Effect of crack length measurement technique and data reduction procedures on the perceived toughness from four-point bend end-notched flexure tests

Results are presented from a study to determine the most appropriate crack length measurement and data reduction techniques for use with the four-point bend end-notched flexure test. Using two different graphite/epoxy materials, the variability in crack length measurements due to different observers and/or measurement techniques, including visual observation and ultrasonic inspection, and their effects on the perceived values of the mode II delamination toughness, G_IIc were evaluated for two different data reduction techniques. It was found that a technique using pretest compliance calibrations is the only approach that will produce accurate non-precracked and precracked values of G_IIc regardless of the material being tested or the nature of crack advance. For those materials where G_IIc values, as obtained by the more common approach that utilizes compliance values obtained during the test, agree with those obtained by compliance calibration, the results from the more common approach may be applied to obtain resistance curve toughnesses.