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Title:A Numerical Investigation of Loading Rate Effects on Pre-Cracked Charpy V-Notch Specimens
Author(s):Koppenhoefer, K.C.; Dodds, Robert H., Jr.
Subject(s):Fracture mechanics
Finite elements
Cleavage fracture
Abstract:Specimen size and loading rate effects on the fracture of ferritic steels remain key issues for the application of pre-cracked Charpy specimens. This investigation employs nonlinear finite element analyses to assess the effects of specimen size and loading rate on cleavage fracture and ductile crack growth in these specimens. To examine loading rate effects on cleavage fracture, plane strain and 3-D' finite element analyses assess crackfront stress triaxiality in quasi-static and impact-loaded CVN specimens. Plane strain analyses utilize J-Q trajectories and the Toughness Scaling Methodology to quantify loading rate effects on near-tip constraint. Crack front conditions in the 3-D analyses are characterized in terms of the Weibull stress which reflects the statistical effects on cleavage fracture. The 3-D computations indicate a less strict size/deformation limit than plane strain analyses 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 methodology based on the Weibull stress is described to remove the effects of constraint loss. This new scaling model also enables prediction of the distribution of quasi-static fracture toughness values from a measured distribution of 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) specimens from impact-loaded, pre-cracked CVN specimens.To quantify the effects of loading rate on ductile crack growth in CVN specimens, plane strain, finite element analyses are used to model ductile crack extension in specimens subjected to quasi-static and impact loading. The Gurson-Tvergaard dilatant plasticity model for voided materials describes the degradation of material stress capacity. Fixed-size, computational cell elements defined 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 impact velocity, 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 background plasticity. Validation of the computational cell approach to predict loading rate effects on R-curves is accomplished by comparison to quasi-static and impact experimental sets of R-curves for three different steels.
Issue Date:1997-08
Publisher:University of Illinois Engineering Experiment Station. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report:Civil Engineering Studies SRS-618
Genre:Technical Report
Sponsor:U.S. Nuclear Regulatory Commission. Office of Nuclear Regulatory Research. Division of Engineering.
Carderock Division, Naval Surface Warfare Center.
NASA-AMES Research Center
Contract No. N00167-92-K-0038
Date Available in IDEALS:2009-11-16
Identifier in Online Catalog:4072114

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