Study On Key Problems In Detection Of Damage In Thin Plate Components Using Nonlinear Lamb Waves

As thin plates are widely used in critical parts of aircraft,their quality reliability naturally attracts more attention.For macro-defects,the conventional nondestructive testing methods have been developed maturely at present.But there is no ideal means to monitor and evaluate the main causes that lead to these macro-defects,such as fatigue,creep,corrosion and so on.In recent years,there are some studies have shown that,there is a close relationship between nonlinear effect of ultrasound and material damage,and the ultrasonic nonlinear parameters can variate obviously with the variation of material damage.For the detection of thin plate,Lamb wave is a suitable method.Due to the restriction of Lamb wave characteristics of dispersion and multi-mode,the development of the research on nonlinear effects of Lamb wave is slowly.However,in recent years,with the development of nonlinear Lamb wave theory and the experimental verification of Lamb wave nonlinear effect,some scholars have carried out researches on the evaluation of material damage by nonlinear Lamb wave.Although nonlinear Lamb wave technology has been developed to some extent,its research is still in its infancy.There are still many problems to be improved and solved in the technology.Therefore,it is of great significance to explore the related problems in the nonlinear Lamb wave technology furtherly.In this paper,some problems in nonlinear Lamb waves are studied,analyzed and discussed.The research includes the selection of mode pairs that satisfy the conditions of cumulative second-harmonic Lamb wave generation,and the application of PVDF comb transducer in the reception of fundamental and second-harmonic signals in nonlinear Lamb waves.And then,the evaluation of high cycle fatigue damage of titanium alloys using nonlinear Lamb waves is studied.The main researches and conludes are as followings:(1)The generation and propagation of linear and nonlinear Lamb waves in solid plates are introduced.The numerical solution of Rayleigh-Lamb equation and the acquisition of general expressions for acoustic field of cumulative second-harmonic Lamb wave are expounded.The generation conditions of cumulative second-harmonic Lamb wave are discussed,and the role of group velocity matching in the excitation of cumulative second-harmonic Lamb wave are still analized.The types of mode pairs that satisfy the generation conditions are introduced.The experimental technology of nonlinear Lamb wave is introduced.All the above work lays a theoretical and experimental foundation for subsequent research.(2)Accoding to the general acoustic field expressions of the cumulative secondharmonic Lamb wave,the variation of second-harmonic Lamb wave displacement amplitude with the propagation distance is studied under the approximate phase velocity matching.An ‘excitation window' of the cumulative second-harmonic Lamb wave is first defined as the basis for mode selection,and its validity is verified in the experiments.The results show that the mode pairs with large excitation window can generate secondharmoic Lamb wave in a larger approximate phasevelocity matcing range.The proposing of ‘excitation window' provides an important basis for the selection of mode pairs.(3)The method of excitation efficiency for mode selection is studied.It was found that the approximate phase velocity matching is lack of discussion in the existing excitation efficiency method.Therefore,an improved excitation efficiency method is proposed.The improved excitation efficiency of different mode pairs is calculated,and the mode pair selection is discussed comprehensively considering the excitation windows.The results show that the selection of mode pairs needs to be confirmed according to the specific detection conditions with the help of excitation window and improved excitation efficiency.That's because that the variations of excitation window and excitation efficiency are inconsistent.(4)A PVDF comb transducer is designed and fabricated,and its experimental verification is carried out.The frequency characteristics of comb transducer are studied by means of output vector addition,and the harmonic frequency characteristics are also obtained.The theoretical and numerical results show that the comb transducer can receive Lamb waves whose wavelength is equal to the finger spacing of comb transducer.And besides fundamental wave,comb transducer can also receive Lamb waves whose frequency is integer times of fundamental waves'.In the experiment,a PVDF comb transducer was fabricated and used to receive Lamb waves from solid plates.It is shown that there is not only fundamental signal but second-harmonic signal in the signals received by the PVDF comb transducer,and there is cumulative effect in the secondharmonic signals.All these results prove that the comb transducer can be used to receive second-harmonic Lamb wave in the nonlinear Lamb wave technique.(5)The high cycle fatigue damage assessment of titanium alloy TC4 that is commonly used in aeronautical thin plate has been carried out using nonlinear Lamb wave technology.Two groups of high cycle fatigue specimen were designed and fabricated.The relative nonlinear parameters of second-harmonic Lamb wave of all specimens were measured by means of wedge piezoelectric transducer.The results show that the relative nonlinear parameter increases with the increasing of loading stress.In addition,the relative nonlinear parameter varies nonmonotonously with the increasing of fatigue life fraction,and the relative nonlinear parameter of the specimen that is in fatigue state is much higher than that in initial state.The PVDF comb transducer is also used in the measuring of relative nonlinear parameters in the experiment,and a same conclusion is obtained as that of the wedge piezoelectric transducer.The application of PVDF comb transducer in fatigue damage assessment provides a new idea for nonlinear Lamb wave measurement technology.And the application of PVDF comb transducer is beneficial to extend the nonlinear Lamb wave technology to the fatigue damage assessment of bending surface components.