Ultrasonic Nondestructive Evaluation Of Damage In Adhesive Structure Based On Nonlinear Guided Waves

Adhesive joints are widely used in aerospace,shipbuilding and automotive industries because of such advantages as lower structural weight,more uniformly distributed stresses and so on.As an important component of the adhesive joints,properties of the adhesive interface determine the integrity of the whole adhesive structure,making the research on damage detection and evaluation of the adhesive interface an extensively concerned topic in the related fields.The nonlinear ultrasonic detection technology shows higher sensitivity in property characterization for the change of microstructure with a spatial dimension much smaller than the wavelength of ultrasonic waves.The ultrasonic detection technology based on nonlinear guided waves is able to excite and receive signals on the ipsilateral side of the structure,and it can detect damages over a long distance,making it a convenient method to inspect large plate and shell structures.Therefore,the internal damage of the adhesive structure was tested and evaluated using ultrasonic detection technology based on nonlinear guided waves in this paper.Firstly,the dispersion curves of multilayer structure were obtained by the method of numerical calculation,and the dispersion characteristics of multilayer structure were analyzed.On that basis,the propagation process of Lamb waves in a multilayer adhesive structure with randomly distributed micro-cracks was studied using the FEM software ABAQUS.And the influence of various parameters of micro-crack on the nonlinear effect of Lamb waves was analyzed.Then,adhesive specimens with local debonding damage were detected using the nonlinear ultrasonic testing system,and the relationship between the nonlinear parameter and the degree of debonding damage was analyzed.Finally,the degradation of mechanical properties of adhesive structures under cyclic temperature fatigue loading was studied and evaluated by finite element simulation and experiment.The acoustic nonlinear parameter versus third-order elastic constant of the adhesive layer,and the acoustic nonlinear parameter versus the fatigue times under low cyclic temperature were obtained respectively.The results are as follows:(1)The dispersion curves of the multilayer structure are different from those of the single-layer structure.The phase velocity of the Lamb wave modes in the multilayer structure changes more dramatically and the relationships between the modes are more complex,including more modal transformation phenomena.Moreover,the phase velocity of Lamb waves in the multilayer adhesive structure will converge to the Rayleigh wave velocity of the aluminum layer and the shear wave velocity of the adhesive layer.In addition,when the elastic modulus and the density of the adhesive layer are changed,the phase velocity changes more significant in some frequency range,while is less affected in other areas.On the contrary,the phase velocity curves are less affected by the change of the Poisson's ratio of the adhesive layer.The results show that,when selecting the excitation modes,the modes and frequencies that are less affected by material parameter changes should be selected as far as possible so as to minimize the influence of material parameter changes on the second harmonic generation effect.(2)For the finite element simulation of the multilayer adhesive structure with micro-cracks distributing randomly,the interaction between Lamb waves and micro-cracks leads to second harmonic generation.The acoustic nonlinear parameters(ANPs)increase linearly with the micro-crack density,while are slightly influenced by the friction coefficient between the surfaces of the micro-cracks.In addition,when the micro-cracks are distributed in the interfaces between the aluminum layers and adhesive layer,the ANPs are slightly bigger in the case of random direction of microcrack than the case in which the directions of microcracks are all horizontal.(3)For the nonlinear ultrasonic test of adhesive structure with local debonding damage,the interaction between S₁ mode Lamb wave and the nonlinear damage leads to second harmonic generation,and the ANPs obtained increase linearly with the increase of the debonding area,indicating that the S₁-S₂ mode pair and the corresponding ANPs can be used to detect and evaluate the nonlinear debonding damage in adhesive structures.(4)For the study on degradation of mechanical properties of adhesive structure under cyclic temperature fatigue loading,the ANPs obtained by the experimental method increase significantly with the increase of temperature fatigue times in the initial stage,then the growth slows down.Under cyclic temperature fatigue load,micro-damage appears in the adhesive layer of the adhesive structure.With the increase of temperature fatigue times,the accumulation and expansion of micro-damage lead to the third order elastic constant of the adhesive layer increasing continuously.Corresponding to this,results obtained based on FEM simulation show that the ANPs grow linearly with the increase of third-order elastic constant of the adhesive layer.The results obtained in this paper provide reference and basis for the non-destructive detection and evaluation of the early material damage in adhesive structures using nonlinear guided waves.