Comparison of Composite Failure Predictions and Experimental Results in a Blind Study

Accurate prediction of failure loads in composite structures is an involved process due to the initiation and progression of complex failure mechanisms leading up to ultimate load. The Air Force Research Lab (AFRL) and partners that make up The Technical Cooperative program (TTCP) performed coupon testing to characterize lamina-level properties of composite materials and then asked for participants to use the provided information to predict part-level ultimate load levels for six configurations with and without bondline defects as part of a blind failure prediction challenge. This paper focuses on quantifying the differences between predictions from ply-level linear elastic analysis methods used in the spaceflight industry and the AFRL-provided part-level ultimate load test measurements. Load predictions for damage initiation in composite laminate regions were assembled using the maximum stress criterion, the Tsai-Wu criterion, a Christensen lamina-level criterion, and Puck’s 2-D strength criterion. Nonlinear behavior and progressive failure mechanisms were purposefully not included in predictions to be consistent with practical analysis methods used in the space industry. Predictions for damage initiation in the adhesive bondline were assembled using a strength criterion from Tong, while the Virtual Crack Closure Technique (VCCT) was also used to evaluate configurations that contained a flaw. While not unexpected, many of the conclusions from the study reinforced that it can be challenging to evaluate all the possible failure modes that can initiate and propagate failure with high degree of certainty. Observations from the comparisons included: strength criteria need to account for interlaminar contributions, fracture-based failure criteria were more consistent with test than strength-based approaches when configurations had flaws, and load levels corresponding to lamina-level strength predictions for damage initiation in matrix and bondline regions were conservative relative to the test-measured ultimate loads in the six part-level configurations examined.