Phased Array Ultrasonic Testing On Radii Of CFRP Laminates With Unidirectional/Fabric Thermoplastic Prepregs

Carbon Fiber Reinforced Plastics(CFRP)is a high-performance composite material owing to the high specific strength,high specific modulus,anti-fatigue,and corrosion resistance.It has been widely used in the manufacture of aircraft load-bearing components.These components usually exist corner parts,i.e.radii,which is prone to defects during the manufacturing and service,threatening service safety.Phased Array Ultrasonic Testing(PAUT),as an advanced non-destructive testing(NDT)technology,is widely used in CFRP component inspection,and showing good potential in applications of radii inspection.In order to improve the load-bearing performance of components and meet the requirements of complex service conditions,the CFRP components will also with laminates with unidirectional/fabric thermoplastic prepregs,and contain transition zones with multiple geometric,which bring new challenges to the radii inspection.In this paper,the inspection ability of the curved array in the radii of unidirectional/fabric laminates is studied by simulation and experiment.On this basis,the feasibility of developing signal processing methods and improving the inspection quality is discussed based on linear arrays.The main research contents and results are as follows:(1)For the radii of unidirectional/fabric laminates,the metallographic structure,elastic constant and layer structure are analyzed.On this basis,the three-dimensional model of PAUT was established based on CIVA simulation platform,which consider the geometric shape of radii,multilayer structure and elastic anisotropy.In addition,a two-dimensional finite element model was established based on COMSOL Multiphysics,which laid the foundation for the study of the wave propagation law and defect response mechanism in the radii.(2)The influence of the central frequency,focus depth,and aperture of the curved array probe on the wave field in the radii was studied by CIVA simulation model.On this basis,a PAUT scheme was designed to inspect the 12×12 mm~2 delaminates,which pre-embed in the subsurface,interlayer and near bottom surface,The results showed that compared to the design value,the depth deviation does not exceed 0.11 mm,the axial quantitative deviation does not exceed 0.30 mm,and the circumferential quantitative deviation does not exceed 0.67 mm,the maximum error 5.6%.It is in good agreement with the experimental anatomical results and meet engineering inspecting requirements.At the same time,the delamination of equal thickness transition of variable curvature and radii of variable thickness transition were studied,and the inspection result was equivalent to the design value.(3)Based on the radii of the unidirectional laminates,the surface adaptive ultrasound(SAUL)was studied and the key inspection parameters were analyzed.The results show that as the distance h increases of the linear array probe and the center of curvature of the radii,the noise imaging on both sides of the radii continues to weaken,and the inspection imaging quality improves significantly,but the inspection range decreases accordingly.On this basis,the inspection range and quantitative methods of SAUL were discussed.The delamination with a circumferential length of 3 mm in the radii was inspected and applied to the inspection of radii of unidirectional/fabric laminates.The quantitative results were equivalent to the metallurgical values,and the circumferential quantitative error was less than 6%.(4)Based on the finite element simulation model of the radii of the unidirectional laminates,the inspection ability of SAUL was studied when two delamination defects existed simultaneously,but the maximum quantitative error of the circumferential length of the delamination reached 17.5%.To the end,a post-processing imaging inspection scheme combining SAUL with receiving focus is proposed,which can superimpose the echo signals of each array element in the receiving focus aperture,and performs imaging processing on the A-scan signal obtained by superposition.The results show that after focusing,the inspection sensitivity is significantly improved.The maximum amplitude at the delamination is increased by 9.5 dB,and the maximum quantitative error of the circumferential length is less than 6%,which is 12%lower than SAUL.On this basis,the multi-defect inspection of radii of unidirectional/fabric laminates was discussed.