Effects of Friction, Geometry and Fixture Compliance on the Perceived Toughness from Three and Four Point Bend End Notched Flexure Tests

摘要

Linear and nonlinear finite element analyses are used to examine the effects of friction, test geometry and fixture compliance on the perceived toughness as obtained from three and four point bend end notched flexure tests. To this end, a newly developed "direct energy balance approach" is used to obtain the "true" energy release rate for any given specimen, test geometry, and coefficient of friction. Finite element analyses are also used in a simulated compliance calibration technique, which is combined with experimental results from fixture compliance tests to obtain a perceived toughness, i.e., the value that would be obtained by experiment. By varying the different parameters, the individual and combined effects of friction, test geometry, and fixture compliance on the ratio of the perceived to true toughness is obtained. The approach is applied to two graphite/epoxy materials for which toughnesses by the three (3ENF) and four point bend end notched flexure (4ENF) tests were obtained experimentally for a range of geometries. These experiments produced larger perceived mode II toughnesses, G{sub}(IIc), by the 4ENF than the 3ENF test, and G{sub}(IIc) values from the 4ENF test were observed to decrease with increasing outer span length. The finite element simulations are shown to accurately recreate the perceived values of G{sub}(IIc) obtained from these tests. Moreover, the finite element simulations indicate that the true toughness values are essentially constant for a given material. These findings are used to make some general recommendations for choosing 3ENF and 4ENF specimen and test geometries, as well as to discuss the relative advantages and disadvantages of the 3ENF and 4ENF test methods.