Multi-Point Rapid Nonlinear Ultrasonic Testing On Fatigue Damage Of Aluminum Alloy

Aluminum alloy has advantages such as anti-oxidation, corrosion resistance and low density. It has been widely used in aerospace industry, train, automotive industry and other fields. Aluminum alloy structures frequently suffer from fatigue due to alternating loads, which often leads to catastrophic consequences. Currently, there are many kinds of ways to evaluate fatigue damage, but most of them need complex calculations. Nonlinear ultrasonic testing method is easy to use compared to other ways, and its characterization of fatigue damage of materials is more intuitive. It has become an important technology in the field of fatigue damage detection.However, nonlinear ultrasonic detection method is mostly used in a single point with fixed position and as a result, it is difficult to give effective early warning of the fatigue damage of materials. Meanwhile, the research on characterization of fatigue damage of materials in different locations is still in its infancy. Therefore, the detection of fatigue crack initiation and propagation in materials during fatigue loading has become an urgent problem in the research of detection of fatigue damage by means of nonlinear ultrasonic testing.In order to solve the problem, a multi-point rapid nonlinear ultrasonic testing system was established to extract the nonlinear signal in multiple locations on 6061 and 7075 aluminum alloy fatigued workpieces. The characterization on fatigue damage and the detection of fatigued zone was completed with the nonlinear coefficient calculated based on the nonlinear signals extracted.A suitable wedge was used to change the incident angle the ultrasound waves. With the wedge designed, oblique incidence method was used to realize the multi-point nonlinear ultrasonic testing on fatigue workpieces. In this experiment, the change of nonlinear coefficients with detection position was discussed. The results showed that the nonlinear coefficient was sensitive to changes in the fatigue area and the system can detect the fatigue region in the workpieces. At the same time, the nonlinear coefficient experienced a trend of firstly increase and then decrease as the cycles of fatigue increased. The characterization of workpieces with fatigue damage was completed by determining the coordinate of nonlinear coefficient in the curve.In order to verify the test results of multi-point nonlinear ultrasonic testing, other methods were used to detect the workpieces such as testing by transducers with angles, ultrasonic C-scan and ultrasound velocity measurement. The results showed the change of sound velocity cannot be used to characterize the fatigue damage. Testing with transducers with angles and ultrasonic C-scan were more sensitive to later fatigue damage. By observing the change of microstructure morphology of the fatigue region, nonlinear coefficient was associated with fatigue characteristics in different stage of materials and realized the assessment of the fatigue damage of materials.