Tension-compression fatigue behaviour of fibre-reinforced ceramic matrix composite with circular hole

The tension-compression fatigue behaviour of a silicon carbide fibre-reinforced glass ceramic matrix composite, SiC/1723, with a circular hole was investigated at room temperature. Two laminate lay-ups were studied: cross-ply, [0/90]_2s, and unidirectional, [0]₈. At first, the fatigue limit based on one million cycles was established for the tension-tension fatigue condition. Then, the fatigue response under fully reversed (tension-compression) cycling loading with a maximum stress equal to the tension-tension fatigue limit was investigated. This tension-compression loading resulted in an increased amount of damage and ultimately led to the specimen failure well before one million cycles. In the cross-ply laminate, the damage mechanisms in the 90° plies involved transverse cracks only during tension-tension cycling, and transverse and longitudinal cracks during tension-compression cycling. In the unidirectional laminate, the longitudinal cracks which initiated at the hole periphery grew longer in tension-compression fatigue than in tension-tension fatigue. On the other hand, no damage and consequently no effect on fatigue life was observed during the compression-compression fatigue condition only.     

LIFE PREDICTION OF CROSS-PLY METAL MATRIX COMPOSITES UNDERGOING THERMOMECHANICAL FATIGUE

A life prediction model that was originally developed for the axial loading of unidirectional metal matrix composites (MMCs) undergoing combined thermal and mechanical loading is extended to the axial loading of cross-ply MMCs by adding an internally initiated matrix fatigue damage term. This new term accounts for the growth of cracks that initiate at the location where fibre–matrix separation occurs in the transversely-oriented plies. A comparison of the model predictions to experimental data on SCS-6/Timetal 21S shows that the model reasonably accounts for the dependence of applied stress, temperature and environment, as well as cyclic frequency. The dominant damage accumulation process for cross-ply MMCs with weak fibre–matrix bonds is described by this internally initiated matrix fatigue damage process for most stress–temperature cycle combinations. However, the fibre-dominated damage accumulation process operates under in-phase TMF when both stress and temperature are high. Environment-enhanced matrix fatigue is the dominant damage accumulation process under isothermal fatigue when stress is low and temperature is high.     

Creep fatigue behaviour of Type 321 stainless steel at 650°C

Abstract

Type 321 austenitic stainless steel has been used in the UK’s advanced gas cooled reactors for a wide variety of thin section components which are within the concrete pressure vessel. These components operate at typically 650°C and experience very low primary stresses. However, temperature cycling can give rise to a creep fatigue loading and the life assessment of these cycles is calculated using the R5 procedure. In order to provide materials property models and to validate creep fatigue damage predictions, the available uniaxial creep, fatigue and creep fatigue data for Type 321 have been collated and analysed. The analyses of these data have provided evolutionary models for the cyclic stress strain and the stress relaxation behaviour of Type 321 at 650°C. In addition, different methods for predicting creep fatigue damage have been compared and it has been found that the stress modified ductility exhaustion approach for calculating creep damage gave the most reliable predictions of failure in the uniaxial creep fatigue tests. Following this, validation of the new R5 methods for calculating creep and fatigue damage in weldments has been provided using the results of reversed bend fatigue and creep fatigue tests on Type 321 welded plates at 650°C in conjunction with the materials properties that were determined from the uniaxial test data.     

An investigation of the damage mechanisms and fatigue life diagrams of flax fiber-reinforced polymer laminates

In this paper we investigated the fatigue damage of a unidirectional flax-reinforced epoxy composite using infrared (IR) thermography. Two configurations of flax/epoxy composites layup were studied namely, [0]₁₆ unidirectional ply orientation and [±45]₁₆. The high cycle fatigue strength was determined using a thermographic criterion developed in a previous study. The fatigue limit obtained by the thermographic criterion was confirmed by the results obtained through conventional experimental methods (i.e., Stress level versus Number of cycles to failure). Furthermore, a model for predicting the fatigue life using the IR thermography was evaluated. The model was found to have a good predictive value for the fatigue life. In order to investigate the mechanism of damage initiation in flax/epoxy composites and the damage evolution, during each fatigue test we monitored the crack propagation for a stress level and at different damage stages, a direct correlation between the percentage of cracks and the mean strain was observed.     

Strain-based multiaxial fatigue damage modelling

A new multiaxial fatigue damage model named characteristic plane approach is proposed in this paper, in which the strain components are used to correlate with the fatigue damage. The characteristic plane is defined as a material plane on which the complex three‐dimensional (3D) fatigue problem can be approximated using the plane strain components. Compared with most available critical plane‐based models for multiaxial fatigue problem, the physical basis of the characteristic plane does not rely on the observations of the fatigue crack in the proposed model. The cracking information is not required for multiaxial fatigue analysis, and the proposed model can automatically adapt for different failure modes, such as shear or tensile‐dominated failure. Mean stress effect is also included in the proposed model by a correction factor. The life predictions of the proposed fatigue damage model under constant amplitude loading are compared with a wide range of metal fatigue results in the literature.     

The influence of particle content on the equi-biaxial fatigue behaviour of magnetorheological elastomers

The equi-biaxial fatigue behaviour of silicone based magnetorheological elastomers (MREs) with various volume fractions of carbonyl iron particles ranging between 15% and 35% was studied. Wöhler curves for each material were derived by cycling test samples to failure over a range of stress amplitudes. Changes in complex modulus (E¹) and dynamic stored energy during the fatigue process were observed. As for other elastic solids, fatigue resistance of MREs with different particle contents was shown to be dependent on the stress amplitudes applied. MREs with low particle content showed the highest fatigue life at high stress amplitudes while MREs with high particle content exhibited the highest fatigue resistance at low stress amplitudes. E¹ fell with the accumulation of cycles for each material, but the change was dependent on the particle content and stress amplitude applied. However, each material failed in a range suggesting a limiting value of E¹ for the material between 1.22 MPa and 1.38 MPa regardless of the particle content and the magnitude of the stress amplitude. In keeping with results from previous testing, it was shown that dynamic stored energy can be used to predict the fatigue life of MREs having a wide variation in particle content.     

Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 10⁷ cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of K_t = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 10⁶ cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.     

Effect of stress ratios on tension–tension fatigue behavior and micro-damage evolution of basalt fiber-reinforced epoxy polymer composites

The tension–tension fatigue behavior and damage mechanism of basalt fiber-reinforced epoxy polymer (BFRP) composites at different stress ratios are studied in this paper. The fatigue experiments were performed under stress ratios, R?=?σ_min/σ_max of 0.1 and 0.5, while the lifetime and the stiffness degradation were monitored and analyzed to investigate the effect of stress ratios. The damage propagation during fatigue loading was periodically monitored by using an in situ scanning electron microscope (SEM). The results show that the fatigue life decreases and the fatigue life degradation rate increases with the decrease of stress ratio for examined BFRP composites. The stiffness degradation is also sensitive to different stress ratios, showing a greater stiffness loss before failure at lower stress ratio. From the SEM images, it is indicated that the micro-damage mode shifts from interface debonding and matrix cracking into fiber breaking with decreasing stress ratios.     

DETERMINATION OF THE POSSIBLE ERROR IN FATIGUE DAMAGE FOR A DISCRETE STRAIN HISTORY

Abstract— The Markov matrix, which is a statistical representation of a service load history, is generally derived using 32 or more strain levels, since this is considered an optimum level between computational effort and accuracy of fatigue life prediction. A recently developed model which predicts the distribution of closed loops and fatigue life from the Markov matrix, is limited in the number of levels which can be analysed because of computational restrictions. This investigation was undertaken to determine the change in the accuracy of fatigue life predictions as the number of strain levels used to represent the strain history is reduced. Since each discretized level is used to represent a range of strain values, a given discretized history will represent a family of histories. A technique is developed which gives the distribution of damage for the family of histories given the discretized history and the distribution of peaks and valleys within each strain interval. By comparing the predicted damage caused by the measured history to a conservative estimate of the distribution of damage for the family of histories, the range of possible error is calculated and used to determine the relationship between the number of strain levels used in the discrete history and the possible error in predicted fatigue damage.     

Ratcheting‐fatigue behaviour of bainite 2.25Cr1MoV steel with tensile and compressed hold loading at 455°C

The ratcheting behaviour of a bainite 2.25Cr1MoV steel was studied with various hold periods at 455°C. Particular attention was paid to the effect of stress hold on whole‐life ratcheting deformation, fatigue life, and failure mechanism. Results indicate that longer peak hold periods stimulate a faster accumulation of ratcheting strain by contribution of creep strain, while double hold at peak and valley stress has an even stronger influence. Creep strains produced in peak and valley hold periods are noticeable and result in higher cyclic strain amplitudes. Dimples and acquired defects are found in failed specimen by microstructure observation, and their number and size increase under creep‐fatigue loading. Enlarged cyclic strain amplitude and material deterioration caused by creep lead to fatigue life reduction under creep‐fatigue loading. A life prediction model suitable for asymmetric cycling is proposed based on the linear damage summation rule.     

Fatigue of beta titanium alloy at 20, 482 and 648 °C

Fatigue life of fibrous metal matrix composites is limited by the distribution of fibre strengths, the fibre‐matrix interfacial strength, and the fatigue resistance of the matrix. The aim of this work is to provide fatigue results for a beta titanium alloy over a range of temperatures and stresses that can be used as input for predicting fatigue life of a titanium matrix composite. Stress controlled tests having fatigue ratios between ?1 and ?0.2 were conducted on a limited number of samples machined from unreinforced laminated Ti‐15Mo‐3Al‐2.7Nb‐0.2Si (TIMETAL^®21S) sheets to represent as closely as possible the in situ matrix material. Stress control was used to enable quantification of strain ratcheting for tensile mean stresses and a fast loading rate was used to minimize time‐dependent (creep) deformation. Stress amplitude‐life data at 20, 482 and 648 °C for fully reversed loading are well fit by a power law. Normalizing the stress amplitude with respect to the power law coefficient appears to account for the temperature dependence of the S–N curves. As the tests had large strains and lives were in the low‐cycle fatigue range, strain range at the half‐life was also correlated to life. For tensile mean stress cycling at 482 and 648 °C, the rate of strain ratcheting per cycle increased to failure; shakedown was not observed.     

Critical plane approach with two families of microcracks for modelling of unilateral fatigue damage

New multiaxial fatigue damage model based on the critical plane approach is proposed. Two different physical mechanisms of the fatigue damage development on each potential failure plane (critical plane) are considered. In general, each critical plane contains two families of a parallel microcracks. The proposed model reproduces simultaneously fatigue damage induced anisotropy, the influence of positive and negative mean stresses, unilateral fatigue damage, microcrack closure effect and fatigue behaviour under variable amplitude loading. The expression for the equivalent stress in the damage evolution equation includes the stress intensity for the amplitudes as well as joint invariants for the mean values of the stress tensor and for the vectors associated with the directions of microcracks. The theoretical predictions are compared with experimental data under uniaxial cyclic loading of brass specimens. The influence of positive and negative mean stresses on the fatigue life of brass is investigated.     

An improved multiaxial high‐cycle fatigue criterion based on critical plane approach

Several groups of fatigue damage parameters are discussed and then an improved multiaxial high‐cycle fatigue criterion based on critical plane defined by the plane of maximum shear stress range is presented in this paper. A compromising solution to consider the mean normal stress acting on the critical plane is also proposed. The new fatigue criterion extends the range of metallic materials which is valid for the ratio 1.25 < f_?1/t_?1 < 2. The predictions based on the presented model show a good agreement with test data.     

预氧化Cf/SiC陶瓷基复合材料及其构件的抗疲劳特性研究

采用在线销钉集成技术实现了二维C_f/SiC复杂构件的近尺寸成型,并考察预氧化C_f/SiC销钉集成构件的高周疲劳寿命及破坏模式。实验结果表明:C_f/SiC构件在不同激振加速度条件下均表现为由销钉断裂所引起的整体分层破坏,层板连接处为C_f/SiC构件的振动疲劳薄弱部位。通过ANSYS振动应力分析和微观组织分析可以推论出,疲劳试验时,裂纹容易沿着层板间的基体扩展,在基体开裂失效后,全部应力施加于销钉处,最终在疲劳应力作用下销钉发生断裂,导致构件整体分层破坏。     

A modification of Smith–Watson–Topper damage parameter for fatigue life prediction under non‐proportional loading

In this paper, the shortcomings of the Smith–Watson–Topper (SWT) damage parameter are analysed on the basis of the critical plane concept. It is found that the SWT model usually overestimates the fatigue lives of materials since it only takes into account the fatigue damage caused by the tensile components. To solve this problem, Chen et al. (CXH) modified the SWT model through considering the shear components. However, there are at least two problems present in CXH model: (1) the mean stress is not considered and (2) the different influence of the normal and shear components on fatigue life is not included. Besides, experimental validations show that the modification by Chen et al. usually leads to conservative fatigue life predictions during non‐proportional loading. In order to overcome the shortcomings of SWT and CXH models, a damage parameter as the effective strain energy density (ESED) is proposed. Experimental validations by using eight kinds of materials show that the ESED model can give satisfactory fatigue life predictions under the non‐proportional loading.     

A simple unified critical plane damage parameter for high-temperature LCF life prediction of a Ni-based DS superalloy

A simple unified critical plane damage parameter (i.e., the modified resolved shear strain range ?γ _mod) based on a slip mechanism-related critical plane concept was proposed in this paper, integrating life prediction of low cycle fatigue (LCF) behavior affected by anisotropy, load ratio and stress concentration into one framework, where the critical plane is determined as the slip plane on which the damage parameter is the maximum during the cycle. For notched specimens, this procedure was specially carried out at the fatigue initiation sites located on the notch surface, which were well predicted by the distribution of Von-Mises stress range ?σ _Mises. The applications of this damage parameter in a directionally solidified superalloy at high temperatures showed that the LCF lives resulting from complicated loading conditions (i.e., variable material orientation, temperature, loading ratio and notch feature) were well simulated consistently, and the predicted fatigue life is within a scatter band of ±3.     

复杂面内应力状态下平面编织高铝纤维增强氧化铝基复合材料强度及疲劳寿命预测方法

针对平面编织氧化铝基复合材料提出了一种复杂面内应力状态下的强度准则和疲劳寿命预测方法。通过拉伸、压缩及纯剪切试验,分别获得了材料的静强度指标。考虑材料拉、压性能的差异和面内拉-剪联合作用对材料强度的影响机制,提出了修正的Hoffman强度理论。采用该强度理论预测得到的偏轴拉伸强度与试验结果基本一致,偏差不超过10%。开展了偏轴角θ=0°、15°、30°、45°,应力比R=0.1,频率f=10 Hz的拉伸疲劳试验,试验结果表明随着偏轴角的增加,相同轴向拉伸载荷下的疲劳寿命逐渐降低。由于面内剪切应力分量的作用,疲劳失效由纤维主导逐渐过渡到纤维和基体共同主导的模式。基于单轴疲劳寿命曲线,采用Broutman-Sahu剩余强度模型表征剩余强度随疲劳循环次数的变化规律,结合剩余强度演化模型和修正的Hoffman强度理论,提出了一种面内复杂载荷条件下的疲劳寿命预测模型,并引入疲劳剪切损伤影响因子表征拉-剪应力联合作用对材料疲劳行为的影响。采用本文提出的疲劳寿命预测模型,预测不同偏轴角拉伸疲劳寿命,预测结果与试验结果基本一致,偏差在1倍寿命范围内。比较结果表明在给定应力比、温度和疲劳载荷频率条件下,该疲劳寿命预测模型可以用来预测平面编织氧化铝基复合材料拉-剪复杂面内载荷条件下疲劳寿命。      

Fatigue crack propagation in microcapsule-toughened epoxy

The addition of liquid-filled urea-formaldehyde (UF) microcapsules to an epoxy matrix leads to significant reduction in fatigue crack growth rate and corresponding increase in fatigue life. Mode-I fatigue crack propagation is measured using a tapered double-cantilever beam (TDCB) specimen for a range of microcapsule concentrations and sizes: 0, 5, 10, and 20% by weight and 50, 180, and 460 μm diameter. Cyclic crack growth in both the neat epoxy and epoxy filled with microcapsules obeys the Paris power law. Above a transition value of the applied stress intensity factor ΔK _T, which corresponds to loading conditions where the size of the plastic zone approaches the size of the embedded microcapsules, the Paris law exponent decreases with increasing content of microcapsules, ranging from 9.7 for neat epoxy to approximately 4.5 for concentrations above 10 wt% microcapsules. Improved resistance to fatigue crack propagation, indicated by both the decreased crack growth rates and increased cyclic stress intensity for the onset of unstable fatigue-crack growth, is attributed to toughening mechanisms induced by the embedded microcapsules as well as crack shielding due to the release of fluid as the capsules are ruptured. In addition to increasing the inherent fatigue life of epoxy, embedded microcapsules filled with an appropriate healing agent provide a potential mechanism for self-healing of fatigue damage.     

Damage evolution in a [±45]2S CFRP laminate under block loading conditions

Cyclic tests were conducted on [±45]_2S angle ply carbon–epoxy specimens using stress ratios, R (minimum/maximum stress) of 0.1 and −1.0. Damage was monitored by measuring progressive strain changes in the loading direction. The fatigue damage parameter was found to satisfactorily describe the evolution of damage throughout life, facilitating fatigue life prediction. Two distinct stages of damage evolution were identified. In Stage I, the fatigue damage parameter and the density of matrix micro-cracking rapidly increased to a level dependent upon the stress (Characteristic Damage State). This was followed by Stage II which was a long period (90% life) of gradual increase in damage, involving crack coalescence, debonding and delamination.On subjecting the specimens to two step block loading tests, synergistic interaction occurred whereby the total fatigue life was greater than that predicted by the summation of the individual blocks of cycles. The effect of crack density and crack closure appeared to play important roles in extending the fatigue life. For the low to high stress level block transition, more cycles were required to reach the Characteristic Damage State, whereas for the high to low sequence, the presence of a large number of cracks and matrix debris within them resulted in closure at the lower stress. Again, the number of cycles to failure increased.