In cross-ply laminates under loading, multiple intralaminar (transverse) cracking in the 90-layers causes laminate stiffness reduction which can be predicted by analytical models studying average of the stress perturbation between transverse cracks. The commonly used analytical models such as shear-lag type or variational-type models assume that stress in damaged 90-layers does not depend on the laminate thickness coordinate. But with increase in crack density, the stress perturbations created by transverse cracks start to interact, generating high stress gradients in through-the-thickness direction in the 90-layers. Location of a new crack and features of its propagation in a high stress gradient region between two cracks in a cross-ply laminate are analyzed in this paper. FEM is used to calculate stress distribution between two cracks. The location of the next crack and its initial orientation is found using principal stresses and orientation of principal axes. Most often these cracks are initiated at ply interface in the middle between already existing cracks. Their propagation across the layer thickness is analyzed using energy release rate based criterion. It is shown that at very high crack density the crack propagation across the layer thickness is unstable in the beginning, but it is stopped when approaching the middle of the layer where the crack opening becomes zero. Presence of local delaminations at the tips of existing cracks changes the energy release rate for propagation of new transverse cracks.

In-plane thermo-elastic constants of symmetric damaged laminates containing transverse cracks in plies and local delaminations starting from crack tip are predicted using a crack opening (COD) and crack sliding displacement (CSD) based approach. An exact elastic analysis shows that the displacement gap on the delamination crack surfaces does not enter the stiffness expressions explicitly. The delamination affects the stiffness via larger COD and CSD of the intralaminar crack. This means that the same expressions for cracked laminates with and without delaminations can be used but with different expressions for COD and CSD. Finite element method is used to analyze the CSD dependence on delamination length and crack density. The obtained approximative expressions for CSD are in a good agreement with FEM. It is shown that in cases when it depends on CSD only, the predicted shear modulus of laminates is in an excellent agreement with direct FEM calculations. The used homogenization over couples of off-axis plies (monoclinic materials) in CSD expressions for balanced laminates is validated.

The effect of test temperature, maximum stress, and stress-ratio on transverse cracking development in cross-ply laminates subjected to tension–tension cyclic loading was analysed. A two-parameter Weibull distribution model was used to predict transverse cracking, wherein the Weibull scale parameter was assumed to be test temperature and number of cycles dependent. By introducing an equivalent stress in the model, it was possible to account for the effect of the stress ratio in cyclic loading over a range of different loading conditions. To verify the model, tests on temperature resistant cross-ply composites were performed at room temperature and at 150 °C with different stress levels and local 90-layer stress ratios. For both test temperatures, increase in stress level increased the transverse cracking tendency. At 150 °C, despite the lower maximum thermo-mechanical ply-stress level compared to room temperature, transverse cracking tendency was found to be higher.

Thermo-mechanical response of [90n/0m]s carbon/epoxy and glass/epoxy cross-ply laminates in 4-point bending is analyzed experimentally and analytically. Intralaminar cracks in surface 90°-plies and local delaminations introduced in one of the 90°-plies at large deflections reduce the laminate bending stiffness and make the laminate asymmetric due to differences in the damage state in the layers. The latter leads to residual thermal curvature that increases with intralaminar crack density and with growing local delaminations. In the present study optical microscopy was used for crack density quantification. It was also found experimentally that small local delaminations develop in the initial stage of damage evolution and under increasing load they grow rapidly from the existing and newly created crack tips. The effect of damage on residual curvature and the bending stiffness was analyzed using an analytical method, where the concept of the effective stiffness of damaged ply is used in the classical laminate theory. Analytical results were validated with a 3-D FEM simulation of the damaged laminate in a 4-point bending test. In the literature a phenomenon that the microdamage in laminate layers causes redistribution of in-plane thermal stresses is often overlooked. The present paper shows that the used analytical approach gives an accurate description of experimental results regarding two independent sets of data: the residual curvature; and the laminate bending stiffness with evolving micro-damage. The present study also renders a better insight in the mechanics of the phenomena and allows estimation of the extent of local delaminations that is difficult to measure in tests.

Accumulation of transverse cracks in carbon fiber heat resistant polymer (with bismaleimide formulation) cross-ply laminates during tensile loading at elevated temperatures and after long heat treatment is analysed. Data shows that both the iso-thermal heat treatment and testing at elevated temperatures reduce the transverse cracking resistance. A two-parameter Weibull failure stress distribution model with scale parameter degrading with heat treatment and elevated temperature is used for crack initiation analysis. The degradation is described by polynomial expansion including interaction terms. Data shows that the scale parameter dependence on the heat treatment time and the test temperature is rather linear. The same expansion parameters have been successfully used for laminates with the same constituents but with a different layup and fiber content.

Presented test results on transverse cracking in cross-ply laminates upon tension–tension cyclic loading show that the increase of crack density depends not only on the maximum transverse stress in the cycle but also on the local cyclic stress ratio RTloc in the analyzed layer. To include the effect of the RTloc in the model with statistical failure stress distribution for crack initiation (based on Weibull distribution) adapted for fatigue, an equivalent stress is introduced in a similar manner as the equivalent strain energy release rate has been used for delamination crack propagation. The equivalent stress in the layer is defined as a power function of the maximum stress and the stress ratio in the layer. It was found, testing laminates with two different fiber contents that higher the local stress ratio in 90-layer, higher the transverse cracking resistance. Transverse crack density simulation using the developed equivalent stress model has been validated against test results. 

Recently, considerable effort has been made to study cellulose/epoxy composites. However, there is a gap when it comes to understanding the post-conditioning anomalous effect of moisture uptake on their mechanical and dynamic-mechanical properties, and on their creep behavior. In this work, up to 10.0 wt% microcrystalline cellulose (MCC) was incorporated into epoxy resin by simple mixing and sonication. Epoxy/MCC composites were fabricated by casting in rubber silicone molds, and rectangular and dog-bone test specimens were produced. The moisture uptake, dynamic mechanical, chemical, tensile, and creep behavior were evaluated. The incorporation of MCC increased the water diffusion coefficient. The changes in storage modulus and glass transition temperature, combined with Fourier-transform infrared spectroscopy analysis, evidenced that water sorption in epoxies causes both plasticization and additional resin crosslinking, although the latter is prevented by the addition of MCC. The creep strain of the composites increased by 60% after conditioning, indicating that plasticization induced by water sorption plays an important role in the long-term properties of the composites.