Embedded flaws for crack path control in composite laminates

An experimental investigation was conducted on using small flaws purposefully introduced into composite laminates to control growth of interlaminar cracks and through-thickness crack branching. Mode I crack growth specimens were used to study branching through 0, 90 and 45 plies. The results showed that crack growth through 0 plies could be promoted by a ply gap, but this was not as controllable as combining a ply gap with a pre-crack to create a 'crack branch flaw'. Crack branching through 45 plies could be controlled using crack branch flaws, and also promoted controllably using ply gaps. Crack branching through 90 plies was seen without any flaws, but was better controlled with embedded delaminations. Using these outcomes, crack branching through two quasi-isotropic laminates was demonstrated. The results have application to improved damage tolerance and fracture toughness, by taking advantage of high toughness crack growth mechanisms.