Standard methods of testing nails include measures of Rockwell hardness, using the C scale. Because of the relatively small cross-sectional area of nail shanks, Rockwell measurements are normally limited to one per nail cross section. Therefore, Rockwell hardness tests cannot detect the presence of any hardness gradients within the nail cross section. Since the hardness is a direct indicator of certain strength characteristics, such gradients could influence nail quality and hence the performance of nailed joints.

Vickers micro-hardness tests of regions within the shanks of threaded non-hardened and hardened nails of several wire diameters and SAE classes were conducted to assess any hardness gradients within the nail-shank cross section. Mean hardness of both non-hardened and hardened increased with increased carbon content over the range of 0.10 to 0.40 wt% (1010, 1022, and 1040 SAE class steel wire).

However, the Vickers hardness number was relatively uniform across the nail shank diameter, indicating a lack of significant hardness gradient. This uniformity was found in all nail-shank diameters within the range of 2.7 to 3.4 mm (0.105 to 0.135 in.) or SAE number classes. Consequently, a single Rockwell macro-hardness measure of several properly prepared nail surfaces should give a reliable indication of nail-shank hardness.