The influence of specimen attachment and dimension on microtensile strength

The higher microtensile bond strength values found for specimens with a smaller cross-sectional area are often explained by the lower occurrence of internal defects and surface flaws. We hypothesized that this aberrant behavior is mainly caused by the lateral way of attachment of the specimens to the testing device, which makes the strength dependent on the thickness. This study showed that composite bars of 1x1x10, 1x2x10, and 1x3x10mm attached at their 1-mm-wide side (situation A) fractured at loads of the same magnitude, as a result of which the microtensile strength (muTS), calculated as F/A (force at fracture/cross-sectional area), significantly increased for specimens with decreasing thickness. Attachment at the 1-, 2-, or 3-mm-wide side (situation B) resulted in equal muTS values (P > 0.05). Finite element analysis showed different stress patterns for situation A, but comparable patterns for situation B. Both situations showed the same maximum stress at fracture.