From macro- to micro-experiments: Specimen-size independent identification of plasticity and fracture properties
Maysam B. Gorji, Jevan Furmanski, Dirk Mohr
Abstract
The stress-strain response of most engineering materials can be conveniently determined using well-established macro-specimens with outer dimensions of several millimeters or even centimeters. However, in the case of curved thin-walled structures or structures with spatial material property gradients, it is often impossible to extract macro-specimens. For the first group of structures the specimen dimensions are greater than the structural dimensions, while for the second group, the assumption of statistically-homogeneous material properties is not valid throughout a volume as large as the specimen gage section. A frequently encountered example is material property gradients around welds in metallic structures or castings in automotive engineering. In view of characterizing the local plasticity and fracture properties at the millimeter scale, a micro-testing technique is discussed in this study. In addition, the microscopic experimental results of 100μm thick steel alloy API X52 are compared to standard macroscopic counterparts with 2mm thickness. A new custom-made micro-testing device is employed to load uniaxial tension, notched, central hole and in-plane shear micro-specimens with a quasi-static nominal actuator strain rate of less than 1μm/s. These micro-scale experiments on a pipeline steel are performed under an optical microscope; a planar digital image correlation is used to compute the surface strain fields. The plasticity and fracture initiation model are then derived using the conventional macro- and micro-experiments. It is concluded that micro-testing provides a powerful means to characterize materials whose properties are homogeneous over a few hundred microns only.