Tensile properties and fatigue characteristics of hybrid composites with non-woven carbon tissue

The mechanical characteristics of hybrid composites with non-woven carbon tissue (NWCT) are investigated under static tensile and tension–tension fatigue loadings. The hybrid composites are made by stacking NWCT and CFRP prepregs. Thirteen kinds of composites are studied; i.e. NWCT composites, CFRP longitudinal [0]₈, transverse [90]₁₂, off-axis [45]₁₂ and angle-ply [±45]_3S, hybrid longitudinal ([0/0]₄, [/0/0/]₄), transverse ([90/90]₆, [/90/90/]₆), off-axis ([45/45]₆, [/45/45/]₆), and angle-ply ([+45/−45]_3S, [/+45/−45/]_3S). The symbol ‘/’ means that the NWCT is located between the CFRP layers. To estimate the stiffness of hybrid composites, the rule of mixtures is used. The effects of NWCT on the tension–tension fatigue life, the residual strength and stiffness of hybrid specimens are evaluated. The fatigue damage and failure mechanisms of the hybrid composites are analyzed with an optical microscope.     

Transverse crack growth behavior considering free-edge effect in quasi-isotropic CFRP laminates under high-cycle fatigue loading

The high-cycle fatigue characteristics focused on the behavior of the transverse crack growth up to 10⁸ cycles were investigated using quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates whose stacking sequence was [−45/0/45/90]_s. To assess the fatigue behavior in the high-cycle region, fatigue tests were conducted at a frequency of 100 Hz in addition to 5 Hz. In this study, to evaluate quantitative characteristics of the transverse crack growth in the high-cycle region, the energy release rate considering the free-edge effect was calculated. Transverse crack growth behavior was evaluated based on a modified Paris law approach. The results revealed that transverse crack growth was delayed under the test conditions of the applied stress level of σ_max/σ_b = 0.2.     

High-cycle fatigue characteristics of quasi-isotropic CFRP laminates over 10 cycles (Initiation and propagation of delamination considering interaction with transverse cracks)

High-cycle fatigue features of over 10⁸ cycles, particularly the initiation and propagation of edge delamination considering the effects of transverse cracks, were investigated using quasi-isotropic carbon-fiber-reinforced plastic (CFRP) laminates with a stacking sequence of [45/0/−45/90]_s in this study. In the relationship between a transverse crack density and initiation and growth of edge delamination, it was found that fatigue damage growth behavior varied depending on applied stress. It was observed that edge delamination initiated and grew at parts where transverse cracks were dense at ordinary applied stress, whereas it was observed that edge delamination grew before or simultaneously with transverse crack propagation at a low applied stress and high-cycle loading. In addition, the critical transverse crack density where delamination begins growing was calculated to evaluate the interaction between transverse crack and edge delamination growth.     

A comparison of as-fatigue and re-consolidation residual properties for notched quasi-isotropic [0/45/90/−45]2S and cross-ply [0/90]4S AS4/PEEK composite laminates

A comparison of the as-fatigued and re-consolidated properties have been made between notched quasi-isotropic [0/45/90/−45]_2S and cross-ply [0/90]_4S AS4/PEEK laminates. For the former, the ±45° plies tend to constrain longitudinal damage development so that damage growth primarily occurred in the transverse direction, causing more widespread damage. This led to prominent mechanical properties degradation, shorter fatigue lives and lower residual strengths. For cross-ply laminates, quick and extensive longitudinal crack tangential to the hole and the corresponding 90° fiber shear off brought about effective stress concentration alleviation. This discouraged further damage development. Hence, their fatigue lives exceeded one million cycles even at high cyclic stress levels and their residual strengths were significantly higher than their virgin strength. On the other hand, the re-consolidation process removed most of the defects that alleviated the stress concentration and thus decreased the strengths. Detailed study of the residual strength changes and damage development history revealed that the residual as-fatigued and re-consolidated strengths were governed by the competition between local structural decay and its resulting stress concentration alleviation.     

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.     

Nonlinear constant fatigue life diagrams for carbon/epoxy laminates at room temperature

Exploration of a full shape of constant fatigue life (CFL) diagram and development of an efficient CFL diagram-based fatigue life prediction method are attempted for multidirectional CFRP laminates. On three kinds of CFRP laminates of [45/90/−45/0]_2s, [0/60/−60]_2s and [0/90]_3s lay-ups, tension–tension, tension–compression and compression–compression fatigue tests are performed at room temperature for two different stress ratios each. Experimental results clearly show that a stress ratio has a significant influence on the fatigue behavior of those CFRP laminates, and the CFL diagrams delineated using alternating stress and mean stress become asymmetric about the alternating stress axis. The alternating stress component of fatigue load for a given constant value of fatigue life turns maximum in the case of fatigue loading at a critical stress ratio that is nearly equal to the ratio of compressive strength to the tensile one. The shape of CFL diagrams progressively changes from a straight line to a nonlinear curve as a given constant value of fatigue life increases. A new and efficient method for accurately predicting an asymmetric nonlinear CFL diagram is then developed which is based on the static strengths in tension and compression and the reference S–N relationship fitted to the fatigue data for the critical stress ratio. The theoretical CFL diagram constructed following the proposed procedure agrees well with the experimental CFL diagram, regardless of the type of CFRP laminate. It is also demonstrated that the S–N relationships predicted using the proposed CFL diagram-based fatigue life prediction method adequately coincide with the experimental results for fatigue loading with a variety of different stress ratios in the range of fatigue life up to 10⁶ cycles.     

Bearing damage evolution of a pinned joint in CFRP laminates under repeated tensile loading

A mechanical pinned joint in the CFRP laminates such as [0/±45/90]_3S, [90/±45/0]_3S, [0/±45/90]_2S and [90/±45/0]_2s is loaded statically and cyclically to finally obtain the critical condition for fatigue. It is derived that in the static loading, the critical damage that yields shear matrix crack is kink and the critical condition to the final failure is the appearance of kink in every inner 0° layer and that in the fatigue loading within the moderate load, the critical damage that yields shear matrix crack is almost always kink-like damage along the collapse front and at high load it is rather kink. Next, the non-elastic elongation of a joint at the maximum load subtracted by the one at 10th cycle is focused on and its capability is figured out for various stacking sequences. The critical value U_NE,F^* for the elongation rate change to the final fatigue failure is around 50–65 μm in the present material. The critical condition to the final fatigue failure and corresponding to U_NE,F^* is roughly the appearance of mostly kink-like damage in every inner 0° layer.     

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.     

Numerical Modeling of Combined Matrix Cracking and Delamination in Composite Laminates Using Cohesive Elements

Sub-laminate damage in the form of matrix cracking and delamination was simulated by using interface cohesive elements in the finite element (FE) software ABAQUS. Interface cohesive elements were inserted parallel to the fiber orientation in the transverse ply with equal spacing (matrix cracking) and between the interfaces (delamination). Matrix cracking initiation in the cohesive elements was based on stress traction separation laws and propagated under mixed-mode loading. We expanded the work of Shi et al. (Appl. Compos. Mater. 21, 57–70 2014) to include delamination and simulated additional [45/?45/0/90]_s and [0₂/90_n]_s {n?=?1,2,3} CFRP laminates and a [0/90₃]_s GFRP laminate. Delamination damage was quantified numerically in terms of damage dissipative energy. We observed that transverse matrix cracks can propagate to the ply interface and initiate delamination. We also observed for [0/90_n/0] laminates that as the number of 90° ply increases past n?=?2, the crack density decreases. The predicted crack density evolution compared well with experimental results and the equivalent constraint model (ECM) theory. Empirical relationships were established between crack density and applied stress by linear curve fitting. The reduction of laminate elastic modulus due to cracking was also computed numerically and it is in accordance with reported experimental measurements.     

Experimental characterization of damage behavior in polycyanate CFRP laminates under high temperature atmospheric exposure

This study focuses on the experimental characterization of damage behavior due to thermo-oxidative-induced matrix shrinkage in carbon fiber reinforced plastics (CFRP) with polycyanate ester. To investigate the effects of laminate configuration on matrix shrinkage behavior, [90]₈ and [0]₈ unidirectional laminates, [±45]_2S angle ply laminates, and [45/0/–45/90]_3S quasi-isotropic laminates were exposed to high temperature atmospheric environment at 180 °C to analyze matrix shrinkage up to 2000 h. These samples were removed from convection oven to observe sample side surface changes. The thermo-oxidative-induced matrix shrinkage was measured on the side surface of CFRP sample by confocal laser microscopy. The results suggested thermo-oxidative-induced matrix shrinkage depended on aged hours, fiber-to-fiber distance, and fiber orientation angle. The matrix shrinkage coefficient could be calculated with a tensorial transformation and empirical formula. The model can predict matrix shrinkage tendency of the 45° intra-lamina layer in quasi-isotropic laminate using the data of 0° and 90° matrix shrinkage in the quasi-isotropic laminates.     

An investigation into the damage development and residual strengths of open-hole specimens in fatigue

An extensive experimental program was carried out to investigate and understand the sequence of damage development throughout the life of open-hole composite laminates loaded in tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/−45₂/0₂]_S, [45/90/−45/0]_2S and [45/90/−45/0]_4S were examined. These were selected on the basis that under quasi-static loading the [45₂/90₂/−45₂/0₂]_S configuration exhibited a delamination dominated mode of failure whilst the [45/90/−45/0]_2S and [45/90/−45/0]_4S configurations showed a fibre dominated failure mode, previously described as “pull-out” and “brittle” respectively. Specimens were fatigue loaded to 1 × 10⁶ cycles or catastrophic failure, which ever occurred first. A number of tests were interrupted at various points as the stiffness dropped with increasing cycles, which were inspected using X-ray computed tomography (CT) scanning. A static residual strength program was carried out for run-out specimens of each configuration.     

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.     

Progressive damage assessment of centrally notched composite specimens in fatigue

The aim of this study is to assess the residual properties and the corresponding damage states within centrally notched quasi-isotropic [0/−45/+45/90]_S T650/F584 (Hexcel) carbon-fiber/epoxy composites subjected to fatigue loading using Digital Image Correlation (DIC), radiography, and a non-contact vibration measurement technique. Quasi-static tests were performed on virgin samples using DIC to determine the full-field in-plane strains at different applied load levels. Fatigue tests were interrupted during the fatigue lifetimes in order to perform quasi-static tests with DIC measurements. Non-contact vibration measurements were performed to investigate the effect of fatigue damage on residual frequency responses. X-ray computed tomography was used to determine the type, location, and extent of fatigue damage development. The results provide an important step in the validation of DIC and vibration response as a powerful combined non-destructive evaluation tool for monitoring the development of fatigue damage as well as predicting the damage level of notched composite materials.     

Effects of seawater absorption on fatigue crack development in carbon/epoxy EDT specimens

Fatigue and static edge delamination tests were performed on [45/0/–45/90]_s CFRP laminates. Both dry and seawater presoaked specimens were tested to examine any effect moisture absorption has on fatigue crack development, which was monitored by optical microscopy, scanning electron microscopy and ultrasonic C-scan. Seawater absorption changed the dominant edge-cracking mode from the -45/90 interlaminar delamination in unaged specimens to intralaminar cracking in 90° plies in aged (seawater saturated) specimens. The edge-crack growth rates in both unaged and aged specimens, however, are similar. The effect of moisture absorption on strain energy release rate has been examined, and the change in dominant edge-cracking mode and edge-crack growth have been discussed in terms of strain energy release rate.     

Fatigue life prediction of carbon/epoxy laminates by stochastic numerical simulation

A three dimensional (3D) finite element model is developed to predict the progressive fatigue damage and the life of a plain carbon/epoxy laminate (AS4/3501-6) based on the longitudinal, transverse and in-plane shear fatigue characteristic. The model takes into account stress analysis, fatigue failure analysis, random distribution and material property degradation. Different cross- and angle-ply laminates including [0₈], [90₈], [0/90₂]_s, [0/90₄]_s, [0₂/90₂]_s, [30₁₆], [45/−45]_2s with the available experimental data are considered for the fatigue life simulation. In order to consider the random distribution of the laminate’s properties from element to element in the model, the laminate’s stiffness, and strength are randomly generated using a Gaussian distribution function. Sudden and gradual material properties degradation are considered during the fatigue simulation. The progressive fatigue damage and failure analysis is implemented in ABAQUS through user subroutines UMAT (user-defined material) and USDFLD (user-defined field variables). The predicted fatigue life of the simulation for different laminates is in good agreement with the experimental results.     

Off-axis fatigue crack growth and the associated energy release rate in composite laminates

Stable matrix crack growth behaviour under mechanical fatigue loading has been studied in a quasi-isotropic (0/90/-45/+45)_s GFRP laminate. Detailed experimental observations were made on the accumulation of cracks and on the growth of individual cracks in +45° as well as 90° plies. A generalised plain strain finite element model of the damaged laminate has been constructed. This model has been used to relate the energy release rate of growing cracks to the crack growth rate via a Paris relation.     

Off-axis Fatigue Crack Growth and the Associated Energy Release Rate in Composite Laminates

Stable matrix crack growth behaviour under mechanical fatigue loading hasbeen studied in a quasi-isotropic (0/90/-45/+45)_s GFRPlaminate. Detailed experimental observations were made on the accumulationof cracks and on the growth of individual cracks in +45° as well as 90° plies. A generalised plain strain finiteelement model of the damaged laminate has been constructed. This model hasbeen used to relate the energy release rate of growing cracks to the crackgrowth rate via a Paris relation.     

Effect of voids on the crack formation in a [45/−45/0]s laminate under cyclic axial tension

In the present work, the influence of manufacturing induced voids on damage mechanisms at the microscopic scale was analysed on [45/−45/0]_s laminates subjected to tension fatigue loading. Microscopic observations of the top surface of the 45° ply revealed that the first event of damage at the microscopic scale was the initiation of multiple micro-cracks in the matrix between the fibres, located preferentially in correspondence of the voids in that layer. The subsequent coalescence of these micro-cracks gave rise to the formation of a crack propagating in the 45° fibres direction. This is qualitatively the same scenario observed in void-free specimens in a recent work by the authors, thus confirming that the same crack initiation criterion can be applied in the absence and presence of voids. In addition, the micro-scale damage is shown to evolve faster and therefore off-axis cracks to initiate earlier and in a larger quantity in the presence of voids.     

A generalized micromechanical approach for the analysis of transverse crack and induced delamination in composite laminates

The previously developed micromechanical approaches for the analysis of transverse cracking and induced delamination are limited for laminates with specific lay-ups such as cross-ply and specific loading conditions. In this paper a new micromechanical approach is developed to overcome such shortcomings. For this purpose, a unit cell in the ply level of composite laminate including transverse cracking and delamination is considered. Then, the governing equations for the stress and displacement fields of the unit cell are derived. The obtained approximate stress field is used to calculate the energy release rate for the propagation of transverse cracking and induced delamination. To show the capability of the new method, it is employed for the analyses of general laminates with [0/90]_s, [45/−45]_s, [30/−30]_s and [90/45/0/−45]_s lay-ups under combined loadings to calculate the energy release rate due to the transverse cracking and induced delamination. It is shown that the obtained energy release rates for transverse cracking and delamination initiation are in good agreement with the available results in the literature and finite element method. Furthermore, the occurrence priority of further transverse cracks and/or delamination at each damage state of the laminates will be discussed.     

Effect of stacking sequence on micro-cracking in a cryogenically cycled carbon/bismaleimide composite

An apparatus was developed to thermally cycle coupon-sized mechanical test specimens to −196 °C. Using this device, IM7/5250-4 carbon/bismaleimide cross-ply ([0/90]_2S and [90/0/90/0/90/0/90/0/90]) and quasi-isotropic ([0/45/−45/90]_S) laminates were submerged in liquid nitrogen (LN₂) and returned to room temperature 400 times. Ply-by-ply micro-crack density (transverse cracks), micro-crack span, laminate modulus, and laminate strength were measured as a function of thermal cycles. The composite micro-cracked extensively in the surface plies followed by sparse micro-cracking of the inner plies. The tensile strength of the two blocked lay-ups (lay-ups with adjacent plies of the same orientation) decreased by 8.5% after 400 cycles. Sectioning of the samples revealed that the micro-cracks in the surface plies spanned the full width of the sample while many of the micro-cracks observed on the edge of the inner plies did not extend to the center of the samples, implying that a rectangular specimen with exposed free edges may result in a significantly different micro-crack density than a sample without free edges.