Estimation of crack driving forces on strength-mismatched bimaterial interfaces

This paper presents a framework to estimate crack driving forces in terms of crack-tip opening displacement and J-integral for mismatched dissimilar joints with interface cracks. The mismatch in elastic, thermal, and plastic hardening properties is not considered, but the mismatch in plastic yield strengths is emphasized here. The main outcome of the present work is that the existing methods to estimate crack driving forces for homogeneous materials can be used with slight modification. Such modification includes: (i) mismatch-corrected limit load solutions; and (ii) evaluating the contribution of each material in dissimilar joints to the total crack driving force, which depends on the strength mismatch of the dissimilar joints.     

Buckling sensitivity of a connecting rod to the shank sectional area reduction

Yield, fatigue and buckling characteristics are often used as evaluation indexes for the performance of engine connecting rods in weight reduction design. There are, however, a limited number of studies on the buckling of connecting rods. Even the widely used field equations for the buckling have limitations in the application since they are derived from ideal support conditions. This study first presents an evaluation procedure for the buckling of a connecting rod via finite element analysis (FEA). The FEA approach allows us to treat the first and second modes of buckling collectively. The buckling stresses from the suggested FEA approach are closer to those measured in rig experiments than those from classical formula are. The stress sensitivities to the area reduction of rod shank are then examined in lights of yield, fatigue and buckling. The stress sensitivity in buckling indicates to be relatively higher than or comparable to those of yield and fatigue. Consequently, when weight reduction of connection rod shank is attempted, buckling should be considered as an essential factor along with the other criteria such as yield and fatigue.     

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.     

A 3D FE model with plastic shot for evaluation of equi-biaxial peening residual stress due to multi-impacts

A 3D multi-impact finite element (FE) model for evaluation of peening residual stress is presented. Combined peening factors by Kim et al. are applied to the 3D symmetry-cell originally contrived by Meguid et al. To describe the feature of multi-impacts, concepts such as FE peening coverage, impact sequence and cycle-repetition are introduced. We successfully extracted the equi-biaxial stress from the simulations of diverse single-cycle and multi-cycle impacts. At four impact locations of FE symmetry-cell, surface and maximum residual stresses converge to equi-biaxial stress, and convergence improves with the number of repetitions of cycle. Impact velocity needed for comparing the FE solution with the XRD result is determined from the Almen arc height and coverage. It is further found that the simulation set with plastic shot produces residual stress consistent with the experimental XRD result.     

Overload failure curve and fatigue behavior of spot-welded specimens

The mechanical behavior of a spot-welded specimen is generally approached in angles of overload and fatigue failures. The primary issue in an overload failure is to establish an overload failure criterion. Fatigue failure of spot-welded specimens can be dealt with a fracture parameter, since a spot-weld forms a singular geometry of external crack type. In this work, we express the limit loads in terms of base metal yield strength and specimen geometries. We then present a master overload failure curve for a single spot-welded specimen in a mixed-mode load domain. The coordinates of the domain are normalized by the limit loads of single spot-welded specimens. Recasting the load vs. fatigue life relations experimentally obtained, we attempt to predict the fatigue life of various spot-weld specimens with a single parameter denoting the equivalent stress intensity factor. This crack driving parameter is demonstrated to successfully describe the effects of specimen geometry and load type in an inclusive manner. The suggested fatigue life formula for a single spot-weld can be used in the assessment of spot-welded panel structures as the fatigue strength of multi-spots is eventually determined by that of each single spot-weld.     

Crack-tip opening angle-based numerical implementation for fully plastic crack growth analyses

This study introduces implementation of a nodal release technique into a FEM/continuum model to enable simulation of fully plastic crack growth. The nodal release technique is implemented in the user-defined element form on the symmetry line of a deeply single-edge cracked specimen so that the force at the crack-tip node on the symmetry line is made zero after several steps upon the satisfaction of a chosen fracture criterion, and an incremental crack extension is achieved. The fracture criterion adopts the crack-tip opening angle (CTOA) which is determined from the specimen’s loading geometry [1]. For evaluation of the present model, the crack growth simulation results from the present FEM model were compared to those from the line-spring model of Lee and Parks [2].     

Enhancement of Drucker-Prager yield model by adding corner points for pressure-dependent materials

Journal of Mechanical Science and Technology - To flexibly describe the pressure-dependent behaviors of materials, Drucker-Prager yield model is enhanced by introducing corner points. We call it...     

Enhancement of the analytical solution for double torsion test using extended finite element techniques

The double torsion (DT) testing technique is not yet standardized for fracture mechanics characterization of brittle materials because previous analytical solutions are insufficient to represent accurate load-displacement (P-h) relationship and stress intensity factor (SIF) in experiments. Therefore, we attempt at enhancing the analytical solutions based on three-dimensional extended finite element (XFE) analysis to obtain reproducible results from DT tests. As a result, the P-h relationship in DT test can be efficiently described by combining the bending deformation of a DT specimen with torsional deformation. Weighting factors are proposed as functions of thickness, moment arm and crack length. DT experiments are conducted with sodalime glass specimens to further validate the proposed weighting factors. Finally, correction terms are provided for SIF and fracturs toughness evaluation in DT specimen with straight and curved crack fronts. Fracturs toughness values of sodalime glass specimens with various thickness are consistent, and in good agreement with literature values.     

A dual conical indentation technique based on FEA solutions for property evaluation

The sharp indenters such as Berkovich and conical indenters have a geometrical self-similarity so that we can obtain only one parameter from an indentation loading curve, which makes different materials have the same load-displacement relation. Most studies to evaluate elastic-plastic properties by using the geometrical self-similar indenter have therefore tried to use dual/plural indentation techniques, on the basis of the concept of representative strain/stress varying with the indenter angle. However, any suggested representative concept is not universally operative for real materials. In this work, we suggest a method of material property evaluation without using the concept of representative strain. We begin the work by studying the characteristics of load-depth curves of conical indenters via finite element (FE) method. From FE analyses of dual-conical indentation, we investigate the relationships between indentation parameters and load-depth curves. The projected contact diameter is expressed as a function of the indenter angle, tip-radius, and material properties, which allows us to simply predict the elastic modulus. Two mapping functions for two indenter angles (45° and 70.3°) are presented to find the two unknowns (yield strain and strain-hardening exponent) via dual indentation technique. The method provides elastic modulus, yield strength and strain-hardening exponent with an average error of less than 5%. The method is valid for any elastically deforming indenters. We also discuss the sensitivity of measured properties to the load-displacement curve variation, and the difference between conical and Berkovich indenters.     

Effect of strength mismatch on fully plastic fields in dissimilar joints under combined loading

Via detailed finite clement limit analyses, plastic limit loads, rotation factors, and crack-tip stress field are investigated for a combined tension and bending of a plance strain single-edge-cracked bimaterial specimen. Limiting bimaterial specimens are considered, consisting of an clastic/perfectly plastic material bonded to an elastic material having the same elastic properties. The limit loads of bimaterial specimens are shown to be very close to those of homogeneous specimens, so that limit load information for homogeneous specimens can be used even for bimaterial specimens. A tractable, approximate elliptical yield locus is proposed, which first the FE. results within 1% for all ranges of tension-to-bending ratios. The plastic rotation factor of bimaterial specimens can be higher than that of homogeneous specimens as much as 25%, when the specimen is subject to small tensile forces. Results from the present analysis is applied to the analysis of typical fracture testing specimens such as compact tension specimens. For both homogeneous and bimaterial specimens, larger tensile forces are associated with substantial loss of crack-tip constrait. Bimaterial specimens have as much as 2 times higher constraint than homogenous specimens, due to plastic strength mismatch. Tractable closed form approximations for crack-tip stresses are proposed in terms of tension-to-bending ratio.