Shear strength measurement of nanostructured bulk metals, or interfaces, in small size metal specimen: equipment, testing, and theoretical analysis

Nanostructured bulk metals have exceptionally high yield stresses and do not present further strain hardening during deformation. In general, it is difficult to measure their yield stress because the samples are usually small, so not suitable for tensile testing. We propose in this work to measure the yield strength of nanostructured materials by full constraints shear testing. The technique is radically different from previous experimental propositions because of the small sample size and the full constraints nature of the test. The new technique is using compression force and transforms it into shear displacement. As the sample is fully constrained, other stress components than the only desired shear stress appear, and contribute to the yield condition of the material. In the proposed testing, their contribution is minimized by finite element (FE) simulations through the geometry of the sample, so that the extra forces are producing mostly hydrostatic stress, leading to nearly a unity correction factor (1.025) between the required nominal shear stress and the equivalent shear stress of the material. FE-informed analytical plasticity calculations produced a simple formula for the equivalent shear stress of the material. The new shear strength measurement technique is also suitable to measure the shear strength of interfaces between two metals that are bonded to each other. The design and its numerical and analytical modeling is validated by experiments carried out on aluminium bulk metal and also on two-layer samples, which had been prepared by the friction-assisted lateral extrusion process.