Specimen size requirements for two-parameter fracture toughness testing

Using a single parameter fracture mechanics theory, a minimum specimen size requirement of min(a, b,B)>200J/σ0 in tension and min(a, b,B)>25J/σ0 in bending, where B is the thickness, b the remaining ligament and a is the crack length of the specimen, were derived [Shih and German (1981), International Journal of fracture 17, 27-43] which have provided the basis for modern fracture toughness testing procedures. Two-parameter fracture toughness testing including the constraint, on the other hand, is desirable since it offers a solution to the transferability issue. A size requirement for a valid two-parameter fracture toughness testing based on the J-A2 three-term solution was determined as min(a, b,B)>11J/σ0 [Chao and Zhu (1998), International Journal of fracture 89, 285-307] in which the limiting case is bend specimens under large scale yielding (LSY). Recent work by Chao et al. (2004, International Journal of fracture, 27, 283-302) has shown that the J-A2 dominance at a crack tip can be significantly enhanced for bending specimens under LSY if a modified J-A2 solution is adopted. This current paper further studies the size of the J-A2 dominant zone using the modified J-A2 solution for deep bend specimens with hardening from low to high and loading from SSY to LSY using finite element analysis. Based on the results, a rather relaxed specimen size requirement min(a, b,B)>6J/σ0 is developed and recommended for a valid two-parameter fracture toughness testing using the J-A2 fracture criterion. Validity of the size requirement is demonstrated by using the experimental J-R curves from non-standard bending specimens for A285 steel.