A unified fatigue failure criterion for unidirectional laminates

A unified fatigue failure criterion using micromechanics related to the fracture plane has been developed to predict fatigue lives of unidirectional fibre reinforced polymer composites subjected to cyclic off-axis tension–tension loading. Since the failure criterion incorporates both stresses and strains it may be characterized as energy based. Accounting for the fibre load angle as well as the stress ratio is the novelty of this fatigue failure criterion. The criterion only requires the stress ratio to be known from the experimental procedure. The relation between the applied load and the micro-stress and micro-strain field can be determined from a numerical method. The fatigue failure criterion has been verified by applying it to different sets of experimental data. Several fibre load angles, different fibre/matrix combinations as well as stress ratios are covered. The predicted fatigue lives are in good agreement with the experimental results for both different fibre load angles and stress ratios.     

Behaviour ofE-glass fibre reinforced vinylester resin composites under impact fatigue

An impact fatigue study has been made for the first time on 63.5% glass fibre reinforced vinylester resin notched composites. The study was conducted in a pendulum type repeated impact apparatus especially designed and fabricated for determining single and repeated impact strengths. A well-defined impact fatigue (S-N) behaviour, having a progressive endurance below the threshold single cycle impact fracture stress with decreasing applied stress has been demonstrated. Fractographic analysis revealed fracture by primary debonding having fibre breakage and pullout at the tensile zone, but a shear fracture of fibre bundles at the compressive zone of the specimen. The residual strength, modulus and toughness showed retention of the properties at high impact stress levels up to 1000 impacts followed by a sharp drop. Cumulative residual stresses with each number of impacts not withstanding the static fatigue failure at long endurances have been ascribed for the composite failures under the repeated impact stresses.     

Tensile fatigue behaviour of tightly woven carbon/carbon composites

The tensile fatigue behaviour of a tightly woven carbon/carbon composite was investigated as a function of stress level. Load-controlled fatigue tests were performed in tension-tension mode with a stress ratio, R, of 0.1 under ambient laboratory conditions. Results of composite behaviour are discussed in terms of the relationship of the stress/strain behaviour to the fatigue life of these composites as well as the effects of applied stress levels. It is shown that these composites exhibit good resistance to cyclic loading. No fatigue failures were obtained after 10⁶ cycles when the maximum tensile load in the fatigue cycle is less than or equal to 80% of the static tensile strength. Evidence of textural changes related to fatigue was observed in the matrix region of these composites.     

Fatigue of boron-aluminium and carbon-aluminium fibre composites

Reversed bending fatigue studies have been performed on boron-aluminium and carbon-aluminium fibre composites. The superior fatigue resistance of the boron composites arises partly from the use of an alloy matrix, but mainly from the more advanced development stage attained in the manufacture of the boron composite. (The carbon-aluminium composites contained many broken fibres and relatively weakly-bonded interfaces which provided easy paths for the propagation of fatigue cracks.)Although matrix fatigue is the primary type of fatigue damage sustained by these composites, some evidence was found for induced fibre failure in boron-aluminium composites under high stress/low cycle conditions.     

In situ analysis of delayed fibre failure within water-aged GFRP under static fatigue conditions

A stress corrosion model has been applied to the microscopic analysis of the delayed fibre failure processes occurring within a water-aged unidirectional glass/epoxy composite under static fatigue loading (i.e. relaxation). By means of in situ microscopic observations, the individual fibre failures within an elementary volume located on the tensile side of the flexural specimens have been quantified as a function of time under various applied strain levels. It was found that the time dependence of the in situ fibre failure processes obeyed a stress corrosion model. From the microscopic observations, it was possible to assess consistent values of the parameters characterising the in situ fibre strength distribution and the subcritical crack propagation law. A comparison with separate static fatigue experiments using unimpregnated fibre bundles demonstrated that the specific physico-chemical environment encountered by the glass fibres within the aged epoxy matrix can induce significant changes in the subcritical crack propagation rates, as compared to stress corrosion cracking data collected in humid air.     

Effects of static–fatigue (tension) on the tension–tension fatigue life of glass fibre reinforced plastic composites

The effects of static–fatigue interaction on tension–tension fatigue life of glass fibre reinforced plastic (GFRP) composites were investigated. This paper proposed a new static–fatigue model, which is capable of predicting residual strength after a period of static loading. Also an algorithm is proposed to calculate fatigue lives with the inclusion of static–fatigue interaction. Predictions from the proposed static–fatigue model show a good agreement with the experimental results. Static–fatigue interaction has shown a considerable effect on fatigue lives of GFRP composites at intermediate and lower applied stress levels possibly due to a longer exposure to applied loads. At higher load levels approximately greater than 65% of ultimate stress, and higher stress ratios range like 0.5 < R < 0.9, fatigue lives shown to be closer to material’s static–fatigue limits which is shorter than the expected lifetime by cyclic fatigue.     

Fatigue strength of tubular carbon fibre composites under bending/torsion loading

Carbon fibre reinforced polymer composites have been increasingly used on structures frequently subjected to biaxial fatigue loadings. This paper studies the fatigue behaviour of tubular carbon fibre composites under in phase biaxial bending/torsion dynamic loadings. Particularly, it was analysed both the torsion stress and mean stress effects on the fatigue strength and failure mechanisms. Fatigue strength decreases significantly with increased torsional/bending stresses ratio, while the damage becomes faster. For the cases in which a torsion loading was applied the effect of the mean stress on the fatigue strength seems to be well fitted by using a quadratic equation.     

Fatigue behaviour of alumina-fibre-reinforced epoxy resin composite pipes under tensile and compressive loading conditions

Fatigue tests have been conducted on composites consisting of epoxy resin reinforced with alumina fibres (AFRP) under cyclic tensile and compressive loading conditions with the variation of fibre orientation. The behaviour of the stress/strain curve for a ±45° sample is different from those for the ±15 and ±25° composite specimens, whereas, the monotonic strength decreases with increase in fibre angle for all specimens, which satisfies the maximum stress failure criterion. Fatigue results show that the applied stress decreases with an increase in the number of cycles to failure under both loading conditions for all composite pipes, but for the ±45° sample the decrease was slow. The results of fatigue tests on a macroscopic level indicate that the matrix crack density slowly increased with increase in the normalized number of cycles to failure in all the specimens. The normalized apparent stiffness therefore falls with an increase of the normalized number of cycles to failure. However, the maximum stress decreased with the increase in the number of cycles to failure in the case of the ±45° pipe. Finally, it is observed that matrix cracking and delaminations are occurring in the ±45° sample whereas delamination and fibre buckling are appearing in the ±15 and ±25° samples.     

Statistical prediction of fatigue failure of fibre reinforced composite materials

A statistical approach is proposed to evaluate the residual strength and life of unidirectional and angle-ply composite laminates subjected to in-plane tensile cyclic stresses. The method is based on the extension of previous static failure criteria describing independently the fibre failure and matrix failure modes, combined with the statistical nature of fatigue failure of fibre-reinforced composites. The static and fatigue strengths of composite laminates at any off-axis angle are evaluated using the fatigue failure functions for the three principal failure modes, which are determined from the fatigue behaviour of unidirectional composites subjected to longitudinal and transverse tension as well as in-plane shear stresses. The evaluations of the fatigue strength of unidirectional E-glass/epoxy laminates under off-axis fatigue loading and angle-ply S-glass/epoxy laminates under in-plane fatigue loading show good agreement between theoretical predictions and experimental results.     

Fatigue behaviour of 3-d SiC/SiC Composites

A tension–tension fatigue damage analysis was performed using 3-d silicon carbide fibre reinforced (orthogonal) silicon carbide matrix (SiC/SiC) composites. Two groups of SiC/SiC specimens were tested. The first group consisted of samples without any oxidation protective top layer coating, whilst the latter one contained samples covered with a well fitting, chemical vapour deposited (CVD) SiC system. This coating is necessary for the material to sustain high temperatures. Both the coated and uncoated material had a fibre volume fraction of about 36% equally distributed in three rectangular directions. Load control fatigue tests were conducted at room temperature. The fatigue life was found to decrease by increasing the cyclic stress level. A power-law equation is proposed, which correlates the applied maximum stress during the fatigue test with the number of cycles to failure. In general, the presence of the coating layer decreases the static strength of the material. However, the nominal maximum cyclic stress for which the endurance fatigue limit appeared, remained unaffected by the presence of the oxidation protective SiC coating. Microstructural examination has also been performed on the fractured specimens and it reveals some of the failure mechanisms of the composite that appeared under quasi-static and dynamic loading.     

Transverse properties of a Ti-6-4 matrix/SiC fibre-reinforced composite under monotonic and cyclic loading

The transverse response of a Ti-6-4/SM1140+ fibre-reinforced composite to both monotonic and cyclic loading has been investigated. Five distinct regions were found in the monotonic stress versus strain curve: (I) elastic deformation of the composite, (II) failure of the fibre/matrix interfaces, (III) elastic deformation of the remaining matrix ligaments, (IV) yielding of the matrix ligaments, and (V) gross plastic deformation, which ultimately leads to specimen failure. The stresses at which interface debonding, matrix yield and final failure occurred rose with increased displacement rate. Stressing to levels above the interface failure stress caused significant damage and limited (0.025%) plastic deformation. A non-linear stress-strain response was observed on unloading/reloading, because the presence within the specimen of constrained holes (containing debonded fibres) resulted in non-homogeneous elastic straining of the matrix. The transverse low-cycle fatigue lives of Ti-6-4/SM1140+composite specimens were strongly dependent on maximum stress for values up to the interfacial failure stress, but less so for maximum stresses greater than 260–265 MPa, where full fibre/matrix debonding had occurred. Fatigue life was also dependent on the uniformity of fibre spacings within the composite.     

Modeling Strength Degradation of Fiber-Reinforced Ceramic-Matrix Composites Subjected to Cyclic Loading at Elevated Temperatures in Oxidative Environments

In this paper, the strength degradation of non-oxide and oxide/oxide fiber-reinforced ceramic-matrix composites (CMCs) subjected to cyclic loading at elevated temperatures in oxidative environments has been investigated. Considering damage mechanisms of matrix cracking, interface debonding, interface wear, interface oxidation and fibers fracture, the composite residual strength model has been established by combining the micro stress field of the damaged composites, the damage models, and the fracture criterion. The relationships between the composite residual strength, fatigue peak stress, interface debonding, fibers failure and cycle number have been established. The effects of peak stress level, initial and steady-state interface shear stress, fiber Weibull modulus and fiber strength, and testing temperature on the degradation of composite strength and fibers failure have been investigated. The evolution of residual strength versus cycle number curves of non-oxide and oxide/oxide CMCs under cyclic loading at elevated temperatures in oxidative environments have been predicted.     

Load controlled low cycle fatigue of [0]8 Sigma fibre reinforced titanium matrix composite

Fibre reinforced titanium matrix composites (TMCs) are being considered for use in future aeronautical gas-turbine compressor discs. Low cycle fatigue is thought to be one of the mechanisms most damaging to such a component. Here, the low cycle fatigue behaviour of Ti-6-4, reinforced with SM1140+ fibre, is investigated over the temperature range 22°C to 600°C. SN curves have a characteristic S shape and can be split into three regions. Fractography, acoustic emission monitoring and cyclic strain recording have elucidated damage mechanisms in each region. In region I (high cyclic stress) damage is caused by matrix creep, that leads to fibre failure. In region III (low cyclic stress), the predominant damage mechanism is matrix crack growth. Cracks initiate at surface machining damage and grow, bridged by intact fibres, into the bulk. The matrix crack growth transfers stress to fibres, eventually causing them to fail in overload, resulting in specimen failure. In region II (intermediate cyclic stress) damage is by a combination of the mechanisms observed in regions I and III. Comparison of Ti-6-4/SM1140+ with Ti-6-4/SCS-6 shows that fatigue lives are similar in regions II and III. In region I it is possible that Ti-6-4/SM1140+ has inferior lives to Ti-6-4/SCS-6.     

The durability of controlled matrix shrinkage composites

During fatigue of aligned fibre pultrusions, the flexural modulus decreases continuously when the applied stresses are tensile and directed along the fibres (R=0.1). In addition, the Poisson's ratio increases continuously, and so does the energy absorbed during the fatigue cycle. Holes in the specimens continuously increase in size in a direction at right angles to the applied stress, but change little in the stressing direction. These effects are enhanced by including compressive stresses in the cycle (R=–0.3) and are reduced by reducing the polymer cure shrinkage pressure. There are notable similarities between fatigue failure and compressive failure in aligned fibre composites, and evidence that matrix stresses are produced at right angles to the fibres, which are probably large enough to cause matrix fatigue failure. These observations lead to the conclusion that the fatigue failure may well originate from misaligned fibres, which generate off-axis stresses. These cause interface failure and polymer fragmentation, which can then lead to fibre failure (and thus composite failure) even when the applied stresses are always tensile.     

The matrix fatigue behaviour of fibre composites subjected to repeated tensile loads

In a fibre/metal matrix composite the mechanical properties of the matrix itself are changed by the presence of the reinforcing fibres. This changed behaviour of the metal is referred to asin situ behaviour, and a phenomenological model is developed to evaluate thein situ plastic stress-strain properties of a metallic matrix containing fibres, from a study of the properties of the composite. The model is based upon the idealised behaviour of the two components of the system. The application of the model to B/Al alloy composites shows that the plastic stress-strain behaviour of the matrix containing fibres varies strongly with the fibre volume content, and also that the matrixin situ cyclic stress-strain behaviour can be approximately described by a power law of the type: where the strength coefficient and the exponent increase with the fibre volume fraction. It also predicts that in the steady state fatigue behaviour of the composites, the fraction of load amplitude carried by the fibres decreases with increasing applied stress amplitude, and is also dependent on the fibre volume fraction. The effect of the applied stress on the damping capacity is established through expressions derived from the basic ideas involved in the model.     

Effect of fibre length on fatigue of short carbon fibre/epoxy composite

The effect of fibre length on the fatigue of a random short carbon fibre/epoxy composite containing 1, 5 or 15 mm length fibres has been studied. All laminates gave a sloping S-N curve with longer fatigue lives obtained at decreasing peak stresses. The fatigue life was independent of fibre length at any peak strain, within experimental variation. Damage accumulation during fatigue cycling is studied in terms of residual strength and modulus reduction. Both techniques suggest that fatigue failure is the result of a ‘sudden death’ mode of failure. Finally, the effect of matrix type on the fatigue life of laminates containing 5 mm length fibres was investigated by adding a greater quantity of flexibilizer to the epoxy matrix. Shorter fatigue lives were obtained for laminates having the more flexible matrix.     

碳纳米管增强开孔铝基复合泡沫的疲劳性能

采用原位化学气相沉积、短时球磨和填加造孔剂法相结合的工艺制备了碳纳米管(CNTs)/Al复合泡沫,研究了其在压缩-压缩循环载荷下的力学性能及失效机制。结果表明,CNTs/Al复合泡沫的应变-循环次数曲线经历线弹性、应变硬化及应变快速增长三个阶段。不同于泡沫铝的逐层坍塌变形失效模式,CNTs/Al复合泡沫疲劳失效的主要原因是大量剪切变形带的形成,试样出现快速的塑性变形。此外,CNTs含量为2.5wt%、孔隙率为60%的复合泡沫试样的疲劳强度相比于泡沫铝提高了92%。CNTs的均匀分布及增强相与基体材料之间良好的界面结合性保证了疲劳载荷能够以剪切力的形式从基体传递到CNTs上,使其充分发挥自身高强度、高韧性的特点,进而提高了疲劳性能。      

Stress transfer in the fibre fragmentation test: Part III Effects of interface debonding and matrix yielding

The micromechanics of stress transfer is presented for the fibre fragmentation test of microcomposites containing debonded fibre–matrix interface and yielded matrix at the interface region. Results from the parametric study are discussed for carbon fibre composites containing epoxy and polyetheretherketone (PEEK) matrices, representing respectively typical brittle debonding and matrix yielding behaviour at the interface region. The stress transfer phenomena are characterized for the two interface failure processes. The sequence of interface failure and fibre fracture as a function of applied stress are also identified. Maximum debonded and yielded interface lengths are obtained above which a fibre will fracture into smaller lengths. There are also threshold fibre fragment lengths above which fibre will fracture without interface debonding or matrix yielding. The applied stresses for these conditions are governed by three strength properties of the composite constituents, namely interface shear bond strength, matrix shear yield strength and fibre tensile strength for given elastic constants of the fibre and matrix, and the geometric factors of the microcomposite. The ineffective length, a measure of the efficiency of stress transfer across the fibre–matrix interface, is shown to strongly depend on the extent to which these failure mechanisms take place at the interface region. This revised version was published online in November 2006 with corrections to the Cover Date.     

FATIGUE LIFE PREDICTION OF NOTCHED COMPOSITE COMPONENTS

Abstract— The local stress/strain approach has been used to predict the fatigue lives of notched composite components. The method was based on a microstress analysis and the application of a multiaxial fatigue parameter incorporating the alternating strain components on the critical plane. This parameter was able to correlate the fatigue lives obtained under a variety of multiaxial loading and geometrical configurations, enabling a generalized fatigue life curve to be determined on the basis of limited experimental data.
The ability of the multiaxial fatigue parameter to relate the fatigue behaviour of composites was illustrated by predicting the locations of crack initiation sites in a unidirectional silicon carbide fibre reinforced titanium plate containing a circular hole tested under constant amplitude cyclic loading. The same approach was also successfully employed to predict the fatigue lives of graphite reinforced epoxy composite tubes with circular holes tested under several combinations of cyclic tension and torsion.     

On the notch sensitivity of flax fibre metal laminates under static and fatigue loading

Damage progression and failure characteristics of open‐hole flax fibre aluminium laminate (flax‐FML) specimens subjected to quasi‐static tensile or tension‐tension fatigue loading were experimentally investigated. Notched and unnotched flax‐FML composites exhibited brittle fracture with little or no fibre pull‐out and minimal delamination at the aluminium/adhesive interface. The flax‐FMLs were tested to failure under tension‐tension fatigue loading conditions (R ratio of 0.1; frequency of 10 Hz; applied fatigue stresses ranging between 30% and 80% of the respective ultimate tensile strength values). The fatigue cycles to failure decreased with the increase in the applied fatigue stress and hole diameter. A phenomenological modelling technique was developed to evaluate the fatigue life of an open‐hole flax‐FML composite. Fatigue tests on specimens subjected to a maximum load equivalent to 35% of the respective tensile failure strength were interrupted at around 85% of the corresponding fatigue life. The accumulated fatigue damage in these specimens was characterised using X‐ray computed tomography. For benchmarking purposes, the fatigue performance and related damage progression in the flax‐FML composite were compared with those of the glass‐FMLs.