Ngoc-Trung Nguyen

Mechanical Behavior of AZ31B Mg Alloy Sheets under Monotonic and Cyclic Loadings at Room and Moderately Elevated Temperatures

Large-strain monotonic and cyclic loading tests of AZ31B magnesium alloy sheets were performed with a newly developed testing system, at different temperatures, ranging from room temperature to 250 °C. Behaviors showing significant twinning during initial in-plane compression and untwinning in subsequent tension at and slightly above room temperature were recorded. Strong yielding asymmetry and nonlinear hardening behavior were also revealed. Considerable Bauschinger effects, transient behavior, and variable permanent softening responses were observed near room temperature, but these were reduced and almost disappeared as the temperature increased. Different stress–strain responses were inherent to the activation of twinning at lower temperatures and non-basal slip systems at elevated temperatures. A critical temperature was identified to account for the transition between the twinning-dominant and slip-dominant deformation mechanisms. Accordingly, below the transition point, stress–strain curves of cyclic loading tests exhibited concave-up shapes for compression or compression following tension, and an unusual S-shape for tension following compression. This unusual shape disappeared when the temperature was above the transition point. Shrinkage of the elastic range and variation in Young’s modulus due to plastic strain deformation during stress reversals were also observed. The texture-induced anisotropy of both the elastic and plastic behaviors was characterized experimentally.

Determination of Anisotropic Yield Coefficients by a Data-Driven Multiobjective Evolutionary and Genetic Algorithm

The texture induced anisotropy of yield strength in cold rolled sheet metals is modeled using anisotropic yield criteria. The classical and other optimization methods used so far to determine the yield coefficients are limited by fixed set of experimental data, initial guess values, and pre-determined weight factors. A robust multiobjective optimization based on evolutionary algorithm proposed in this paper minimizes the error in yield stress and plastic strain ratio simultaneously and thereby overcomes the limitations in the approaches used before. The new approach is tested using Hill48 and Barlat89 yield criteria for five different materials from literature. The new approach is observed to improve the prediction capability of yield coefficients when compared to earlier approaches. The Pareto frontier obtained in the new approach can serve as a comparative tool to evaluate the accuracy of different yield criteria.

Application of Failure Criteria in Aluminum Sheet Forming Analysis

Abstract The numerical simulation of the Forming Limit Diagram(FLD) test was carried out to calculate the limiting dome height(LDH: ISO12004-2) for aluminum alloy sheet Al6061-T6. The finite element analysis was used as an effective method for evaluating formability and diagnosing possible production problems in sheet stamping operations. To predict fracture during the stamping process, several failure models such as Cockcroft-Latham, Rice-Tracey, Brozzo and ESI-Wilkins-Kamoulakos(EWK) criteria were applied. The predicted results were discussed and compared with the experiments for Al6061-T6. Key Words : Sheet Metal Forming, Failure Model, Formability, Finite Element Analysis 1. 서 론 재료 판재성형에서 설계인자의 도출이나 설계조건의 결정은 기본적으로 소재의 성형성 평가에 근거를 두고 있다. 스탬핑 공정에서 판재는 여러 가지 변형 모드가 조합된 형태로 성형되므로, 성형의 성패 여부나 난이도를 체계적으로 규명하는 것은 사실상 어려운 실정이다[1]. 일반적으로 판재 성형해석 과정에서 파단예측을 위한 방법으로는 단순 인장시험에서 얻은 시편의 파단 변형률(failure strain)을 이용하는 방법, 소성 불안정 이론에 근거하여 한계변형률을 이용하는 방법, 파괴역학에 기초한 파단모델(failure model)을 적용한 방법이 있다[2]. 상당응력(equivalent stress)이나 상당변형률(equivalent strain), 두께변화(thinning)로 평가하는 방법은 재료역학적인 접근 방법으로서 단순 인장시험에서 얻은 재료데이터를 해석에서얻은결과와단순비교하는방법이다. 간단한시험만으로파단을평가할수있다는장점이 있으나, 파괴역학적인 측면을 고려하지 않기 때문에 실제 현상을 반영하지 못할 뿐만 아니라 파단과정을 모사할 수 없다. 그리고 현재 가장 많이 사용되는 성형한계도(forming limit diagram)를 이용한 방법은 많은 양의 시험으로 인해 시간, 비용, 인력의 측면에서 어려움이 따른다. 또한, 변형률 경로 변화에 대한 영향성은 하중경로가 선형적인 과정에서는 크지 않지만, 복잡한 형상의 다공정 성형(multistage forming)의 경우, 하중경로가 바뀌기 때문에 그 영향성이 크다. 따라서 실제 성형공정에서 성형한계도를 적용한 방법은 현실적이지 않을 수 있음을 의미한다[3]. 본 논문에서는 응력과 변형률의 함수로서 계산되는 몇가지 파단 모델들을 적용하여 성형해석을 수행하였고, 성형성 평가에 적용 가능 여부를 판단하 #

Constitutive model of AZ31B sheet at various pre-strains and temperatures

Due to their high specific strength, vibration absorption capability, and excellent corrosion resistance, Mg alloys have been potential alternative to other lightweight materials in the automotive industry. Mg alloys are known to have unique mechanical properties; i.e., yielding asymmetry, anisotropy, unusual hardening behavior at room temperature. Usually, Mg alloy sheets have inferior formability at room temperature, but the formability increases when the temperature increases. Moreover, the asymmetry and anisotropy become less significant due to the activation of non-basal slip systems at higher temperature. Utilizing this unique properties, the forming of Mg alloy sheets has been frequently conducted at the temperature of 200 °C or higher, at which twinning effect is less dominant. However, the forming process at elevated temperature lowers production speed due to the additional heating and cooling stages. To resolve this problem, studies on technology that maximizes the formability of Mg alloy sheets...