Direct damage quantification techniques – a comparative study

The introduction of advanced high strength steels, e.g., into the automotive industry, initiated huge interest in understanding ductile fracture of sheet metals, and prediction of damage evolution by advanced forming simulations. This instigated the development of experimental methodologies that provide microvoid evolution parameters. Previously, the authors demonstrated that the popular microindentation-based damage characterization, which relates degradation of elastic modulus and/or hardness to damage evolution, is fundamentally flawed. The current research carefully evaluates damage quantification techniques that measure damage directly through the volume of voids: (i) digital processing of SEM images of etched or polished internal surfaces, (ii) high-resolution X-ray tomography using a state-of-the-art table-top micro-CT-scanner, and (iii) newly-developed highly-sensitive density measurements using surface profilometry. For techniques (i) and (ii), operator-controllable contrast/brightness and threshold settings seriously hamper the absolute accuracy; X-ray tomography is limited to voids larger than a few micrometers; and a density accuracy of ~0.1% bounds the spatial resolution of technique (iii) to ~1mm. More fundamentally, the void density does not capture correctly the degree to which a material is damaged because, e.g., void interaction and damage morphology effects can only be probed mechanically. This calls for an improved mechanical damage quantification technique that is better defined than microindentation.