Monitoring Of Damages In Composites Under Fatigue Loads By Using Stress Waves

The usage rate of composite materials in aerospace industries is increasing due to their advantages of high specific strength and specific stiffness.However,fatigue damages could inevitably occur in composite structures under long-term cyclic loads.These fatigue damages,combined with other structural defects,may continue to grow under fatigue loads until structural failure,posing a major threat to structural safety.Therefore,it is crucial to apply structural health monitoring system(SHMS)to monitor the damage states of composite structures under fatigue loads in real time,and utilize the monitored information to assess the fatigue damages and make decisions to maintain the damaged structures to ensure their safety.In this study,firstly the propagation of stress waves in composite structures is studied and the dispersion relation of stress waves in anisotropic composite laminates is established by using semi-analytical finite element(SAFE)method.Meanwhile the change of group velocities with the growth of fatigue damages is simulated by numerical examples to provide a basis for stress waves-based monitoring of fatigue damages.Then,the processing of stress wave signals is studied.Wavelet transform is used to analyze the stress wave signals excited and received by the piezoelectric elements in the time-frequency domain and extract the characteristic features related to the fatigue damages,and measure the actual propagation velocities of the stress waves in the structure to characterize the occurrence and development of fatigue damages.Two pattern recognition methods,i.e.,outlier analysis and least squares support vector machine(LSSVM),are introduced to determine whether there are fatigue damages and the growth of fatigue damage in composite specimens by combining the extracted multiple features.Finally,experimental studies on damage monitoring for composite structures by using stress waves under fatigue loads are performed.The tensile fatigue test for glass fiber laminates and the compression fatigue test for carbon fiber laminates after low velocity impact are considered.The stress wave signals are analyzed by the two pattern recognition methods to judge the occurrence and growth of fatigue damages.The experimental results have verified the feasibility and effectiveness of damage monitoring of composite structures based on stress waves method under fatigue loads.