Ring-core method in determining the amount of non-uniform residual stress in laminated composites: experimental, finite element and theoretical evaluation

Residual stresses in composite constructions are created because of non-uniform contraction of layers during cooling from curing to ambient temperature. The aim of this study is to investigate the feasibility of incremental ring-core method to estimate non-uniform residual stresses in various laminates of polymer matrix composites. To carry out this test, glass/epoxy specimens were selected in two different types of symmetric cross-ply laminate ([0 degrees(4)/90 degrees(4)](S)) and symmetric quasi- isotropic laminate ([0 degrees(2)/+/- 45 degrees/90 degrees(2)](S)) The samples were created using hand layup and vacuum bagging. By experimental tests, released strains measured during milling process and coefficient factors for every annular groove machining operations were obtained by finite element modeling. As well, residual stresses were calculated by classical lamination plate theory. For symmetric cross-ply, the maximum difference between experimental and theoretical results was about 16%, which was observed in the first and eighth layers. Symmetric quasi- isotropic specimen showed also good agreement between the results and the maximum measurement error of 8% was observed. For the mentioned two cases, summation of components of the residual stress in all directions is near to zero. The results of this study show that in the ring-core method, because of the nature and higher volume of material removal than other methods, values of residual stress released are higher and the results have a good consistency compared with the theoretical values.