Indentation failure in cross ply laminates, comparison between observations and 3D field solutions

A numerical as well as an experimental study of indentation damage in thick laminates is presented. Static indentation of a spherical steel ball into thick laminates, one of GFRP type and one of CFRP, is performed. The resulting fracture is in both material systems dominated by a bottom centre crack, inclined matrix cracks, accompanied by delaminations. The order of appearence is material dependent. The fracture initiates in the layer opposing the point of contact in the case of a GFRP laminate, whereas the first evidence of fracture is found in the layer nearest the loaded surface, in the case of a CFRP laminate. Full 3D finite element analyses of the elastic contact problem are performed, using a newly developed finite element specifically suited for the purpose. The numerical solutions indicate that the matrix cracking is governed by the failure criterion of maximum tensile stress perpendicular to fibres. Using this criterion the numerical solutions reveal a correlation between the ratio of moduli and the mode of failure. A low value of E, 1/E2 results in failure initiation in the lower layer, whereas a high value of El /b2 results in crack initiation and onset in the upper layers. The experimentally found divergence in terms of failure initiation between the GFRP laminate and the CFRP laminate supports this. The influence of cracks and delaminations on the overall stress distribution is studied qualitatively by inserting cavities of appropriate size and shape into the finite element model.