On J Integral and JIc Material Testing (With Special Reference to Welded Structures)

The introduction of J integral for crack significance evaluation enabled the extension of fracture mechanics approach to wider range of mechanical behaviour of materials and structures. In addition to brittle fracture, controlled by plane strain fracture toughness, K_Ic and J_Ic, J integral can also desribe the crack behaviour in plastic range. Standardized methods ASTM E399, E813 and E1152, and JSME 001 are shortly described with comments on their application. Special attention is paid to J integral direct measurement method, that enabled the evaluation of crack behaviour not only on tensile panel specimens with surfac crack, but also on the real structure such a pressure vessel. The crack behaviour in welded structure is strongly depended on structural heterogeneity of welded joints and the differences in mechanical properties of base metal, weld metal and heat-affected-zone.The effect of weldment heterogeneity on its strength and ductility is studied on SM80P high-strength low-alloyed steel of 800 MPa ultimate tensile strength and three different welded joints, performed by sumerged are welding. The strain distribution in welded join is depended on matching effect of weld metal, that was performed as normalmatched (slightly lower yield strength of weld metal compared to 800 MPa base metal), undermatched and overmatched. For the same weldments combinations, the effect of crack size and position on strength and ductility was analyzed and the benefits of overmatching effect for cracked weldment safety were clearly recognized, Direct measurement of J integral for cracks positioned in weld metal and heat-affected-zone enabled the comparison with the same value on small three-point-bend specimens. Pop-in effect was observe in small specimens, with the crack tip in HAZ. Practical application of fracture mechanics parameters is demonstrated for residual strength evaluation of cracked pressure vessel by use of Ratvani, Erdogan and Irwin elasto-plastic model for crack driving force and comparison with materials J resistance curves.