Research On Mechanical Properties And Damage Modes Of Thin-ply Composites

The thesis is focused on the optimal design of thin-walled wing structures made of carbon fiber reinforced polymer(CFRP),aiming at improving the interlaminar properties,damage resistance and design flexibility of the CFRP laminates.Mechanical property of the CFRP with varied ply thickness was investigated by experiment and Finite Element Modeling(FEM)method.Effects of ply thickness on the mechanical properties of the composite laminate was obtained,which helped in predicting the failure strength and failure mode of the laminated composite in service.The interlaminar property of laminates with different ply thicknesses and ply angles was studied by short-beam shear tests.The result showed that as the ply thickness decreased,the short-beam shear strength of the laminate increased and the coefficient of variation(CV)of the data decreased.For example,when the ply thickness decreased from 0.2mm to 0.02 mm,the short beam shear strength of the unidirectional laminates,orthotropic laminates and quasi-isotropic laminates increased by 26.3%,31.6% and 57.2% respectively.SEM images of fracture surfaces showed that the delamination of the thin-ply laminates was more and smoother than conventional ply laminates with the fracture of plies,which suggest that the shear resistance and reliability of the laminates have been effectively improved by thinning the ply thickness.The damage resistance of the laminates with different ply thicknesses and ply angles was studied by open hole tensile tests and acoustic emission(AE)detection.According to the test,thinning the ply thickness would decreased the open hole tensile strength,and one possible reason is that the stress concentration around the hole can not be released in time.Moreover,excellent interlaminar performance of the thin-ply laminates made the material prone to quasi-brittle fracture at the hole.The AE signal in the tensile loading indicated that the thinner the ply is,the closer the occurrence of strong AE signal and complete material fracture are.With prefabricated damage,although the tensile strength of thin-ply laminates was lower than the thick-ply laminates,their reliability is better in initial damage stress range.Finally,finite element model of the laminates were established using a solid element which contains a resin layer between plies,and the model was loaded in short-beam shear mode.Under the same loading condition,thin-ply laminates showed a small high-shear-stress area,more uniform stress distribution as well as higher maximun values of shear stress and equivalent elastic strain.Meanwhile,the load corresponding to initial delamination of thin-ply laminates was higher than that of conventional ones,which indicates a better interlaminar property of the thin-ply laminates.For the failure mode,the thin-ply laminates was found to fracture progressively under the maximum stress.