In: Civil Engineering
The results of torsion test on two shafts are shown in the figure. The top shaft shows the torsion failure in the cast iron which is a brittle material and the bottom figure is the test result in a shaft made of ductile steel. Considering the fact that brittle material are controlled by tension (fail in tension) and ductile materials are controlled by shear stress (fail in shear), explain why the brittle cast iron has a 45 degree fracture surface, while the ductile steel shaft has a flat fracture surface. Provide detailed logic based on stress transformation.