An assemble consisting of two cylindrical portions AB and BC is fixed at the rigid wall at A. Section AB is made of steel (Es = 200 GPa, αs = 12 × 10−6 /°C) and section BC is made of aluminum (Eal = 73 GPa, αal = 23 × 10−6 /°C). If the gap between C and the rigid wall at D is 0.15 mm when the temperature is 10 °C, determine the average normal stress in section BC when the temperature rises to 50°C