D3 Mechanical Tests: Traction and Hardness

Topic

The tensile test on this metal—the first of the two experimental methods described by this model—stretches a standardized specimen of the metal at a controlled speed while continuously recording the applied force and the resulting elongation; based on this force and elongation—both referenced to the specimen's original dimensions—the test determines the engineering stress and engineering strain of the metal at each moment of the test. A higher applied force on the specimen (for a given original cross-section) results in a higher recorded engineering stress at that moment.

In contrast, the hardness test—the second experimental method described by this model—presses a standardized indenter against the metal's surface using a known load and measures the indentation left on that surface; depending on the indenter's geometry and the scale used, the test determines the metal's Brinell or Vickers hardness. A smaller indentation left by the indenter (for a given applied load) results in higher measured hardness on either the Brinell or Vickers scale, whereas a larger indentation results in lower measured hardness.

This hardness value, measured on either scale, correlates with the metal's strength according to Tabor's relationship: the indentation resistance measured by the hardness test—within the small volume of metal deformed beneath the indenter—reflects approximately the same strength that the tensile test measures across the specimen's entire cross-section. Consequently, a higher measured hardness value implies, according to Tabor's relationship, a higher estimated strength. Thus, this model—utilizing the tensile and hardness tests with which it began—generates the input data required by two other models in the set: the one describing the metal's strength, based either on the engineering stress directly measured by the tensile test or on the hardness (which estimates that same strength via Tabor's relation when a tensile specimen is unavailable); and the one describing the metal's strain hardening, based on the evolution of that same engineering stress during the tensile test as engineering strain continues to increase beyond the yield point.

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