A simple model is presented for the effect of size and geometry on fracture toughness when represented as J-R curves in the fully plastic region. The model, which is little more than dimensional analysis, refers to growth beyond anyJ-controlled regime. It suggests that the abscissa of a J-based R-curve should be Δa/c, where c may be thickness, ligament, or material toughness (expressed as a plastic zone size) according to circumstance. The analysis is consistent with the well known picture in the LEFM regime where thickness is the only geometric term that affects toughness.
Comparison is made with many published results, in most cases satisfactorily. The agreement is best for tests of various ligament widths with constant thickness. The reason for this is the limited understanding of how the shear lips develop before a steady-state pattern of growth is reached and the likelihood that the controlling factor may change when thickness is a variable. Since for the fully plastic state the increment of work from which toughness is derived involves only the limit load and dq/da, the relation between an increment of displacement and the corresponding crack growth, the latter seems to be the factor that contributes most to the differing trends of fully plastic J-R curves as initial geometry is varied. The near total lack of published data for dq/da needs urgent remedy if these geometric effects are to be understood.