Study On Eigenfrequency Method And Acoustoelastic Effect Of Ultrasonic Guided Wave In Waveguide Structures

As an efficient nondestructive testing method, Ultrasonic guided wavetechnology can detect concurrently defects and stress, it has a potential advantage thatthe main components of steel structure (e.g., rectangular tube, h-beam, steel rod, etc.)are detected. However, because of the complexity of cross section shape, the wavecharacteristics of steel structures are difficult solved by the traditional wave theory,the numerical method is also less, and the research of prestressed waveguide structureis more rare.To investigate wave characteristics of the special section waveguideds andprestressed structures, a finite element eigenfrequency method is established toanalysis dispersion of arbitrary cross-section waveguideds base on continuum theoryof vibration. The propagaton characteristics of plate strip are studied by this method,and it is applied to analysis wave motion of prestressed structure, the optimal modesand excitation frequencies are obtained for stress measurement. On the basis oftheoretical analysis, the effect factors are study, and acoustoelastic experiment systemis constructed. With steel bar and rectangular tube as the research object, theacoustoelastic effect of longitudinal modes and SH0mode are studied, demonstratesreliability of acoustoelastic experimental system and feasibility of this method. Themain work is as follows:(1) The finite element eigenfrequency method is put forward to solve dispersioncharacteristic which cross section is consistent along the axial, the convergence isanalysed for mesh size and length of the model. The dispersion of common structuresand two kinds of special waveguide structures are calculated. The results show thatfree degree numbers and type of model can be choosed by the suitable boundaryconditions. It is difficult that the dispersion is calculated for complex waveguidestructure and high frequency band.(2) The wave characteristics of plate strip are studied, and the affect of widthboundary is analyzed for dispersion. The results show that the low order mode canform high order modal by in-order superposition, the cutoff frequency is determinedby the boundary dimensions for new mode. A0and SH0mode can be generated inplate strip by numerical simulation combined with the infinite element method, therest of the Lamb wave modes is not easy to be incentive and recognition, the reason is explained that S0mode is difficult to excitate. The engineering structure, rectangulartube, h-beam and elliptical tube as the research object, the dispersion characteristicsare analyzed, it is found that the boundary constraint is obvious in cutoff frequency.The antisymmetric mode is easiest generated, the local modal analysis can besubsituted for plate strip.(3) The prestressed waveguide structure is analysed by finite elementeigenfrequency method. It is presented that acoustoelastic effect is described byacoustoelastic constant dispersion curve. The acoustoelastic effect sensitive mode andexcitation frequency are obtained for plate, rod, pipe and plate strip. The frequencyblind spot is existent that no acoustoelatic effect. It is provided with the sameacoustoelasticity effect for the same dispersion in different structures.(4) The affect of acoutoelstic effect is clarify for material composition,third-order elastic constants and texture effect, the secondary factors and thesignificance level of third-order elastic constants is obtained by orthogonal analysismethod for longitudinal modes. the temperature effect, the effects of tensile strain,poisson’s effect is analyzed, and the precision of time delay estimation method areevaluated. The results show that the poisson effect can be ignored, thecross-correlation method is best suited for time delay estimation. Finally, themeasuring system is calibrated.(5) The acoustoelastic test system is established. The acoustoelstic effect oflongitudinal modes and SH0mode were studied with rod and rectangular tube astesting object. The results show that: the error of acoustoelastic constant is bigger inlow stress areas. The reliability of experimental system and the feasible of theorymethod are demonstrated. The SH0mode test results show that the method is adaptivefor rectangular tube, but it is prerequisite to improve experiment technology.