Strength-probability-time (SPT) diagram- an adjunct to the assessment of dental materials?

This investigation sought to construct and compare strength-probability-time (S-P-T) diagrams for four dental materials. Three of these were resin composites and one was dental plaster. In the case of dental plaster a total of 90 compressive specimens was fabricated whereas for each of the other materials a total of 75 specimens was prepared. The compressive strength of equal sized groups of each material was then determined at the crosshead (XHD) speeds of 1, 5 and 10 mm min^-1 respectively. The data was subjected to Weibull analysis to relate the probability of failure to the applied stress. Where strong correlations were found between the (i) mean compressive strength and crosshead speed, (ii) individual compressive strengths and failure times, the data was used to determine the crack velocity exponent (n) and produce a S-P-T diagram. Although only one of the materials (P-50) evaluated fulfilled all the necessary criteria and yielded a value of n = 16.13 (7.22), it is suggested that this method may enable comparisons to be made amongst other materials satisfying the required conditions. As such diagrams are based upon a crackgrowth law they may be of value in assessing the likely clinical wear resistance of new formulations. Consideration, however, should always be given to what levels are deemed acceptable for the intended clinical application of the material. Thus, before this technique can be employed fully, to the evaluation of new restorative materials, further work is necessary to determine appropriate design criteria.