Research On Non-contact Ultrasonic Guided Wave Scan Imaging Technology For Composite Laminates

Properties such as high specific strength, high specific stiffness and low weightmake composite materials key manufacturing material for many applications such aswind turbine blades, aerospace, shipbuilding and other industries. However, defectmay appear in composite materials during manufacturing and service procedure.The existence of defects makes composite materials’ service procedure unsafe. Inorder to ensure the safety of the materials’service procedure, nondestructive testing&evaluation (NDT&E) technologies for quality testing must be performed aftermanufacturing process.According to literature review on the conventional composite materials NDTtechnology, the composite materials ultrasonic testing technology and air-coupledultrasonic guided wave technology, air-coupled non-contact ultrasonic guided wavetechnology is determined to detect defect for composite laminate plate. Time reversalmethod is employed to eliminate the guided wave dispersion and achieve signal focusand signal amplification. Reconstruction Algorithm for Probabilistic Inspection ofDamage (RAPID) is applied to imaging test results. Theoretical analysis is performedto verify the feasibility of these methods.Lamb wave A0mode is employed for testing according to the wave shape ofLamb wave. A three-dimensional finite element (FE) model is created to predictpropagation characteristics of Lamb wave and the interaction of Lamb wave withdelamination, and some unique mechanisms of interaction between Lamb wave A0mode and delamination are revealed in detail. It is concluded that little reflectionoccurs when the waves propagate from the main laminate to sub-laminates and largereflection will happen when the waves propagate from the sub-laminates to the mainlaminate. S0mode propagated in the same direction with reflected wave will beconverted when reflection take place. The amplitude of converted signal is dependedon the amplitude of reflected wave. It is also observed that there is little or noconverted S0mode when A0mode encounters the delamination located atsub-symmetric interface and symmetric interface. Experimental results obtained withair-coupled ultrasonic transducers are well in accordance with finite elementsimulation results.Transmission and reception of the air-coupled transducers is investigated from theoretical analysis and experimental study. Research results show that incident angleis directly linked to the velocity of ultrasonic in air and the phase velocity of excitedLamb wave mode by the Snell’s law. Air-coupled receiving transducer is not sensitiveto waves whose propagation direction is the same as its inclination direction and moresensitive to waves whose propagation direction is opposite to its inclination direction.Time reversal method is theoretical analyzed from the point of view of signalsand systems analysis. A virtual time reversal method that can realize time reversalfrom mathematical operations is proposed. Air-coupled ultrasonic guided wave scansystem is constructed. According to the transmitting and receiving characteristics ofthe air-coupled transducers, an air-coupled guided wave scan imaging algorithm isdeveloped through the modification of RAPID. Composite laminate plates withcircular, rectangular, and trapezoidal delamination are tested using the constructedair-coupled ultrasonic guided wave scan system. Testing results obtained withair-coupled ultrasonic guided wave scan system are well in accordance with thatobtained with commercial immersion C-scan system.