Study On Fatigue Damage Detection And Evaluation Of Composites Based On Ultrasonic

Composite laminate structures typically face a complicated load environment during their service life, and the repeated action of the fatigue loading may easily damage structural components. The accumulation of damage can lead to a change in stiffness and strength of the material, which then usually seriously affects the service life and security coefficient of the component. Therefore, it is vital to detect and evaluate the fatigue damage of composite materials especially when part of structural. Meanwhile, the accumulation of fatigue damage also cause the change in ultrasonic velocity and ultrasonic attenuation coefficient, in this paper, the fatigue damage of composite laminates was detected by ultrasonic nondestructive testing, and the ultrasonic velocity and ultrasonic attenuation coefficient were used to evaluate the degree of fatigue damage. To sum up, the main research contents are as follows:The static three-point bending test was conducted to obtain the ultimate bending strength, bending modulus and load-deflection curves of composite laminates. The stress level of fatigue loading was determined according to the ultimate bending strength. Three-point bending cyclic loading tests were performed under different stress levels and different loading frequencies. The fatigue mechanical response and damage mechanism corresponding to the progressive failure process were investigated according to the stiffness degradation curve, deflection curve and the ultimate failure morphologies. It was found that the stiffness degradation curves and the deflection curves under different stress levels and different loading frequencies show obvious three stages characteristics, the stress level and load frequency influence the duration of the three stages and the ultimate fatigue failure morphologies.The specimen with an artificial prefabricated flaw was detected by immersion ultrasonic scan imaging through the combination of conventional ultrasound and slice C scan method. Based on the principle of ultrasonic C scan the bottom wave amplitude imaging, through one scan, ultrasonic reflected signals were acquired in different depth position to form slice C scan imaging and slice C scan images were used for observe defects inside the material. Experimental results have shown that, complex defects in composite materials can be multi angle imaging and full range of clearly displayed by combined ultrasonic A, B, C scan imaging method with slice C scan imaging method.Slice C scan imaging experiments were performed on the composite laminates with different fatigue damage levels, then the ultrasonic signals were extracted from the fatigue damage region, and the values of ultrasonic velocities and ultrasonic attenuation coefficient were calculated. The results show that:with the increasing degree of fatigue damage, the ultrasonic velocity decreases and the ultrasonic attenuation coefficient increase. Their overall trend in change trend can be divided into three stages. The overall trends in change of the ultrasonic velocity and the ultrasonic attenuation coefficient are consistent with the three-stage trend in change characteristics of the development of the fatigue damage in composite structures. The computing results were fitted by polynomial functions and exponential function. From the correlation coefficient and relative error of fitting curve of view, the fatigue damage degree of composite materials can be evaluated by the exponential function. The residual stiffness of the specimens was measured and decreased with the increasing change rate of ultrasonic velocity and change rate of ultrasonic attenuation coefficient. The bending fatigue damage was found to rapidly increase once the change rate of the ultrasonic velocity or the change rate of the attenuation coefficient had reached 6.2% or 351.25%, respectively, and these values can be used as critical values for evaluating the bending fatigue damage.