Figure 2. Mechanical testing of cortical bone in tension and compression.

A typical mechanical test (monotonic load-to-failure in tension) generates force vs. displacement data that is converted to an engineering stress vs. strain curve from which mechanical properties are determined. Toughness is the area under the entire curve up to the point of fracture (ultimate stress), while post-yield toughness is the area under the curve after a yield point that is conventionally identified by the 0.2% offset method (intersection of the curve and a linear line with a slope equal to apparent modulus and extending from 0.2% strain) (A). In load-to-failure tests of human cortical bone at 5 mm/min, bone typically has a lower ultimate stress in tension than in compression. To mark the yield point on each curve, the long dashed lines are parallel to the apparent modulus of each test and start from 0.2% strain (B).