Effects Of Variable Amplitude Loading Research Articles

Crack layer modeling of overload-induced slow crack growth retardation of high-density polyethylene

The existing crack layer model theoretically mimics the discontinuous slow crack growth behavior of high-density polyethylene; however, it can only be applied to cyclic loads of constant amplitude or constant load conditions. Most load-bearing components, such as pressurized pipes, are subjected to variable-amplitude loads with unexpected overloads under their service conditions. Thus, the effect of variable amplitude loading on the slow crack growth behavior and lifespans should be understood. In this study, a modified crack layer model was developed to simulate the overload-induced retardation behavior of discontinuous slow crack growth of pipe-grade high-density polyethylene for the first time. The developed model was verified by conducting a sensitivity study on several input parameters and comparing the results with experimental results. The measured retardation results were successfully reconstructed using the proposed model in high accuracy. This study expands the applicability of the crack layer model for the reliable use of pipe-grade high-density polyethylene under various fatigue loading conditions, including unexpected overloads.

Residual stress stability of HFMI-treated transverse attachments under variable amplitude loading with the P(1/3) and the linear spectrum

The potential of HFMI treatment to increase fatigue life under service loading remains in debate. However, some recent studies show that even under variable amplitude loading (VAL), fatigue strength is increased compared to untreated welds. Discussions generally focus on the stability of initial compressive residual stresses, which may be reduced during VAL due to high peak stresses. In this context, the potential of HFMI treatment is often only attributed to residual stress stability. This study presents further results on the effect of VAL with a P(1/3) and a linear load spectrum on the residual stress stability of HFMI-treated transverse stiffeners (TS) made of mild steel (S355) and high-strength steel (S700M). The impact of random, High-Low and Low-High loading sequences on the fatigue strength as well as on the residual stress behaviour of HFMI-treated joints will be discussed.

Experimental validation of railway axle fatigue crack growth using operational loading

Prediction of fatigue crack growth is an important tool for determining inspection intervals for railway axles. Previously, we described the correspondence between the theoretical crack growth behaviour exhibited by compact tension specimens and the observed results for full-scale axles under constant amplitude loading. In this study, the crack growth behaviour of full-scale axles was experimentally investigated under variable amplitude loading, which was based on the operational load–time history. Crack growth models were compared with full-scale testing results. Moreover, the effect of variable amplitude loading on crack growth behaviour is discussed.

Investigation on the mode of failures and fatigue life of laser-based powder bed fusion produced stainless steel parts under variable amplitude loading conditions

Additive Manufacturing (AM) is a method of joining metal/non-metals or composites layer by layer using different energy sources. Among the various AM processes, laser-based powder bed fusion (LPBF) is very popular, in which geometrically complex structures can be manufactured directly from CAD models. One of the least investigated areas in LPBF is the fatigue property of LPBF produced stainless steel parts, which find a variety of engineering and medical applications. In actual service conditions, many engineering components undergo variable cyclic loadings. Therefore, in order to widen industrial applications of LPBF process, effects of variable amplitude loading under both zero and tensile mean stresses on the fatigue life of LPBF produced 15-5 precipitation hardened stainless steel parts have been examined in the present study. Further, different modes of failure, effects of load sequences on fatigue life and the cumulative damage during the process have also been studied. For a typical case under tensile mean stress, results showed that the number of cycles to failure with low to high loading sequence was almost double of that with the sequence reversed. Also, the cumulative damage was more in the first case than that of the second case. Fracture surfaces were studied using Scanning Electron Microscopy to investigate the mode of failures and completely different fracture surface morphologies for these two cases explain the observed difference in number of cycles to failure with the reversal of the load sequence.

Characterization of fatigue delamination growth under mode I and II: Effects of load ratio and load history

The study of fatigue delamination growth in composite materials aims to develop a slow-growth approach for composite materials that would provide conservative and reliable results. The present work focuses on the study of the load ratio effect on fatigue crack growth at constant amplitude and on the effect of variable amplitude loading on crack propagation. With this in mind, a Crack Driving Force (CDF) is chosen to attempt to overlap delamination growth curves obtained from different load ratio values for mode I and mode II. It is shown that the CDF collects the effect of the load ratio on the crack growth curves and allows to build a crack growth master curve for mode I and II. Variable amplitude loads are then considered for delamination propagation in mode I. However, variable amplitude loads bring to light a load history effect during fatigue crack growth that the CDF does not take into account.

Fatigue crack growth in the 2050-T8 aluminium alloy

Fatigue crack growth (FCG) is usually studied using ΔK, however several limitations have been found in the use of da/dN-ΔK curves. Therefore, the FCG in the 2050-T8 aluminium alloy is studied here using the plastic CTOD. Experimental work was developed to obtain FCG in M(T) specimens. The experimental procedure was replicated numerically in terms of geometry, loading and material behaviour. The cyclic plastic behaviour of the material was modeled using stress-strain curves obtained in low-cycle fatigue tests. Numerical predictions of plastic CTOD were obtained for different crack lengths and a da/dN versus plastic CTOD model was built. This model was used to predict the effect of variable amplitude loading, namely, overloads and load blocks. All the predictions agreed qualitatively with the known effect of the load patterns studied. Crack closure was found to be a main mechanism behind the variations of plastic CTOD and therefore of da/dN.

Thickness Effect Analysis on Fatigue Crack Propagation of 7475 Plate Material under Variable Amplitude Loading

In the crack growth prediction models, the effect of variable amplitude loading is taken into account by different ways. One of them the Newman’s model is used very often, but in its original form it is not able to take into account the effect of specimen thickness. The new modification which involves the specimen thickness is presented. The study of variable thickness impact is based on the FE – analysis of M(T) specimen. The variability of constrained factor α was analysed for several load levels and specimen thicknesses. The value of α is governed by the ratio of thickness B versus plastic zone size rp. The effect of overloads on the plastic zone and relevant constraint factor value is analysed as well. During loading, it was found, that the constraint factor value is lower after overloads than when creating monotonic plastic deformation on the same load level in a large part of the cycle. The influence of thickness effect based on different α value after overload was successfully implemented into the strip yield model. Simulation of crack growth taking into account the thickness of the specimen under variable amplitude loading shows similar behaviour like the experimental data.

New experimental methods for investigating variable amplitude loading effects in HCF and VHCF regimes

AbstractFor the investigation of variable amplitude loading effects, it is necessary to increase the test frequency of conventional testing facilities because, for the evaluation of many components, test results with more than 107 cycles are also needed. However, due to the restriction of conventional testing facilities and the associated costs and required time, variable amplitude testing is almost limited to 107 cycles. Service loading with more than 107 cycles to failure is characterized by low amplitudes with a high number of cycles. The challenge in investigation of material fatigue in the regimes of high cycle and very high cycle fatigue is to apply this large number of cycles to failure in an acceptable time frame. For this reason, it is essential to use a machine, which is able to operate at high frequencies. In this paper, two testing machine concepts with piezo actuators are presented. In the first concept, a high performance piezo stack actuator is presented, in which the specimen and the load cell are mechanically assembled in series. This set‐up applies forces up to 10 kN and testing frequencies up to 1000 Hz. The second testing facility is a hybrid testing system, which consists of an inertial mass actuator and a servo hydraulic actuator connected in parallel. Both systems are capable of testing normal specimen dimensions and provide the possibility to work with variable amplitude loading as well as constant amplitude loading.

Polycrystalline numerical simulation of variable amplitude loading effects on cyclic plasticity and microcrack initiation in austenitic steel 304L

This paper presents a numerical simulation of variable amplitude loading effects on cyclic plasticity and microcrack initiation in 304L steel based on the polycrystal plastic theory. A volume of the material corresponding to a real microstructure, containing about 70 grains, has been modeled with the crystal plasticity approach. The model takes account of dislocation densities on the 12 slip systems, isotropic and kinematic hardening, grain sizes, crystal orientations and elastic anisotropy. The material parameters used in this polycrystal simulation for 304L steel were obtained through experimental measurements and the literature or identified through an inverse procedure. In this work, two load conditions were considered, in the first case, total strain amplitude changed from ±0.3% to ±0.7% (overload) and back to ±0.3% and in the second case, it changed from ±0.1% to ±0.5% (overload) and back to ±0.1%. Results demonstrated that overload effects on cyclic plasticity and microcrack initiation are different in these two cases and depend on specific loading conditions.

Delamination in Fiber Metal Laminates (GLARE) during fatigue crack growth under variable amplitude loading

This paper presents a study on the influence that load variations have on delamination shapes in Fibre Metal Laminates. Previously fatigue tested centre-crack tension specimens have been chemically etched to obtain the final delamination shapes. In addition, fatigue crack growth tests on similar specimens have been performed to investigate the formation of delamination shapes. For this purpose, digital image correlation has been used as strain measurement technique to record the delamination shapes in situ testing. An explanation is put-forward in order to understand the effects of variable amplitude loading on the formation of delamination shapes. A transition in delamination shapes was observed, but evaluating this observation using an analytical fatigue crack growth prediction model including the observed change in delamination shapes revealed no significant effect on subsequent crack growth.

Effects of intermittent overloads on fatigue of PMMA bone cement

PMMA bone cements are widely used in orthopaedics, but questions remain as to their long term durability. Although fatigue of bone cements has been widely studied, the effects of variable amplitude loading are not fully understood. This paper presents a novel method using a crossover experimental design, in which a crack is propagated under ΔK control and a series of different overload patterns are applied. This method allowed accurate and repeatable measurements of the effects of overloads on crack growth. It was found that single overloads every ten to 100 cycles caused a pronounced increase in the crack growth rate compared to Paris Law predictions; there was no evidence of retardation as would be expected in most materials. Overloads may fracture intermittently propagating crazes and help the crack past micro-structural obstacles, allowing disproportionately faster crack growth. The observed increase in crack growth rate over a wide range of different overload intervals and for single and block overloads could be accounted for by an eight-fold increase in crack growth rate for approximately fifty cycles after each overload.

Elastic–plastic fatigue crack growth analysis under variable amplitude loading spectra

Most fatigue loaded components or structures experience a variety of stress histories under typical operating loading conditions. In the case of constant amplitude loading the fatigue crack growth depends only on the component geometry, applied loading and material properties. In the case of variable amplitude loading the fatigue crack growth depends also on the preceding cyclic loading history. Various load sequences may induce different load-interaction effects which can cause either acceleration or deceleration of fatigue crack growth. The recently modified two-parameter fatigue crack growth model based on the local stress–strain material behaviour at the crack tip [1,2] was used to account for the variable amplitude loading effects. The experimental verification of the proposed model was performed using 7075-T6 aluminum alloy, Ti-17 titanium alloy, and 350WT steel. The good agreement between theoretical and experimental data shows the ability of the model to predict the fatigue life under different types of variable amplitude loading spectra.

Modelling the Behavior of Short Fatigue Cracks under Variable Amplitude Loading Using FEM

The behavior of short fatigue cracks under variable amplitude loading (VA) was investigated by FEM. The crack closure induced by the crack surface roughness was taken into consideration by using the contact between these crack surfaces. The effects of variable amplitude loading on the performance of short cracks are demonstrated with factors such as grain orientation and misorientation, crack length and the friction efficient between the contacted crack surfaces. Through the two indicators, crack tip opening displacement represented by "CTOD and "CTSD and the plastic strain range of crack tip, the characteristics of short cracks affected by loading blocks are discussed in detail. It is shown from the numerical results that the significance of the design of loading blocks in the fatigue experiments is evident and the performance of short cracks from the variable amplitude loading is more effective due to the closer to practice.

Fatigue crack propagation under variable amplitude loading in PMMA and bone cement

Fatigue failure of PMMA bone cement is an important factor in the failure of cemented joint replacements. Although these devices experience widely varying loads within the body, there has been little or no study of the effects of variable amplitude loading (VAL) on fatigue damage development. Fatigue crack propagation tests were undertaken using CT specimens made from pure PMMA and Palacos R bone cement. In PMMA, constant amplitude loading tests were carried out at R- ratios ranging from 0.1 to 0.9, and VAL tests at R = 0.1 with 30% overloads every 100 cycles. Palacos R specimens were tested with and without overloads every 100 cycles and with a simplified load spectrum representing daily activities. The R- ratio had a pronounced effect on crack propagation in PMMA consistent with the effects of slow crack growth under constant load. Single overloads caused pronounced crack retardation, especially at low da/dN. In Palacos R, similar overloads had little effect, whilst individual overloads at low da/dN caused pronounced acceleration and spectrum loading retarded crack growth relative to Paris Law predictions. These results demonstrate that VAL can have dramatic effects on crack growth, which should be considered when testing bone cements.

Variable amplitude fatigue of bonded aluminum joints

The effect of variable amplitude loading on the fatigue life and failure mode of adhesively bonded double strap (DS) joints made from clad and bare 2024-T3 aluminum was investigated. Under constant amplitude loading, the clad specimens failed through the adhesive or the substrate at high and low stress levels, respectively. The presence of overload cycles shifted the failure mode towards adhesive failure. The specimens made from bare aluminum failed only in the adhesive. Effective stress range vs. failure life curves developed for the bare and clad specimens were used in conjunction with a linear damage summation to predict the failure lives of double strap specimens subjected to two variable amplitude aircraft load spectra with reasonable accuracy.

The effect of variable amplitude loading on stress distribution within a cylindrical contact subjected to fretting fatigue

A finite element model of a cylindrical Hertzian contact on a test sample subjected to alternating shear loading has been developed. The model has been used to investigate shear stress distributions at the contact during variable amplitude fretting fatigue for a load configuration in which the sample cyclic stress is applied in phase with shear force on the cylindrical contact. It has been found that during constant amplitude cyclic loading, shear stress distributions and positions of the stick-slip boundary at load maxima and minima remain fixed. Application of overloads changes the stress distribution and the position of the stick-slip boundary attained by loading of subsequent cycles. The largest cycle maximum stress determines the position of the stick-slip boundary adopted by subsequent smaller amplitude cycles. In general variable amplitude fretting fatigue the position of the stick-slip boundary will be changing with each load cycle. Hence fatigue initiation processes will occur at locations dispersed over an extended region over the contact. The implications of this behaviour for models for fretting fatigue life calculation are explored.

Investigation Into Cumulative Damage Rules to Predict Fretting Fatigue Life of Ti-6Al-4V Under Two-Level Block Loading Condition1

The effects of variable amplitude loading on fretting fatigue behavior of titanium alloy, Ti-6Al-4V were examined. Fretting fatigue tests were carried out under constant stress amplitude and three different two-level block loading conditions: high-low (Hi-Lo), low-high (Lo-Hi), and repeated block of high and low stress amplitudes. The damage fractions and fretting fatigue lives were estimated by linear and non-linear cumulative damage rules. Damage curve analysis (DCA) and double linear damage rule (DLDR) were capable to account for the loading order effects in Hi-Lo and Lo-Hi loadings. In addition, the predictions by DCA and DLDR were better than that by linear damage rule (LDR). Besides its simplicity of implementation, LDR was also capable of estimating failure lives reasonably well. Repeated two-level block loading resulted in shorter lives and lower fretting fatigue limit compared to those under constant amplitude loading. The degree of reduction in fretting fatigue lives and fatigue strength depended on the ratio of cycles at lower stress amplitude to that at higher stress amplitude. Fracture surface of specimens subjected to Hi-Lo and repeated block loading showed the clear evidence of change in stress amplitude of applied load. Especially, the repeated two-level block loading resulted in characteristic markers which reflected change in crack growth rates corresponding to different stress amplitudes.

SHORT CRACK GROWTH BEHAVIOUR UNDER VARIABLE AMPLITUDE LOADING OF SHOT PEENED SURFACES

Abstract— The effect of variable amplitude loading on the initiation and propagation of fatigue cracks from shot peened and unpeened surfaces was investigated. The results confirm the well‐known effect of shot peening, i.e. the increase of fatigue life in comparative tests under constant amplitude loading. Shot peening increases the time to crack initiation and also decreases crack propagation rate, particularly in the earlier stages of growth.Variable amplitude tests were of various kinds: (i) half‐life at one stress level, followed by testing at the second stress level until failure, (ii) sequence loading at two stress levels, and (iii) a different number of overload cycles (1, 10, 100) following a given number of base amplitude cycles.The results of the two amplitude tests showed different behaviour depending on whether the first half‐life cycles were of higher or lower stress levels. These results are explained in terms of crack density and crack coalescence. The extent of damage accumulation during block loading depends on the stress levels employed. The benefits of shot peening are compromised by high stress levels.Results from the overload tests showed that the interpretation of results is not straight forward. A combination of the theories of crack retardation and microstructural‐fracture mechanics seems a necessary prerequisite in an explanation of the resultant behaviour.

A Non-Linear Fatigue Damage Model for Concrete in Tension

Although concrete is used primarily because of its properties under compressive loading, a knowledge of its tensile behavior is essential in situations in which cracking, and the resulting change in stiffness, are critical for the service and ultimate load behavior of a structure. One important aspect of the tensile behavior of concrete is its performance under cyclic loading. The majority of past studies on the tensile fatigue performance of concrete have focused on the behavior under constant amplitude loads. However, in recent years, the effect of variable amplitude loading has also been emphasized. In this paper a model is presented for fatigue damage accumulation for concrete subjected to nonconstant tensile cyclic loads. The model is based on the concepts of Continuum Damage Mechanics and is able to account for the nonlinear propagation and accumulation of fatigue damage. Calibration of the model is done using existing experimental data, and its validity is checked against recently published tests.