Assessing the relative conservatism of the unit circle failure envelope

Tsai’s Modulus, an invariant of the stiffness tensor Q, and the unit circle failure criterion, a strain normalized failure envelope, can both be used in preliminary design. Tsai’s Modulus can be used to predict the elastic properties of the material while the unit circle can predict the likely failure of the material. However, to date the influence of these approaches on each other has not yet been established. In addition, the predictions of the unit circle have not been compared with other well-established failure criteria or experimental data. Therefore, this work determined how the use of experimental or Tsai’s Modulus derived material data affected the failure predictions before comparing the unit circle and other failure criteria for a variety of lamina and laminate material systems. Results showed that the use of experimental or Tsai’s Modulus derived material data significantly affected the first-ply failure (FPF) envelope but not the last-ply failure (LPF) or unit circle envelopes. The unit circle envelope was generally conservative for lamina and laminates but the level of conservatism varied with material system.