| With the continuous development of China's power industry,the thermal power unit move towards large capacity,high parameters and so on.At the same time,the domestic production of thick-walled pipe is not so qualified,compared with the imported products,due to the lagging standards and manufacturing process.Thus,the defects of thick-walled pipe from manufacturing process are of serious risk.At present,The traditional non-destructive testing technology uses spot-by-point inspection for spot inspection of thick-walled pipelines,which is difficult to meet the requirements of actual inspection.Ultrasonic guided waves,as a new type of non-destructive testing technology,can improve the detection efficiency of thick-walled pipe.Hence,this dissertation presents a new type of flexible broadband Rayleigh wave transducer based on PZT,which realizes the detection of the inner and outer wall defects of thick pipe with different diameter.The main research contents include:(1)On the basis of line loading simulation,using the finite element method combined with the mutual admittance principle,the element spacing p,the array element width a,array number n and the array element length l of the equidistant comb Rayleigh wave transducer based on PZT-5H material on steel plates and thick-walled pipes are investigated.The influence of n and l on the directivity of the transducer were studied too.By optimizing the selection of the backing material and the thickness of the backing,the tailing of the transducer signal is improved.Optimization results show that when p is equal to?R(?R is the wavelength of the corresponding frequency Rayleigh wave),a is equal to?_R/2,and when n is 5,the transducer signal is less tailed and the energy is concentrated.Experiments show that the comb transducer developed according to the simulation parameters has good directivity and can detect the defects of the inner and outer walls of thick-walled pipelines.(2)According to the optimization results of equidistant transducer,PZT-5H and piezoelectric composite materials were used to design non-equidistant transducer and compared with each other.For the piezoelectric PZT-5H,in combination with the multi-resonance coupling theory,a non-equidistant transducer design is performed.When the length of the transducer element is equal to 30mm and the thickness is 1.2mm,there is a continuous uninterrupted resonant mode in the design band between 500kHz–1MHz,and its impedance value is the smallest,which is good for sensor excitation.The directivity and bandwidth characteristics of non-equidistant transducer with 30mm array element length are studied.Then,the directivity and bandwidth characteristics of the 30 mm array element length non-equidistant transducer are studied.Its?-3dB beamwidth is 30°and has good directivity.The-3dB bandwidth is 417kHz,meeting the design requirements.(3)The basic performance of non-equidistant transducer made of piezoelectric composite material of type 1-3 is studied by finite element method.Firstly,the optimum ratio of piezoelectric phase width to height and the volume percentage of piezoelectric phase are determined.Then the parameters of the non-equidistant surface wave transducer made of this material are studied.The results show that the non-equidistant broadband transducer fabricated by piezoelectric composite has good directivity and the-3dB bandwidth is about 420 kHz.But compared with the transducer made of PZT-5H materials,the performance advantages are not obvious,and the excitation performance is weak.(4)Finally,non-equidistant transducers are fabricated according to the simulation results.The defects of the thick steel plate and inner/outer walls of thick-walled pipe were tested by using the fabricated transducer.Experiments show that the developed non-equidistant comb wideband Rayleigh wave transducer can detect the crack defects in pipeline and on the thick steel plate.Also,it can recognize the corrosion defects. |
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