Experimental study on type and cause of defects in variable thickness composite laminates during hot pressing process

The glass fiber/epoxy resin 648 variable thickness laminates were prepared with the hot press and the autoclave methods. The mechanism of defect formation in the two processings was investigated, and the effects of the lay-up type and ply-drop gradient on the extent and distribution of defects were studied. The results show that fiber asymmetric distribution, rich resin and delamination are the major types of defects. Two dimensional resin flow results in fiber uneven distribution, which is influenced by the fiber discontinuous area, permeability of the fiber bed and the uniformity of applied pressure. Rich resin results from the fiber discontinuous area. Delamination has close relations with fiber distribution and anisotropy in thermal expansion. The results are valuable for eliminating the defects and controlling the processing quality in variable thickness laminate structures.