Fatigue crack growth and load redistribution in Ti/SiC composites observed in situ

Sequential synchrotron X-ray microtomography and diffraction have been applied to follow the growth of fatigue cracks and the associated load redistribution in a Ti/SiC fibre composite. A sequence of micron resolution tomographs reveal for the first time how the cracks progress from ply to ply around the fibres. Complementary high spatial resolution (40 μm) diffraction scans interleaved between the tomographic image acquisitions during the fatigue experiment have enabled the fibre strains and thereby the interfacial shear stress to be mapped as a function of crack growth. The matrix crack front was found to bow out between fibres, eventually reconnecting further downstream. This leads to the prolonged retention of bridging matrix ligaments and increased crack path tortuosity. The rate of crack growth was found to slow somewhat as a fibre is approached. As the crack grew past the fibres under observation the extent of the sliding region and the level of the fibre bridging stress increased. The interfacial shear strength after fatigue was around 60 MPa in the crack-tip region, in common with previous experiments.     

The effect of fibre fractures in the bridging zone of fatigue cracked Ti–6Al–4V/SiC fibre composites

The aim of the paper is to study the partitioning of stress between bridging and broken fibres and the nearby matrix in the region around a fatigue crack in the matrix of a Ti–6Al–4V/SCS–6 SiC fibre composite. This was achieved by using synchrotron X-ray radiation to perform a combination of high spatial resolution tomographic imaging and strain mapping. The average elastic fibre strain for each ply was mapped with distance from the crack, ply by ply. Two samples were examined; one in which there were no broken fibres and one in which some fibres in ply 1 had broken. The contributions of broken and bridging fibres were separated using a double peak fitting routine. The interfacial stress variation and the extent of interfacial debonding were deduced from the fibre strain profiles. Contrary to most micromechanical models the interfacial frictional sliding stress was not found to be constant along the fibre length, but to decrease approximately linearly towards the crack plane. Upon unloading the fibres were found to undergo reverse sliding at the interface.     

Measurements of fibre bridging during fatigue crack growth in Ti/SiC fibre metal matrix composites

High spatial resolution synchrotron X-ray strain mapping has been used to map the elastic matrix and fibre strains in the vicinity of a fatigue crack in a Ti–6Al–4V/SCS6 SiC fibre composite. A 0.61 mm fatigue crack was initiated and grown in three-point-bending. By using an in-situ loading stage it was possible to map the crack opening (longitudinal) strain distribution at K_appl=K_max and K_appl=0. In the far field region, significant thermally induced stresses were evident, being compressive in the fibres and tensile in the matrix. Around the notch and in the wake of the crack tip essentially no residual strain and only small interfacial shear stresses were found in the unloaded case, indicative of a debonded/damaged interface. At K_max the maximum tensile stress in the matrix is in the vicinity of the crack tip, whereas for the SiC fibres the maximum stress is in the bridging zone in the wake of the crack. The perturbed zone extends about ±1.5 mm either side of the crack. It was at the boundary of this zone that the maximum interfacial shear stresses (∼80 MPa) were measured in the loaded stage. A small area of tensile strain in front of the crack tip in the unloaded condition suggests frictional resistance from the bridging fibres acts to keep the crack slightly open. A simple three-dimensional finite element model has been developed to help interpret the results. The crack is introduced statically by node release and the Coulomb friction law governs the interface strength. The results of the model are compared to the synchrotron strain measurements. This comparison confirms the degradation of the interface strength in the wake of the crack.     

Interfacial shear strength behaviour of Ti/SiC metal matrix composites at room and elevated temperature

Synchrotron X-ray strain measurements have been used to follow the distribution of elastic strain, and hence interfacial shear stress, along single SCS-6 and Sigma SM2156 Ti–6Al–4V matrix coated SiC monofilaments sandwiched between Ti–6Al–4V foils during single-fibre-fragmentation testing. The interfacial shear strength behaviours were characteristically different. For the SCS-6 system, the interfacial response was dominated by classical frictional sliding, initiating near the ends and progressing along most of the length of the fragments, in common with previous observations. By contrast, for the SM2156 fibre system, a significant threshold stress was evident which must be exceeded before sliding could occur. As a result, sliding was only found near the ends of the fibre fragments. Upon unloading, reverse frictional sliding was found to take place, initiating from the fibre ends at a shear stress somewhat lower than that for forward sliding. Finite element modelling suggests that this is due to a reduction in the radial fibre clamping stress upon unloading rather than a change in the friction coefficient. For both the systems, the frictional sliding strength fell approximately linearly with increasing temperature towards zero at ～600–700 °C, consistent with a Coulomb friction coefficient of ～0.4 and thermal clamping residual stresses that approach zero at these temperatures. By contrast, the threshold stress required to initiate sliding for SM2156 falls at a slower rate with increasing temperature, such that it would still be significant at these temperatures. Some evidence was found for a decrease in interfacial shear stress with decreasing fragment length.     

A micromechanical bridging law model for CFCCs

In the present work, a methodology is presented for the assessment of bridging laws for continuous fibre-reinforced ceramic matrix composites based on material properties as well as micromechanics of fibre deformation and failure. A load–displacement model is initially formulated that utilizes weakest-link statistical concepts to analyse and relate the individual contributions of matrix, intact/bridging and failed/pull-out fibres during the composite fracture process. The total and individual contributions to the bridging law and crack growth resistance of the material are determined by identifying the non-elastic part of displacement as crack opening. The model is validated against the experimentally recorded load–displacement behaviour of a notched SiC-fibre-reinforced glass–ceramic matrix composite tested under monotonic tension. The output parameters of the converged regression procedure remain within a small scattering range from the corresponding mean values that compare favourably with known material properties. A parametric analysis of the effect of fibre volume fraction, Weibull modulus of fibres and interfacial shear stress in overall composite performance is presented in view of the ability of the model to serve as an a priori fracture prediction tool.     

Fatigue crack initiation and multiplication of unnotched titanium matrix composites

Fatigue crack initiation and multiplication of the unnotched SCS-6 silicon carbide fiber-reinforced titanium matrix composites with different matrix and interfacial properties have been investigated experimentally and analytically. Ti–15V–3Al, Ti–6Al–4V, and Ti–22Al–23Nb were chosen as matrix materials. The initiation and propagation of each individual matrix crack as a function of fatigue cycles and applied stress levels were carefully monitored. The statistical distribution of crack growth rates in each composite has been constructed and analyzed. The evolution of normalized matrix crack density and stiffness reduction of these composites under fatigue loading also has been characterized. A modified shear-lag model, coupled with the strain-life equation and a fiber bridging model were used to predict the fatigue crack initiation life, matrix crack growth rate, normalized matrix crack density, and residual stiffness of the composites. The predicted fatigue properties correlated well with experimental results.     

In situ observation on fatigue crack growth in SCS-6/Ti-15-3 composite at elevated temperatures

Direct observation on fatigue crack growth behavior in SiC (SCS-6) fiber-reinforced Ti-15-3 alloy matrix composite subjected to a constant tension–tension loading mode was performed by scanning electron microscope using a single edge-notched specimen in vacuum at room temperature and 550 °C. The fatigue crack growth rate at 550 °C was lower than that at room temperature, and the difference between the fatigue crack growth rates at room temperature and 550 °C increased with increasing fatigue cycles. The crack opening displacement at 550 °C was smaller than that at room temperature when the crack length exceeded a definite value, though the interface friction stress between the fiber and matrix at elevated temperature was much smaller than that at room temperature. The above results were explained qualitatively by a residual stress mechanism at the crack front and the crack closure behavior at crack wake, which could be produced by matrix creep asymmetry in tension and compression at elevated temperature during each fatigue cycle.     

Effect of fatigue on the interface integrity of unidirectional Cf-reinforced epoxy resin composites

The current study reports on the fatigue behaviour, micromechanics of reinforcement and stress transfer efficiency of the interface of an autoclave-processed C_f-epoxy laminate. Throughout fatigue loading at a maximum strain below the critical fatigue limit of the matrix material, testing was interrupted at discrete fatigue levels of 10⁰, 10³, 10⁴ and 10⁵ cycles to allow for laser Raman microscopy measurements of the axial stresses developing along natural and induced discontinuities on fibres in the composite. The established axial stress profiles were regressed to establish the corresponding interfacial shear stress (ISS) profiles at each fatigue level. The effect of fatigue on the stress transfer efficiency of the interface was calculated through the evolution of three interfacial performance parameters (maximum ISS, length required for its attainment and transfer length) as a function of loading cycles. Additional measurements on “random” locations helped identify the main damage mechanisms developing at the microscale.     

Fatigue Behavior and Damage Evolution of SiC Fiberreinforced Ti-6Al-4V Alloy Matrix Composites

研究SiC纤维增强钛基复合材料(SiCf/Ti-6Al-4V)室温疲劳行为和损伤演化机制。疲劳试验条件:载荷控制、应力比0.1和加载频率10 Hz。采用疲劳断裂试验建立最大加载应力为600~1200 MPa内SiCf/Ti-6Al-4V的S-N曲线。采用疲劳中止试验以及SEM显微分析研究应力水平对SiCf/Ti-6Al-4V疲劳损伤演化的影响。结果表明,SiCf/Ti-6Al-4V疲劳损伤萌生模式与演化过程与应力水平密切相关。在高应力水平(Smax=1000 MPa),纤维开裂是主要损伤萌生模式。一旦2或3根纤维断裂后,纤维裂纹和基体裂纹开始联接并形成宏观扩展裂纹。在中等应力水平(Smax=800 MPa),基体裂纹萌生与扩展是主要损伤模式。多条基体裂纹萌生于试样外表面棱边和离外表面附近试样内部开裂的纤维基体界面处。基体裂纹均沿垂直于加载方向扩展,且大部分纤维未断裂并纤维桥接基体裂纹。在低应力水平(Smax=600 MPa),仅在C涂层和界面反应层之间和C涂层内部观察到局部界面脱粘现象。     

Fatigue crack growth in a Tiβ21s/SCS-6 composite

The fatigue crack growth resistance of a Tiβ21s/SCS-6 composite at room temperature has been characterised experimentally and analysed theoretically. Particular attention has been paid to the transition from crack-arrest (CA) to catastrophic specimen failure (CF), and emphasis has been placed on the influence of the extrinsic factors of initial notch length, specimen width and loading configuration, and of the intrinsic fibre strength distribution on this transition. Experimental results show a marked influence of extrinsic factors on the CA/CF transition, and they also reveal the importance of fibre strength on the fatigue crack growth resistance of the composite. The stress in bridging fibres has been analysed theoretically, and the results have been used to explain the experimental observations.     

Torsional Fatigue Cracking and Fracture Behaviors of Cold-Drawn Copper:Effects of Microstructure and Axial Stress

The fatigue cracking and fracture behavior of cold-drawn copper subjected to cyclic torsional loading were investigated in this study.It was found that with increasing stress amplitude,the fracture mode of cold-drawn copper gradually changes from a shear fracture on transverse maximum shear stress plane to a mixed shear mode on both transverse and longitudinal shear planes and finally turns to the shear fracture on multiple longitudinal shear planes.Combining the cracking morphology and the relationship between torsional fatigue cracking and the grain boundaries,the fracture mechanism of cold-drawn copper under cyclic torsional loading was analyzed and proposed by considering the effects of the microstructure and axial stress caused by torsion.Because of the promotion of the grain boundary distribution on longitudinal crack propagation and the inhibition of axial stress on transverse crack grown,the tendency of crack propagation along the longitudinal direction increases with increasing stress levels.     

短纤维增强复合材料的仿生模型──Ⅰ 哑铃状短纤维增强复合材料的应力分析

受生物体构件的启发，提出了哑铃状短纤维增强复合材料的模型．分析了带球短纤维和基体中的应力分布，发现端头半径粗化将改善纤维中轴向应力分布，使之趋于均匀化，同时可减少纤维端部界面的剪应力．研究表明纤维长径比对纤维端头应力影响较小．讨论了纤维－基体模量比对纤维轴向最大拉伸应力和界面最大剪应力的影响．     

In situ flow stress of pure aluminium constrained by tightly packed alumina fibres

The in situ matrix flow stress of continuous fibre-reinforced aluminium is measured in tension along the fibre axis. We use a new, tighter, estimate for the effect of differential Poisson contraction between fibres and matrix and take into consideration nonlinear elastic fibre behaviour; these improvements remove inconsistencies found in earlier work. Resulting in situ matrix flow stress curves are characterized by a substantial gain in hardness of the matrix as compared to the unreinforced alloy, and a strong Bauschinger effect. These effects are caused by dislocation emission during cooldown by matrix/fibre thermal strain mismatch. The surprising insensitivity of hardening to the prior rate of composite cooldown suggests that thermal dislocational hardening starts already at temperatures where unreinforced pure aluminium would creep rapidly. The absence of significant recovery during furnace cooldown is attributed to a small amount of iron in supersaturated solution, and/or to subgrain boundary pinning at the fibres.     

High cycle fatigue behaviour of Ti–6Al–4V alloy at elevated temperatures

High cycle fatigue behaviour of Ti–6Al–4V alloy was studied at 623 K and 723 K. Fatigue strength decreased at elevated temperatures compared with at ambient temperature. In the short life regime, fatigue strength was lower at 723 K than at 623 K, but in the long life regime it was nearly the same at both temperatures. At elevated temperatures, cracks were generated earlier at applied stresses below the fatigue limit at ambient temperature, indicating lowered crack initiation resistance. Small cracks grew faster at elevated temperatures than at ambient temperature, which became more noticeable with increasing temperature. After allowing for the elastic modulus, small cracks still grew faster at elevated temperatures.     

团聚颗粒对SiC/AZ91D复合材料裂纹萌生和扩展行为影响的研究

基于复合材料真实微观结构,在颗粒与基体界面引入内聚力单元,建立了4种不同颗粒团聚分布的有限元模型(均匀分布、三处团聚、两处团聚和一处团聚),研究了颗粒团聚对SiC/AZ91D复合材料裂纹萌生和扩展机制的影响。结果表明:当裂纹萌芽时,应力在基体中分布很不均匀,最大应力值出现在颗粒群尖角处,颗粒团聚程度越严重,裂纹萌生最大应力值越大;当裂纹扩展时,颗粒团聚程度越严重,基体内最大应力值越大,裂纹扩展程度越高;当裂纹完全断裂时,随着颗粒团聚程度的加剧,颗粒应力最大值逐渐增加,而基体应力最大值变化不大。颗粒团聚加速裂纹萌生扩展过程,颗粒应均匀分布于基体中。复合材料裂纹萌生扩展机制是由于SiC颗粒群边界和尖角处应力集中严重,导致基体损伤,萌生微裂纹,微裂纹沿着切应力最大方向扩展汇集成主裂纹。     

Time-dependent micromechanical behavior in graphite/epoxy composites under constant load: a combined experimental and theoretical study

We present an integrated theoretical and experimental study on the localized creep behavior around fiber breaks in model unidirectional graphite fiber/epoxy matrix composites under constant axial stress at room temperature. Micro Raman spectroscopy (MRS) and classic composite shear-lag models were coupled to examine the time evolution of fiber and matrix strain/stress distributions around a single fiber break in planar low volume fraction graphite fiber–epoxy matrix composites. In-situ MRS micro-scale measurements show that strain redistribution around the fiber fracture is time-dependent and localized. We observe decreases in peak interfacial shear stress and concomitant increases in load recovery length and interfacial inelastic zones from the fiber fracture point. These results showing the time dependence of load transfer are related to creep tests on the monolithic matrix material at various stress levels. The translation of monolithic to in-situ matrix creep is achieved using two viscoelastic matrix composite models, a multi-fiber and a single fiber model. MRS results show that the load recovery length increases at the rate of (T/T_c)^α/2 and the maximum interfacial shear stress relaxes at the rate of (T/T_c)^−α/2, where T is time, T_c and α are parameters obtained from matrix creep tests. These results are in good agreement with the multi-fiber model predictions. The single fiber model gives similar results for these samples where the fiber spacing is relatively large (5∼7 fiber diameters).     

A BIOMIMETIC MODEL OF SHORT FIBRE REINFORCED COMPOSITE MATERIALS── I Stress Distribution of Composite Reinforced by Dumbbell Fibres

受生物体构件的启发，提出了哑铃状短纤维增强复合材料的模型．分析了带球短纤维和基体中的应力分布，发现端头半径粗化将改善纤维中轴向应力分布，使之趋于均匀化，同时可减少纤维端部界面的剪应力．研究表明纤维长径比对纤维端头应力影响较小．讨论了纤维－基体模量比对纤维轴向最大拉伸应力和界面最大剪应力的影响．     

High temperature high cycle fatigue behavior of new aluminum alloy strengthened by (Co,Ni)3Al4 particles

High cycle fatigue (HCF) behavior of a new heat-resistant aluminum alloy at elevated temperature was investigated. This alloy consists of an α-Al matrix, a small amount of precipitated Mg₂Si, and distributed (Co, Ni)₃Al₄ strengthening particles. HCF tests were conducted with a stress ratio of (R)=0 and a frequency of (F)=30 Hz at 130 °C. The fatigue limit (maximum stress) of this alloy was 120 MPa at 10⁷ cycles. This is a value superior to that of conventional heat-resistant aluminum alloys such as the A319 alloy. Furthermore, regardless of the stress conditions, the new heat-resistant Al alloy has an outstanding fatigue life at high temperatures. The results of fractography observation showed that second phases, especially (Co, Ni)₃Al₄ particles, were effective to the resistance of fatigue crack initiation and propagation. On the other hand, Mg₂Si particles were more easily fractured by the fatigue crack. This study also clarifies the micromechanism of fatigue deformation behavior at elevated temperature related to its microstructure.     

Influence of Elevated Temperature and Stress Ratio on the Fatigue Response of AM60B Magnesium Alloy

The fatigue response of a high pressure die-cast AM60B Mg alloy is studied at room and elevated temperatures. The fatigue tests are conducted with stress ratio of R?=?0.1 and frequency of 30?Hz. The main objective is to determine whether elevated temperature would affect the fatigue response of the alloy. In addition, fatigue crack growth characteristics of the alloy is investigated at room temperature. The purpose of this test is to ascertain the capability and accuracy of a finite element approach coupled with the Walker model in assessing the life cycle of the alloy, in consideration of the influence of stress ratio.     

Micromechanical analysis of internal stress development during single-fibre fragmentation testing of Ti/SiCf

An axisymmetric finite element model of a single fibre composite tensile specimen having a Ti–6Al–4V matrix reinforced with one SCS6 SiC fibre has been constructed. The sliding of the fibre is modelled using a constant critical shear stress and fibre fragmentation introduced using the element removal technique. In this way, the fibre and matrix strains have been computed as the fibre fractures progressively. The results are compared to synchrotron X-ray strain scanning measurements made in-situ during the test and critical interfacial parameters are extracted. Good agreement is found between numerical and experimental results based on a critical shear stress of 200 MPa for loading and unloading including both forward and reverse slip.