Fatigue Damage Process Research Articles

Overview of Structural Life Assessment and Reliability, Part I: Basic Ingredients of Fracture Mechanics

Overview of Structural Life Assessment and Reliability, Part I: Basic Ingredients of Fracture Mechanics

Overview of Structural Life Assessment and Reliability, Part I: Basic Ingredients of Fracture Mechanics

Overview of Structural Life Assessment and Reliability, Part I: Basic Ingredients of Fracture Mechanics

Fatigue damage of collagenous tissues: experiment, modeling and simulation studies.

Mechanical fatigue damage is a critical issue for soft tissues and tissue-derived materials, particularly for musculoskeletal and cardiovascular applications; yet, our understanding of the fatigue damage process is incomplete. Soft tissue fatigue experiments are often difficult and time-consuming to perform, which has hindered progress in this area. However, the recent development of soft-tissue fatigue-damage constitutive models has enabled simulation-based fatigue analyses of tissues under various conditions. Computational simulations facilitate highly controlled and quantitative analyses to study the distinct effects of various loading conditions and design features on tissue durability; thus, they are advantageous over complex fatigue experiments. Although significant work to calibrate the constitutive models from fatigue experiments and to validate predictability remains, further development in these areas will add to our knowledge of soft-tissue fatigue damage and will facilitate the design of durable treatments and devices. In this review, the experimental, modeling, and simulation efforts to study collagenous tissue fatigue damage are summarized and critically assessed.

Non contact inspection of the fatigue damage state of carbon fiber reinforced polymer by optical surface roughness measurements

This work presents the evaluation of a new non-contact technique to assess the fatigue damage state of CFRP structures by measuring surface roughness parameters. Surface roughness and stiffness degradation have been measured in CFRP coupons cycled with constant amplitude loads, and a Pearson׳s correlation of 0.79 was obtained between both variables. Results suggest that changes on the surface roughness measured in strategic zones of components made of the evaluated CFRP, could be indicative of the level of damage due to fatigue loads. This methodology could be useful for other FRP due to similarities in the fatigue damage process.

Fatigue Life Prediction of Composite Laminates Based on Progressive Damage Analysis

A three dimensional analysis model is developed on the fatigue life prediction of composite laminates based on a progressive damage analysis. This model consists of stress analysis, fatigue failure analysis and material property degradation. Teserpe’s failure criteria is used to fatigue damage analysis. Fiber tensile/compressive breakage, matrix tensile/compressive cracking, matrix/fiber shear failure and tension/compression delamination are considered in fatigue damage analysis. The methodologies of sudden degradation and gradual degradation are both applied in the material property degradation. The stiffness and strength gradual degradation is based on the Shokrieh fatigue model, which is based on fatigue test for unidirectional laminates. In order to consider the scatter of the material in the practical structures, the stiffness and strength of the material are randomly distributed using normal distribution in the numerical model. The progressive fatigue damage model is developed in finite element code ABAQUS through user subroutine UMAT, which can simulate the fatigue damage process. Fatigue life of different ply stacking sequences and geometries composite laminates under different cycle loading are predicted. The predicted fatigue life is in good agreement with the experimental results.

Multiphysics computational analysis of white-etch cracking failure mode in wind turbine gearbox bearings

Wind energy is currently one of the most promising and the fastest-growing installed alternative-energy production technologies. It is anticipated that by 2030, at least 20% of the US energy needs will be provided by this form of energy. The economics of wind energy require that wind turbines (convertors of wind energy into electrical energy) be able to operate for at least 20 years, with only regular maintenance. Unfortunately, many wind turbines require major repairs after only 3–5 years in service, mainly due to the failure of their gear boxes. It is this lack of gear box reliability which is currently compromising wind energy economics. In the present work, a multiphysics computational methodology has been developed and used to analyze the problem of white-etch cracking, one of the key processes responsible for the premature failure of gearbox roller bearings. The multiphysics methodology includes: (a) quantum-mechanical calculations of the hydrogen dissolution and the accompanying grain boundary embrittlement phenomena; (b) atomistic-level kinetic Monte Carlo analysis of hydrogen diffusion from the crack wake into the adjacent unfractured material; (c) cohesive zone type modeling of the intergranular fracture processes; and (d) a conventional displacement-based finite element analysis of the kinematic and structural response of the bearing under service loading conditions. The results obtained clearly revealed the operation of the white-etch cracking phenomena and their possible interaction with the conventional rolling-contact fatigue damage processes.

Monitoring Corrosion Fatigue Damage Process of LY12CZ Aluminum Alloy with and without Coating by Acoustic Emission

Corrosion fatigue (CF) of aluminum alloy is an important form of aircraft structural damage. Considering the real service conditions, it is necessary to study the effects of the coating on the CF process. The CF lifetime tests were conducted in 3.5% NaCl solution. The coating could well protect specimens before it damaged and markedly improved the resistibility of CF. According to results of S-N fatigue curves, at σmax=250MPa, real-time detection of the CF process for LY12CZ aluminum alloy with and without coating was performed using digital acoustic emission (AE) technique. Three damage modes, namely, coating damage, corrosion and crack propagation, were determined using AE parameters analysis, that is the distribution of amplitude and peak frequency.

Microstructure and Strain-Based Fatigue Life Approach for High-Performance Welds

Microstructure and pre-existing surface flaws in smooth notch geometries significantly affect the fatigue life of welded joints. Traditionally, a welded joint is assumed to incorporate crack-like defects and the crack propagation dominates the total fatigue life. For a smooth weld notch geometry, the macro crack initiation period becomes more significant, and this difference cannot be modelled with the existing stress or fracture mechanics ‑based approaches. In this paper, a microstructure and strain ‑based fatigue life approach is presented. In the approach, the fatigue damage process is modelled as a repeated crack initiation process within a material volume related to the microstructure. The novelty of the developed approach is that the size of the damage zone is defined from the grain size statistics without using fracture mechanics. The approach is able to consider the changes in the stress gradient, stress triaxiality and plasticity during the fatigue crack initiation and growth. The developed approach has been validated with experiments on submerged-arc and laser-hybrid welded joints. The predicted fatigue life, crack growth path and rate showed good agreement with the experiments. For a welded joint with smooth and favourable notch shape, the short crack growth, i.e. macro crack initiation period is dominant and it has a significant influence on the fatigue life.

Fatigue damage behavior of a surface-mount electronic package under different cyclic applied loads

This paper studies and compares the effects of pull–pull and 3-point bending cyclic loadings on the mechanical fatigue damage behaviors of a solder joint in a surface-mount electronic package. The comparisons are based on experimental investigations using scanning electron microscopy (SEM) in-situ technology and nonlinear finite element modeling, respectively. The compared results indicate that there are different threshold levels of plastic strain for the initial damage of solder joints under two cyclic applied loads; meanwhile, fatigue crack initiation occurs at different locations, and the accumulation of equivalent plastic strain determines the trend and direction of fatigue crack propagation. In addition, simulation results of the fatigue damage process of solder joints considering a constitutive model of damage initiation criteria for ductile materials and damage evolution based on accumulating inelastic hysteresis energy are identical to the experimental results. The actual fatigue life of the solder joint is almost the same and demonstrates that the FE modeling used in this study can provide an accurate prediction of solder joint fatigue failure.

Overview of Structural Life Assessment and Reliability, Part V: Joints and Weldments

Overview of Structural Life Assessment and Reliability, Part V: Joints and Weldments

Fatigue Strength Reduction Factor for Polycrystalline Nickel Base Superalloy with and without Non-Metallic Inclusions

Polycrystalline nickel base superalloy is popular for its wide applications; it is used in hot sections of power generation turbines, rocket engines and other challenging conditions due to its high strength and good creep, fatigue, and corrosion resistance at high temperature. However, the presence of inclusions introduced into the superalloy during the fabrication processes can significantly degrade the fatigue life. This paper utilizes a new probabilistic method which captures both the essence of microstructure and notch root stress gradient to determine the notch size and inclusions effects on the fatigue strength of notched nickel base superalloy with and without non-metallic inclusions. Notched cylindrical specimens of nickel base superalloy of varying notch root radii are modeled using microstructural-sensitive crystal plasticity finite element codes. The stresses extracted from the fatigue damage process zone around the notch are used in determining the fatigue strength reduction factor and the associated probability of failure of the notched specimens. The numerical results obtained are in direct correlation with the experimentally obtained value for the different notch root radii.

Deformation Analysis of Asphalt Mixtures with Digital Image Correlation Method

Based on digital image correlation method (DICM), this paper promotes a new method for tracking the movements of coarse aggregates of asphalt mixtures. The four-point bending fatigue test is applied as the observed test. The fatigue damage process of AC-16 asphalt mixture is observed by CCD camera. Results show that the new method is able to well reflect the exact displacement of coarse aggregates under different loading repetitions. And the movement trends also reflect the influences of cracks in the specimens. Compared to the original method, the new method provides more detail movement information of different materials (aggregates and asphalt binder). This method helps to the research of asphalt mixtures in meso-scale.

ATOMIC FORCE MICROSCOPY INVESTIGATION OF THE D16AT ALLOY DEFORMATION RELIEF

The possibility of atomic force microscopy application for the investigation of fatigue damage process of aluminum alloy D16AT has been substantiated. It has been shown the possibility of atomic force microscopy application for the analysis of the surface deformation relief, which is formed under cyclic loading. Deformation relief is external feature of local micro plastic deformation formed by the dislocation motion and dislocation transformation. As a result of these we can watch the extrusions, intrusions and persistent slip bands on the surface of material. Presented results of the investigation of deformation relief morphology under cyclic loading by the AFM gives the ground to suppose that the height of extrusions is grows during all stage before fatigue crack initiating. The height of extrusions rising under the cyclic loading and can be considered as a quantitative parameter of fatigue damage of structural elements made of alclad aluminum alloys.

Mechanical Stability of Monolithic Catalysts: Scattering of Washcoat Adhesion and Failure Mechanism of Active Material

Alumina washcoated cordierite monoliths are prepared to investigate mechanical stability of monolithic catalysts by ultrasonic vibration and thermal shock. It is shown that mechanical failure of monolithic catalysts is a stress-induced fatigue damage process, i.e., microcracks are first initiated at the pre-existing flaws and then propagate to form macrocracks, leading to the washcoat surface cracking, interconnection between large cracks, and consequent spalling of active material. It is also found that the washcoat adhesion of mini-monolithic catalysts is distributed over a wide range of values, resulting from fatigue failure of active material. Statistics indicates that the measured weight losses follow a bimodal Weibull distribution, attributed to the transition in the type of the flaws that control fatigue, i.e., the transition in the two spalling modes, interior spalling (cohesive failure) and interface spalling (adhesive failure). Furthermore, it is recommended that at least five monolithic specime...

Flexural behaviour of UHTCC‐layered concrete composite beam subjected to static and fatigue loads

ABSTRACTAs an engineered material, ultra‐high toughness cementitious composite (UHTCC) exhibits the characteristics of pseudo strain hardening and multiple cracking under uniaxial tension. It can be applied as the reinforcing and protective material of concrete structures. In this paper, static and fatigue flexural tests were carried out on UHTCC‐layered concrete composite beams, for which UHTCC layer was used on the tension side. Under both static and fatigue loads, plane section assumption was suitable for such composite beams, and a good bond strength was achieved between the two layers. For static specimens, the UHTCC layer enhanced the ductility of the concrete layer. While under cyclic loads, because of the reinforcing effect of UHTCC, more than one crack were formed in the concrete layer, which led to a ductile deformation. Furthermore, the fatigue damage process of the composite beam was analysed.

Uniaxial ratcheting and fatigue failure behaviors of hot-rolled AZ31B magnesium alloy under asymmetrical cyclic stress-controlled loadings

Uniaxial ratcheting and low-cycle fatigue failure behaviors of the hot-rolled AZ31B magnesium alloy are studied by uniaxial asymmetrical cyclic stress-controlled experiments at room temperature. The effects of mean stress, stress amplitude and stress rate on the uniaxial ratcheting response and ratcheting life of the studied magnesium alloy are analyzed. The microscopic observations near fracture surface are carried out. In addition, a fatigue parameter is introduced to describe the ratcheting damage and fatigue damage processes, and a new model is established to describe the relationship between the linear density of twins and the fatigue parameter. Results show that (1) the mean stress and stress amplitude has a great influence on the ratcheting strain and fatigue failure life. When the mean stress or stress amplitude is increased, the ratcheting strain and its rate rapidly increase. So, the fatigue failure life is reduced. (2) The increase of mean stress and stress amplitude can accelerate the generation of twins, which decreases the fatigue resistance and increases the ratcheting strain. However, the stress rate has little effect on the twinning. So, the effects of stress rate on ratcheting and low-cycle fatigue failure behaviors are not obvious under the tested conditions. This is because the studied magnesium alloy shows highly rate-independent property at room temperature within the tested strain rate range. (3) The established model can accurately describe the relationship between the linear density of twins and the fatigue parameter. Furthermore, the threshold fatigue parameter for twinning is found.

表面粗さ測定に基づくSUS316NGの疲労損傷評価と適用範囲

Changes in the surface roughness of SUS316NG during cyclic loadings were investigated, and the relationship between those changes and the fatigue damage processes, including evolution of surface relief due to active slip systems, were discussed on the basis of microscopic observations. Strain-controlled fatigue tests were conducted at three constant strain ranges Δε = 8, 4, and 1%. During the tests, surface roughness was measured periodically at cycles determined with respect to the usage factor UF, and then the surface was observed directly to clarify the surface morphology. As a result, until the middle of fatigue life, the arithmetic mean roughness Ra increased linearly with the number of cycles regardless of the strain range conditions. Clear changes in surface roughness were obtained even at a strain range as small as 1%. The results suggest that the surface roughness measurement can probably be used to assess fatigue damage because it increases linearly with the number of cycles until the middle of fatigue life. The rate at which surface roughness increased with UF became smaller with decreasing applied strain range. In the damage process, the surface became uneven because of active slip systems and the asperity became larger when the number of cycles increased. Surface observations revealed that the number of slip bands decreased and the development of the asperity became more difficult with decreasing applied strain range. The change tendencies of surface roughness were similar to those of the density of slip bands at each strain range. This shows that the rate at which surface roughness increased with UF is affected by the number of slip bands at each strain range condition.

Modelling of Surface Initiated Rolling Contact Fatigue Damage

A parametric study was performed to investigate the contribution of surface roughness, friction and a constant residual surface stress to the rolling contact fatigue damage process. The effects on initiation, crack path and fatigue life were examined for a gear application. The asperity point load model could predict effects on rolling contact fatigue that are observed with experiments. The study of a ring/cone crack subjected to a rolling contact fatigue load allowed to explain the v-shaped cracks typical for surface initiated spalling. The results further support the asperity point load mechanism as a source behind surface initiated rolling contact fatigue.

Chloride ion transport in fly ash mortar under action of fatigue loading

In order to study the chloride ion transport performance in fly ash addition mortar, a new method, in which the fatigue loading and chloride diffusion are undertaken simultaneously, was developed. This method realizes coupling the fatigue damage process and the process of chloride transporting of fly ash mortar. The transport performance of chloride in fly ash mortar specimens was studied under different stress levels. Moreover, the effect of fly ash content on transport performance of chloride ion in mortar was investigated. AE (Acoustic Emission) and SEM were used to acquire the damage distribution of mortar specimens under action of fatigue load. The results show that the diffusion coefficient of chloride in mortar specimens increases with stress level of fatigue loading. The addition of fly ash can mitigate the penetration of chloride ion. The results of microcrack 3D location acquired by AE, accompanied with crack characterizing from SEM, indicate that the damage degree of mortar specimen increases with stress level of fatigue loading. Furthermore, higher damage degree of mortar leads to more the chloride ion content in the sample.

Magnetomechanical behavior for assessment of fatigue process in ferromagnetic steel

In this work, the change of magnetization as a function of applied stress has been investigated for test specimens of AISI 1018 steel. The various stages of fatigue damage process are characterized by the magnetomechanical measurements recorded by an APS 428D fluxgate magnetometer. Of great significance is the fact that the stress-magnetic field hysteresis loop area changes systematically with the progression of fatigue. The magnetomechanical hysteresis demonstrates conspicuous changes in the initial stage of fatigue loading, then reverts to a relatively stable phase, and finally, drastic variations appear again as the cyclic loadings approach terminal failure. This work demonstrated that it is possible to correlate the progress of fatigue in ferromagnetic steels with the nondestructive evaluation technique of the magnetomechanical effect.