Which characteristic typically defines softer materials in hardness testing?

In hardness testing, softer materials are characterized by their ability to create deeper indentations when subjected to an identical load compared to harder materials. This behavior occurs because softer materials have less resistance to deformation and indentation. When a fixed load is applied, softer materials yield more readily, leading to a more significant impression or indentation depth.

The relationship between hardness and the formation of indentations is fundamental in understanding material properties. The depth or size of the indentation is a direct reflection of the material's hardness, which is why this characteristic is a reliable indicator of softness.

In terms of the other options, harder materials would typically show less deformation under load and resist indentation more effectively, which are qualities associated with their increased hardness. The thickness of the test piece is more related to specific testing methods and not inherently a characteristic defining the softness or hardness of materials.