Two rods of different materials having coefficients of thermal expansion α1 and α2 and Young's moduli Y1 and Y2 are fixed between two rigid and massive walls. The rods are heated to the same temperature. If there is no bending of the rods, the thermal stresses developed in them are equal provided

When a rod is heated to a temperature T, the increase in its length l is given by I=α LTwhere l, = len8th of the rodand α=coefficient of thermal expansion∴  strain =lL=αTand stress = Y x strain = YαTSince the value of T is same for the two rods and hence the stresses in them will be equal ifY1α1T=Y2α2T  or  Y1Y2=α2α1