Nonlinear Ultrasonic Testing For Fatigue Damage Of Aluminum Alloy Welding Joint

6061aluminum alloy is widely used in various fields because of its excellentcomprehensive performance, and welding technology is irreplaceable in processingof aluminum alloy. Usually, structure failure of aluminum alloy is caused by fatiguewhich often begins with welding joints. So, it is great significant that to evaluatefatigue damage of aluminum alloy and welding joint.For the moment, the assessment method of fatigue damage is limited. Errorsbetween the results and the practical are large. However, in nondestructive testingfield, nonlinear ultrasonic testing technology show the obvious advantage onevaluation of fatigue damage.This paper takes the nonlinear ultrasonic method to evaluate fatigue damageand finds that the nonlinear parameter has a certain relationship with fatiguedamage. The content includes:Through testing the equipment performance and choosing proper transducers,this paper evaluates the fatigue damage through nonlinear ultrasound, and acquiresthe characterization of fatigue characteristic value–the nonlinear parameter r.Using three kinds of transmission mode of different angle, this paper got therelation curves between nonlinear parameter r and fatigue damage. Comparing thethree kinds of mode, this paper finds that nonlinear ultrasound can detect closedcrack when the direction of ultrasonic transmission and the direction of the loadhave a certain angle. Particularly, it has the best effect when the angle is vertical.Through the best testing angle, fatigue specimen of welding joint are detected.The nonlinear effect factors and their influence are analysed. Also the relation curvebetween nonlinear parameter r and fatigue damage is acquired. The maximum of thecurve can be taken as a reference value which characterizes fatigue damage to acertain degree.In signal processing, pulse reverse technology and HHT are applied tononlinear ultrasonic testing. By the pulse reverse technology, the nonlinearultrasonic effect is more apparent, which is more advantageous to evaluate fatiguedamage. By HHT, the time discrimination of signal is improved, and partial featureof the signal can be seen. Through change rule of the instantaneous amplitude, the relation curve between the instantaneous amplitude and fatigue damage is acquired.From this curve, the time when fatigue crack begins can be approximately detected.