Progressive Fatigue Damage Research Articles

Simulation and measurement of the self-heating phenomenon of carbon/epoxy laminated composites under fatigue loading

In the present research, the self-heating phenomenon in laminated composites under fatigue loading was studied. The generated-dissipated energy (GDE) model was developed to calculate the generated energy due to the viscoelastic nature of the matrix, and the dissipated energy through the convection and conduction mechanisms. To consider the effects of irreversible fatigue failure modes in laminated composites, the GDE model was coupled with the progressive fatigue damage (PFD) model. By coupling these two models, the self-heating (S–H) model was developed to simulate the self-heating temperature rise in laminated composites under fatigue loadings. An experimental program was conducted to validate the present model. The temperature at the surface of laminated composites was monitored by an infrared thermography camera during the fatigue tests. The experimental results show that the present model can precisely predict the self-heating temperature rise in composite specimens with an arbitrary stacking sequence under fatigue loading conditions.

An investigation into fatigue behaviour and damage progression in pseudo-ductile thin-ply angle-ply laminates

This paper presents a detailed investigation on the fatigue response and damage progression in pseudo-ductile thin-ply angle-ply laminates with two configurations: an intermediate modulus-high modulus fibre combined IM-HM [±277/0]s and a standard modulus SM-SM [±266/0]s laminate. 80% of the “yield” stress was found to be the maximum stress for both laminates, where they can withstand 106 cycles without obvious damage or stiffness reduction. When laminates containing pre-fractured 0° plies were loaded in fatigue, both of the laminates showed a delamination dominated damage mode: the delamination initiated from the pre-fractured 0° plies and propagated at the interface between the 0° and angle plies with increasing number of cycles. An estimated delamination growth rate based on the stiffness reduction and delamination initiated from 0° ply fracture are also discussed. The IM-HM laminate with broken 0° fibres can still withstand fatigue loading at 80% severity without further modulus reduction or damage progression, whilst the SM-SM failed in delamination within a small number of cycles.

Degradation of elastic characteristics of the CFRP used in the design of a gas turbine engine as a result of high-cycle fatigue damage

Carbon fiber reinforced polymers (CFRP) are increasingly being used in heavily loaded parts of aircraft engines. CFRP fan blades, vanes must successfully resist fracture due to high-cycle fatigue. One of the consequences of fatigue damage is a decrease in the rigidity of the material and a drop in the natural vibration frequencies of parts. These effects are of interest when developing fatigue fracture models and predicting durability. The purpose of this article is to develop a method and obtain experimental data on the decrease of elastic characteristics of CFRP as a result of progressive fatigue damage. The developed technique consists of two stages. During the first one, the natural frequencies and eigenmodes of the samples during their fatigue testing are experimentally obtained. During the second stage, the four elasticity parameters of the CFRP laminate monolayer are identified via the natural frequencies. The inverse numerical/experimental technique for material properties identification is applied. The dependences of elastic characteristics on the relative fatigue life are obtained as experimental results of both modal and fatigue tests. The results can be useful in studying the fatigue behavior of the examined materials and in creating methods for calculating fatigue life.

Change of the Elastic Characteristics of a Fiber-Reinforced Laminate as a Result of Progressive Fatigue Damage

It is a widely known fact that the stiffness of polymer composite materials decreases with the accumulation of fatigue damage under cyclic loading. The purpose of this article is to develop a method and obtain experimental data on decrease of the elastic characteristics of a fiber-reinforced laminate, as a result of progressive fatigue damage. The developed technique consists of two stages. At the first one, the natural frequencies and eigenmodes of the samples during their fatigue testing are experimentally obtained. The dependences of the natural frequencies of the samples on the number of loading cycles are found. At the second stage, the four elasticity parameters of the laminate monolayer (two Young modules, the shear module and Poisson's ratio) are identified via the natural frequencies. The inverse numerical/experimental technique for material properties identification is applied. The dependences of the natural frequencies and mentioned elastic characteristics on the relative fatigue life are obtained as experimental results of both modal and fatigue tests. The results can be useful to study the fatigue behavior of the investigated materials and to create methods for calculating fatigue life.

Short-beam shear fatigue life assessment of thermally cycled carbon–aluminium laminates with protective glass interlayers

Fibre metal laminates (FMLs) are attractive construction materials, especially for use in aerospace and transport facilities. Throughout their service life, thin-walled structures made of FMLs are exposed to static and dynamic loads, as well as corrosion and the unfavourable influence of environmental conditions. The paper presents an experimental analysis of the combined mechanical and environmental long-term behaviour of carbon-based fibre metal laminates and their variants with protective glass layers. The Al alloy/CFRP and Al alloy/GFRP/CFRP laminates in a 3/2 configuration were used. The tested laminates were subjected to 1500 thermal cycles with a temperature range of 130 °C. The static and fatigue interlaminar shear strengths were tested before and after thermal conditioning. It was shown that the stable stiffness reduction in the tested laminates was observed with increasing fatigue cycles, due to the progressive fatigue damage accumulation. The thermally cycled laminates feature slightly smoother stiffness loss, while a more rapid decrease was observed in thermally untreated laminates. Moreover, the fatigue life of the tested laminates subjected to thermal cycling revealed nine times fewer fatigue cycles of laminates with glass protectors after thermal cycles in comparison to the laminates not subjected to thermal cycling.

Strain-controlled fatigue life prediction of Flax-epoxy laminates using a progressive fatigue damage model

A finite element model that incorporates the progressive fatigue damage model (PFDM) is developed to predict the fatigue life of Flax-epoxy composites with [0]16, [90]16, [0/90]4S, [±45]4S and Quasi-isotropic [0/+45/90/−45]2S layups under strain-controlled conditions. A stochastic distribution method is used to consider the manufacturing-induced variation of the material properties. The PFDM analysis includes stress analysis, sudden material property degradation as identified by the Hashin failure and maximum stress criteria, and gradual material property degradation in the longitudinal, transverse and in-plane shear directions. The gradual stiffness degradation is coupled with gradual strength degradation to avoid excessive testing. The PFDM is implemented via a user-defined material subroutine UMAT in ABAQUS that is written in FORTRAN. The predicted strain-life curves and residual stiffness degradation show a good agreement with the experimental results at different strain levels.

Numerical life prediction of unidirectional fiber composites under block loading conditions using a progressive fatigue damage model

Practical applied components made of Fiber-Reinforced Plastics (FRPs) are usually subjected to cyclic loading with variable load amplitudes and arbitrary load orientation in the course of their service life. For a cost-effective design process of composite structures, it is essential to use computationally efficient and detailed fatigue damage design tools. This contribution focuses on the extension and application of a progressive Finite-Element-based Fatigue Damage Model (FDM) for unidirectional FRPs for lifetime prediction under different block loading conditions. The employed FDM relies on a layer- and energy-based approach, and it is capable of including basic fatigue phenomena such as load sequence effects and stress redistributions. First, the damage evolution laws on which the FDM is based are extended to block loading conditions with arbitrary load sequences. The effects of passive damage, which occur in this context under alternating cyclical tensile and compressive loading, are also taken into account. Secondly, appropriate block loading patterns are defined for the numerical prediction of the lifetime. Finally, the calculation results are presented and critically evaluated based on experimental findings from literature. The comparison of simulations with experiments demonstrates the high predictive capacity of the presented FDM.

Fatigue Response of Cold-Rolled Type-304 Stainless Steel Foil

This study presents the fatigue response of 304 stainless steel foil, cold-rolled to a thickness of 3.2 μm with 87 percent cold work at orientations of 0, 45, and 90 degrees to the direction of rolling. Fatigue specimens were fabricated by laminating a supportive layer of 20-μm polyimide film to one side of the foil and patterning 242 crack initiation features by photolithographic process. Progression of fatigue damage was determined through electrical resistance measurement. The fatigue response was demonstrated to be largely affected by anisotropy existing between the rolling direction and the off-axis orientations. Fatigue cracks that traveled in a direction parallel to the elongated grains (cyclic loads applied at 90-degree orientation to foil rolling direction) had the most fatigue response (undesirable characteristic). The construction of the specimens with thin foil supported by a film backing contributed to high fatigue threshold.

Progressive fatigue damage modelling of fibre-reinforced composite based on fatigue master curves

In this paper, fatigue progressive damage of fibre-reinforced composite is modelled by the present fatigue model, in which the fatigue damage incorporates two parts of the delamination and in-plane damage. The in-plane fatigue life is determined by the method of fatigue master curves. Further, Hashin and cohesive fatigue failure criteria are adopted for the determination of in-plane and delamination failure initiation, respectively. The fatigue master curve successfully predicts the fatigue life of fiber-reinforced composite with arbitrary layup sequence under fatigue loading of arbitrary stress ratio. Moreover, the delamination of interlayer subjected to arbitrary stress ratio is simulated based on the modified cohesive element. Finally, the proposed fatigue finite element model shows good agreements with the experimental data of notched glass fibre-reinforced plastic (GFRP) composite laminate and carbon fibre-reinforced plastic (CFRP) composite bolted joints.

Novel method for in situ damage monitoring during ultrasonic fatigue testing by the advanced acoustic emission technique

Ultrasonic fatigue testing (USFT) is an effective method for the rapid characterisation of the high cycle fatigue properties of structural materials. However, the process of initiation and progression of fatigue damage remains uncertain in this way of testing due to the limitations of existing measuring techniques. The acoustic emission (AE) method was developed in the present work to pave a new way to monitor the fatigue process during USFT. The proposed new methodology revealed the AE activity related to fatigue damage, allowing to distinguish between surface and internal fatigue crack initiation and to follow the development of fatigue damage.

Low-cycle fatigue behavior and damage progression of a fiber reinforced titanium matrix composite

The low-cycle fatigue behavior and damage progression of a SiC fiber reinforced Ti–6Al–2Sn–4Zr–2Mo-0.1Si matrix composite was investigated systematically. The fatigue life increased from 4 to 52388 cycles when the maximum stress decreased from 1500 MPa to 1100 MPa. The modulus calculated from the stress-strain response exhibited little variation for all the specimens during the fatigue process. Fractographic observations showed that the fatigue cracks initiated internally and propagated preferentially in the internal sputtered matrix instead of the unreinforced matrix in the external layer for the cylindrical specimen. The proportion of the fatigue region in the fracture surface increased as the applied stress level decreased. The internally-progressed damage, especially breaks of bridging fibers, was well characterized in situ by the acoustic emission system.

Characterization of impact fatigue damage in CFRP composites using nonlinear acoustic resonance method

Accidental impacts may induce impact fatigue damage in carbon fiber reinforced plastic (CFRP) composite and the progressive fatigue damage is difficult to detect by traditional non-destructive techniques. In this work, a nonlinear acoustic resonance method was proposed for characterizing the impact fatigue damage in CFRP composites. Based on the hysteretic nonlinearity of heterogeneous materials, a fatigue model was constructed to evaluate the impact fatigue damage and predict the residual service life of the composite material. Thereafter, experiments of pendulum impact were conducted on the CFRP composites and nonlinear acoustic resonant were measured to evaluate the impact fatigue damage. Additionally, the impact fatigue damage in the CFRP composites was inspected by an infrared thermography and an optical microscopy. The results indicate that the shift of resonance frequency occurs due to the stiffness degradation and the relative frequency shift has a good linear relation with excitation voltage and strain change. In addition, the damage index rises slowly at the beginning of the impact cycle and rapidly increases when the impact fatigue life exceeds 70%. The experimental results demonstrate that characterizing the impact fatigue damage in CFRP composites using nonlinear acoustic resonance method is an effective and reliable approach.

Fatigue damage prediction in the annulus of cervical spine intervertebral discs using finite element analysis

Neck pain is a major inhibitor affecting the performance of U.S. military personnel. Repetitive exposure to cyclic loading due to military activities over several years can lead to accumulation of fatigue damage in the cervical intervertebral disc annuli, leading to neck pain. We have developed a computational damage model based on continuum damage mechanics, to predict fatigue damage to cervical disc annuli over several years of exposure to military loading scenarios. By integrating this fatigue damage model with a finite element model of the cervical spine, we have overcome the underlying assumption of a uniform stress distribution in the annulus. The resulting element-wise damage prediction gives us insight into the location of damage initiation and pattern of fatigue damage progression in the cervical disc annulus.

Revealing the competitive fatigue failure behaviour of CFRP-aluminum two-bolt, double-lap joints

The fatigue failure behaviour of CFRP-metal fastened joints in highly-loaded structures differs from that of CFRP-CFRP or metal-metal fastened joints because of the significant dissimilarity between fatigue damage mechanisms of CFRP and metal materials. In this paper, the competitive fatigue failure behaviour of a CFRP-aluminum two-bolt, double-lap joint was revealed via experiments and a novel numerical prediction method. In the proposed method, fatigue failures of the CFRP and aluminum joined plates were predicted by a novel progressive fatigue damage model and the nominal stress method, respectively. The predicted competitive fatigue life and failure mode of the joint were consistent with the experimental results, which validated the efficiency of the proposed method. It was found that, the collapse failure of the joint is caused by the failure of the CFRP laminate at high loading levels, whereas caused by the failure of the aluminum plate at low loading levels. Furthermore, influences of the thickness ratio of the joined plates on the competing fatigue failure of the joint were determined. Based on the critical thickness ratios corresponding to the transition of failure mode, the fatigue failure mode diagram of the joint was established, which is helpful for the design of the CFRP-aluminum bolted joint.

Fatigue of graphene.

Materials can suffer mechanical fatigue when subjected to cyclic loading at stress levels much lower than the ultimate tensile strength, and understanding this behaviour is critical to evaluating long-term dynamic reliability. The fatigue life and damage mechanisms of two-dimensional (2D) materials, of interest for mechanical and electronic applications, are currently unknown. Here, we present a fatigue study of freestanding 2D materials, specifically graphene and graphene oxide (GO). Using atomic force microscopy, monolayer and few-layer graphene were found to exhibit a fatigue life of more than 109 cycles at a mean stress of 71 GPa and a stress range of 5.6 GPa, higher than any material reported so far. Fatigue failure in monolayer graphene is global and catastrophic without progressive damage, while molecular dynamics simulations reveal this is preceded by stress-mediated bond reconfigurations near defective sites. Conversely, functional groups in GO impart a local and progressive fatigue damage mechanism. This study not only provides fundamental insights into the fatigue enhancement behaviour of graphene-embedded nanocomposites, but also serves as a starting point for the dynamic reliability evaluation of other 2D materials.

Numerical study of the effect of torsional fatigue loading on the energy absorption of composite tubes

AbstractA three‐dimensional finite element model is developed to predict the effect of torsional fatigue loading on the energy absorption of unidirectional carbon/epoxy (AS4/3501‐6) composite tubes. Progressive fatigue damage model for fatigue loading and progressive failure model for axial velocity impact are implemented separately in the Abaqus software through user subroutine VUMAT. Each model is validated by the experimental data available in literature and good agreement is observed between the simulation results and experimental data. Then, these two models are sequentially applied to the composite tube with three different layups. The tube is first placed under torsional fatigue loading and a number of predefined loading cycles is applied. During this loading, the mechanical properties of the material are degraded based on sudden and gradual material properties degradation rules. Next, the fatigued tube is placed under axial impact loading. The results show that by increasing the number of fatigue cycles, the tube's ability to absorb energy is first reduced uniformly but in a particular cycle, due to the sudden drop in the shear mechanical properties of the material, the amount of energy absorption drops sharply and the tube loses its ability to absorb energy before it's catastrophic fatigue failure.

Progressive fatigue damage analysis of composite bolted joint using equivalent stress model.

Composite bolted joints are quite necessary for composite structures connection, which has become the main limit for the use of composites in main load-bearing structures. In this article, a fatigue model of composite bolted joint based on equivalent stress is established by programming in ABAQUS USDFLD subroutine to simulate the progressive failure of composite bolted joints. By introducing three-dimensional Tsai-Hill static failure criterion, equivalent stress is calculated for investigating effects of multiaxial stress on fatigue life. In the subroutine of progressive failure for fatigue model, fatigue life of composite bolted joint and damage state of elements that are meshed in the process of modelling are connected by defining field variable. Different fatigue modes are predicted here by changing stress amplitude and ratio loading, in which simulation results agree well with that obtained in corresponding experiments.

Assessment of a Progressive Fatigue Damage Model for AS4/3501–6 Carbon Fiber/Epoxy Composites Using Digital Image Correlation

This paper presents an experimental study for the assessment of a three-dimensional Progressive Fatigue Damage Model (PFDM) with a carbon fiber/epoxy (AS4/3501–6) material system. Specimens with a central circular hole were selected to represent notched composite structures. Digital Image Correlation (DIC) was used to monitor the surface strain evolution throughout the fatigue lifetime. The PFDM model was implemented in the commercial finite element software ABAQUS using the user material (UMAT) utility. Residual stresses from the composite manufacturing cycle are included in the fatigue damage modeling. Results are presented that compare experimental and predicted strain distributions and fatigue lives. Comparisons between the experimental and simulated response highlight the value of the PFDM model while demonstrating its shortcomings.

Fatigue damage behavior in carbon fiber polymer composites under biaxial loading

An investigation into the damage accumulation and propagation behavior in carbon fiber reinforced polymer (CFRP) composites under complex in-phase biaxial fatigue loading has been conducted. The goal is to capture early stage damage and obtain an improved understanding of damage propagation and associated degradation in material properties. Both cross ply and quasi isotropic laminate configurations have been studied and the tests were conducted under constant amplitude in-phase biaxial loading. An optimization technique was used to design the cruciform specimens for each stacking sequence. To understand the propagation of damage from the micro-to the macroscale, the fractured surfaces were analyzed, during various stages of fatigue, using electron microscope assisted fractography and a high-resolution camera. Material property degradation was determined by measuring the change in specimen stiffness to analyze the progression of fatigue damage and is correlated to the micro- and macroscale damage mechanisms and the biaxial fatigue loading parameters. The results provide insight into the initiation and propagation of damage mechanisms in CFRP composites which is essential to understanding the fatigue behavior of composite materials under complex multiaxial loadings.

Continuous damage-based Voronoi finite element analysis of contact fatigue of a wind turbine gear

Contact fatigue failures of gears, especially those used in heavy duty conditions such as wind turbine gears, become important issues in mechanical transmission industry. In the present work, a continuous damage mechanism and Voronoi-based finite element model is developed to investigate the contact fatigue of a wind turbine gear. Plane strain assumption is adopted to simplify the gear contact model. Voronoi tessellations are utilized to represent the microstructure topology of the gear material, and continuous damage mechanism is implemented to reflect the material degradation within critical substrate area. With the developed framework, the contact pressure distribution, intergranular mechanical response and the progressive fatigue damage at the grain boundaries during repeated gear meshing are evaluated and discussed in detail. The depths of the maximum shear stress reversal and the crack initiation agree well with previously reported findings. The influence of microstructure on the gear contact fatigue behavior is also investigated.