Constraint effects on reference temperature, To, for ferritic steels in the transition region

This study presents the methodology and key results derived from stochastic simulations to examine the effect of constraint loss on the reference temperature, F0 at which the median fracture toughness of 1 T size specimens equals 100 MPa for ferritic steels in the transition region. Using the Weibull stress as a local fracture parameter for transgranular cleavage, nonlinear three dimensional finite element analyses provide a quantitative description of constraint loss along crack fronts in deeply-notched SE(B) specimens in terms of a toughness scaling model. The simulations employ a Monte Carlo procedure to generate trial sets of small-scale yielding fracture toughness data which follow the three parameter Weibull distribution prescribed in the ASTM draft standard to determine T0. Reserve corrections of the SSY values using the toughness scaling model predict large-scale yielding values (that would be measured in a fracture test). The differences in reference temperatures for the SSY and LSY toughness values represent the shift due to constraint loss. Results of extensive experimental testing programs, complemented by these simulations, provide key information needed by testing standard committees to specify appropriate censoring (deformation) limits on measured fracture toughness data relative to specimen size and material flow properties.