APEX Inspections - Hardness (HB, HRB) Hardness (HB, HRB) is a characteristic of a solid material expressing its resistance to permanent deformation. Hardness can be measured on the Mohs scale or various other scales. When testing metals, indentation hardness correlates linearly with tensile strength. This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment. We provide Brinnel, Vickers and Knoop testing to the Aviation and Industrial machine shops.