An experimental/analytical comparison of three-dimensional deformations atthe tip of a crack in a plastically deforming plate - III: Comparison of numerical and experimental results

A comparison between the three-dimensional experimental and numerical displ acement fields surrounding a notch/crack in a ductile 4340 steel tested in three-point bending is presented. Excellent agreement between computed and measured deformations exists at load levels below 50 to 75 percent of ultim ate loads. Experimentally determined crack tunnel profiles are included in the finite element model through nodal release; the evidence of the crack t unnel appears in the displacements at the surface. It is shown that surface measurements of unloading reveal specimen-internal failure initiation in t he form of tunneling. Out-of-plane deformations deviate from analytical val ues earlier than in-plane values; this observation compromises the accuracy with which predictions of in-plane crack tip variables can be made when th ey are based on measured out-of-plane deformations (caustics, gradient sens ing) once significant plasticity arises. Comparison is made between J-integ ral values calculated from the external boundary conditions and from a doma in integral. The tunneling tests provide a method of estimating a critical value of J. The stress intensity factor governs the deformation in the elas tic regime, but, because of the finite notch-tip radius underlying the expe rimental configuration, the HRR field does not describe the deformation wel l under plastic conditions. Comparison of numerical simulations with and wi thout tunneling provide insight into criteria that could be used to impleme nt an implicit crack propagation scheme into the numerical model.(1)