Iranian Journal of Materials Forming
Volume:3 Issue: 1, Spring and Summer 2016

Finite element modeling has been used to study the development of surface deformation during indentation with a Vickers indenter. A wide range of materials with different elastic modulus and yield stresses are examined. Results show that in a pyramidal indentation process, for a perfectly plastic material, sinking-in during loading can change to pile-up in unloading. This phenomenon depends on the elastic modulus to yield stress ratio. Results also show that the amount of pile-up cannot be related solely to the strain-hardening exponent, as often assumed. Rather, after initially sinking-in at small depths of penetration, the pile-up for many materials evolves and increases gradually as the indenter is driven into the material. It is shown that the ratio of the plastic volume radius to the indentation depth is nearly constant during loading and it is a function of the yield stress and the Young modulus. Experimental verification in loading and unloading is carried out with the results of Alcala et al. (Acta Materialia, 2000, pp. 3451).

In the current research the maximum deflection of circular plates made of AA5010 and AA1100 alloys under blast load was investigated. Shock waves were produced by exploding a spherical charge in different distances from the center of plates. The ABAQUS software uses conwep equation for blast loading analysis. It was found the results of these simulations have about 30% to 40% inaccuracy in comparison with experimental results. To improve the accuracy of the simulations the Friedlander equation was used that considers the positive phase of blast wave as exponential and the negative phase as bi-linear function. To this goal, the vdload subroutine was developed. Results were shown the difference between the experimental and simulation was decreased to 8%. Also, the effect of uniform and non-uniform shock waves on the deformation of structure and various types of failure were investigated. It was observed that uniform shock waves can be achieved when the minimum distance between the exploding charge and plate is about 3 times of the radius of plate.

Modeling the flow curves of materials at elevated temperatures is the first step in mathematical simulation of the hot deformation processes of them. In this work a comparative study was provided to examine the capability of three different constitutive equations in modeling the hot deformation flow curves of AZ91 magnesium alloy. For this, the Arrhenius equation with strain dependent constants, the exponential equation with strain dependent constants and a recently developed simple model (developed based on a power function of Zener-Hollomon parameter and a third order polynomial function of ε power a constant number) were examined. Root mean square error (RMSE) criterion was used to assess the modeling performance of the examined constitutive equations. Accordingly, it was found that the Arrhenius equation with strain dependent constants has the best performance for modeling the hot deformation flow curves of AZ91 magnesium alloy. The results can be further used in mathematical simulation of hot deformation manufacturing processes of tested alloy.

In this study, at the first stage, the rupture criterion of bubbles wall in Aluminum metal foam liquid was investigated by using Lattice Boltzmann. The two phases modeling were accomplished by using a modified Shan-Chen model. This model was run for several bubbles in A356�.%SiC melt system. Then, bubbles morphologies (virtual metallographic) for A356�.%SiC foams were simulated. Results showed that simulation data and the virtual metallographic have a good agreement with the metallographic empirical results after solidification. In the second stage, several cubic A356�.%SiC foams were compressed under uni-axial compression load base on ASTM E9 standard. Stress-strain curves of the foams were determined by a data acquisition system with gain 10 samples per second. Then foams plastic deformation behavior simulated based of a new asymptotic function by ABAQUS software. Discretized digital solid-model of the solid bubbles was prepared by using virtual metallographic images which obtained from present code. Then load-displacement curves were plotted for simulation and experimental results. Results show both curves obtained from experimental and simulation have a good agreement with approximately 1.8% error. Therefore present software could be useful tool for predicting of metal foams plastic deformation behavior without experimental try and error.

The purpose of this investigation is to examine the viability of using the sheet metal for forming applications. Forming limit diagram (FLD) composed of negative and positive minor strain with respect to major strain which occurs at directional zero strain with the critical thickness of sheet metal. The negative minor strain region of FLD is predicted by localized necking. However there is no directional zero strain in the positive minor region of FLD is predicted with help of Marcinaik-Kuczynski assumption. The present work aims to determine the stretchability in terms of limiting strain of Austentic stainless steel 316L using M K analysis and hemi spherical dome stretching. Strain hardening exponent was derived from uni axial tensile test of Austentic stainless steel 316L under different in homogeneity conditions. C programme was developed to predict the theoretical FLD and results were compared with experimental value. The limiting strain of material is found as 0.4 in experimental and Marcinaik - Kuczynski analysis. Fractography shows the large amount of cleavage fracture and evidence for cleavage initiating because of other inclusions.

The strain rate sensitivity (SRS) of Mg–1.5 wt% Gd processed by conventional extrusion and 2 passes of simple shear extrusion (SSE) was investigated by shear punch testing. Shear punch tests were conducted at initial shear strain rates in the range of 0.003–0.3 s-1 and at temperatures in the range of 573–773 K. A fine-grained microstructure with an average grain size of about 2.5 µm, obtained after 2 passes of SSE, resulted in high SRS index (m-value) of 0.4 at 723 K. The calculated activation energy for 2 passes deformed alloy is 116 kJ/mol which is higher than the activation energy of grain boundary diffusion in magnesium (75 kJ/mol). This higher amount of activation energy can be attributed to the presence of gadolinium in this alloy. This SRS index together with an activation energy of 116 kJ/mol are indicative of a superplastic deformation behavior dominated by grain boundary sliding accommodated by grain boundary diffusion at 723 K.