| Carbon Fiber Reinforced Plastic(CFRP)has been increasingly used as an alternative to metallic material in load bearing components of aircraft due to its advantages of light weight,high strength and outstanding designability.These large-scale components usually have complex shapes.Their corner parts,i.e.radii parts,are weak points in the stages of manufacturing and service.Timely inspection of internal defects within the radii part using nondestructive testing(NDT)technique is an important mean for quality control of CFRP components.Phased Array Ultrasonic Testing(PAUT)is considered as one of the most promising NDT techniques.In comparison to homogeneous isotropic metallic materials,complex shaped CFRP components show an obvious elastic anisotropy and heterogeneity.To be specific,wave velocity is not constant,but varies with propagation direction from approx.3000 to 9000 m/s.Acoustic property mismatch occurs at the interface between two different plies.An obliquely incident ultrasound will change its propagation direction and velocity.Moreover,fibers in the radii part also change their directions continuously along the curved shape,which leads to the variation of wave velocity with position.Consequently,ultrasonic ray path through one ply is a curved line.Due to the coupled interaction between complex shape,multilayered structure and elastic anisotropy of material,ultrasonic ray path gets complex and is difficult to be predicted.Wave propagation mechanism in the radii part remains unclear.Direct application of PAUT techniques for homogeneous isotropic materials to CFRP components results in a poor image quality and missed defects.To overcome the above difficulties,three aspects,i.e.acoustic modeling,ray tracing method and Total Focusing Method(TFM)imaging of CFRP components,are studied in this paper.Firstly,the influence of material on wave propagation behavior is analyzed by FEM.On this basis,further research is performed on ultrasonic ray tracing method for inverse calculation of complex ultrasonic ray paths.Finally,ultrasonic travel time predicted by the proposed ray tracing method is used for total focusing method imaging with the aim of improving both image quality and detectability of defects.The main research contents and results are as follows:(1)As foundation of this study,metallographic observation and elastic constants measurement are first made.In view of the microstructure feature of CFRP,an acoustic modeling method is proposed for CFRP components,which considers the coupled interaction of complex shape,multilayered structure and elastic anisotropy of material.Quantitative description of the material elasticity of CFRP components is achieved.The finite element modeling and simulation is performed for both planar and L-shaped multidirectional CFRP laminates(the planar laminate has a 5.79 mm thickness,the radii part has an 8.64 mm thickness,90-degree angle and 8.80 mm concave-surface radius).A comparative analysis on the wave propagation rules and source of noise between the two laminates is conducted.Results show that the multilayered structure and elastic anisotropy of CFRP pose more impacts on obliquely incident ultrasound.Consequently,structural noises get improved due to wave reflection at ply interfaces.Wavefront also bends with the shape of radii part,which further worsens the oblique incidence of ultrasound at ply interfaces.The extent of oblique incidence can be weakened by using an acoustic coupling medium which has an acoustic property close to CFRP.Noise reduction is thus realized.(2)Conventional ultrasonic ray tracing method based on Snell's law is unable to perform inversion calculation of 'ray path between two specific points' for CFRP components,and suffers from low computational efficiency.According to the idea of 'discretization-integration',an ultrasonic ray tracing method based on Fermat's principle is proposed which applies Dijkstra's shortest path searching algorithm originating from graph theory in computer science,and considers the coupled interaction of complex shape,multilayered structure and elastic anisotropy of CFRP components.This method not only settles the difficulty in inversion calculation of ray path passing through the radii part,but also is high in calculation accuracy and quick in calculation speed,thus meeting the demand of PAUT where inversion calculation of a large number of ray paths is required.Results show that the predicted ultrasonic travel times coinside well with the actual values measured by the back-wall reflection method.Compared with conventional methods,the proposed ultrasonic ray tracing method improves the computational efficiency by three orders.The above results confirm the validity of the proposed method.(3)Current PAUT methods as applied to CFRP components has a poor image quality and low defect detection sensitivity.To solve these problems,an ultrasonic imaging method is proposed,which combines the Dijkstra's algorithm based ultrasonic ray tracing method with TFM technique.Ultrasonic travel time between every acoustic source and image point combination is first calculated by ultrasonic ray tracing method.Then,TFM is introduced to synthesize a high-resolution focused image of the region of interest by performing 'delay and sum' post-processing on full matrix capture data.PAUT experiment is performed for the radii part of an L-shaped CFRP specimen which has an 8.64 mm thickness,90 degree angle and 8.80 mm concave-surface radius.Results show that when compared with conventional TFM,the proposed method has a higher image quality.All?1.5 mm side-drilled holes distributed in the depth and circumferential directions within the radii part can be found at the right positions.Detection sensitivities of the SDHs are improved by 3.5 dB. |
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