Improving the material model of CGI in experimental and analytic ways

After understanding the presence of dislocations, humankind has started to use metals as an industrial material. Cast irons are preferred due to its competitive low prices compared to other materials in most industries, ease of accessability and their wide range of achievable mechanical and physical propeties as well. Cast Irons, generally, consist of an iron matrix (such as pearlite, ferrite, etc.), graphite and some other additives. Some graphite morphology types are flaky, compact or nodular, mostly effected on the mechanical and physical properties. Some of cast irons which are frequently used in industries; grey cast iron (GCI) which has flaky graphite phases, nodular cast iron (NCI) which has nodular graphite phases and compact graphite iron (CGI) which has both flaky and nodular graphite phases, it's called compacted graphite phases. Compacted Graphite Iron, which is also known as CGI or Vermicular Graphite Iron, is a form of cast iron in which the shape of the graphite particles is between that grey iron where the graphite is in flake and ductile iron where the graphite is in spheres. The worm-like compacted-graphite shape provides physical properties between grey and ductile irons. In order to flaky graphite morphology, Grey Cast Iron has a good damping capacity and preferred by castability, machinability, and heat conductivity features. In order to nodular graphite morphology, Nodular Cast Iron has a better mechanical properties. In most applications of automotive industry, thermal conductivity is the main reason for the material selection especially for internal combustion engine components and brake systems. Due to combining good mechanical and physical properties with thermal conductivity, Cast Irons have been used for cylinder heads, brakes, pistons, and disks. Although the CGI was explored in 1940s, which was nearly same with the Grey Cast Iron's exploration; it has taken great attention in recent years with meeting the supply. However, because of some problems in its machinability, it requires to have more research. The increasing burn pressure of diesel engines is required to achieve better performance, less emission and more productive burn. Due to these requirements, machinability problems of the CGI has studied by the academia. CGI is the proper material providing for the needs of automotive industries. In recent years, there has been a great deal of effort for studying behaviour of materials at high strain rate and high temperature conditions. In the literature, there are some type of finite element methods. Many different constitutive models exist to describing material's behaviour in plastic region. Most common constitutive material models are Zerilli-Armstrong, Bodner-Partom and Johnson-Cook (JC). JC is the simplest material model in order to determine the parameter easier than the others and consider the equivalent stress as a function of plastic strain, strain rate and temperature. Johnson-Cook material model is used to describe material's behaviour in the plastic region over large strains, at high strain rates and at high temperatures. The flow stress has three main parts; first bracket is elasto-plastic term, second bracket is viscosity term, and third part is thermal softening term. Johnson and Cook presented the flow stress in a formula; 𝜎=(𝐴+𝐵𝜀𝑛)[1+𝐶𝑙𝑛(𝜀̇/𝜀0̇)][1−((𝑇−𝑇𝑅𝑜𝑜𝑚)/(𝑇𝑚𝑒𝑙𝑡−𝑇𝑅𝑜𝑜𝑚))𝑚] where 𝜎 is the flow stress of material, 𝜀 is the plastic strain, 𝜀̇ is the strain rate, 𝜀0̇ is the reference strain rate, T is the temperature of the material, 𝑇𝑚𝑒𝑙𝑡 is the melting point of the material and 𝑇𝑅𝑜𝑜𝑚 is the room temperature. The empirical constants are as follows: A is the yield stress, B is the pre-exponential factor, C is the strain rate factor, n is the work-hardening exponent and m is the thermal-softening exponent. Commercial finite element softwares support some constitutive material models, and one of them is Johnson-Cook. In a real life application, material modelling is too expensive and takes so much time. Various researchers have conducted some tests to find Johnson-Cook parameters; A, B, n, C, and m. By using these parameters many ortogonal cutting tests can be conducted and no necessary to prepare samples, no necessary to use machines and no necessary a heater units, no sensors, no measuring devices. In this thesis, at first, it is decided to use compacted graphite cast iron as a materials in order to best appropriate material for automobile industries especially engine blocks. There are many sources and information about gray cast irons and nodular cast irons. In order to compacted graphite cast iron's mechanical and physical properties between gray cast iron and nodular cast iron, it is easier to compare the data after finding parameters. There are available the johnson-cook parameters of gray cast iron and nodular cast iron in the literature. For determining the johnson-cook parameters, it is done a quasi-static tensile test at 20 0C. In the true stress-true strain graph, the yield point gives the A parameter. In order to determine the A parameter, test's graph should supply the yield point. However, it is necessary the data between yield point and ultimate tensile strength determining B and n parameters. After subtracting the yield point value from stress datas, draws a log-log graphs and that graphs give the B and n parameters. In order to determine C parameter, it is necessary to conduct high strain rate tests, so Split Hopkinson Pressure Bar (SHPB) Tests conducted at 1530, 1550, 2000 and 2200 s-1 strain rates. To determine the C parameter, first and third brackets are not necessary, so they were eliminated by dividing quasi-static flow stress and high strain rate stress. In order to determine m parameters, in addition to 20 0C quasi-static test, it is conducted at 300, 400, 500, and 700 0C quasi-static tests. In addition, for determining m parameter, first and second brackets of Johnson-Cook flow stress formula are not necessary. Being eliminated them by dividing reference temperature flow stress and high temperature flow stress formula. The m parameter determined with the slope of drawn the graph. Due to pearlitic structure, CGI is a brittle material, so determining a better solution, it has conducted compression tests at 1s-1 strain-rates and 20 0C conditions. Thus a wider plastic region data obtained. In order to redetermine of Johnson-Cook parameters, being used the quasi-static compression test datas and were taken average then finalized CGI's JC parameters.