(Solved):   6) Materials Aspects [ 30 marks total] (a) Which mechanical property most controls the for ...

 

6) Materials Aspects [ 30 marks total] (a) Which mechanical property most controls the formability of the steel sheet (so it can be easily shaped): Stiffness, E, or ductility (\% elongation)? Describe the role of \( \mathrm{C} \) content in the steel in controlling the balance between formability and strength in service through the balance of phases formed in the microstructure. [6 marks] (b) The steel sheet that has been used to make the chassis \( \mathrm{C} \)-beam is predicted to have surface defects with a mean value of \( 1 \mathrm{~mm} \pm 0.2 \mathrm{~mm} \), from the forming tools, whereas the internal defects (formed by microvoid coalescence around secondary phase particles) are found to be \( 0.5 \mathrm{~mm} \pm \) \( 0.12 \mathrm{~mm} \). The stress calculations indicate the C-beam will be loaded to \( 330 \mathrm{MPa} \) on starting the engine. The fracture toughness is determined to be \( 32 \pm 2 \mathrm{MPaVm} \) and the yield strength is \( 345 \mathrm{MPa} \) \( \pm 7 \mathrm{MPa} \). You can assume the shape factor \( \mathrm{Q}=1.1 \) for a surface defect and \( \mathrm{Q}=1 \) for an internal defect and the \( \pm \) error values stated are equivalent to one standard deviation. Will the \( \mathrm{C} \)-beam be able to withstand the maximum predicted operating stress? (Explain your reasoning) [8 marks] (c) To improve the toughness of the steel after forming, it is annealed at varying temperatures (for the same time period) It is found that the final grain size of the steel sheet is controlled by the temperature used during the annealing process. The data in Table Q7 below shows how the final grain size varies with this temperature and how this grain size then affects the subsequent room temperature yield strength. The annealing temperature does not change any other microstructural features. What underlying processes control: (i) the variation of grain size with temperature and (ii) the effect of grain size variation on strength? Justify your answers in terms of the underlying materials processes occurring. Based on your proposed underlying mechanisms, write an equation for each case that describes the physics occurring. Prove whether these relationships exist for the data given in Table Q7 using a graphical method. Based on your analyses, identify the annealing temperature required to obtain a room temperature yield strength of \( 315 \mathrm{MPa} \). Table Q7: Annealing temperatures used, the resultant grain sizes produced and the corresponding yield stress values at room temperature [12 marks] (d) What benefit might there be in making the C-beam chassis from Al sheet or a C-fibre reinforced composite?