Mechanism Based Damage Model for Linerless Thin-Ply Composite Pressure Vessels

Thin-ply composites are being considered by NASA for space exploration applications, where the suppression of microcracks could give rise to linerless cryogenic tanks. In this proposed Phase I STTR effort, material testing coupled with health monitoring techniques will be used to quantify damage accumulation within composite materials, both standard ply thickness and of the thin-ply design. A multi-scale physics based approach, verified with empirical data, will be used to develop a design tool capable of predicting the useable life of a composite structure subjected to cyclic loads. A fracture mechanics based model in a multi-scale framework is proposed as a design tool for modeling thin-ply laminates. The key variable of the model, the microcracking critical energy release rate (CERR), is to be calibrated to quasi-static and fatigue testing. Acoustic emission (AE) monitoring will be used to quantify the crack density as a function of load history. The model will be interrogated with CERRs to best match the crack density as a function of load observed during the experiments. If the CERR is indeed a material property, the same value should exist regardless of ply thickness and fiber architecture. The design tool will include a stand-alone program to perform this calibration of the CERR for cross-ply laminates. Additionally, a User Material (UMAT) will be written to link the microcracking model to a structural level model in a commercial finite element code.