Fatigue behaviour of hybrid composites

A study has been made of the fatigue behaviour in repeated tension of unidirectional and [(±45, 0, 0)₂]_s hybrid laminates composed of XAS carbon fibres and E-glass fibres in the same 913 epoxy resin. The ordinary mechanical properties of these composites are close to those predicted by simple, conventional models of hybrid behaviour. For the unidirectional materials, the fatigue stress for a given life is not a linear function of composition, showing a substantial positive deviation from the rule of mixtures. This behaviour closely mirrors that of unidirectional carbon-Kevlar hybrids reported in Part 1 of this work. In terms of strain-life comparisons, results for all hybrid compositions and plain carbon fibre reinforced plastic fall within a single scatter-band. These features are also reflected in the fatigue behaviour of the more complex hybrid laminate.     

Prediction of initiation of transverse cracks in cross-ply CFRP laminates under fatigue loading by fatigue properties of unidirectional CFRP in 90° direction

A fatigue life to the initiation of transverse cracks in cross-ply carbon fiber-reinforced plastic (CFRP) laminates has been predicted using properties of the fatigue strength of unidirectional CFRP in the 90° direction. In the experiments, unidirectional [90]₁₂ laminates were used to obtain a plot of maximum stress versus number of cycles to breaking, and two types of cross-ply laminates of [0/90₄]_S and [0/90₆]_S were used to evaluate the initiation and multiplication of transverse cracks under fatigue loading. Transverse cracks were studied by optical microscopy and soft X-ray photography. Analytical and experimental results showed good agreement, and the fatigue life for transverse crack initiation in cross-ply laminates was predicted successfully from the fatigue strength properties of the unidirectional CFRP in the 90° direction. The prediction results showed a conservative fatigue life than the experimental results.     

Damage evolution of angle-ply SCS-6/Ti composites under static and fatigue loading

The effect of fibre orientation and laminate stacking sequence on the tensile and fatigue behaviour of SCS-6/Ti 15-3 composites were investigated. The laminates used in this study were: (90)₆, (0/ ± 45)_s, (0/90)_s, and (90/ +-45)_s. The initiation and progression of microstructural damage at various stress levels was thoroughly characterized. It was found that fatigue life at high applied stresses were controlled by fibre fracture; progressive damage involving fibre fracture, interfacial debonding and matrix cracking became dominant at low applied stresses. Observation of the damage mechanisms in the angle-ply laminates under cyclic loading suggests that increasing the fibre-matrix bonding strength may improve the load carrying capability and fatigue life of laminates containing off-axis plies.     

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.     

Damage Assessment of CFRP [90/±45/0] Composite Laminates over Fatigue Cycles

The present paper develops a stiffness-based model to characterize the progressive fatigue damage in quasi-isotropic carbon fiber reinforced polymer (CFRP) [90/±45/0] composite laminates with various stacking sequences. The damage model is constructed based on (i) cracking mechanism and damage progress in matrix (Region I), matrix-fiber interface (Region II) and fiber (Region III) and (ii) corresponding stiffness reduction of unidirectional plies of 90°, 0° and angle-ply laminates of ±45° as the number of cycles progresses. The proposed model accumulates damages of constituent plies constructing [90/±45/0] laminates by means of weighting factor η ₉₀, η ₀ and η ₄₅. These weighting factors were defined based on the damage progress over fatigue cycles within the plies 90°, 0° and ±45° of the composite laminates. Damage model has been verified using CFRP [90/±45/0] laminates samples made of graphite/epoxy 3501-6/AS4. Experimental fatigue damage data of [90/±45/0] composite laminates have fell between the predicted damage curves of 0°, 90° plies and ±45°, 0/±45° laminates over life cycles at various stress levels. Predicted damage results for CFRP [90/±45/0] laminates showed good agreement with experimental data. Effect of stacking sequence on the model of stiffness reduction has been assessed and it showed that proposed fatigue damage model successfully recognizes the changes in mechanism of fatigue damage development in quasi-isotropic composite laminates.     

Fatigue of bolted joints in (0/90) CFRP laminates

The results of an experimental study of the fatigue behaviour of double-lap bolted joints using 1 mm thick (0/90)_2s and 2 mm thick (0/90)_4s laminates from the XAS/914 CFRP system are described. The damage development under fatigue loading was monitored qualitatively by microscopy, and quantitatively by recording the bearing stiffness and permanent joint deformation with cycling. The possibility of a stiffness-based failure criterion is investigated.The importance of the bolt clamp-up torque on the fatigue performance of the laminates was also studied It is shown that damage development under fatigue loading is considerably reduced at higher clamp-ups.     

Fracture and fatigue study of unidirectional glass/epoxy laminate under different mode of loading

Interlaminar fracture is the dominant failure mechanism in most advanced composite materials. The delaminating behaviour of materials is quantified in terms of the strain energy release rate G. In this paper, the experimental measurements of the fatigue delaminating growth for some combinations of energy release rate mode ratio have been carried out on unidirectional glass/epoxy laminates. On this base the constants in the Paris equation have been determined for each G_II/G_T considered modal ratio. The fatigue threshold strain energy release rate Δ G_Tth , below which delaminating doesn't occur, were measured. Three type specimens were tested, namely: double cantilever beam (DCB), end‐loaded split (ELS) and mixed‐mode bending (MMB) under mode I, mode II and mixed‐mode (I + II) loading, respectively. Scanning electron microscopy techniques were used to identify the fatigue delamination growth mechanisms and to define the differences between the various modes of fracture.     

The effect of temperature on fatigue damage of FRP composites

The present study intends to investigate the effect of temperature on cumulative fatigue damage (D) of laminated fibre-reinforced polymer (FRP) composites. The effect of temperature on fatigue damage is formulated based on Ramkrishnan–Jayaraman and Varvani-Farahani–Shirazi residual stiffness fatigue damage models. The models are further developed to assess the fatigue damage of FRP composites at various temperatures (T). This task is fulfilled by formulating the temperature dependency of Young’s modulus (E) and ultimate tensile strength (σ_ult) as the inputs of the models. Temperature-dependant parameters of Young’s modulus and ultimate tensile strength are found to be in good agreement with the experimentally obtained data when used for unidirectional, cross-ply and quasi-isotropic FRP laminates. The proposed fatigue damage model is evaluated using six sets of fatigue damage data. The proposed temperature-dependent model was also found promising to predict the fatigue damage of unidirectional (UD) and orthogonal woven FRP composites at different temperatures.     

Simulation of fatigue performance of cross-ply composite laminates

The fatigue life of cross-ply composite laminates was evaluated using a statistical model. A modified shear-lag analysis was applied to describe the cycle-number-dependent stiffness reduction and consequent stress redistribution processes in the laminates resulted from both progressive transverse matrix cracking in transverse plies and local delamination at tips of transverse cracks. From the strength degradation behaviour and the static strength distribution of 0° plies as well as the fatigue behaviour of 90° plies, the fatigue life of cross-ply laminates with various types of lay-up can be simulated from the model. Predictions of fatigue performance are compared with experimental data for [0/90₂] _s , [02/90₂] _s and [0₂/90₄] _s graphite/epoxy cross-ply laminates: good agreements are obtained.     

Impact properties of sisal-glass hybrid laminates

Work of fracture (WOF) of unidirectionally aligned sisal-glass hybrid laminates, especially glass core — sisal shell (SGS) laminates, have been studied using flat charpy test. Keeping the volume fraction of sisal at about 0.4, the WOF of SGS laminates linearly increased from 80.2 to 228 kJ m^–2 by varying the volume fraction of glass at the core (V _{g core}) from 0 to 0.2. However, further enhancement in WOF has been observed when the glass core is shifted from the midline towards the tensile side of the laminate with respect to impact. The specific work of fracture (165 kJ m^–2) of SGS laminate atV _{g core} 0.2 is identical to that of glass-reinforced polyester composites containing 0.6 volume fraction of glass. Addition of glass facings to SGS laminates increased its WOF by 20 to 30 kJ m^–2 when the volume fraction of glass in the facings (V _{g facings}) was about 0.04. The increase in resistance to compressive yielding that occurs by placing glass facings above a critical size (V _{g facings} 0.04 in the present case) adversely affected the WOF of SGS hybrid laminates.     

Effect of off-axis ply orientation on 0°-fibre microbuckling

The aim of the present work is to study both experimentally and theoretically the compression failure mechanisms in multi-directional composite laminates, and especially the effect of the off-axis ply orientation on fibre microbuckling in the 0°-plies. The critical mechanism in the compressive fracture of unidirectional polymer matrix composites is plastic microbuckling/kinking. In multi-directional composites with internal 0°-plies, catastrophic failure also initiates by kinking of 0°-plies at the free-edges or manufacturing defects, followed by delamination. When 0°-plies are located at the outside, or in the case of cross-ply laminates, failure rather tends to occur by out-of-plane buckling of the 0°-plies. T800/924C carbon-fibre–epoxy laminates with a [(±θ/0₂)₂]_s lay-up are used here to study the effect of the supporting ply angle θ on the stress initiation of 0°-fibre microbuckling. Experimental data on the compressive strength of laminates with θ equal to 30, 45, 60 or 75° are compared to theoretical predictions obtained from a fibre kinking model that incorporates interlaminar shear stresses developed at the free edges at (0/θ) interfaces. Initial misalignment of the fibres and non-linear shear behaviour of the matrix are also included in the analysis.     

Behaviour of unidirectional and crossply ceramic matrix composites under quasi-static tensile loading

Experimental results are presented for the quasi-static tensile behaviour of unidirectional, (0/90)_s, (0₂/90₄)_s and (0/90)_3s silicon carbide fibre (Nicalon) reinforced calcium aluminosilicate glass-ceramic matrix laminates. The stress-strain behaviour and associated damage development is described in detail for each laminate. The damage development is quantified by counts of crack density (in both the longitudinal and transverse plies) and stiffness reduction as functions of applied strain. The damage initiation and growth (and its effect on residual properties) are discussed with reference to the Aveston-Cooper-Kelly (ACK) theory for unidirectional ply cracking and crossply laminate shear-lag (originally developed for polymer matrix composites) to describe the transverse ply cracking behaviour.     

Hybrid composites based on polyethylene and carbon fibres. Part 6: Tensile and fatigue behaviour

The tensile and fatigue behaviour of unidirectional carbon-high-performance polyethylene/epoxy hybrid composites has been studied, including the effect of hybrid design and surface treatment of the high-performance polyethylene (hp-pe) fibres. Results indicated that the tensile behaviour of carbon-hp-pe hybrids in both monotonic and fatigue testing can be interpreted, adopting the conventional ‘constant strain’ model for hybrid composites. Deviations from this constant strain model, so-called hybrid effects, were observed in monotonic tensile testing for those hybrid systems with the highest degree of fibre dispersion, incorporating either untreated or treated hp-pe fibres, whereas only the latter displayed synergistic fatigue performance. Hybrid effects under tensile loading conditions were in reasonable agreement with calculations accounting for statistical effects and stress concentrations as determined by finite element analyses.     

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.     

In-plane shear properties of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid and non-hybrid composites

The in-plane shear properties (shear strength τ_xy and shear modulus G_xy) of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid and non-hybrid composites have been investigated experimentally and theoretically. The effect of stacking sequence and random fiber relative volume fraction (V_fR/V_fT) in hybrid composites were reported. Laminates were fabricated by hand lay-up technique with a total of 5 plies, by varying the number and position of random glass layers so as to obtain four different hybrid laminates; i.e. [0.5R/U/U]_S, [U/0.5R/U]_S, [U/U/0.5R]_S, and [U/R/U/R/U]. All unidirectional fiber laminate [U]₅ and another of all random fiber laminate [R]₅ were also fabricated for comparison purpose. The average thickness of the manufactured laminates is 5.5 ± 0.2 mm and the total fiber volume fraction (V_fT) is 37%. Failure modes of all specimens were investigated. Results indicated that the in-plane shear properties (shear strength τ_xy and shear modulus G_xy) of unidirectional fiber composite can be considerably improved by incorporation of random glass fiber and forming hybrid composites.     

Jute and glass fibre hybrid laminates

Hybrid laminates have been fabricated from randomly oriented jute fibre mats and woven glass fabrics with a common polyster resin matrix. Hand lay up techniques were used to simulate practical production methods in the field. A variety of laminate constructions were mechanically tested and some laminates were in addition assessed for environmental stability. Modified rule of mixtures expressions successfully predicted the tensile properties of the laminates and the jute plies were seen to control the failure of hybrid laminates at about 0.8% strain. Fracture toughness measurements of G_IC andK _IC indicate that hybrid laminates have maximum toughness (G _IC 12 kJ m^–2 when jute plies are sandwiched between glass fabric facings. All the hybrid laminates were found to be tough in impact, although here fabric plies used as the laminate core maximize the work of fracture at a value of approximately 45 kJ m^–2. Hybrid laminates with jute facings are, as expected, least able to withstand hot moist environments. However, significant moisture uptake by the polyester resin matrix was measured for all laminates. Optical and scanning electron microscopy have been used to explain the mechanical performance and environmental resistance of the hybrid laminates.     

Fatigue damage and hysteresis in wood-epoxy laminates

Wood-epoxy laminates were subjected to constant amplitude fatigue tests in tension-tension (R = 0.1), compression-compression (R = 10) and reverse loading (R = –1) in order to follow property changes and fatigue damage accumulation. Hysteresis loops were captured during these tests and the form of stress versus number of cycles to failure (S-N) curves was established. Reversed loading is the most damaging mode of cyclic stress application. In terms of static strengths, the wood laminate is weaker in compression than in tension. However at low levels of stress, following many fatigue cycles, the fatigue life is greater in compression-compression than in tension-tension. The shape of captured hysteresis loops is strongly influenced by loading mode. As subcritical damage develops, loop area increases and dynamic modulus falls. In reversed loading, loop bending and distortion is observed depending on whether the damage is tension- or compression-dominated or both. Maximum and minimum fatigue strains, the dynamic modulus and loop area have been plotted as a function of the number of fatigue cycles. The majority of damage occurs towards the end of the sample life but property changes can be detected throughout fatigue tests. Normalisation of fatigue data demonstrates that the fatigue behaviour of wood-epoxy laminates is consistent.     

Quantification of flexural fatigue life and 3D damage in carbon fibre reinforced polymer laminates

Carbon fibre reinforced polymer (CFRP) laminated composites have become attractive in the application of wind turbine blade structures. The cyclic load in the blades necessitates the investigation on the flexural fatigue behaviour of CFRP laminates. In this study, the flexural fatigue life of the [+45/−45/0]_2s CFRP laminates was determined and then analysed statistically. X-ray microtomography was conducted to quantitatively characterise the 3D fatigue damage. It was found that the fatigue life data can be well represented by the two-parameter Weibull distribution; the life can be reliably predicted as a function of applied deflections by the combined Weibull and Sigmodal models. The delamination at the interfaces in the 1st ply group is the major failure mode for the flexural fatigue damage in the CFRP laminate. The calculated delamination area is larger at the interfaces adjacent to the 0 ply. The delamination propagation mechanism is primarily matrix/fibre debonding and secondarily matrix cracking.     

Interlaminar shear behavior of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid and non-hybrid composite laminates

The interlaminar shear behavior of unidirectional glass fiber (U)/random glass fiber (R)/epoxy hybrid composites was studied with short beam shear bending test. Random glass fiber (R)/epoxy means chopped fiber composite having short discontinuous fiber randomly dispersed in epoxy matrix. The effect of stacking sequence and unidirectional glass fiber relative volume fraction (V_fU/V_fT) on the interlaminar shear strength (ILSS) of the manufactured composites has been investigated experimentally and theoretically. The laminates were fabricated by hand lay-up technique with 5 plies. Two non-hybrid composite laminates [R]₅ and [U]₅ were fabricated using the same fabrication technique for the comparison purpose. The average thickness of the manufactured laminates is 5.5 ± 0.2 mm and the total fiber volume fraction (V_fT) is 37%. Failure modes of all specimens were investigated. Experimental results indicated that the ILSS of [U]₅ is higher than those of hybrid and [R]₅ composite. Hybrid composites have higher ILSS than that of random composites. The stacking sequence and (V_fU/V_fT) ratio have a detectable effect on ILSS of the investigated composites.     

About the creep-fatigue interaction on the fatigue behaviour of off-axis woven-ply thermoplastic laminates at temperatures higher than Tg

The influence of plasticity and viscous effects on the fatigue behaviour of off-axis C/PPS laminates was investigated at temperatures higher than glass transition temperature. The obtained results clearly show that creep and fatigue are mutually influencing phenomena. Compared to the reference fatigue behaviour (with no prior loading), the fatigue life can be significantly extended with prior creep depending on loading conditions. Indeed, the strain accumulation seems to slow down after a long time creep preload, as if the time-dependent mechanisms were “evacuated” during this preload. The same conclusion can be drawn for the damage accumulation when the prior creep stresses are higher than the damage threshold or when the hold time is long enough, inducing significant plastic deformations. In angle-ply laminates, such deformations are associated with the reorientation of fibres. They contribute to the reduction of stress intensities, which results in increasing both fatigue life and maximum strain ɛ_max at failure during fatigue loadings.