Tension and compression of flat $[\pm45^\circ]_{2s}$ specimens from fiber reinforced plastic: Numerical and experimental investigation of forming stresses and strains

Based on the qualitative analysis of the geometric pattern of deformation of flat specimens from cross-ply fibrous composites with the $[\pm45^\circ]_{2s}$ lay-up when they are under tension and compression and using the physical dependencies compiled earlier for this class of composites, the relationships have been derived that made it possible to determine the components of the strain and stresses in the material orthotropy axes through the axial strain of the specimen measured in the experiment. In the linear formulation of two- and three-dimensional problems, numerical experiments have been carried out to determine the parameters of the stress-strain state of specimens consisting of one and two laminas of unidirectional fibrous plastic with $[+45^\circ]$ and $[\pm45^\circ]_2$ structures, respectively. The cases of tension of long and compression of short specimens have been considered; the analysis of the forming stress components along the fibers located in the central part and in the vicinity of the corner points of the specimens has been carried out. Experiments on the tension of flat specimens from a fiber composite with a $[\pm45^\circ]_{2s}$ lay-up to determine the deformed state using a non-contact strain measurement system have been performed. The obtained results allow to indicate areas in which the implementation and continuous change of internal non-classical buckling modes of structural elements of fibrous composites are possible during the loading process, which is probably one of the reasons for the physically nonlinear behavior of specimens with the $[\pm45^\circ]_{2s}$ lay-up under tension and compression.