The fatigue of aluminum and steel samples was studied by using positronarmihilation technique(PAT) and ultrasonic decay method

respectively. When thenumber of fatigue cycle is small

some of positron parameters increase with theincrement of fatigue cycles because of the extreme sensitivity of positron to verysmall vacancies. But some structural transformation might happen when the numberof fatigUe cycle is large enough. For example some vacancies might aggregate andform vacancy clusters. Some dislocations might annex or tangle. These kinds oftransformation in structure will reduce the density of vacancies and dislocations

which is responsible for the decreasing trend of the curve after the average lifetimeof positron achieves its maximum. The result of ultrasonic decay constantmeasurement also shows similar trend.