It is not so stiff nor as strong as natural wood along the grain, but it is substantially stronger and stiffer than wood across the grain. Modulus of rupture, the ultimate unit strength of a material in flexure or bending, has become a common measurement of hardboard strength. Even though the figure obtained is not mathematically correct, it is valuable in that it combines tensile, compressive and bending properties. It is frequently used in comparing one material with another. The height of drop in inches that produces visible failure on the opposite face is recorded as the index of resistance to impact.
Abrasion resistance is resistance afforded by a material to wear by friction.
AR is not normally measured on unfinished hardboard, but can be determined by means of a Taber Abraser. This machine measures wear by standard abrasive filled wheels, and measures resistance to scratching under a known load. 100 Initial Caliper Thickness swelling is critical where hardboard is subjected to large variations in humidity or long exposure to high humidity or water. Impact Strength under sudden localized load applied against the face of a panel held between supports. This value is usually determined by dropping a 2-inch diameter steel ball from increasing heights at the same point, the center, in the panel, until the panel fails.