Crack growth and cleavage in mismatched welds: a micromechanics study using a cell model

摘要

In the ductile/brittle transition regime, fractureresistance of steel weldments is controlled by the competitionbetween ductile tearing and cleavage fracture. Under typicalconditions, a crack in a weld initiates and grows by ductiletearing but, ultimately, failure occurs by catastrophic cleavagefracture. In this study, the transition to cleavage fracture in theweld metal is based on a weakest link mechanism in conjunctionwith Weibull statistics. This model leads immediately to theWeibull stress, σw, as the measure of propensity for cleavagefracture. The Weibull stress depends on the level of the stress andthe volume of the sampled materials. The successful applicationof this cleavage fracture model hinges on the accurate descriptionof the evolving stress field due to interplay between plasticyielding, weld geometry and ductile tearing prior to cleavage.This interplay can be captured using a cell model of the weldmaterials. A cell is a basic materials unit endowed with theappropriate micro-separation characteristics. This model isapplied to compute the behavior of understated and overmatchedwelds. The fracture resistance due to ductile tearing and the onsetof unstable cleavage fracture are calculated for different cracklengths, specimen geometries and weld widths.