Control of composite material crack branching for improved fracture toughness

An experimental investigation was conducted to develop techniques for controlling crack branching in composite laminates. Double cantilever beam specimens were tested to study mode I dominated crack growth. Embedded flaws were generated using ply gaps and strips of non-stick film, both individually and in combination as a "branch flaw". Crack branching in 0° plies was generated in an inconsistent manner using ply gaps, but in a consistent manner using branch flaws. Branching through 90° plies occurred automatically due to their orientation, and could be further controlled using embedded delamination flaws. Crack branching in 45° plies was more complex, but could be controlled using ply gaps as well as branch flaws. These discoveries were combined to demonstrate crack branch control through a quasi-isotropic laminate. The results have application to design of future high toughness and damage tolerant aerospace composites.