Laminate Failure
To determine whether a laminate will fail due to applied loading, the program first calculates stresses across the different plies. It next applies a failure criterion based on these stress levels using a failure theory. A laminate is considered to fail when a first ply or a first group of plies fails.
Failure of composites occurs in multiple steps. When stress in the first ply or a first group of plies is high enough, it fails. This point of failure is the first ply failure (FPF) beyond which a laminate can still carry the load. For a safe design, laminates should not experience stress high enough to cause FPF. The point where the total failure occurs is termed the ultimate laminate failure (ULF). Failure of composites occurs on a micromechanical scale due to fiber damage, matrix cracking, or interface or interphase failure. These local failure modes cannot predict global laminate failure satisfactorily.
Composite failure theories predict global laminate failure. These failure theories can be interactive, non-interactive or partially interactive. The non-interactive theories do not consider the interaction between different stress components, whereas the interactive theories do.
The three theories available for laminate failure criteria are:
- Tsai-Hill Failure Criterion
- Tsai-Wu Failure Criterion
- Maximum Stress Criterion