Stage Of Fatigue Life Research Articles

Performance evaluation of a magnetic NDE technique to assess residual fatigue life of the retired hydraulic cylinders

Metal magnetic memory (MMM) method is a novel and potential nondestructive evaluation (NDE) technique to assess residual fatigue life of the ferromagnetic structure, such as hydraulic cylinder. In this paper, an evaluation framework was proposed. Firstly, the MMM signals under different fatigue cycles were recorded in the tensile-tensile experiment. A representative signal characteristic Hpp was defined as an evaluation index. The monotonic change trend and abrupt change point was investigated. Statistical results show that Hpp can be used to predict residual fatigue life. Except for a few specimens, almost all specimens produced an abrupt change at the normalized fatigue life coefficient k = 0.91. This can be used to distinguish different stages of fatigue life. The sensitivity, specificity, and diagnostic performance were analyzed using Receiver Operating Characteristic (ROC) curves. The statistical results show that when the coefficient k = 0.91, the diagnostic performance is relatively best with an area under the curve (AUC) is about 0.85, and Jorden's index is about 0.75.

TKD/EBSD and TEM analysis of microstructural changes ongoing in AISI 304L steel exposed to the cyclic loading

The study deals with microstructural evolution of the 304L steel exposed to the low cycle straining. The two states of material were studied, the as-received and solution annealed state. Solution annealing caused grain growth from 29 ± 10 μm to 48 ± 30 μm, reduction of chemical heterogeneity and decrease of overall dislocation density. Cyclic hardening/softening curves were measured and three stages of fatigue life were distinguished. Initial cyclic hardening was followed by cyclic softening. The pronounced secondary hardening was measured in the last stage of fatigue life. Advanced techniques of electron microscopy were used to enlighten origin of the material behavior in particular stages. The material response in individual stages is thoroughly discussed in terms of microstructure evolution. Electron channeling contrast imaging visualized the fine microstructural features and revealed structure evolution, however was not able to identify the features. Transmission electron microscopy is useful for individual structural features characterization, however, it is quite complicated to be performed. Transmission Kikuchi diffraction provided similar results to transmission electron microscopy while characterization of evolved microstructural features revealed the deformation mechanism acting under loading at lower time and relatively smaller effort. Particular damage mechanisms operating in material are linked to the mechanical data.

Study on tensile and fatigue failure in the low-hardness zone of AA2519-T62 FSW joint

The aim of this research was to investigate the basic performance and failure of AA2519-T62 friction stir welded joint in tensile test and in low cycle fatigue regime. It has been reported that at the retreating side, the layer of overgrowth grains undergoes deformation in the TMAZ and forms a characteristic large-grain band partly surrounding the SZ. The reported UTS is very high and it equals 405 MPa, what corresponds to 86.5% joint efficiency value. The failure occurred in the LHZ at the retreating side with the fracture mechanism characterized by simultaneously cracking in several parallel planes. The LCF behavior of the tested joint indicates three stages of fatigue life: a relatively long period of cyclic hardening (up to 500–1000 cycles), the longest period of cyclic stabilization, followed by cyclic softening until failure. The fatigue crack initiation takes place in the near-surface layer of overgrown grains and then propagates through the low-hardness zone.

The influence of tool traverse speed on the low cycle fatigue properties of AZ31 friction stir welded joints

This paper describes the basic low cycle fatigue properties of AZ31 friction stir welded joints obtained by different welding velocities: 150, 300, 600, and 900 mm/min with the constant value of 1600 rpm tool rotation speed. The performed LCF tests at various strain amplitudes: 0.25, 0.35, 0.5 (for R=-1) together with fractography analysis allowed to state that the value of welding velocity has no significant impact on the LCF behavior of AZ31 FSW in the investigated range. The tests also revealed that the AZ31 FSW joint is characterized by three stages of fatigue life: a very short period of cyclic hardening (up to 100 cycles), cyclic stabilization, and a relatively long period of cyclic softening until failure. All joints are free of imperfections and tend to fail in the heat-affected zones.

Nonlinear Properties Analysis of Metallic Fatigue Damage Process Based on the Combined Nonlinear Parameter

To effectively extract the nonlinear properties of fatigue crack extension and evaluate the fatigue damage of metallic specimens, the chaos and fractal theory is applied to analyze the nonlinear output signal in a nonlinear ultrasonic experiment. A combined nonlinear parameter using the Lyapunov exponent, Kolmogorov entropy, and correlation dimension is proposed to evaluate the nonlinearity of fatigue damage. The nonlinear ultrasonic experiment is carried out on specimens with different stages of fatigue life, wherein the ultrasonic nonlinear output signal is collected and its combined nonlinear parameters are calculated. With the increase of fatigue damage in metallic specimens, the combined nonlinear parameter increases monotonically and tends to become saturated when a macroscopic crack appears, which indicates that complexity and randomness gradually increase during the accumulation of fatigue damage. According to microcrack propagation, the combined nonlinear parameter is very sensitive to the evolution of fatigue damage in metallic specimens, especially earlier fatigue damage. The relationship between the combined nonlinear parameter and the fatigue damage of specimens is established, which can provide an effective analytical method for evaluating fatigue damage and predicting the service life of specimens.

Accumulation and evolution of elastically induced defects under cyclic loading: Quantification and subsequent properties

This article quantifies the evolution of bulk material defects in α-iron during cyclic elastic loading and investigates its subsequent effect on quasi-static and dynamic mechanical response. Fatigue test specimens were cycled at a constant stress amplitude, 70% of the yield stress, to cause microscopic defects throughout the material due to a macroscopic elastic load. Afterwards, wire-cut electrical discharge machining was used to obtain multiple sub-tensile specimens from within the gauge section of the fatigued material. X-ray computed tomography of the sub-tensile specimens was used to quantify distribution and location of voids that were induced at different stages of fatigue life. Then, tension tests were performed on the sub-tensile specimens at low (10−3 s−1) and high (103 s−1) strain rates to study changes in mechanical properties. Yield stress and ultimate tensile stress were found to change under both quasi-static and dynamic strain rates based on the amount of prior cyclic loading. Changes in mechanical behavior were correlated to the characteristics of observed defects that exhibited distinct grouping at a certain point during the fatigue life. Mechanical properties of the sub-tensile specimens after fatigue loading displayed different trends depending on the strain rate. Low strain rate deformation resulted in decreases of approximately 20% in the yield stress, ultimate tensile stress, and ductility due to increased fatigue loading. Conversely, during high strain rate deformation some samples with more fatigue loading displayed higher maximum flow stresses. Results demonstrate the significance of bulk material changes which occur at mechanical loads below the yield stress. The systematic experimental procedures developed here for investigating the accumulation, evolution, and effect on subsequent properties due to elastically induced microscopic defects could potentially be applied to a wide range of material systems.

Effect of metallurgical variables on the austenite stability in fatigued AISI 304 type steels

Low-cycle fatigue behavior, destabilization of austenite and emergence of deformation induced martensite (DIM) was investigated in wrought AISI 304 type (304, 304L, 348) austenitic stainless steels of diverse semi-product forms. The specimens fabricated from cylindrical bars, plates or thin sheets were cyclically strained with high strain amplitudes under total strain control in symmetrical push-pull at room and depressed temperatures to different stages of fatigue life. Quantitative and qualitative data on the formation, morphology and specific distribution of DIM in the volume of the material within the whole gauge part of the specimens are confronted with the respective cyclic stress-strain response and discussed in terms of characteristic chemical heterogeneity. This heterogeneity arises in the form of chemical banding due to the complex solidification behavior of AISI 304 type steels and its contemporary production route. Possible effect of additional solution treatment (various cooling rates due to quenching in different media) on DIM formation is discussed.

Mechanical properties of concrete to cyclic uniaxial tensile loading using variable waveforms

The mechanical properties of concrete under cyclic tensile loading using square waveform, sine waveform and ramp waveform are studied. The experiments are performed on a closed-loop electro-hydraulic servo-controlled material testing system (MTS). The axial strain, dissipated energy per loading cycle, the damage evolution law and deformation modulus are mainly studied. The results show that the three-stage evolution law of axial strain and damage variable of concrete under ramp waveform and sine waveform are more obvious than those under the square waveform. The dissipated energy changes at different stages of fatigue life. At the beginning and end of the fatigue life, the rate of dissipated energy is higher than that at the medium stage of the fatigue time, which is attributed to the formation of cracks. The evolution of deformation modulus of concrete subjected to cyclic tensile loading using three loading waveforms also shows three stages: fast increase in the damage—increase at a slow constant rate—and accelerated increase in damage until failure.

A nonlinear continuous damage model based on short‐crack concept under variable amplitude loading

AbstractIn this paper, a modified nonlinear damage accumulation model is proposed by using intrinsic crack size as the damage variable in the stress‐control condition. The model's development is based on the Chaboche nonlinear damage law and the short‐crack theory. The validations are confirmed by using the experimental data of Ti–6Al–4V and 2024‐T3 collected from tests and literature. The model capabilities of predicting damage accumulation and crack growth rate in the multi‐level loading condition as well as the variable amplitude loading condition with single and multiple over‐load are investigated and discussed in detail. Comparison results show that the proposed model is able to consider the loading ratio, the loading sequence and the over‐load effect on damage accumulation correctly. Meanwhile, the damage accumulation in the last stage of fatigue life can be described more clearly by the proposed model attributed to the use of crack size as the damage variable.

Slip Activity of Persistent Slip Bands in early Stages of Fatigue Life of Austenitic 316L Steel

Flat specimen of 316L steel was cyclically pre-deformed with constant plastic strain amplitude to early stage of fatigue life relevant to the period of cyclic strain localization and fatigue crack initiation. To document slip activity and reversibility/irreversibility of persistent slip bands (PSBs) in situ experiments in the high-resolution SEMFEG under special imaging conditions were performed. The half-and full-cycle slip activity and distribution of plastic strain within PSBs in individual grains were investigated via slip steps generated in half-and full-cycle deformation after intermediate vibration polishing. After completion of in situ tests the surface topography in identical locations was quantitatively documented using atomic force microscopy (AFM).

Micro-CT investigation on fatigue damage evolution in short fibre reinforced polymers

Abstract The development of predictive models for fatigue behavior of materials is closely related to a quantitative assessment of damage nucleation and propagation phenomena. Conventional methods used for this purpose, such as microscope observations, conventional radiographs, ultrasonography techniques, either are destructive or do not reach an adequate resolution. Imaging techniques using synchrotron radiation, and in particular X-ray micro-computed tomography (micro-CT), however, combine the advantages of a non-destructive technique with a high spatial resolution. In this paper, damage evolution in samples extracted from PA6.6GF35 specimens subjected to fatigue tests interrupted at different stages of fatigue life is investigated by means of synchrotron radiation micro-CT. Although preliminary, our results suggest that the application of this technique for micro-voids detection within the glass short fibre polyamide reinforced samples offers unique possibilities of investigating the micro-scale internal structure of the samples but, at the same time, presents several criticalities at different stages of the analyses, ranging from specimen size to set-up configuration and threshold selection. A frank discussion of the critical issues encountered during the analyses supplements the presentation in an effort to help defining general guidelines for future developments in this field.

Effect of Artificial Defect and Mean Shear Stress on Torsional Fatigue Behaviour

The aim of this work is to study the influence of artificial defect and mean stress on fatigue strength under torsion loading. Spherical artificial defects have been machined at the surface of gauge length of fatigue samples. Experimental investigations conducted on both defective and defect free materials. The crack initiation mechanisms have been identified based on several observations on Scanning Electron Microscope (SEM) at different stage of fatigue life. It is observed that the defect free material subjected to torsion loading allows relatively earlier initiation. Experimental results show that mean shear stress has no effect on fatigue behaviour under torsion loading. It is also concluded that defects are much more damaging in high cycle fatigue regime than in low cycle fatigue one.

Nonlinear ultrasonic characterizing online fatigue damage and in situ microscopic observation

A robust experimental procedure was developed, by which the evolution of fatigue damage in AZ31 magnesium alloy was tracked online with the ultrasonic nonlinearity parameter β. β values of three sets of samples under different stress levels were measured. Microstructures of specimens at different fatigue stages were observed in situ by optical microscopy. The experimental results show that there is a significant increase in β linked to the accumulation of persistent slip bands (PSBs) and micro-cracks at the early stages of fatigue life and reaches the maximum, about 55% of fatigue life. Ultrasonic attenuation coefficient increases with the expanding of micro-cracks and leads to β decrease slightly after 55% of fatigue life. The variation of β with fatigue cycles is in good agreement with the growth of PSBs and micro-cracks. In addition, it has no significant effect on the experimental results for the changes of low- and high-cycle fatigue and the fatigue mode with tension-tension and tension-compression.

Static and Fatigue Behaviour of Hexagonal Honeycomb Cores under In-plane Shear Loads

Due to their high specific strength and high specific stiffness properties the use of honeycomb panels is particularly attractive in spacecraft structures. However, the harsh environment produced during the launch of a satellite can subject the honeycomb cores of these sandwich structures to severe quasi-static and dynamic loads, potentially leading to static or early fatigue failures. Knowledge of the static and fatigue behavior of these honeycomb cores is thus a key requirement when considering their use in spacecraft structural applications. This paper presents the findings of an experimental test campaign carried out to investigate the static and fatigue behaviors of aluminum hexagonal honeycomb cores subject to in-plane shear loads. The investigation involved carrying out both static and fatigue tests using the single block shear test method. These results are also discussed in relation to the observed damage and failure modes which have been reported for the statically tested specimens and for the fatigue tested specimens at various stages of fatigue life. As well as conducting tests for the more conventional principal cell orientations (L and W), results are also presented for tests carried out at intermediate orientations to investigate the variation of core shear strength with loading orientation. The results are further investigated using explicit non-linear finite element analysis to model the buckling failure mechanisms of the tested cores.

Effect of Static Pre-Strain on Low Cycle Fatigue Life of AISI 316L Stainless Steel

In order to clarify the effects of a great amount of static pre-strain (1/4 cycle) on fatigue life of AISI 316L stainless steel, strain controlled low cycle fatigue tests were carried out. Hardness and fracture ductility of specimens were measured. Results showed that in the case of a specimen with p=0.01, static pre-strain of 0.16 noticeably reduced a fatigue life. To clarify the reasons for the decrease in fatigue life with pre-strain, the maximum stress amplitude of the hysteresis loop was measured. In the case of the specimen with p=0.01, work softening was observed in the early stage of fatigue life. On the other hand, when the cyclic plastic strain ranges were p=0.10, no softening was observed. These results suggest that work softening is significant in determining the fatigue life of the specimens at a relatively low level of applied cyclic strain.

Numerical Simulation of Fretting Wear in Press-Fitted Shaft Considering the Running-In Period

In this paper the fretting wear simulation technique with the press-fitted specimens have been developed, which can consider the running-in period of total wear process by adopting nonlinear wear coefficient. The amount of microslip and contact variable at press-fitted and at bending loaded condition of press-fitted shaft was analysed by applying finite element method. With the finite element analysis result, a numerical approach was applied to predict fretting wear based on modified Archard's equation and updating the change of contact pressure caused by local wear with influence function method. The predicted wear profiles of press-fitted specimens at the contact edge were compared with the experimental results obtained by rotating bending fatigue tests. It is shown that the predicted wear profiles considering the running-in period with nonlinear wear coefficent is consistent with experimental results than that with constant wear coefficient. Therefore, the fretting wear simulation technique proposed is feasible and efficient for numerical simulation of fretting wear on press fits at the initial stage of fatigue life.

Fatigue initiation in C35 steel: Influence of loading and defect

The aim of this work is to study the influence of both defect and tension and torsion loading on stress number of cycles ( S– N) 25 curves for C35 steel. A spherical artificial defect has been machined at the surface of gauge length of fatigue samples. The crack initiation mechanisms have been identified based on several observations on Scanning Electron Microscope (SEM) at different stage of fatigue life. The initiation crack length definition is proposed for defect free and defective material. For defect free material, torsion loading allows relatively earlier initiation compared to tension loading. In the case of defective material, it is observed that, for the both types of loading, initiation fatigue life is not negligible by comparison to total fatigue life. It is also concluded that defects are much more deleterious to fatigue life in the range of high cycle fatigue regime. However, for the limited fatigue lives (between 10 4 and 10 5 cycles), the defect free and defective material S– N curves are relatively comparable.

Persistent Slip Bands

Abstract The slip activity and shear strain of persistent slip bands (PSBs) in polycrystalline nickel were studied after half-cycle deformation on well-polished specimen surfaces at different stages of fatigue life using the combination of atomic force microscopy (AFM) and scanning electron microscopy (SEM). Although the local shear strain of PSBs seems to be nearly independent of the stage of fatigue life, the half-cycle slip activity of PSBs significantly depends on it. In-situ deformation experiments in the scanning electron microscope (SEM) showed that during half-cycle loading the slip steps do not proportionally develop to the applied plastic strain and not the whole accumulated PSB volume is active. The reactivation of the cumulative PSB volume is different for individual PSBs and grains, and evolves more slowly than the reactivation of grains. PSBs in polycrystals are subjected to a development history — their activity changes both temporally as well as spatially. Possible reason for this behaviour could be hardening effects due to secondary slip within the PSBs. Detailed investigations of the dislocation structure developed at different stages of fatigue life using the electron channelling contrast (ECC) in the SEM give more information about it.

FATIGUE LIFE PREDICTION OF ROLLED AZ31 MAGNESIUM ALLOY USING AN ENERGY-BASED MODEL

Fatigue behavior of rolled AZ31 magnesium alloy, which shows an anisotropic deformation behavior due to the direction dependent formation of deformation twins, was investigated by carrying out stress and strain controlled fatigue tests. The anisotropy in deformation behavior introduced asymmetric stress-strain hysteresis hoops, which make it difficult to use common fatigue life prediction models, such as stress and strain-based models, and induced mean stress and/or strain even under fully-reversed conditions; the tensile mean stress and strain were found to have a harmful effect on the fatigue resistance. An energy-based model was used to describe the fatigue life behavior as strain energy density was stabilized at the early stage of fatigue life and nearly invariant through entire life. To account for the mean stress and strain effects, an elastic energy related to the mean stress and a plastic strain energy consumed by the mean strain were appropriately considered in the model. The results showed that there is good agreement between the prediction and the experimental data.

Effect of Humidity on Fatigue Strength of Squeeze Cast Al Alloy

Fatigue properties of a squeeze cast Al-Si-Mg alloy were examined under rotating bending and the effect of humidity on the crack initiation and propagation behavior was investigated in controlled humidity. The fatigue strength was decreased in high humidity (RH85%) in comparison with the one in low humidity (RH25%). Cracks initiated at the early stage of fatigue life from slip bands and eutectic Si particles in many cases and partially casting defects, and most of fatigue life was occupied by the growth life of a small crack regardless of humidity. The propagation of a crack were promoted by humidity, especially the growth rate of a crack larger than about 100 µm, which propagated in tensile mode, was strongly accelerated.