ELEVATED-TEMPERATURE X-RAY-MEASUREMENT OF RESIDUAL-STRESSES IN A FIBER-REINFORCED AL-ALLOY

Thermal residual stresses have been measured using X-ray diffraction i n an Al-2% Mg matrix with 10, 20 or 26 vol % Al2O3 Short fibres. Stres s measurements were made at room temperature as well as in situ at ele vated temperatures up to 250 degrees C. The thermal stresses arise due to the difference in coefficient of thermal expansion (CTE) between t he matrix and the reinforcement. The largest CTE is found in the matri x, resulting in tensile residual stresses after a temperature drop, e. g. after processing or annealing. A high fraction of reinforcement imp lies higher matrix stresses than a low fibre content. The stresses dec rease with increasing temperature for all fibre volume fractions. Meas urements are compared with calculations using a modified Eshelby model for equivalent inclusions. Parameters taken into account in the model are coefficient of thermal expansion, Young's modulus, and volume fra ction and geometric shape of the reinforcing phase. A good correlation between calculations and experimental results has been found, bearing in mind that no plasticity is taken into account in the Eshelby model . The plastic behaviour of the composites has been described using a m odel based on a rigid spherical cavity in an elastic-plastic matrix.