Modelling of tensile deformation curve

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The component studied

The component studied in this thesis is a CH11 cylinder head from a Volvo truck diesel engine, weighing in at around 220 kg. It is cast in grey iron with the material composition as stated in Table 2. A cylinder head is a component which is bolted on top of the cylinder block of an engine and it is housing the valves for the inlet air and the exhaust gas, the fuel injector and cooling channels. The cylinder head is subjected to high temperatures during the combustions but also fluctuating temperatures and pressures.[24] A key aspect for the material in a cylinder head is that it must transport away the generated heat efficiently to limit thermal fatigue, i.e. have high thermal conductivity, while also maintaining the correct dimensions i.e. have low thermal expansion. The thermally induced stresses are also reduced by low thermal expansion, small thermal gradients and low stiffness of the material. The findings in this research is not limited to cylinder heads but could also be of interest in other components where the physical properties are of great interest, e.g. brake discs, cylinder blocks and furnaces.

Physical property sample extraction

Samples were also needed for the physical property tests, dilatometer test to measure thermal expansion, differential scanning calorimetry testing to determine the specific heat and laser flash apparatus testing to measure thermal diffusivity. All these test procedures have been described in section 2.5. The purpose of the physical testing was to test the material’s thermal properties at different locations, solidifying at different rates, in the component. It was decided to test at three different solidification times; fast, intermediate and slow, and with a sample size of three for each solidification condition making a total of nine physical test locations. These locations were determined by studying images from casting simulation performed in MAGMA[47] of the cylinder head showing the solidification times in sections throughout the whole component, see Figure 25. The three different solidification times were chosen to be:  Fast solidification – around 200 seconds.  Intermediate solidification – around 600-700 seconds.  Slow solidification – around 1700-1800 seconds. The three samples from the intermediate solidification were taken at the surface of the cylinder head which is in direct contact with the cylinder block, which is the source of the heat. The physical properties of the material at this location were of great interest for the manufacturer, Volvo. The three samples from the slow solidification was taken from the center of the component. These six samples, from slow and intermediate solidification, were taken from cylinder section 3, 4 and 5 and after the mechanical sample extraction was finalized it was clear that the best decision was to take the physical test samples from part B which still had a lot of material to take samples from.

Mechanical test results

The data extracted from the tensile test were imported into a Matlab [55] script to generate tables and stress-strain diagram with the properties of e.g. ultimate tensile strength, yield strength and Young’s modulus (E). The values eb and ep represents the maximum engineering strain at fracture and the maximum plastic strain respectively. The script also provides the values for the strain hardening exponents nH and nL and the strength coefficients KH and KL as seen in Figure 49, which are used to calculate the Hollomon and Ludwigson equations, equation (7) and (8). Those equations are then being used to model the tensile test curve. In Figure 49 the Hollomon, KH and nH, parameters are determined in the plastic strain interval 0.002-0.01 (0.2 % to 1 %) while the Ludwigson parameters, KL and nL, are determined in the interval 0.0005-0.002 (0.05 % to 0.2 %). In Figure 50 the true stress-strain is shown, modelled using only the Hollomon equation with the plastic strain interval 0.0005-0.01 (0.05% to 1 %).

Microstructure analysis

The results from the image analysis software of the images from the nine physical samples are presented in this section followed by analyses of the results. The graphite microstructures of the three solidification conditions are compared both visually and by the values for graphite fraction and the size of the graphite particles. The matrix is also confirmed to be pearlitic. Visual comparison of microscope images When studying the images from the optical microscope it is quite clear that the graphite particles are smaller and shorter in intermediate solidification condition compared to slow solidification. Also the images from samples from fast solidification conditions shows even smaller and shorter graphite particles. One typical image from each solidification condition is shown in Figure 71. Some of the images from the fast solidification also exhibits areas of organized orientation of the graphite particles, type E as described in section 2.2.2, which can be seen in the right image in Figure 71.

Contents :

• 1 Introduction
  • 1.1 BACKGROUND
  • 1.2 PURPOSE AND RESEARCH QUESTIONS
  • 1.3 DELIMITATIONS
  • 1.4 OUTLINE
• 2 Theoretical background
  • 2.1 RESEARCH APPROACH
  • 2.2 GREY IRON
  • Composition
  • Microstructure
  • 2.3 MECHANICAL BEHAVIOR
  • Modelling of tensile deformation curve
  • 2.4 PHYSICAL PROPERTIES
  • Thermal expansion
  • Density
  • Specific heat
  • Thermal conductivity
  • 2.5 MATERIAL TESTING
  • Tensile testing
  • Dilatometer (DIL)
  • Density determination
  • Differential Scanning Calorimetry (DSC)
  • Laser Flash Apparatus (LFA)
• 3 Method and implementation
  • 3.1 THE COMPONENT STUDIED
  • 3.2 SAMPLE EXTRACTION
  • Mechanical property sample extraction
  • Physical property sample extraction
  • 3.3 TESTING
  • Mechanical property testing
  • Physical property testing
  • 3.4 SAMPLE PREPARATION FOR OPTICAL MICROSCOPE ANALYSIS
  • 3.6 CONNECTING CASTING SIMULATION AND FE SOFTWARE
• 4 Results & Analysis
  • 4.1 MECHANICAL TEST RESULTS Comparing the UTS-values between the three cylinder heads Microstructure analysis comparing six selected samples
  • 4.2 PHYSICAL TEST RESULTS
  • Dilatometer – thermal expansion
  • Density
  • DSC – Specific heat
  • LFA – Thermal diffusivity
  • Thermal conductivity
  • Microstructure analysis
  • 4.3 COMPARING MODELLING- AND EXPERIMENTAL VALUES FOR THERMAL CONDUCTIVITY
  • 4.4 FEA RESULTS
  • Experimental values
  • Extreme values
• 5 Discussion and conclusions
• 6 References
• 7 Search terms
• 8 Appendices
  • 8.1 APPENDIX
  • 8.2 APPENDIX
  • 8.3 APPENDIX

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Connecting casting simulation and FE software including local variation of physical properties.