Laminate final failure prediction

Most of the criteria discussed so far focus on predicting some form of failure onset, typically first-ply failure. As referred to above, this does not necessarily coincide with final failure or collapse of a laminate. There may be significant additional load carrying capability, which can be very important for damage tolerance considerations. 

The stress predicted by Eqn (6.38) is very conservative. A laminate will have damage onset when the right-hand side of Eqn (6.38) exceeds the stress at which first damage will occur, but this damage onset almost never coincides with final failure. As mentioned above, some stress redistribution takes place as damage starts and evolves. This is shown schematically on the right of Figure 6.8. Therefore, Eqn (6.38) cannot be used to predict final failure of a laminate with a hole. For this reason, alternative methods have been proposed. 

Alterna five methods to predict final failure of laminate with a hole... 

Many reports have shown that for a tc/4 quasi-isotropic laminate, when the interlaminar normal stress is tensile and is the dominant component among the free edge stresses, then open mode delamination may occur. Examples of such laminates include [ 45/90/0]s, [ 45/0/90]s, and [0/ 45/90]s laminates. In these laminates, the open-mode delamination crack may propagate into the laminate before final failure of the laminate. For these laminates, classical failure criteria are not suitable for laminate strength prediction. 

FIGURE 9.3 Comparison between the predicted and measured final failure stresses for (07+45°/90°) AS4/3501-6 carbon-fiber/epoxy laminates subject to biaxial loads (Test Case No 6-range of biaxial stress ratios). (Reprinted from Failure Criteria in Fibre Reinforced Polymer Composites The World-Wide Failure Exercise, Hinton, M. J., A. S. Kaddour, and R D. Soden, eds., Elsevier, London, 2004, with permission from Elsevier.)... 

These recent developments show significant improvement in the accuracy and apphcabdity of the failure criteria and point to the importance of incorporating physical mechanisms in the failure model. These models may, currently, require some parameters that are experimentally determined and may come at a significant computational cost as criteria such as the LaRC 03 are extended to post-first-ply failure of laminates, but are the most promising for predicting final collapse of the material in reahstic composite structures. 

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