Multi-point Rapid Nonlinear Ultrasonic Inspection on Fatigue Damage of 7075 Aluminum Alloy

Nonlinear ultrasonic technology is a non-destructive testing method which is very sensitive to fatigue damage. A multi-point rapid testing system is designed to study the non-linear ultrasonic effect in fatigue process with different levels of fatigue damage of aluminum alloy, the different positions of the specimen were tested, the ultrasonic fundamental wave and the second harmonic amplitude of different fatigue loading cycles of the specimen are extracted, the nonlinear parameters is calculated by them, and the results are compared with the metallographic analysis results, the relationship between changes of tissue and nonlinear parameters is studied. The results show that nonlinear parameters increase first and then decrease with the accumulation of fatigue damage, and reached the critical value when the fatigue cycles reached 45 000 times, corresponding to the crack initiation stage. However, the nonlinear parameters begin to decrease when the macro cracks appear, and the results of multi-point scanning showed that there exists great difference between the nonlinear parameter of fatigue damage region and original ones in single specimen, which indicated that the nonlinear parameters were very sensitive to the change of fatigue damage, therefore, with this method, the fatigue region could be tested and positioned preliminary.