A Numerical Investigation of Loading Rate Effects on Pre-Cracked Charpy V-Notch Specimens

摘要

Specimen size and loading rate effects on the fracture of ferritic steels remain key issues for the application ofpre-cracked Charpy specimens. This investigation employs nonlinear finite element analyses to assess the effectsof specimen size and loading rate on cleavage fracture and ductile crack growth in these specimens. Toexamine loading rate effects on cleavage fracture, plane strain and 3-D' finite element analyses assess crackfrontstress triaxiality in quasi-static and impact-loaded CVN specimens. Plane strain analyses utilize J-Q trajectoriesand the Toughness Scaling Methodology to quantify loading rate effects on near-tip constraint. Crackfront conditions in the 3-D analyses are characterized in terms of the Weibull stress which reflects the statisticaleffects on cleavage fracture. The 3-D computations indicate a less strict size/deformation limit than plane strainanalyses to maintain small-scale yielding conditions at fracture under quasi-static and impact loading conditions.For impact analyses which violate these size/deformation limits, a modified toughness scaling methodologybased on the Weibull stress is described to remove the effects of constraint loss. This new scaling model alsoenables prediction of the distribution of quasi-static fracture toughness values from a measured distributionof impact toughness values (and vice versa). This procedure is applied to experimental data obtained from a CrNi-Mo-V pressure vessel steel and accurately predicts quasi-static fracture toughness values in 1 T-SE(B) specimensfrom impact-loaded, pre-cracked CVN specimens.To quantify the effects of loading rate on ductile crackgrowth in CVN specimens, plane strain, finite element analyses are used to model ductile crack extension inspecimens subjected to quasi-static and impact loading. The Gurson-Tvergaard dilatant plasticity model forvoided materials describes the degradation of material stress capacity. Fixed-size, computational cell elementsdefined over a thin layer along the crack plane provide an explicit length scale for the continuum damage process.Parametric studies focusing on numerically generated R-curves quantify the relative influence of impactvelocity, material strain rate sensitivity, and properties of the computational cells (thickness and initial cell porosity).In all cases, impact loading elevates significantly the R-curve by increasing the amount of backgroundplasticity. Validation of the computational cell approach to predict loading rate effects on R-curves is accomplishedby comparison to quasi-static and impact experimental sets of R-curves for three different steels.