Linear And Nonlinear Lamb Waves Techniques Towards NDT For Composites

Composite is of high strength-to-weight ratio and stiffness-to-weight ratio,which enables it a young but proficient material for military development,especially in aerospace and astronautic industries.Work loads and environmental factors may cause damages in composites,typically like a delamination,leading to the weakened performance of the structure,imposing unsafety risks on the overall structure and threating citizens' safety and even national security.As an efficient non-destructive evaluation(NDT)technique,ultrasonic Lamb waves are sensitive to types of defects and thus are prevalent for damage estimation in composites structures.Aiming at identifying accumulated fatigue damages as well as the micro-defects in the early fatigue life for composites,this thesis proposed a linear and a nonlinear Lamb wave technique,with the main research contents and innovative ideas as following:(1)Theoretical modeling of Lamb waves in composites: Different approaches are proposed and compared with regards to modeling Lamb waves in composites,including 3D elasticity theory,plate theories,semi-analytical finite element method and finite element methods.A novel third-order shear deformation theory(To SDT)was developed,providing more accurate results for lower Lamb wave modes than other existing plate theories.In addition,this To SDT runs about ten times faster than the 3D elasticity theory.The propagating characteristics of Lamb waves in composites are investigated,which gives the required information for later experimental design and conduction.(2)Nonlinear second harmonic analysis in composites: A hyperelastic material model for a nonlinear transversely isotropic material was developed.This thesis studied the two criteria for generating cumulative second harmonics in composites and selected primary-secondary modes pairs for second harmonic generation.Numerical simulation was carried out to verify the theoretical results of second harmonic existing criteria and the possible mode pairs.This part provided theoretical guidelines for experimental conduction of micro-defects detection in composites.(3)Measurement of phase velocity dispersion curves for Lamb waves in composites: An advanced laser generation based imaging(LGBI)system was set up to visualize the propagation characteristics of Lamb waves in composite structures.Based on the position-time diagram method and 2D Fourier transform technique,phase velocity dispersion curves of different modes along any propagation direction can be obtained from the laser-scanned ultrasonic data.(4)Reconstruction of the stiffness coefficients in composites: This thesis proposed a genetic Lamb algorithm based optimization method to inverse the measured Lamb wave phase velocities and thus to reconstruct the nine equivalent-single-layer independent stiffness coefficients in composites.Sensitivity analysis indicated that phase velocity dispersion curves of only two guided wave modes and along only three propagation directions are required for material reconstruction.(5)Qualitatively evaluation of accumulated fatigue damages in composites: A NDT method for qualitatively estimating the accumulated fatigue damages in composite structures was established.Experiments were conducted to measure Lamb wave phase velocities for composites under different fatigue states,together with the stiffness properties of the structure.The degradation of phase velocities and stiffness parameters during the fatigue process for composites were observed,which enables NDT for accumulated fatigue damages and thus the remaining life prediction in composite structures.