Determination of Lüders Strains and Flow Properties in Steels from Hardness/Microhardness Tests

This is the foundational paper establishing the core ABI® concept. Published in 1983 in Metallurgical Transactions A, it demonstrates for the first time that ball indentation can quantitatively extract Lüders strain and the full true-stress/true-plastic-strain curve from small steel specimens — the key intellectual basis for all subsequent ABI® development.

The work was conducted at Oak Ridge National Laboratory under the NRC fusion reactor materials program, where the need to characterize irradiated materials from small volumes of material drove the development of miniature specimen techniques. Haggag and Lucas showed that by carefully analyzing the relationship between hardness measurements at multiple loads and the underlying stress-strain behavior, it was possible to reconstruct the complete flow curve including the Lüders plateau — a feature that most hardness-based methods could not capture.

This paper laid the theoretical groundwork for the automated, multi-cycle indentation approach that would become the ABI® test method. The ability to extract not just a single hardness number but the full stress-strain curve from indentation data represented a paradigm shift in mechanical property measurement, opening the door to nondestructive evaluation of structural components in service.