On the Development of New Test Techniques to Measure the Tensile Response of Materials at High and Ultra-high Strain Rates

Abstract

Background There is a lack of reliable methods to obtain valid measurements of the tensile response of high performance materials such as fibre composites, ceramics and textile products at high rates of strain. Objective We propose and assess two new test techniques aimed at measuring valid tensile stress versus strain curves at high and ultra-high strain rates. Methods We conduct detailed, non-linear explicit Finite Element (FE) simulations of the transient response of the test apparatus and specimen during the tests and we develop simple analytical models to interpret the test measurements. We consider two test techniques: one based on the split Hopkinson bar apparatus, and suitable for strain rates of up to 1000 /s, and a second technique relying on projectile impact and aimed at measurements at strain rates higher than 1000 /s. Results The simulations are successfully validated using test data at strain rates of order 200 /s and then used to predict the test performance at strain rates up to approximately 5500 /s. We find that both techniques can give valid stress versus strain curves across a wide range of strain rates. Conclusions We identify the limits of both techniques and recommend optimal measurement strategies for dynamic testing of materials with different ductility.

Dynamic response of Advanced Placed Ply composites

“…The proximity of the failure plane to the specimen shoulders, and the strain concentrations measured using DIC, suggest that this lower strength may be a consequence of the geometry of the specimens, rather than strain rate dependency at the material level. As mentioned previously, brittle carbon fiber composites are sensitive to mechanical gripping at high strain rates, potentially reducing their measured strength [20]. Moreover, while stress concentrations arising from the through-thickness reinforcements in the cross-ply AP-PLY laminates could trigger failure in the specimens at relatively low loads, the fact that the strength of the baseline laminates was reduced to a similar or greater extent suggests that the fiber undulations are not the primary cause of the low dynamic tensile strength.…”

“…Second, a large pulse duration with a progressive rising time is required to avoid the premature failure of the brittle composite [19]. An additional consequence of the brittle nature of carbon fiber composites is that they are more sensitive to mechanical gripping, which may result in measurements of strength that underestimate the actual material strength [20]. Furthermore, since the gauge length and distance between bar ends are much greater in large SHBs, it is more difficult to achieve force equilibrium, requiring more detailed analysis to ascertain the validity of the experimental data [21].…”