Crack growth behavior of internal titanium plies of a fiber metal laminate

Hybrid Titanium Composite Laminates (HTCL) offer many attractive properties for use on aerospace structures. They have many of the advantages of traditional composites such as good strength and stiffness to weight ratios. Further, the fiber ply orientations can be varied to give desired orthotropic properties. The HTCL also retain some of the advantages of metallics (titanium in particular) such as electrical conductivity, toughness, and bolt-bearing capacity. In an effort to further optimize this class of materials, the objectives of this research was to investigate several potential heat treatments of Ti-15-3 as to their mechanical properties and how they may translate into a more crack resistant HTCL. Stress–strain curves to failure were developed for each of the four heat treatments of Ti-15-3. Then fatigue tests were conducted on the HTCL specimens containing an open center circular hole. Specimens were made using a foil of one of the four heat treatments placed at the mid-section of the laminate. The fatigue tests determined the number of cycles to crack initiation and the subsequent crack growth behavior of the Ti foil. Fatigue testing was conducted at four different stress ranges, all at a stress ratio of 0.1. Dye penetrant enhanced radiography was used to detect the cracks in the internal ply of titanium. Observations were made on the effect of the foil heat treatment on crack growth behavior.