Novel methods for advancing reliability testing of photovoltaic (PV) modules and materials have recently been developed. Combined-accelerated stress testing (C-AST) is one such method which has demonstrated reliable reproduction of some field-failures which were not reproducible by standard certification tests. To increase confidence and assist in the development of C-AST, and other new testing protocols, it is important to validate that the failure modes observed and mechanisms induced are representative of those observed in the field, and not the product of unrealistic stress conditions. Here we outline a method using appropriate materials characterization and modelling to validate the failure mechanisms induced in C-AST such that we can increase confidence in the test protocol. The method is demonstrated by applying it to a known cracking failure of a specific polyamide (PA)-based backsheet material. We found that the failure of the PA-based backsheet was a result of a combination of stress factors. Photo-oxidation from ultra-violet (UV) radiation exposure caused a reduction in fracture toughness, which ultimately lead to the cracking failure. We show that the chemical and structural changes observed in the backsheet following C-AST aging were also observed in field-aged samples. These results increase confidence that the conditions applied in C-AST are representative of the field and demonstrates our approach to validating the failure mechanisms induced.
The performance of body armor made of ultra-high molecular weight polyethylene (UHMMPE) is dependent on the tensile strength of UHMMPE fibers. UHMMPE is known to exhibit different mechanical properties when tested at different strain rates and different temperatures. The high tensile strength and low surface energy make it difficult to securely grip single fibers while testing without any slippage. Consequently, making accurate measurements of the tensile strength of this material is challenging. Efforts to simultaneously interrogate strain-rate and temperature conditions have been complicated by the difficulty in gripping the fiber, and in consistently heating a small (≈ 15 µm diameter) fiber. In this paper, a method is described for tensile testing single UHMMPE fibers at high rates and high temperatures, focusing on an innovative clamp design and a specialized single fiber heater that can be coupled to traditional tools for measuring mechanical properties of fibers. Metrics of success for the single fiber clamps are described based on the percentage of failures observed at the jaw grips and the strain to failure.
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