Evaluation of precracking methods for the end-notched flexure test

Results are presented from a study to evaluate and develop static mode II precracking methods for use with the end-notched flexure test. Precracking followed by fracture toughness testing was performed on specimens from two different materials. Precracking was performed using both the four-point bend and conventional three-point bend end-notched flexure geometries. All testing was performed in the latter geometry. There was no difference in the precracks created by the two geometries. However, delamination toughness was observed to strongly depend on the amount of dynamic crack advance that occurs during the precracking process. This was hypothesized to be due to the faster crack speeds that are associated with larger amounts of advance. To address this, various precracking geometries were evaluated in order to determine those that would produce precracks that were essentially straight, perpendicular to the direction of crack advance, and of sufficient length to produce a precracked toughness at or near the minimum value that occurs with increasing precrack length. This results in a recommended geometry for use with the end-notched flexure test that is appropriate for both precracking and testing, and which therefore allows for non-precracked and precracked toughnesses to be obtained from the same test specimen.