Nonlinear Lamb Wave In Composites And Its Application To The Detection Of Impact Damage

Composite materials have been widely used in the field of aerospace and aeronautics due their advantages of high specific strength,high specific stiffness and light weight.However,it is inevitably that composite materials suffer from damages during manufacturing and service.The invisible micro-damage caused by low-velocity impacts are one of the most prevalent type of defects found in composite materials,which can significantly affect the reliability and service life of the structure,and may finally lead to the catastrophic incidents due to sudden failure.Thus,it is necessary to detect impact damage in order to ensure the safety use of composite materials.Nonlinear guided wave method has become one of the effective techniques to detect damage in composite materials due to its superiorities of wide inspection range,micro-damage recognition and high cost performance,compared with the conventional ultrasonic testing technique.In this thesis,various methods based on nonlinear Lamb waves have been investigated to detect a 1 mm thick carbon fiber reinforced polymer(CFRP)composite with different degrees of impact damage.The second-harmonic generation(SHG)method of Lamb wave has been used to detect the damage in composite materials.The excitation mode which satisfied the phase matching conditions is determined according to the dispersion curves,and the relevant data are extracted from frequency domain after experiments.The results show that the relevant nonlinear parameters increase with the impact damages.Compared to the intact specimen,the relevant nonlinear parameter associated with 15J damage specimens increase by 200%.Then a phase-reversal nonlinear Lamb wave approach is proposed to improve the signal-to-noise ratio of SHG of Lamb waves by counteracting the fundamental waves,and meanwhile increasing the efficiency of SHG of Lamb wave propagation.The experiment results are consistent with those obtained by former method.In order to avoid the interference of nonlinearity from the experiment instruments,a nonlinear guided wave method with co-directional mixing has been investigated.The excitation mode pairs have been determined based on the resonant conditions before the corresponding measurement system is proposed.Two Lamb wave modes with different excitation frequencies are mixed in the same direction by controlling the time delay,and the magnitude of fundamental frequency and components at sum and difference frequency are extracted to calculate the relevant nonlinear parameters.Result shows that the associated nonlinear parameter increase with the impact energy.In addition,different excitation mode pairs have also been investigated to optimize the excitation modes.Finally,the sideband peak count(SPC)technique has been conducted in this dissertation.Combination of SPC approach and nonlinear Lamb wave mixing technique is proposed to characterize the impact induced material nonlinearity in composites.Optimization of excitation frequency and mode selection in composite material are examined to detect the impact damages.It is found that optimized normalized maximum value of threshold line is around 1-10%of that of primary waves at fundamental frequencies of 1.95 MHz and 2.25 MHz,respectively.The number of side peaks is linear proportional to the energy of impact damage under the designed condition.