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.

The program will directly benefit the advancement towards linerless cryogenic tanks for space exploration applications. Structural components in both manned and unmanned vehicles will benefit from the thin-ply composites by increasing design allowables resulting in thinner and lighter structures. Additionally, Structural Health Monitoring Systems (SHMS) and Health and Usage Monitoring Systems (HUMS) are both technologies that aim to improve component life prediction through the analysis of operational data collected by sensors. The correlation between AE signal accumulation and crack density within composite parts is a powerful tool for any industry currently using composite materials. This technology can give real time information regarding the health of the composite part, allowing for efficient servicing and/or replacement of parts.

Any industry utilizing composite materials can benefit from the advancement of thin-ply composites, which includes the automotive industry, fixed-wing aircraft, rotorcraft, industrial pressure vessels, and recreational sports equipment. Benchmarking the relationship between acoustic emissions and composite damage enables Structural Health Monitoring Systems for any application where safety is of a concern (i.e. commercial aircraft).

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