechanical components to characterise an induced residual stress field. This research work's originality was aimed at highlighting fracture mechanics' role in detecting possible component destruction by energetic analysis of crack propagation and evaluating service-life to be used as a technique for characterising the effect of prior loading history regarding a given material. The technique presented in this work is known worldwide as the crack compliance method, based on linear elastic fracture mechanics principles developed by Vaidyanathan and Finnie. Three studies are shown (bent beam, pressurised pipe and modified SEN specimen) where components were induced with a residual stress field. The way non-homogeneous loading could introduce a residual stress field is also presented; if residual stress field acting on a specimen has been characterised, then the mechanical process can be manipulated and a beneficial effect induced into the material.

Shear localization in ice: Mechanical response and microstructural evolution of P-faulting

Systematic experiments on laboratory-grown polycrystalline granular ice and columnar S2 ice loaded triaxially under a high degree of confinement at T=−10°C to T=−40°C at applied strain rates ε˙11=1×10-5s-1 to ε˙11=2×10-1s-1 show the mechanical response and microstructural evolution leading to terminal failure. Terminal failure is characterized by a sudden brittle-like drop in load, localized heating and the development of a narrow shear band, consisting of recrystallized grains, oriented on a plane of maximum shear. This mode of failure, termed plastic (P) faulting, is consistent with the idea of adiabatic heating leading to localized mechanical instability and shear deformation. The microstructural state of the material, including the development of dynamic recrystallization and any prior loading history, does not have a significant affect on the character of shear localization or the levels of deformation required to generate P-faulting.

Differentiating between damage and thixotropy in asphalt binder’s fatigue tests

When testing asphalt binders under cyclic conditions, a drop in modulus is observed. This decrease has been explained by an increase in the temperature of materials due to energy dissipation, thixotropy, and damage. This study presents the application of a cyclic uniaxial tension–compression test to analyze the causes and effects of that behavior on the fatigue properties of asphalt binders.The authors conducted both time and strain sweep tests and concluded that the steady-state complex modulus depended only on the strain applied and was not influenced by the prior loading history, i.e. both data sets converged to the same complex modulus values after a short number of cycles when applying the same strain. In addition, by increasing and decreasing strain alternatively it was found that the initial complex modulus drop is mostly caused by thixotropic effects rather than irreversible damage and can be recovered. The recovery of material properties was faster and complete when the binder was mechanically forced to its initial state instead of allowing it to rest (healing).

Crack-compliance method for assessing residual stress due to loading/unloading history: Numerical and experimental analysis

The understanding of how materials fail is still today a fundamental research problem for scientist and engineers. The main concern is the assessment of the necessary conditions to propagate a crack that will eventually lead to failure. Nevertheless, this kind of analysis tends to be more complicated, when a prior loading history in the material is taken into consideration and it will be extremely important to recognize all the factors involved in this process. In this work, a numerical simulation and experimental evaluation of the induction of residual stresses, which change the crack initiation conditions, in a modified compact tensile specimen is presented. Several analyses were carried out; an initial evaluation (numerical and experimental) was performed in a specimen without a crack and this was used for the estimation of a residual stress field produced by an overload; three more cases were simulated and a crack was introduced in each specimen (1 mm, 5 mm and 10 mm long, respectively). The overload was then applied to set up a residual stress field into the component; furthermore, in each case the Crack Compliance Method (CCM) was applied to measure the induced residual stress field. By performing this numerical simulation, the accuracy of the CCM can be evaluated and later corroborated by experimental procedure. On the other hand, elastic–plastic finite element analysis was utilized for the residual stress estimation. The analyses were based on the mechanical properties of a biocompatible material ( AISI 316L). The obtained results provided significant data about diverse factors, like; the manner in which a residual stress field could modify the crack initiation conditions, the convenient set up for the induction of a beneficial residual stresses field, as well as useful information that can be applied for the experimental implementation in this research. Finally, some beneficial aspects of residual stresses are discussed.

Effective spring stiffness for a planar periodic array of collinear cracks at an interface between two dissimilar isotropic materials

Explicit analytical expressions are obtained for the longitudinal and transverse effective spring stiffnesses of a planar periodic array of collinear cracks at the interface between two dissimilar isotropic materials; they are shown to be identical in a general case of elastic dissimilarity (the well-known open interface crack model is employed for the solution). Since the interfacial spring stiffness can be experimentally determined from ultrasound reflection and transmission analysis, the proposed expressions can be useful in estimating the percentage of disbond area between two dissimilar materials, which is directly related to the residual strength of the interface. The effects of elastic dissimilarity, crack density and crack interaction on the effective spring stiffness are clearly represented in the solution. It is shown that in general the crack interaction weakly depends on material dissimilarity and, for most practical cases, the crack interaction is nearly the same as that for crack arrays between identical solids. This allows approximate factorization of the effective spring stiffness for an array of cracks between dissimilar materials in terms of an elastic dissimilarity factor and two factors obtained for cracks in a homogeneous material: the effective spring stiffness for non-interacting (independent) cracks and the crack interaction factor. In order to avoid the effect of the crack surface interpenetration zones on the effective spring stiffness, the range of the tensile to transverse load ratios is obtained under the assumption of small-scale contact conditions. Since real cracks are often slightly open (due to prior loading history and plastic deformation), it is demonstrated that for ultrasound applications the results obtained are valid for most practical cases of small interfacial cracks as long as the mid-crack opening normalized by the crack length is at least in the order of 10 −5.

Cyclic plastic behaviour of primary heat transport piping materials: Influence of loading schemes on hysteresis loop

Cyclic plastic deformation behaviour of 304LN stainless steel and SA333 Gr.6 C–Mn steel has been investigated employing a constant, increasing and decreasing strain amplitude cycling. Influence of loading scheme on cyclic plasticity characterizing parameters such as cyclic hardening/softening and Masing behaviour are examined. Additional hardening is identified with change in loading scheme and the hardening/softening sustains a dependency of prior load history in both the materials. Masing behaviour is not a common phenomenon in many engineering materials and both the materials in this investigation exhibited a non-Masing behaviour, however, the degree of deviation from Masing varied with the loading scheme. Microscopic evidences to explain the non-Masing behaviour are presented. The experimental investigations show that change in yield stress alone could not be used to explain the hardening/softening behaviour. However, sufficient experimental support is gathered to show that the strain range effect (non-Masing behaviour) is due to change in yield stress and can be explained by a continuous distribution of yield levels.

Mechanical behavior of nylon 66 fibers under monotonic and cyclic loading

The deformation and fracture behavior of partially oriented nylon 66 fibers with a diameter approximately 33 μm exposed to uniaxial monotonic and cyclic loading was studied using a unique test technique. Prior cyclic deformation resulted in a ratchet strain as well as a decrease in residual strength and ductility of the fibers. A description of the fracture process is hypothesized and supported by critical flaw size and energy calculations. The critical flaw size at fracture did not depend on the prior cyclic loading history.

Failure strength of the bovine caudal disc under internal hydrostatic pressure

The structure of the disc is both complex and inhomogeneous, and it functions as a successful load-bearing organ by virtue of the integration of its various structural regions. These same features also render it impossible to assess the failure strength of the disc from isolated tissue samples, which at best can only yield material properties. This study investigated the intrinsic failure strength of the intact bovine caudal disc under a simple mode of internal hydrostatic pressure. Using a hydraulic actuator, coloured hydrogel was injected under monitored pressure into the nucleus through a hollow screw insert which passed longitudinally through one of the attached vertebrae. Failure did not involve vertebra/endplate structures. Rather, failure of the disc annulus was indicated by the simultaneous manifestation of a sudden loss of gel pressure, a flood of gel colouration appearing in the outer annulus and audible fibrous tearing. A mean hydrostatic failure pressure of 18±3 MPa was observed which was approximated as a thick-wall hoop stress of 45±7 MPa. The experiment provides a measurement of the intrinsic strength of the disc using a method of internal hydrostatic loading which avoids any disruption of the complex architecture of the annular wall. Although the disc in vivo is subjected to a much more complex pattern of loading than is achieved using simple hydrostatic pressurization, this latter mode provides a useful tool for investigating alterations in intrinsic disc strength associated with prior loading history or degeneration.

A Model for the Inclusion of Notch Plasticity Effects in Fatigue Crack Growth Analysis

Abstract Notches or other stress concentrations are by far the most common sites for the initiation and growth of fatigue cracks in aircraft structures. The growth of these cracks is directly influenced by the material stress-strain response in the vicinity of the notch. Specifically, when the applied (remote) stress is sufficient to cause local plastic deformation at the notch, the response (local) stresses can no longer be found using elastic stress concentration factors, and they become dependent on the prior loading history. This is to say that the response stresses can no longer be treated as state variables. The occurrence of fatigue crack growth at notches which experience local yielding one or more times during their design lifetime is, in fact, quite common in many cyclically loaded structures. Some of the assumptions inherent in “traditional” Linear Elastic Fracture Mechanics (LEFM) based fatigue crack growth analysis may be inappropriate for such problems. In particular, the assumption that the stress distribution on a critical plane remains proportional to the elastic distribution throughout the loading history becomes incorrect when one or more of the applied loads causes plastic deformation and introduces or alters a residual stress field in this region. This paper first describes an elastic-plastic stress-strain response algorithm which may be used to estimate response stress distributions on a critical plane on a cycle-by-cycle basis. This is followed by a discussion of the manner by which stress intensity factors may be calculated based on these response stress distributions using Green's functions. Finally, the use of these stress intensity factors for the calculation of crack growth rate and, ultimately, crack growth life, is demonstrated.

Effect of Construction and Service Loads on Reliability of Existing RC Buildings

For existing structures, information about “safe” past performance (such as prior load history) may be used to update the probability distribution of resistance and hence reliability. This technical note investigates the effect of realistic construction loading with sustained dead and live loads and extraordinary live loads on time-dependent probabilities of failure. Structural configurations considered include RC floor beams and slabs designed to ACI 318, for various live-to-dead load ratios and construction periods for typical RC multistory buildings. The results show, for example, that the probability of failure for an existing structure after 5–10 years of occupancy is one or more orders of magnitude lower than that prior to the application of construction loads. This suggests that, if a structure survives construction and early service (occupancy) loads, then its reliability is increased significantly. Finally, for cases where prior service live loads are unknown, the inclusion of construction dead l...

Spinal posture and prior loading history modulate compressive strength and type of failure in the spine: a biomechanical study using a porcine cervical spine model

Objective. The purpose of this study was to investigate the effect of posture and loading history on the compressive strength and site of failure in the spine. Design. An in vitro experiment was performed using a porcine cervical model that provided a homogenous population of young healthy spines. Background. The distribution of stresses amongst the many load bearing tissues of the spine is altered throughout the day by posture and the history of loading, but it is not clear how this modulates tissue damage or the risk of injury. Methods. 48 porcine cervical spines were harvested and dissected into motion segments containing two vertebrae and the intervening disc (C3/4 and C5/6). Compressive loads and rotational torques (flexion/extension) were applied so that the effects of four loading histories (hydrated, neutral dehydration, flexed dehydration, superhydrated) and two failure postures (neutral, flexed) could be examined. Levels of dehydration were based on those reported over the course of a day. Dissection techniques and X-rays were used to document tissue damage. Results. Specimens had a lower yield point (43–63%) and ultimate compressive strength (23–47%) when in a flexed posture than when in a neutral posture. When injured in a neutral posture, superhydrated specimens had a lower strength (22–29%) than dehydrated specimens. Loading history also modulated the site of failure. Conclusions. The spine may be more prone to injury early in the morning when the discs are at their greatest level of hydration and/or when they are in a fully flexed posture. Relevance Knowledge of the modulators of spinal injuries, which vary on a daily basis, and throughout the day, will assist in the development of useful injury avoidance strategies.

Modeling of Cyclic Ratchetting Plasticity, Part I: Development of Constitutive Relations

The existing plasticity models recognize that ratchetting direction strongly depends on the loading path, the stress amplitude, and the mean stresses, but their predictions deviate from experiments for a number of materials. We propose an Armstrong-Frederick type hardening rule utilizing the concept of a limiting surface for the backstresses. The model predicts long-term ratchetting rate decay as well as constant ratchetting rate for both proportional and nonproportional loadings. To represent the transient behavior, the model encompasses a memory surface in the deviatoric stress space which recalls the maximum stress level of the prior loading history. The coefficients in the hardening rule, varying as a function of the accumulated plastic strain, serve to represent the cyclic hardening or softening. The stress level effect on ratchetting and non-Masing behavior are realized with the size of the introduced memory surface. Simulations with the model checked favorably with nonproportional multiaxial experiments which are outlined in Part 2 of this paper.

Creep-fatigue damage assessment by subsequent fatigue straining

A series of creep-fatigue tests has been conducted with modified 9Cr-1Mo steel at 873 K in a high vacuum environment of 0.1 mPa. In order to investigate the accumulation of creep-fatigue damage, the creep-fatigue test programme includes changes in strain waveform during the test: from creep-fatigue type to fatigue type and from fatigue type to creep-fatigue type. The conventional linear cumulative damage rule for fatigue and/or creep-fatigue damage fails in evaluating the creep-fatigue life under the present complicated strain wave history. The linear summation of the life fraction is smaller than unity when the prior loading is creep-fatigue type and larger than unity when the prior loading is fatigue type. Scanning electron microscope (SEM) observation of the fracture surface was also conducted. In the case where the strain waveform changes from prior creep-fatigue type to subsequent fatigue type, the crack mode changes from transgranular to intergranular with an increase in the prior creep-fatigue loading history. In the case where the strain waveform changes from prior fatigue type to subsequent creep-fatigue type, the primary crack mode is generally intergranular regardless of the prior fatigue loading history.