Abstract: 31Si2MnCrMoVE steel is ultra-high strength steel which is specially developed for manufacturing solid rocket motor shell. The fracture toughness increases continuously, correspondingly, the steel plate thickness can be reduced due to development of the smelting technology. However, the increasing of fracture toughness and reduction of plate thickness result in the difficulty of measuring its surface crack fracture toughness because too small ligament sizes of specimens could not satisfy the validity criterion. So the linear elastic fracture mechanics can not be used to evaluate the fracture toughness of thin ultra-high strength steel, but the ductile fracture toughness( JIC) of thin ultra-high strength steel can be evaluated using the elasto-plastio fracture mechanics. The fracture mechanics test and finite element method had been conducted to calculate the J-integral in the vicinity of the crack tip under the conditional load in the condition when the conditional fracture toughness (KIQ)can not satisfy the validity criterion. The computed J-integral can be used as the ductile fracture toughness JIC of the material. Surface-crack fracture toughness KIe is computed on the fracture mechanics method from JIC . However, if JIC is used as the fracture parameter, the validity of J-integral must be analyzed. So, the conservation of loop J-integration and the J-dominant characteristic of ultra-high strength steel surface crack are studied. These are important technical parameters for design of aerospace solid rocket motor.

Key words: Finite element analysis, Fracture toughness, J-dominant zone, Surface crack, Ultra-high strength steel