## 3.9. Guide to Composite Failure Criteria

ACP-Post provides a number of failure criteria for the strength assessment of composites. Both established and basic as well as recent and advanced failure criteria are included in the program. This section provides some guidance on the selection of failure criteria.

- It is advised that you use failure criteria that distinguish between different failure modes (for example, fiber failure or matrix failure).

- The use of all-inclusive quadratic failure criteria is not advised (for example, Tsai-Wu, Tsai-Hill, or Hoffman). In most situations, these criteria are less accurate than others and provided minimal information on any failure.

- It is more conservative to combine different failure criteria (Puck, Max Stress, and LaRC) than using any single criterion.

- In general, failure criteria that consider all in-plane stresses (s1, s2, s12) and the out-of-plane interlaminar shear stresses (s13, s23) should be used, as these results are available in a shell model.

- 3-D stresses (s3) can often be ignored in thin laminates with moderate curvature. Otherwise, Puck 3-D can be used to investigate delamination.

- 3-D solid models can be used to get more accurate results, especially if out-of-plane stresses are being investigated.

- Wrinkling and Core Failure should be evaluated for sandwich structures.

- The use of Puck 2-D is recommended over the use of Puck Simplified.

For more information, see Failure Analysis and Postprocessing. A comprehensive comparison of composite failure criteria was carried out in the worldwide failure exercise [ 38 ].

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