Open Hole Compression Strength and Failure Characterization in Tape Laminates

Bau, H., Hoyt, D.M.

presentation to ASTM D-30, Technical Symposium, May, 1999.

Open hole compression (OHC) design criteria often size the acreage of composite aircraft skin structures. Therefore, there can be a significant payoff in improving OHC allowables through better characterization of OHC strength and behavior. This study, using progressive damage analysis and empirical methods, provides new insight into OHC strength and failure behavior. Open hole compression failure behavior was modeled using PDHOLEC, a progressive damage 2-D finite element code. Ultimate strength predictions were combined into carpet plots over a wide range of layups for Hexcel IM6/3501-6, Hexcel IM7/8552, and Toray T800H/3900-2 tape laminates at room temperature/ambient and 180°F/ wet environments. A detailed evaluation of PDHOLEC progressive failure output resulted in the identification of several distinct predicted failure mechanisms. Predicted failure mechanisms and ultimate strengths are compared to OHC test data over a range of layup, geometry, and environmental variables. In addition, PDHOLEC sensitivity studies were conducted for mechanical properties and geometry input parameters.

Two distinct failure mechanisms for OHC configurations were identified based on the detailed PDHOLEC failure study and strength trend studies of IM6/3501-6 test data: a 0° ply kinking/buckling failure mechanism and a matrix cracking mechanism. The 0° ply kinking/ buckling failure mechanism was also identified in IM7/8552 and T800H/3900-2 tape laminates. The PDHOLEC predictions and test data were grouped by the identified failure mechanisms. Then curve fit equations were generated to characterize the ultimate strength behavior for each failure mechanism. Statistical methods were used to assess the accuracy of the progressive damage analysis results.