A novel benchmark test for composites under complex loading sequences resulting in non-self-similar damage evolution

Transport industry is increasingly moving towards more optimized and lightweight structures. Cost- efficient and sustainable designs must meet structural integrity and reliability. Experimental testing is the basis in which state of the art design tools are developed, providing material properties and validation data to those tools. Monotonic, fatigue, and durability characterization methodologies under different loading modes in composite materials have been widely studied in literature over the past decades [1]–[3]. Characterization tests are usually performed in geometrically simple specimens, and data reduction schemes are used trying to synthesize the whole experiment into some experimental constants, assuming that they are representative of a material property. However, some simplifications which are not representative of the in-service loading conditions are sometimes made. Among others, the transient the effects of alternating different loading modes, monotonic, and fatigue loading is not considered in characterization tests. Moreover, in fracture characterization tests, self-similar damage evolution is generally assumed, collapsing the whole fracture process zone with linear elastic fracture mechanics approaches. Sometimes, characterization tests are taken as a reference to validate predictive models [4]. This practice may result in not considering some important effects that are present in-service. In this work, a novel fracture benchmark test that quantifies the transient effects of alternating different loading modes, monotonic, and fatigue loading is presented. Moreover, it does not just give information from the crack tip location, it also gives qualitative information of the fracture process zone evolution using X-ray tomography data. The benchmark test is based on the double cantilever beam concept, but some modifications were made to achieve non-self-similar damage evolution. To demonstrate the capabilities of the benchmark test, a case study was performed on a AS4D/PEKK- FC thermoplastic composite. Different combinations of loading modes, monotonic, and fatigue loading were applied. Load-displacement, X-ray tomography, and fractography data was obtained and correlated, being able to monitor the non-self-similar damage evolution of different failure mechanisms as brittle fracture, plastic deformation, and large-scale fibre bridging. The results may be used as reference to validate predictive models that deal with monotonic damage, fatigue damage and fracture. It also provides new information for understanding the mechanics underlying the fracture processes ​
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