مهرداد آقایی خفری

Forming limit diagram (FLD) is a useful tool for investigation of sheet's formability for designing industrial products. Experimentally extracting FLD requires exact experimental tests, and is time consuming, and expensive. Therefore, several studies have been carried out on the usage of theoretical methods and finite element software for determining these diagram. In this study, FLD for AA3105 aluminum alloy sheet were obtained by simulating the Nakazima and modified Marciniak test in ABAQUS software. In order to numericaly determine FLD of AA3105, Hill yield criterion, Hosford yield criterion and GTN damage model based on Hosford criterion and Voce and power law hardening rules were investigated. Due to the lack of the Hosford yield criterion and the GTN damage model based on Hosford criterion in the ABAQUS software, VUMAT subroutines has been developed and used to determine the behavior of the AA3105 aluminum alloy. The results showed that the predicted FLD based on the Hill criterion, shows large deviation from experimental results. The usage of the Hosford criterion and GTN damage model for aluminum alloys showed a better correlation with experimental results. Also, due to the presence of voids in metals, the GTN damage model which is based on the void volume fraction has a greater physical justification than other yield criterion. Furthermore, by comparing numerical FLDs that obtained from Nakazima and modified Marciniak tests, it was concluded that the limit strains in modified Marciniak test is lower than the Nakazima test.

In this research, recrystallization behavior and coarsening kinetics of 7075 alloy were investigated during conventional and newly modified SIMA processes. The conventional SIMA process consisted of applying 10-55% uniaxial compression strain at ambient temperature and subsequent semi-solid treatment at various temperatures and times. A new modified SIMA process was developed by introducing repetitive upsetting-extrusion (RUE) for the first time for semi-solid processing of 7075 Al alloy. Specimens of 7075 alloy were subjected to different RUE cycles at 250 °C, and then the semi-solid treatment was carried out. Microstructural studies revealed that the recrystallization rate is accelerated, the average grain size is reduced considerably and the sphericity of solid grains is improved by the newly modified SIMA process. Lifshitz-Slyozov-Wagner (LSW) theory was used to describe the coarsening process of the semi-solid slurries. Coarsening rate of the solid grains in the newly modified SIMA process was slower than the conventional SIMA process.

The aim of this study is to investigate the life-span according to the damage caused by the main mechanisms of damage development in turbine blades and to model the growth of the damage. For this purpose, the low cycle fatigue test on martensitic 410 stainless steel was immersed in tempered glass at 565°C in three strain gauges 0.8, 1 and 1.5 with a constant temperature of 500°C and 15 seconds per cycle. The effect of creep-fatigue interaction on life and also damage to turbine blade in different conditions was investigated. The results showed that with the variation of the strain amplitude from 0.8 to 1.5, the life of the piece varies from 205 to 65 cycles and this is while the level of failure of the samples varies. In the next step, the modified Coffin-Manson model was used to indicate the damage and its simultaneous effect on the life of the piece. The results showed that decreasing the number of grain boundaries and its effect on the cavities created in the piece decreases the damage and thus the life of the turbine blade increases. High-temperature tensile tests and low-tensile fatigue-temperature control were also performed in different tempering modes for 410 and 420 steel stainless steel and The results showed that, under the same conditions, the temperature increase from 200 to 565°C resulted in a decrease in life from 2218 to 1952 cycles.

In this study, severe plastic deformation of 7075 aluminum alloy was investigated using a new method, based on the combination of conventional upsetting and direct extrusion. In this process, which is called repetitive upsetting-extrusion, cylindrical samples were first subjected to upsetting and were subsequently subjected to extrusion at 250 °C with various processing cycles. Die design was carried out considering the possibility of conducting both upsetting and extrusion by using a single die and the maximum of four RUE cycles were successfully performed on the samples. Finite element method was used to simulate the deformation behavior of 7075 alloy during repetitive upsetting-extrusion processing and the strain distribution was obtained for the deformed samples. The finite element simulation results correlated fairly well with the microstructural observations. Based on the simulation results, the maximum effective strain was observed at the central region of the samples. The deformation behavior and the flow pattern were discussed based on the experimental and the simulation results. In addition, the effect of applied strain on mechanical properties of processed samples was studied. Tensile strength and elongation of deformed samples increased with extending the number of repetitive upsetting-extrusion cycles.

Experimental and numerical analysis of a single overload on the fatigue life of AISI 4140 CT specimens was studied. Fatigue tests were conducted on base line CT specimen under single overload ratios of 1. 5 and 1. 75. Numerical analyses were performed on 2D incorporating the stress intensity factor and the J-integral as driving force. Furthermore، ABAQUS commercial software was used to simulate elastic-plastic crack growth and crack closure. Overload-induced retardation effects on the crack growth rate are considered based on the crack closure concept. A novel model for considering pre-overload and post overload effect on the crack growth is introduced. This model is based on the effective stress intensity factor and the effective J value. Overload increases fatigue lives by factors of 1. 24 and 1. 5 for overload ratios of 1. 5 and 1. 75، respectively. Two dimensional FEM results are in good agreement with the experiment with a maximum error of 10% using stress the intensity factor based method and -6% using the J integral based method. Comparing present paper method with Harmain model indicates that this model requires fewer experiments and Harmain model requires less calculation.