Determination of material constitutive models using orthogonal machining tests
The aim of present research is to fine-tune and improve Johnson-Cook (JC) constitutive model parameters for American Iron and Steel Institute (AISI) 1045 carbon steel and Oxygen-free high conductivity (OFHC) copper using orthogonal machining tests. This dissertation is divided into two parts. Part one is concerned with improving Johnson-Cook (JC) constitutive model parameters for AISI 1045 carbon steel by comparing results of finite element (FE) simulations of orthogonal machining with experimental results. A large number of FE simulations are carried out using different values for each of the five unknown parameters of the JC model. Second-order polynomials are fitted to the results of FE simulations to describe how the peak strain rate, strain rate distribution in the primary shear zone (PSZ) and chip thickness vary with the changes in the material parameters. The peak strain rate and strain rate distribution in the PSZ obtained experimentally using Digital Image Correlation (DIC) of the high-speed microphotographs of the PSZ and the measured chip thickness are available. The material constants that minimize the error between experimental observations and FEA results for each of these quantities are found by non-linear optimization using the sum-squared errors of the objective function. The efficacy of this approach is assessed by comparing FEA results obtained using this set of parameters to experimental measurements under Assessment of Machining models (AMM) cutting condition 2. Part two first quantitatively evaluates and compares the performance of different material models for OFHC copper widely used in machining with reference to the experimental results obtained from cutting tests. Unlike other research, the strain and strain rate along the nominal shear plane are experimentally measured using Digital Image Correlation (DIC) of high-speed photographic images of the side surface of the chip. Since the average strain and strain rate directly measured from cutting tests, an analytical estimate is required only for the temperature, hence reducing the uncertainty in the proposed approach for identification of the constants used in different material models. The determination of the material constants used in the Johnson-Cook and Zerilli-Armstrong constitutive models for OFHC copper from carefully designed machining experiments is the focus of part two.
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