Analysis of mesostructure and fracture kinetics of elements of lattice composite structures under transversal compression using stochastic FEA micromechanics

The paper analyses the mesostructure of the structural elements of lattice aircraft shells - ribs consisting of alternating layers of equal thickness and made from unidirectional CFRP and pure matrix material. In experimental studies, the elastic characteristics of unidirectional CFRP were obtained under three-point bending and transversal compression. As a result, the longitudinal modulus of elasticity of the layered composite turned out to be 101 GPa, and the shear modulus was 2.95 GPa. Numerical modeling of the meso- and micromechanics of the interaction of the noticed layers under transversal compression has been performed up to failure. The ANSYS FEA software (explicit and implicit formulations) was used. The regular and stochastic stacking of fibres in the cross section under compression is considered. The fiber diameters in the composite element were measured on thin sections using a Zeiss Axio Observer D1m digital microscope and were equal to 5.1 ± 0.8 µm. Layers with a fiber volume fraction of about 60 % alternate with layers of pure epoxy. It is proposed to use only the first principal stress in the matrix as a micromechanical criterion for failure under com-pression and tension. At the first stage of calculations, the problem of transversal compression of a cell with a regular laying of fibres was solved (the error in the value of the transversal modulus of elasticity was less than 2 %). At the second stage, an assessment was made of the strength and accumulation of microdamages under compression in a model of a layered structure with stochastic fibre stacking. The analysis of stress-strain state of a layered mesostructure under compression made it possible to explain the reason that the rib has a trans-verse strength twice lower than that of a homogeneous CFRP. The calculated values of the ultimate strength in transversal compression of a layered rib are in good agreement with the experimental ones.