An evaluation of data reduction methods for the mixed mode bending test

Results are presented from a combined numerical and experimental study to assess the accuracy of six different methods of data reduction for the mixed-mode bending test. These include two methods in the literature that use only the load data from the test, a modification to one of these methods to improve accuracy, two variations of compliance calibration, and a newly proposed "load-deflection method." First, the accuracy of the various methods were evaluated by comparison to finite element predictions for a typical laminate. Second, the various methods were applied to double cantilever beam and end-notched flexure test data that had previously been reduced by well-established techniques. These "preliminary studies" were used to obtain the five most accurate methods. Finally, five laminates were tested in the mixed-mode bending fixture at each of five mode ratios: G_II/G = 0.2, 0.4, 0.6, 0.8 and 1.0. The data from these tests were reduced by the five data reduction methods. The mean value of critical energy release rate, G_c, at G_II/G = 0.4 was compared to the mean Gc obtained by compliance calibration of a separate set of five single leg bending test specimens, and G_c at G_II/G =1.0 was compared to the mean G_c obtained by compliance calibration of a separate set of five end-notched flexure test specimens. By these comparisons, by physical considerations of the test results, and by examinations of the standard deviations of the various data pools, it was concluded that a modification to a method that uses only load data from the test is the most accurate. This modification involves only the experimental determination of the bending rigidities of the cracked and uncracked regions and the use of these results in the reduction of data.