Why the Hardness Test Cannot Determine Yield Strength

This technical presentation explains a fundamental limitation of conventional hardness testing that is directly relevant to pipeline integrity management: hardness values correlate with ultimate tensile strength (UTS) — not yield strength (YS).

The core technical argument is straightforward. A hardness measurement is made at a high indentation strain of 8–10% (depending on indenter geometry), where the stress state corresponds to the material's necking point or UTS. However, yield strength is defined at a much lower strain — 0.2% offset per ASTM E8, the same offset ABI® uses to determine yield strength. Because different pipeline steels have different stress-strain curve shapes (different work-hardening behaviors and ductilities), there is no reliable correlation between UTS and YS.

Haggag supports this argument with data from a 1999 Battelle/ASME report that tested numerous pipeline steels and found no useful correlation between hardness values and yield strength. The report's findings were significant enough that PHMSA did not accept hardness testing as a method for pipe grade or yield strength determination.

Three steel materials might have the same UTS but three different yield strength values because the shape of their stress-strain curves varies with pipeline grade and thermo-mechanical condition. This is why the ABI® test — which measures the full stress-strain curve through progressive multi-cycle indentation rather than a single-point hardness measurement — can accurately determine yield strength while conventional hardness testing cannot.

This presentation directly addresses a common misconception in pipeline integrity management and explains why ABI®/SSM® satisfies the nondestructive testing requirements of PHMSA's 49 CFR §192.607 while portable hardness testing does not — PHMSA does not accept hardness results as replacements or estimates of tensile properties.