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

In the ductile/brittle transition regime, fracture resistance of steel weld ments is controlled by the competition between ductile tearing and cleavage fracture. Under typical conditions, a crack in a weld initiates and grows by ductile tearing but, ultimately, failure occurs by catastrophic cleavage fracture. In this study, the transition to cleavage fracture in the weld m etal is based on a weakest link mechanism in conjunction with Weibull stati stics. This model leads immediately to the Weibull stress, a,, as the measu re of propensity for cleavage fracture. The Weibull stress depends on the l evel of the stress and the volume of the sampled material. The successful a pplication of this cleavage fracture model hinges on the accurate descripti on of the evolving stress field due to interplay between plastic yielding, weld geometry and ductile tearing prior to cleavage. This interplay can be captured using a cell model of the weld material. A cell is a basic materia l unit endowed with the appropriate micro-separation characteristics. This model is applied to compute the behavior of undermatched and overmatched we lds. The fracture resistance due to ductile tearing and the onset of unstab le cleavage fracture are calculated for different crack lengths, specimen g eometries and weld widths.