Composites are very sensitive to the load in the direction of thickness, especially out-plane low velocity impact (LVI). In this article, the detailed failure mechanisms of composite laminates with four material systems (CCF300/Epoxy, CCF300/Bismaleimide, CCF800/Epoxy, and CCF800/Bismaleimide) under the loading of LVI and compression after impact (CAI) were studied by experiment and finite element analysis. In finite element model, a newly proposed multi-scale failure criterion (MMF3) and cohesive elements are used to determine the intralaminar damage and interlaminar delamination, respectively. Through experiment and finite element analysis, it can be concluded that the damage tolerance performance of epoxy resin composites are better than that of bismaleimide resin composites, and the damage tolerance performance of CCF300 carbon fiber composites are better than that of CCF800 carbon fiber composites. The proposed multi-scale failure analysis method can effectively simulate the impact and CAI damage process of composite laminates.

Computational mechanics has been carried out to study the microscopic failure mechanisms of unidirectional fiber-reinforced polymer composites. A representative volume element of fiber random distribution based on molecules random collision model is established, with two dominant damage mechanisms, matrix plastic deformation and interfacial debonding included in the simulation by the extended Drucker–Prager model and cohesive zone model, respectively. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the transverse tension fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation, while the compression failure is dominated by matrix plastic damage. The longitudinal tension and compression are both dominated by fiber breakage, but longitudinal tension initiates as matrix plastic damage and longitudinal compression initiates as fiber microbuckling. Finite element method appropriately simulated the process of progressive damage of the fiber buckling failure, which is consistent with the observed result under scanning electron microscopy.

The governing equations for the buckling of honeycomb cores with various cell geometries under combined compression and shear are established and three types of core including rectangular, hexagonal and triangular cores are under consideration. After invoking the Bloch wave representation form, the equations are simplified by the periodicity and the hypothesis that the out-of-plane displacement remains zero at the intersections. Different cell geometries and load cases are taken into account and numerical results offer validation for the analytical solutions. Moreover, the results of Finite Element (FE) models show that the fine results can only be acquired by models with appropriate cell numbers. Experimental study is conducted on the regular hexagonal honeycomb structures. Both the results of the numerical benchmarks and the experiments prove the effectiveness of the proposed analytical method and the hypothesis for predicting the buckling load of honeycomb structures.

In this paper, a longitudinal compression experiment of composites was conducted and the macroscopic failure mode was obtained. Also, the microscopic failure morphologies of longitudinal compression and kink band were observed by using scanning electron microscopy. It can be seen that, under compression, fibers bend and form a kink band, which is the most typical failure mode. Then a micromechanical model of fiber random distribution based on the random collision algorithm, which can reveal the progressive failure mechanism of longitudinal compression considering the kink-band deformation, was established, with two dominant damage mechanisms – plastic deformation and ductile damage initiation of the polymer matrix and interfacial debonding included in the simulation by the extended Drucker-Prager model and cohesive zone model, respectively. Through numerical simulation, the loading and failure procedures were divided into three stages: elastic domain, softening domain and fiber failure domain. It can be concluded that the kink band was a result of fiber instability (micro-bulking), which is caused by the elastic bending of fibers. The fibers rotate and break into two places, forming a kink band. Then the fibers rotate further until the matrix between the fibers fails and the kink-band breaks and, hence, the composite loses its load-bearing capability.