Constraint Effects in Weld Thermal Simulated Steel Specimens

The brittle fracture constraint effect in two different weld thermal simulated microstructures of a 500 MPa steel is investigated through testing of bending specimens with different a/W ratios. It is found that on average the constraint effect is higher in the CGHAZ microstructure compared to the ICCGHAZ. The procedure for calibration of Weibull parameters proposed by Gao et al. [1] has been applied to the results. It is found that a lower m-value is calibrated for the ICCGHAZ microstructure due to the lower constraint effect observed. Three-parameter representations of the Weibull distribution better represent the distribution of the fracture toughness values compared to two-parameter representations.

Bending Modified J-Q Theory and Its Application to Fracture Constraint Analysis

The J-Q theory [1,2] can characterize the crack-tip fields and quantify fracture constraints for various geometric and loading configurations in elastic-plastic materials, but it fails to do so for bending-dominant geometries at large-scale yielding (LSY). This issue significantly restricts its applications to fracture constraint analysis. A modification of the J-Q theory is thus proposed in this paper as a three-term solution with an additional term to address the global bending stress to offset this restriction. The nonlinear global bending stress is linearly approximated in the region of interest at LSY. To verify the bending-modified J-Q solution, detailed elastic-plastic finite element analysis (FEA) is carried out under plane strain conditions for three conventional bending specimens, i.e., single edge notched bend (SENB), single edge notched tension (SENT) and compact tension (CT) specimens for X80 pipeline steel. Deformation considered varies from small-scale yielding (SSY) to LSY. The results show that the bending modified J-Q solution can well match FEA results of crack-tip stress fields for the bending specimens at all deformation levels from SSY to LSY, and the modified parameter Q is a load- and distance-independent constraint parameter at LSY. Thus, the modified parameter Q can be effectively used to quantify the crack-tip constraint for bending geometries. Its application to fracture constraint analysis is demonstrated by ranking crack-tip constraint levels for fracture specimens and by determining constraint corrected J-R curves for the X80 pipeline steel.