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Completed TRL 3 (started at 2, targeting 3)
Project Objective
The purpose of this project was to evaluate the efficacy of monitoring for creep crack growth over long periods of time in polymeric/nonmetallic structural materials including high-density polyethylene (HDPE) and Kevlar composites.
Project Description
Interest in the use of polymers in spaceflight and habitat structures is increasing due to their light weight (and in the case of inflatable structures, their ability to be stowed in a reduced volume). This presents unique challenges from a damage tolerance perspective, as polymeric materials are subject to crack growth from both fatigue and creep mechanisms. In addition, analytical tools are not available to consider the combined effects of these crack growth mechanisms. Using structural health monitoring methods to constantly monitor for crack growth in polymeric structures offers a potential alternative to the current practice of performing nondestructive evaluation only at the start of a component's lifetime and predicting subsequent crack growth analytically. One potential non-destructive monitoring method is acoustic emission monitoring, which “listens” for the elastic waves generated in a material due to redistribution of stresses due to crack extension. While acoustic emission monitoring is well established for crack growth in metals, its application to polymetric structures has been limited. The purpose of this project was to evaluate whether acoustic emission monitoring is a viable method for monitoring slow crack growth in polymeric structures through a series of coupon and full-scale laboratory tests. The previously limited investigation of acoustic emission use on polymers placed the entry TRL at approximately 2, while this project aimed to demonstrate a TRL 4 through the test of a full-scale pressure vessel in a laboratory environment.
Project Results and Conclusions
Tests were performed on both HDPE and Kevlar coupons and a full-scale HDPE pressure vessel. Cracks were induced in each test article via a razor blade to control the crack growth initiation such that it fell between the acoustic emission sensors. Although some promise was shown in the initial coupon testing at relatively high loading rates, with acoustic emission hits observed concurrently to observed crack extension, the slow rates of crack growth and high attenuation of HDPE resulted in limited to no crack growth detectability during long-term tests. Notably, in the HDPE pressure vessel test, the acoustic emission system did not begin to pick up any signals until after the vessel began leaking despite optically-observed crack growth occurring before the leak began. This suggests that acoustic emission monitoring may not be a viable method for long-term structural health monitoring of polymeric structures although there is certainly more research that can be done to better evaluate its use (looking at items such as different sensor arrangements, signal processing, and transducer mounting). Due to the failure of acoustic emission in monitoring slow crack growth in a full-scale article, the exit TRL is approvimately 3 (instead of the targeted 4) due to some proof-of-concept being demonstrated in high-load rate tests.
The intent of this project was to provide a benefit to spacecraft and habitat structures made from polymeric materials to investigate a potential means of monitoring for crack growth to ensure the safety of these structures (preventing leaks, depressurization, rupture, etc.). Unfortunately, the tests performed in this effort showed little success in monitoring slow crack growth, likely due to the slow crack growth rates and high attenuation of the materials in question. While further study of acoustic emission monitoring may provide additional benefits for polymeric structures, investigating other structural health monitoring methods may also be valuable to provide more reliable indication of slow crack growth in polymers.
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