Hyungyil Lee

Interfacial crack-tip constraints and J-integrals in plastically mismatched bi-materials

Abstract The effect of the T -stress and plastic mismatch on interfacial crack-tip stress under small scale yielding condition is investigated in this paper via detailed finite element analyses. The mismatch in plastic strength/hardening is focused, and plane strain elastic–plastic crack-tip fields have been modeled with modified boundary layer formulation. Plastic mismatches as well as compressive T -stresses in bi-material are shown to affect the interfacial crack-tip constraint substantially. Simple patching of slip-line fields is presented to characterize the interfacial crack-tip stress fields. Effects of T -stress and plastic hardening mismatch on asymmetry of the J -integral for bi-materials, and its implication on interfacial toughness are also discussed.

Mathematical Expressions for Stress-Strain Curve of Metallic Material

Stress-strain curves based on Ramberg-Osgood and Hollomon relations are strongly dependent upon the regressed range of strain. This work investigates mathematical expressions of true stress-strain curves of metallic materials. We first observe the variation of yield strength, strain hardening exponent and stress-strain curve with regressed range of stain. Based on sectional regression and expression using one or two parameters, we propose an optimal strain range for which yield strength and nonlinear material behavior are quite appropriate.

Line-spring finite element for fully plastic crack growth—I. Formulation and one-dimensional results

Abstract A line-spring finite element model is developed to resolve fully plastic, quasi-steady, through-thickness crack growth in plane strain single-edge-cracked (SEC) specimens and surface-cracked plate/shell structures. The plane strain sliding-off and cracking model of McClintock et al. (1995) is adopted to obtain the instantaneous crack-tip opening angle (CTOA) in terms of material parameters and the instantaneous slip-line angle and stress triaxiality at the crack-tip. The slip-line angle and crack-tip stress triaxiality are calculated approximately using the least upper bound method of Kim et al. (1996a). Utilizing these approaches, the generalized forces transmitted by the line-spring finite element provide the constraint-dependent CTOA. The increment of crack extension is then determined from the kinematic relation with incremental crack-tip opening displacement through CTOA. Detailed description of the model, as incorporated into the ABAQUS (1993) finite element code in the form of a user-defined element, is given in Part I. Parametric studies in plane strain SEC specimens are carried out to examine the effects of loading type, ductility and strain hardening on plane strain crack extension behavior, and the effects of elastic surroundings on structural stability. Applications of the line-spring model to problems of surface-cracked plate and pipe are presented in Part II (Lee and Parks, 1998).

Line-spring finite element for fully plastic crack growth—II. Surface-cracked plates and pipes

Abstract Crack-tip opening angle (CTOA) is generally considered as the most operable fracture descriptor for fully plastic, quasi-steady, extensive crack extension. In Part I, based on the CTOA crack growth criterion, a line-spring finite element model was presented to resolve through-thickness crack growth in plane strain single-edge-cracked specimens and surface-cracked plate/shell structures under fully plastic loading. With constraint-dependent CTOA and some supplemental kinematic relations given, the line-spring model ultimately monitors crack extension from the history of generalized displacements. The model was implemented in the implicit ABAQUS finite element code (1993b) in a user-defined element form. Following the plane strain parametric studies in Part I, we further apply the line-spring model to the problems of surface-cracked plates and pipes in Part II here. Effects of material hardening, configuration and location of the surface crack on the histories of penetration-displacement/pressure are examined in a remotely-stretched plate and a pressurized cylindrical vessel. As the most plausible exercise of a stable crack propagation leading to leak-before-break failure, a circumferentially cracked pipe subject to pure bending is selected. Evolution of CTOA along the crack-front and surface crack enlargement pattern are examined for each case. Experimentally-observed CTOA values for the remotely-stretched plate are interpreted in light of the model prediction.

Evaluation of Material Characteristics by Micro/Nano Indentation Tests

The present work reviews the methods to evaluate elastic-plastic material characteristics by indentation tests. Especially the representative stress and strain values used in some papers are critically analyzed. The values should not only represent the load-depth curve, but also represent the whole of deformed material around the impression. We briefly introduce other indentation techniques to evaluate residual stresses, creep properties, and fracture toughness. We also review some technical problems that are related to the accuracy issues in indentation tests.

Software and Hardware Development of Micro-indenter for Material Property Evaluation of Hyper-Elastic Rubber

In this work, effects of hyper-elastic rubber material properties on the indentation load-deflection curve and subindenter deformation are examined via finite element (FE) analyses. An optimal location for data analysis is selected, which features maximum strain energy density and negligible frictional effect. We then contrive two normalized functions, which map an indentation load vs. deflection curve into a strain energy density vs. first invariant curve. From the strain energy density vs. first invariant curve, we can extract the rubber material properties. This new spherical indentation approach produces the rubber material properties in a manner more effective than the common uniaxial tensile/com-pression tests. The indentation approach successfully measures the rubber material properties and the corresponding nominal stress-strain curve with an average error less than 3%.

Design enhancements for stress relaxation in automotive multi-shell-structures

Abstract Recent attempt to enhance the safety against collision has reshaped the simple single-shell structure into the integrated multi-shell structure. Moreover, due to various regulations continuously tightened for environment, weight reduction of automobile becomes an increasingly important issue. Weight reduction is mainly accomplished by better redesign, adoption of lighter materials, and small-sizing of auto (parts). Focusing on the local redesign among three, we suggest local patching methods. We also present, as another way, a method of determining thicknesses of subpart-shells in an integrated multi-shell structure. Those redesign methods successfully bring a preset amount of stress relaxation. More specifically, we first select a cross member as local patching model. Based on the finite element stress calculations, we relieve the stress of cross member by patching in two ways––non-uniform thickness patching and optimized uniform thickness patching. The latter is more effective and practical. Selecting a box type subframe as other redesign model, we determine the thickness of each subpart-shell by axiomatic design approach. The patching methods and the axiomatic approach of this work can be extended to the other multi-shell structures such as center member and lower control arm.

Energy Absorption Characteristics and Optimal Welding Space of Square Hat Type Thin-walled Tube

In this work, energy absorption characteristics and optimal welding space of spot-welded square hat type tube are investigated via quasi-static crush experiments and finite element (FE) analyses. A FE model reflecting the crush characteristics is established based on the experimentally observed crush mechanisms of specimens with welding spaces (20, 30 & 45 mm) and (25,40 & 55 mm) respectively for two specimen widths (60, 75 mm). The established FE model is then applied to other crush models of widths (50, 60 & 75 mm) with various welding spaces (20, 25, 30, 40, 45, 55, 75, 150, 300 mm) respectively. We examine the energy absorption characteristics with respect to the welding space for each specimen width. The outcome suggests an optimal spot welding space of square hat type thin-walled tube. Energy absorption is also presented in terms of yield strength of base metal, specimen thickness, width, and mean crushing force of spot-welded square hat type thin-walled tube.

Ductile Fracture Model in the Shearing Process of Zircaloy Sheet for Nuclear Fuel Spacer Grids

Features of sheared edges are predicted based on material properties of Zircaloy obtained from the tensile test and ductile fracture model such as the Gurson-Tvergaard-Needleman (GTN) and Johnson-Cook models. The sheared edges formations are numerically analyzed in each ductile model. An appropriate ductile fracture model is selected to study the relative depth of sheared edges with respect to process parameters. The tendency of failure parameters that are affected by sheared edges and fracture duration is investigated. We applied changes on parameters of failure models to show that the punch force curve and the ratio of characteristic lengths could be coincided, which led us to conclude that the GTN and Johnson-Cook models are equivalent. In the Johnson-Cook model, however, the characteristic length of the sheared edges does not change as each failure parameter reaches a critical value. Hence, the FE prediction model for forming defects is developed using the GTN failure model. Finally, the characteristic length of sheared edges have been measured using the FE prediction model for shearing process parameters such as punch velocities, clearance, and tool wear. Our results showed that the punch-die clearance is the most significant factor that affects forming defects when compared to other factors.

Acquirement of True Stress-strain Curve Using True Fracture Strain Obtained by Tensile Test and FE Analysis

In this work, we predict a true fracture strain using load-displacement curves from tensile test and finite element analysis (FEA), and suggest a method for acquiring true stress-strain (SS) curves by predicted fracture strain. We first derived the true SS curve up to necking point from load-displacement curve. As the beginning, the posterior necking part of true SS curve is linearly extrapolated with the slope at necking point. The whole SS curve is then adopted for FE simulation of tensile test. The Bridgman factor or suitable plate correction factors are applied to pre and post FEA. In the load-true strain curve from FEA, the true fracture strain is determined as the matching point to test fracture load. The determined true strain is validated by comparing with test fracture strain. Finally, we complete the true SS curve by combining the prior necking part and linear part, the latter of which connects necking and predicted fracture points.