Ductile Fracture by VoidSheet Coalescence in HY100 Steel
Ductile Fracture by Void-Sheet Coalescence in HY-100 Steel: A Modeling Study Friday, July 2, 1999 James P. Bandstra, Ph. D. Candidate Metals Science and Engineering Dr. Donald A. Koss, Advisor
Ductile Fracture in Metallic Systems Basic mechanisms of ductile fracture • Nucleation of voids - particle cracking or debonding • Growth of voids • Coalescence / linking Region of scrutiny
Ductile Fracture depends on: • Material structure: crystalline, microstructure • Material properties: s-e • Temperature - low temperature considered here • Inclusion “microstructure” + Second phase particles • Stress state
Void coalescence / linking How does coalescence occur? Least known area of ductile fracture • By void impingement - “generalized” • By void-sheet mechanism - localized
HY-100 Steel - Experimental Study Dana Goto, Ph. D. research, 1997 HY-100: commercial, hot-rolled, quenched and tempered, low carbon Ni-Cr-Mo steel 25. 4 mm thick plate Military Specification, MIL-S 16216 K: Sy min = 100, 000 psi , % RA min = 45%.
HY-100 Experimental Study (Goto, 1997) HY-100 plate • austenitized at 905 C (1660 F) for 1 hour, quenched, tempered at 638 C (1180 F) • martensite-bainite microstructure, sub-micron carbide (Fe 3 C) particles • Mn. S inclusions: <1. 0 to 2. 0 mm diameter “spherical” inclusions AND larger, elongated (~30 -100 mm long and 2 -3 mm thick) inclusions and stringers • average Af for larger Mn. S inclusions = 0. 00015 ( 0. 00005).
HY-100 Experimental Study (Goto, 1997) HY-100 plate directions and planes
HY-100 - Experimental Study (Goto, 1997) Notched bar tensile tests: Stress Triaxiality = sm/seq
HY-100 - Experimental Study (Goto, 1997) Failure Limit Diagram shows two distinct regions
Failure Mechanism Region II failure characterized by: • low ductility • high stress triaxiality, sm/seq (high constraint) • weak sensitivity to stress triaxiality • presence of large, elongated Mn. S inclusions + smaller, secondary inclusions/particles • void-sheet failure mechanism
Computational Modeling Key issue: Can a pair of 2. 5 mm diameter voids, nucleated at elongated Mn. S inclusions, lead to an intense strain localization between the voids, which in turn trigger nucleation of microvoids at a secondary population of smaller particles/inclusions, leading to material failure?
Computational Modeling Model concept showing region of interest 45° 2. 5 mm DIA 70 mm spacing
FEM Two-Void Model and boundary conditions Constraint equations for sides and top
Two-Void Model Definitions for stress/strain terms Stress biaxiality Stress triaxiality
Two-Void Model Applied stress conditions analyzed
Two-Void Model Results Local strain localizes over a narrow band at higher stress biaxialities
Two-Void Model Results Strain localizes rapidly with applied strain at high stress biaxiality
Six hole vs. Twohole models Six holes in favorable orientation increases strain localization somewhat
Predictions of Material Failure Local equivalent plastic strain = 0. 5 is nucleation strain value at secondary population of carbides / sulfides Values in literature range from 0. 3 to 0. 7 Failure initiation occurs when secondary population nucleates voids across intervoid region Interfacial stress criterion:
Predictions of Material Failure Applying local nucleation strain criteria leads to macroscopic failure strain (ef)
Failure Limit Diagram Computational predictions and Experimental data are similar
Effect of Gurson Material Response of porous plastic material (dilatational) Yield condition: Void growth: Case a: Sulfides - Vf = 0. 0002, enuc = 0. 01 Case b: Carbides - Vf = 0. 021, enuc = 0. 5
Failure Limit Diagram Gurson material lowers failure strains - softening effect
Void Growth Void growth increases with strain and appears exponential with stress triaxiality
Void Growth Mc. Clintock void growth equations
Void Growth Void growth data determined to fit Mc. Clintock type growth equations and constants determined
Void Growth Void growth generally follows form for isolated voids - except at high stress triaxiality
Influence of Strain-Hardening HY-100 stress-strain response and strain-hardening
Influence of Strain-Hardening Increasing “n” decreases tendency to localize strain
Shear Band Limit Load Analysis Applying limit load analysis to a shear band allows comparison to experimental and FEM predictions
Critical Void Size Void size at critical value of local strain indicates a critical void size for strain localization
Critical Void Size Strain hardening results confirm critical void size
Critical Void Size / Spacing Critical void size/spacing ratio found formation of intense strain localization and failure initiation Size/spacing ratio = 0. 3 at which point intense strain localization occurs Based on analyses with void spacing 1 X, 2 X and 3 X and strain-hardening variations
Effect of Pre-Strain and Strain Path Changes Experiments by Chae: Pre-strain at high stress triaxiality, fail at low stress triaxiality - showed continuation of void-sheet failure to lower stress triaxialities
Effect of Pre-Strain and Strain Path Changes Computational FLD agrees well with experimental data
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