Fatigue Damage Propagation Research Articles

Warp direction fatigue behavior and damage mechanisms of centrally notched 2.5D woven composites at room and elevated temperatures

Fatigue behavior, notch sensitivity and environmental effect are three of the principal concerns in composite applications, especially those for aeroengines that require mechanical fastening and fatigue resistance at elevated temperatures. As a new generation of woven composites, 2.5D woven composites (2.5DWC), have drawn increasing amounts of interest due to the excellent fatigue resistance of these materials. However, the lack of temperature-dependent fatigue explorations into centrally notched 2.5DWC (N-2.5DWC) significantly limits the engineering applications of these materials. Therefore, the objective of this paper is to investigate the temperature-dependent fatigue behavior of N-2.5DWC. Warp direction tension-tension fatigue tests of N-2.5DWC were performed at 20 °C and 180 °C for the first time. The fatigue lives and the related failure mechanisms were obtained experimentally. Interestingly, the fatigue life at the same temperature exhibited a sudden increase with a decreasing stress level. A temperature-dependent fatigue life prediction model was established, and the fatigue lives and damage propagation processes at various temperatures and stress levels were predicted. The predicted damage propagation process was a “L” shaped, which was consistent with the corresponding experimental fracture morphologies. Finally, the effects of temperature and warp arranged density on the warp direction fatigue life were discussed.

Improvement of Low-Cycle Fatigue Resistance in TWIP Steel by Regulating the Strength Distribution

Unlike the stress controlled fatigue in high-cycle regime, low-cycle fatigue (LCF) properties can hardly be improved by traditional cold working process, e.g. cold rolling, drawing. It remains an intricate problem enhancing LCF performance, in the premise of guaranteeing strength (or cyclic stress) which is also vital to long-time serve for engineering components. In this study, a material design philosophy regarding architecting strength gradient is proposed to improve the LCF properties of twinning-induced plasticity (TWIP) steels. Sharp and gradual strength gradients were introduced into an Fe-18Mn-0.6C TWIP steel by appropriate pre-deformation & annealing process, and the influences of strength and its distribution on the LCF performance were investigated. It is found that the TWIP steels with negative strength gradients (including sharp and gradual gradients) show better LCF performances than both homogeneous strengthened and unstrengthened congeners. While the samples with positive strength gradients exhibit the shortest fatigue lives at the same strain amplitudes in all the tested samples. The excellent LCF properties of materials with negative strength gradient are believed to originate from the formation of strain gradient and their structural characteristic of suppressing fatigue crack growth at various stages during cyclic loading. The significant influences of strength distribution on fatigue damage and crack propagation mechanisms may provide new and important implications for the design of the materials with high fatigue performance. Meanwhile, the hetero-microstructures with strength gradients can be easily prepared in TWIP steel (or maybe other engineering structural materials) by the present process in a cost-effective and large-scale way and therefore is expected to be conducive to industrial production.

Assessment of Fatigue Damage and Crack Propagation in Ceramic Matrix Composites by Infrared Thermography

The initiation and propagation of damage in SiC fiber-reinforced ceramic matrix composites under static and fatigue loads were assessed by infrared thermography (IRT). The proposed thermographic technique, operating in lock-in mode, enabled early prediction of the residual life of composites, and proved vital in the rapid determination of the materials’ fatigue limit requiring testing of a single specimen only. IRT was also utilized for quantification of crack growth in the materials under cyclic loads. The paper highlights the accuracy and versatility of IRT as a state-of-the art damage assessment tool for ceramic composites.

Mechanisms of fatigue crack initiation and early propagation in cyclically strained nickel superalloy

In the present work, the mechanisms of fatigue crack initiation and early crack propagation of a γ′ strengthened nickel-based superalloy have been investigated. Low cycle fatigue tests have been performed on polycrystalline superalloy MAR-M247 under symmetrical strain controlled loading at ambient temperature in air. The nature of early fatigue damage, crack initiation and subsequent fatigue crack propagation was assessed for applied total strain range of 1%. Test was interrupted at given numbers of cycles corresponding to 1%, 2%, 5%, 10%, 20%, 40%, and 80% of fatigue life. The evolution of pronounced surface relief, also known as persistent slip markings (PSMs) was observed using atomic force microscopy (AFM) and high resolution scanning electron microscopy (SEM-FEG). The kinetics of extrusion growth was quantitatively measured. The regions of fast and slow extrusion growth were identified. Sharp surface PSMs arise in areas where lamellae of localized cyclic slip – persistent slip bands (PSBs) – intersect the original flat surface. Thin PSMs cut both γ matrix and γ′ precipitates. PSMs consist of extrusions accompanied by parallel intrusions. Fatigue cracks were initiated at individual PSMs. Focused ion beam (FIB) revealed fatigue crack nucleation from sharp intrusion and its subsequent propagation parallel to one of the active {111} 〈110〉 slip systems. Transmission electron microscopy (TEM) observation showed plastic strain localization into dislocation rich slabs running parallel to {111} slip planes.

Meso-scale progressive damage modeling and life prediction of 3D braided composites under fatigue tension loading

3D braided composites have broad potential applications in the high-tech industries because of their superior mechanical properties. Fatigue is an essential design factor for their use in those engineering applications. The fatigue damage accumulation during cyclic loading should be involved in the numerical models in order to predict the fatigue life accurately. In this paper, a unit-cell based finite element model in conjunction with continuum damage mechanics (CDM) is developed for simulating the fatigue damage evolution process and predicting the fatigue life of 3D braided composites under fatigue tension loading. This meso-scale fatigue modeling, including stress analysis, failure criteria and material property degradation scheme, is implemented via a user-material subroutine UMAT based on ABAQUS/Standard platform with FORTRAN code. The fatigue damage initiation and propagation processes of 3D braided composites with typical braiding angles on the unit-cell model as a function of number of cycles are presented in detail. The fatigue life of 3D braided composites is predicted from the computed S-N curve and the stiffness degradation process is also investigated. The obtained numerical results indicate that the present model can provide a suitable reference to the numerical study of the fatigue issues in other textile composites.

Pressure-sensitive bond fatigue model with damage evolution driven by cumulative slip: Thermodynamic formulation and applications to steel- and FRP-concrete bond

In this paper we introduce a thermodynamically consistent bond-interface pressure-sensitive damage model with cumulative sliding strain measure as a fundamental source of fatigue damage. The modeling approach provides a clear physical interpretation of the dissipative mechanisms governing the propagation of fatigue damage within the concrete-steel/FRP interface so that it is possible to reproduce both the monotonic and the cyclic behavior of the bond with a consistent set of material parameters. The model has been applied for simulation of the degradation process in the bond between concrete and reinforcement bars and between concrete and FRP sheets under pullout fatigue loading. The paper presents numerical studies of the fatigue pullout behavior with detailed analysis of the damage propagation during loading and unloading stages, and at different levels of imposed lateral pressure. To validate the ability of the model, simulations of pullout tests published in the literature have been conducted.

Predictive airframe maintenance strategies using model-based prognostics

Aircraft panel maintenance is typically based on scheduled inspections during which the panel damage size is compared to a repair threshold value, set to ensure a desirable reliability for the entire fleet. This policy is very conservative since it does not consider that damage size evolution can be very different on different panels, due to material variability and other factors. With the progress of sensor technology, data acquisition and storage techniques, and data processing algorithms, structural health monitoring systems are increasingly being considered by the aviation industry. Aiming at reducing the conservativeness of the current maintenance approaches, and, thus, at reducing the maintenance cost, we employ a model-based prognostics method developed in a previous work to predict the future damage growth of each aircraft panel. This allows deciding whether a given panel should be repaired considering the prediction of the future evolution of its damage, rather than its current health state. Two predictive maintenance strategies based on the developed prognostic model are proposed in this work and applied to fatigue damage propagation in fuselage panels. The parameters of the damage growth model are assumed to be unknown and the information on damage evolution is provided by noisy structural health monitoring measurements. We propose a numerical case study where the maintenance process of an entire fleet of aircraft is simulated, considering the variability of damage model parameters among the panel population as well as the uncertainty of pressure differential during the damage propagation process. The proposed predictive maintenance strategies are compared to other maintenance strategies using a cost model. The results show that the proposed predictive maintenance strategies significantly reduce the unnecessary repair interventions, and, thus, they lead to major cost savings.

Effect of notch on static and fatigue performance of multilayered composite structures under tensile loads

Fatigue failure of laminated composites containing circular or elliptical (vertical and horizontal) holes and subjected to tension was studied. This research investigates the stress components, the initiation and propagation of static and fatigue damage and failure in notched composite laminates by analytic method and FE model, both of which are validated by experimental tests. The major focus was to understand the failure mechanisms of composites induced by stress concentrations, and to determine the effects of ply orientations, material properties and geometry on the fatigue damage. It was demonstrated that the crack initiation and growth was connected with the interaction of the stress intensity factors (SIFs) KI and KII.

Wet and dry flexural high cycle fatigue behaviour of fully bioresorbable glass fibre composites: In-situ polymerisation versus laminate stacking

Fully bioresorbable phosphate based glass fibre reinforced polycaprolactone (PCL/PGF) composites are potentially excellent candidates to address current issues experienced with use of metal implants for hard tissue repair, such as stress shielding effects. It is therefore essential to investigate these materials under representative loading cases and to understand their fatigue behaviour (wet and dry) in order to predict their lifetime in service and their likely mechanisms of failure. This paper investigated the dry and wet flexural fatigue behaviour of PCL/PGF composites with 35% and 50% fibre volume fraction (Vf). Significantly longer flexural fatigue life (p < 0.0001) and superior fatigue damage resistance were observed for In-situ Polymerised (ISP) composites as compared to the Laminate Stacking (LS) composites in both dry and wet conditions, indicating that the ISP promoted considerably stronger interfacial bonding than the LS. Immersion in fluid (wet) during the flexural fatigue tests resulted in significant reduction (p < 0.0001) in the composites fatigue life, earlier onset of fatigue damage and faster damage propagation. Regardless of testing conditions, increasing fibre content led to shorter fatigue life for the PCL/PGF composites. Meanwhile, immersion in degradation media caused softening of both LS and ISP composites during the fatigue tests, which led to a more ductile failure mode. Among all the composites that were investigated, ISP35 (35% Vf) composites maintained at least 50% of their initial stiffness at the end of fatigue tests in both conditions, which is comparable to the flexural properties of human cortical bones. Consequently, ISP composites with 35% Vf maintained at least 50% of its flexural properties after the fatigue failure, which the mechanical retentions were well matched with the properties of human cortical bones.

A new algorithm for prognostics using Subset Simulation

This work presents an efficient computational framework for prognostics by combining the particle filter-based prognostics principles with the technique of Subset Simulation, first developed in S.K. Au and J.L. Beck [Probabilistic Engrg. Mech., 16 (2001), pp. 263-277], which has been named PFP-SubSim. The idea behind PFP-SubSim algorithm is to split the multi-step-ahead predicted trajectories into multiple branches of selected samples at various stages of the process, which correspond to increasingly closer approximations of the critical threshold. Following theoretical development, discussion and an illustrative example to demonstrate its efficacy, we report on experience using the algorithm for making predictions for the end-of-life and remaining useful life in the challenging application of fatigue damage propagation of carbon-fibre composite coupons using structural health monitoring data. Results show that PFP-SubSim algorithm outperforms the traditional particle filter-based prognostics approach in terms of computational efficiency, while achieving the same, or better, measure of accuracy in the prognostics estimates. It is also shown that PFP-SubSim algorithm gets its highest efficiency when dealing with rare-event simulation.

TSA based evaluation of fatigue crack propagation in steel bridge members

Fatigue crack propagation in aging steel bridges has become a serious problem. Nondestructive evaluation of fatigue damage propagation is necessary to ensure safety and to estimate the remaining life of the bridges. Conventionally employed nondestructive testing (NDT) techniques such as visual testing, magnetic particle testing and ultrasonic testing are time- and labor- consuming techniques, further these NDT techniques cannot be used to directly evaluate the remaining strength of the bridges. Thermoelastic stress analysis (TSA) using infrared thermography has been widely used as an effective full-field experimental stress measurement technique. In this study, TSA was applied for on-site measurement of stress distributions around fatigue cracks, and the future crack propagation behavior was estimated by the fracture mechanics approach. Experimental studies were conducted for laboratory specimens which modeled a part of welded structure in steel bridges. The stress intensity factors were calculated from stress distributions measured by TSA technique. Relationship between stress intensity factor ranges and crack propagation rates was obtained. It is found that the obtained relationship shows a good correspondence with the Paris law. Further TSA technique was applied to evaluate the effectiveness of repair or reinforcement for defective portions. Severity reduction in stress distribution around the fatigue crack after treatment was confirmed for actual steel bridge members by TSA. Crack propagation rate was estimated from the stress intensity factor calculated from on-site stress measurement data. As the result, 55% reduction in crack propagation rate was ascertained indicating the positive effect of the crack repair.

Statistical Performance Assessment of an NDE-Based SHM-DP Methodology for the Remaining Fatigue Life Prediction of Monitored Structural Components and Systems

Integrated structural health monitoring and damage prognosis (SHM-DP) methodologies, coupled with sensor-based nondestructive evaluation (NDE) techniques, are becoming increasingly important for the near-real-time condition assessment (i.e., SHM) and future performance predictions (i.e., DP) of aging mechanical systems, civil structures, and infrastructure networks, as well as automotive, naval, and aerospace vehicles. A successful SHM-DP strategy, capable of identifying all critical damage mechanisms while accounting for all relevant sources of uncertainty, can be used as an advanced tool to effectively and optimally manage the life-cycle of the monitored system, recursively forecast its remaining useful life (RUL), and ultimately reduce the overall ownership cost through dynamic reliability-based inspection and maintenance (RBIM) plans, system downtime minimization, catastrophic failure prevention, and potential RUL extension. In this perspective, fatigue damage propagation is one of the most critical and unpredictable deterioration processes for a large variety of structural and mechanical systems that are subjected repeatedly to cyclic and/or random operational loading during their service life. Within this limited scope, the authors developed a comprehensive NDE-based SHM-DP framework for recursively predicting the time-varying system reliability and the remaining fatigue life (RFL) of monitored systems subjected to deterioration by multi-site fatigue damage propagation. This paper provides a brief overview of the proposed framework and then uses a set of experimental fatigue test data to perform a thorough statistical performance assessment of the developed methodology at the local reliability component level (i.e., single damage mechanism and single damage location) including NDE detectability and measurement uncertainty as well as both load and model parameter uncertainty.

Fatigue damage propagation models for ductile fracture of ultrahigh toughness cementitious composites

Ultrahigh toughness cementitious composites are a kind of high-performance cementitious material with a characteristic of ductile fracture. Based on the continuum damage mechanics theory and flexural fatigue damage model, two damage propagation models of ultrahigh toughness cementitious composites are built. One is a linear bilogarithmic model with J-integral range as its independent variable, while the other one is a linear model on a semilogarithmic scale with fatigue stress level as its independent variable. However, according to former research, the J-integral depends strongly on specimens’ geometry, so the first damage propagation model is deeply influenced by material dimension. As a result, the second damage propagation model is more convenient in application, shows the material fatigue property in comparison with the first model. In order to prove these two models and obtain the parameters, a three-point flexural fatigue experiment on single-edge-notched fracture specimens is carried out. The results shows that the two models fit better with the experimental results, rather than the crack propagation law of ultrahigh toughness cementitious composites.

Probabilistic inference of fatigue damage propagation with limited and partial information

A general method of probabilistic fatigue damage prognostics using limited and partial information is developed. Limited and partial information refers to measurable data that are not enough or cannot directly be used to statistically identify model parameter using traditional regression analysis. In the proposed method, the prior probability distribution of model parameters is derived based on the principle of maximum entropy (MaxEnt) using the limited and partial information as constraints. The posterior distribution is formulated using the principle of maximum relative entropy (MRE) to perform probability updating when new information is available and reduces uncertainty in prognosis results. It is shown that the posterior distribution is equivalent to a Bayesian posterior when the new information used for updating is point measurements. A numerical quadrature interpolating method is used to calculate the asymptotic approximation for the prior distribution. Once the prior is obtained, subsequent measurement data are used to perform updating using Markov chain Monte Carlo (MCMC) simulations. Fatigue crack prognosis problems with experimental data are presented for demonstration and validation.

How proper similitude can improve our understanding of crack closure and plasticity in fatigue

The appropriateness of some common similitude principles with respect to describing and predicting fatigue damage propagation is discussed. Linear elastic fracture mechanics have provided a basis to describe damage growth using stress intensity factors or strain energy release rates, both related to the work of Griffith and Irwin. The fatigue crack growth equations presented in the literature are discussed, and it is demonstrated that the principles of similarity in current methodologies have not yet been well established. As a consequence, corrections for the stress ratio effect are misunderstood. An alternative principle of similitude using cyclic work and strain energy release is proposed.

Fatigue Life Prediction of Composite Pin Joints

In this research, new progressive fatigue damage models are established to calculate the fatigue life and simulate damage process of composite pin joints. The proposed models based on residual strength and residual stiffness of unidirectional laminates, have three parameters to present the different damage state, which can accurately describe the growth process of fatigue damage propagation by the mathematical method. The fatigue damage models combining with stress analysis, failure analysis, and material property degradation process, can predict the fatigue life, damage state and residual material properties of composite structures under arbitrary loading conditions. Using the models, composite pin joints with different stacking sequence are analyzed, fatigue life and damage quantification are concluded simultaneously. The proposed models and the process of analysis provide a way to solve the fatigue durability of composite structures.

Effects of compressive stresses on torsional fatigue

Rolling contact fatigue (RCF) is the dominant failure mode in properly installed and maintained ball and roller element bearings. Lundberg and Palmgren in their seminal publication indicated that this failure is due to the alternating component of shear stress. Thus, torsional fatigue experiments have been used to predict the RCF behavior of bearing materials. In non-conformal contacts, due to Hertzian pressure the contact experiences large compressive stresses. Hence, it is critical to take into account the effect of these large compressive stresses in torsional fatigue to better simulate RCF conditions. This paper presents an investigation of torsional fatigue of bearing steels, while the effects of combined compressive stress and its relevance to material behavior in rolling contact fatigue is examined. An MTS test rig was used to investigate the fatigue life of several bearing steels and their failure mechanisms were evaluated through fractography. Then the effects of compressive stresses on torsional fatigue were investigated. A set of custom designed clamp fixtures were designed, developed and used to apply Hertzian pressures of up to 2.5GPa on the torsion specimens. The experimental results indicate that at high cycle fatigue, a combination of shear and biaxial compression, by application of Hertzian contact, is more detrimental to fatigue life than shear alone; however, as expected it has little to negligible effects in the low cycle fatigue regime. Also the failure mode changes such that fracture planes form a cup and cone pair with multiple internal cracks as opposed to helical planes observed in pure torsion which are formed by a single crack. A 3D finite element model (using ABAQUS) was developed to investigate the fatigue damage accumulation, crack initiation, and propagation in the material. The topology of steel microstructure is modeled employing a randomly generated Voronoi tessellation wherein each Voronoi cell represents a material grain and the boundaries between the cells are assumed to represent the weak plane in the steel matrix. Continuum damage mechanics (CDM) was used to model material degradation during the fatigue process. A comprehensive damage evolution equation is developed to account for the effect of mean stress on fatigue. The model predicts the fatigue lives and crack patterns successfully both in presence and absence of compressive stresses.

Fatigue damage propagation in unidirectional glass fibre reinforced composites made of a non-crimp fabric

Damage progression in unidirectional glass fibre reinforced composites manufactured of a non-crimp fabric subjected to tension-tension fatigue is investigated, and a quantitative explanation is given for the experimentally observed stiffness degradation. The underlying damage-mechanisms are examined using three distinct microscopic analyses, and the transverse crack density is measured. It is documented that the stiffness loss in fatigue is directly related to fibre fractures in the load-carrying axial fibre bundles, initialised by interface debonding and cracking in the transverse backing bundles. A simple stiffness spring model validates the stiffness loss observed. A fatigue damage scheme is presented, which suggests that damage initiates due to failure of the backing bundle causing a stress concentration in the axial load-carrying fibres. This stress concentration, along with fretting fatigue, gives rise to axial fibre fractures and a loss of stiffness, eventually leading to final failure. The uniqueness of the present work is identification of the mechanisms associated with tension fatigue failure of unidirectional non-crimp fabrics used for wind turbine blades. The observed damage mechanisms need further attention and understanding in order to improve the fatigue life-time of unidirectional glass fibre reinforced non-crimp fabrics.

An adaptive Cu trace fatigue model based on average cross-section strain

This study focuses on quasi-static mechanical cycling durability of copper traces on printed wiring assemblies (PWAs). PWA specimens populated with Land Grid Array (LGA) components on copper-defined pads were cycled to failure under zero-to-max, three-point bending. Failure is defined in terms of electrical opens due to fatigue damage propagation through the entire cross-section of the trace. Failure statistics were collected and failure analysis was conducted to identify fatigue failures in the copper traces, near the connection to the solder pad.Cyclic bending of this assembly was modeled with 3D, elastic–plastic, finite deformation (geometrically nonlinear) finite element analysis. Due to the complexity of the geometry, a two-step global–local approach was used to identify the cyclic strain history and the mean-stress at the copper trace failure site.A generalized strain-based fatigue model is proposed, to characterize the fatigue durability in terms of the amplitude of cyclic strain and the cyclic mean of the hydrostatic stress. To be consistent with the failure criterion defined above, the strain and stress values are averaged over the entire cross-section. Average cross-sectional model constants are iteratively estimated by ensuring that they are simultaneously compatible with both the durability test data and the copper stress–strain curves used in the finite element analysis (FEA). To minimize the computational burden, a Response Surface (RS) was created for the actual iteration. The key RS variables and their functional inter-relationships were determined from a fractional factorial FEA parametric study.This study is important because it provides insight into copper trace failure in PWAs under mechanical cycling, a quantitative model to predict its occurrence, and validated guidelines to prevent it by design.

Stacking Sequence Effects on Fatigue Intra-laminar Damage Progression in Composite Joints

In this paper the effects of the stacking sequence on the fatigue intra-laminar damage accumulation in pinned composite joints is investigated. A fatigue damage propagation numerical model based on gradual material degradation rules and Hashin fatigue failure criteria is formulated, implemented in a finite element platform and then used to simulate the intra-laminar fatigue damage evolution in the analysed composite joints. The model has been preliminary validated against literature experimental data in terms of s-n curves providing confirmation of its effectiveness in predicting the joints fatigue life. Different stacking sequences: zero-dominated, quasi-isotropic, cross-ply with different 0 ° and 90 ° plies distributions, have been considered when investigating the influence of the stacking sequence on the fatigue behaviour of the joints. The simulation of the joints’ fatigue life provided detailed information on the intra-laminar damage mechanisms on-set and evolution related to fatigue gradual degradation of material stiffness and strength for different values of the applied maximum stresses.