Linear and nonlinear finite element analyses of unidirectional, symmetric single leg four-point bending tests

Geometrically linear and nonlinear finite element analyses are used to determine the energy release rate and mode ratio in simulated tests of unidirectional, symmetric, single leg bending specimens under four-point bending. It is shown that the finite diameter loading rollers that are typically used in practical test set-ups cause this test to be inherently nonlinear. The differences between the linear and nonlinear results are presented parametrically as a function of material properties, specimen thickness, roller diameter, crack length, and inner and outer span length. The perceived advantages and disadvantages of this test are compared to those of the more commonly used three-point single leg bending test. It is concluded that the four-point test provides an attractive alternative, as it can use the same type of test specimens and will produce toughnesses with essentially the same accuracy. Moreover, it allows non-precracked and precracked toughnesses, as well as R-curve data, to be obtained from each specimen tested.