Jidong Kang

Microstructural Aspects of Damage and Fracture in AZ31 Sheet Materials

There are considerable data in the literature dealing with deformation mechanisms in AZ31 sheets. However, there is little information on the damage and fracture processes in this material. In this contribution, digital image correlation is used to follow deformation patterns occurring during tensile and v-bending tests at room temperature. A variety of surface analysis techniques and three-dimensional x-ray tomography have been used to examine the relationship between deformation, damage initiation, and the final fracture processes. The results show that premature diffuse necking occurs in the tensile tests without transit into localized necking. Deformation twins cluster by an autocatalytic process to form shear bands serving as preferential sites for strain localization and crack initiation. Damage appears in the form of microcracks within the shear bands at a late stage of necking and lead to the final fracture. The presence and the distribution of second-phase particles and their distributions help accelerate the final fracture processes.

Effect of Mg content on Portevin–Le Chatelier band strain in Al–Mg sheet alloys

Experimental measurements of stress drops and band strains in type-B Portevin–Le Chatelier (PLC) bands were carried out for 5xxx series Al–Mg sheets with Mg content between 1.8 and 4.5 wt%. While the stress drops increase with global strain, the band strain values in all the samples follow a common linear relationship with global strain. The results indicate that the type-B PLC band strain is independent of solute content at given strain rate.

Local Strain Measurement in a Strip Cast Automotive Aluminum Alloy Sheet

Local strain measurement based on digital image correlation at both macroscopic and microscopic scales is presented. A speckle pattern was used for the macroscopic strain mapping to reveal the inhomogeneous deformation processes occurring during tensile deformation of a strip cast automotive aluminum sheet. Moreover, a novel microscopic strain mapping technique based on scanning electron microscopic (SEM) topography image correlation was introduced for strain mapping down to the grain level. The SEM images taken from an in-situ tensile sample of the same material within a field emission SEM chamber are used to demonstrate the validity of the method. The results clearly reveal the evolution of local strain of order of one as well as the formation of shear band in the material.

The Effect of Porosity on Fatigue of Die Cast AM60

AM60 high-pressure die castings are known to contain significant porosity which can affect fatigue life. We have studied this using samples drawn from prototype AM60 shock towers by conducting strain-controlled fatigue tests accompanied by X-ray computed tomography analysis. The results show that the machined surface is the preferential location for fatigue crack development, with pores close to these surfaces serving as initiation sites. Fatigue life shows a strong inverse correlation with the size of the fatigue-crack-initiating pore. Pore shape and pore orientation also influence the response. A supplemental study on surface roughness shows that porosity is the dominant factor in fatigue. Tomography enables the link between porosity and fatigue crack initiation to be clearly identified. These data are complemented by SEM observations of the fracture surfaces which are generally flat and full of randomly oriented serration patterns but without long-range fatigue striations.

Measuring the Mechanical Properties of Aluminum Sheets and Their Resistance Spot Welds at Large Strains Using Digital Image Correlation Coupled with a Modified Shear Test

Constitutive behavior of aluminum sheets and their resistance spot welds at large strains is critical for light weigh vehicle design analysis. However, data from uniaxial tensile tests are usually limited to small strains or material instability. A novel technique was developed using digital image correlation coupled with a modified shear test to directly measure stress – strain curves of aluminum sheets at large strains. The modified shear sample prevents end rotation of the shear zone compared to the ASTM standard B831 test. The results show that the effective stress and effective strain curves from shear tests match those obtained from uniaxial tension, but only by incorporating the material anisotropy using the Barlat–Lian yield function. The technique was then further applied to aluminum resistance spot welds to allow the first time to determine both the shear strength and stress-strain curves of spot welds at large strains. The results show that while there is slight change in yield strength and work hardening behavior for non heat treatable AA5754 spot welds, there is significant decrease in both yield strength and work hardening rate for heat treatable AA6022-T4 spot welds. Technique details and experimental validation will be discussed.

Porous TiNi Biomaterial Produced by Self-Propagating High-Temperature Synthesis: Pore Structure, Mechanical Property and Application

Porous TiNi shape-memory alloy (TiNi SMA) bodies were produced from the (Ti+Ni) powder mixture by self-propagating high-temperature synthesis (SHS) method. Various processing variables such as ignition temperature and heating schedule were used to control the pore structure. Relationship between pore structure and mechanical property was also investigated. An in vivo test was performed to evaluate bone tissue response and biocompatibility of porous TiNi SMA. It showed no apparent adverse reactions such as inflammation and foreign body reaction. Bone ingrowth was found in the pore space of all implanted blocks. Introduction Titanium-nickel intermetallic compound is well known as one

Inhomogeneous Deformation of AZ31 Magnesium Sheet in Uniaxial Tension

Inhomogeneous plastic deformation during uniaxial tensile test of AZ31 magnesium sheet has been studied using digital image correlation and electron backscatter diffraction techniques. It is shown that large strain gradients exist on the sheet surface parallel and perpendicular to the loading direction and very little deformation occurs in the thickness direction. The lack of thinning leads to abrupt fracture right after the formation of a premature but profound diffuse neck without transitioning to any localized neck. Such inhomogeneous deformation arises from the strong basal texture of the starting sheet and the resultant need for contraction and double twinning to accommodate strain. The strain distribution on the sheet surface evolves nonlinearly with strain, impacting the measured r-value.

Influence of the restored work-hardening rate on ductility studied by X-ray computed tomography

Abstract Void growth and coalescence in a model material containing an artificial three-dimensional void array (the estimated local void volume fraction was 1.1%) were investigated by X-ray computed tomography (=XCT). The initially tapered cylindrical voids became elongated along the principal straining direction by some amount of tensile plastic deformation, and then the work-hardening capacity in the intervoid matrix was restored by an intermediate annealing. The annealed model material was pulled in tension all the way up to fracture to see if the existing void coalescence models can predict the void coalescence strain.

A Novel Approach for the Prediction of Shear Localization in Sheet Forming

In this work, a novel approach has been proposed to predict shear band formation under uniaxial tension. The approach postulates a heterogeneous distribution of mechanical properties through the starting material. The model is used to study the effect of such heterogeneity on the development of shear localization with increasing strain. A correlation between diffuse and localized necking, and strain‐hardening rate has been found. The predicted force‐displacement curves and the limit strains to failure agree well with experimental observations.