Failure Criterion
The maximum stress criterion applies to composite shells.
Failure occurs according to the maximum stress criterion when the stress in one of the principal material directions exceeds the strength in that direction. The overall state of stress in the global coordinates is first computed by the program. Then, the program computes stress along the principal material directions for each lamina by applying a coordinate transformation.
The program assumes a state of plane stress (2D assumption) for a lamina with σ3 =0, τ13 =0, τ23 =0. The failure index is computed as follows:
The program reports the factor of safety (FOS) as 1 / (F.I.). The FOS should be greater than 1 for laminates to be safe.
where:
X1 is the tensile strength in material direction 1
X2 is the tensile strength in material direction 2
S12 is the shear strength
See Composite Ply Material Directions for a definition of material directions 1 and 2.
Further,
X1 = X1T if σ1 > 0
X1 = X1C if σ1 < 0
X2 =X2T if σ2 > 0
X2 = X2C if σ2 < 0
Superscripts T and C denote tensile and compressive strengths.
The maximum stress criterion:
- Does not take the interactions between different stress components into account as seen from the above equations.
- Predicts specific failure modes since stress in each material principal direction is compared with strength in that direction.