Burak Bal

Twinning activity in high-manganese austenitic steels under high velocity loading

Deformation temperature and manganese content dependencies of twinning activity in two types of high Mn austenitic steels were investigated upon high velocity tensile loading. It was observed that nanotwin formation within previously formed twins dominates at subzero temperatures and significantly contributes to work hardening.

A New Venue Toward Predicting the Role of Hydrogen Embrittlement on Metallic Materials

Abstract This paper presents a new crystal plasticity formulation to predict the role of hydrogen embrittlement on the mechanical behavior of metallic materials. Specifically, a series of experiments were carried out to monitor the role of hydrogen interstitial content on the uniaxial tensile deformation response of iron alloyed with hydrogen, and the classical Voce hardening scheme was modified to account for the shear stresses imposed on arrested dislocations due to the surrounding hydrogen interstitials. The proposed set of physically grounded crystal plasticity formulations successfully predicted the deformation response of iron in the presence of different degrees of hydrogen embrittlement. Moreover, the combined experimental and modeling effort presented herein opens a new venue for predicting the alterations in the performance of metallic materials, where the hydrogen embrittlement is unavoidable.

Experimental and Numerical Evaluation of Thickness Reduction in Steel Plate Heat Exchangers

A multiscale modeling approach was utilized to predict thickness reduction in steel plate heat exchangers (PHEs) utilized in combi boilers. The roles of texture and microstructure were successfully accounted for by properly coupling crystal plasticity and finite element analysis (FEA). In particular, crystal plasticity was employed to determine the proper multiaxial hardening rule to describe the material flow during the forming of PHEs, which was then implemented into the finite element (FE) metal-forming simulations. The current findings show that reliable thickness distribution predictions can be made with appropriate coupling of crystal plasticity and FEA in metal forming. Furthermore, the multiscale modeling approach presented herein constitutes an important guideline for the design of new PHEs with improved thermomechanical performance and reduced manufacturing costs.

DETERMINATION OF MATERIAL RESPONSE AND OPTIMIZATION OF JOHNSON-COOK DAMAGE PARAMETERS OF ALUMINIUM 7075 ALLOY

The effects of rolling direction and notch radius on the mechanical response of aluminium 7075-T651 alloy were investigated and the Johnson-Cook damage parameters of aluminium 7075-T651 alloy on both rolling directions were determined. Specifically, mechanical responses of aluminium 7075-T651 along the rolling direction and perpendicular to the rolling direction were obtained from monotonic tensile tests. 56 tensile tests in total were performed on notched specimens with 3 different notch radiuses and smooth specimens. Tensile tests were repeated 7 times for each case to ensure the consistency and to obtain the closest mechanical response to the real mechanical response with minimum error. Experimental findings revealed that being perpendicular to the rolling direction deteriorates the elongation at failure dramatically but can increase the mechanical properties in elastic region. The final areas of the fractured samples, used for the calculation of Johnson-Cook damage parameters, were measured by an optical microscope. The Johnson-Cook damage parameters of aluminium 7075-T651 alloy for different applications were computed by Levenberg-Marquardt optimization method. Collectively, this study opens the venue for accurate damage simulations of aluminium 7075-T651 along the rolling direction and perpendicular to the rolling direction for different applications.

Numerical Investigation of the Role of Volumetric Transformation Strain on the Relaxation Stress and the Corresponding Hydrogen Interstitial Concentration in Niobium Matrix

The effects of relaxation stress on the hydrogen concentration in Niobium- (Nb-) H media were investigated by iterative numerical modeling approach. To calculate the transformation strain, relaxation stress, and corresponding relaxed hydrogen concentration around an edge dislocation, a new third-order polynomial formulation was utilized in the model. With the aid of this polynomial, hydrogen induced relaxation stress never exceeds the dislocation stress, which indicates that the total stress field never turns to compressive state and diverges the results. The current model calculates the hydrogen concentration not only in the vicinity of an edge dislocation but also far away from the dislocation. Furthermore, the effect of relaxation stress on the interaction energy was also captured in the model. Overall, the current findings shed light on the complicated hydrogen embrittlement mechanisms of metallic materials by demonstrating that hydrogen induced relaxation has a significant effect on the hydrogen atom concentration and the interaction energy between the existing internal stress field and the solute hydrogen atom.