Detection Of Surface Micro-defects By Nonlinear Guided Wave Tomographic Method

During the processing and service of metal and non-metallic components,microscale or macroscale damage can be caused by external loads and other factors.Among them,crack is the most typical and extremely easy to occur failure mode.The fracture failure of components caused by crack is a gradual process.The micro cracks that are initially generated will gradually expand and generate macroscale cracks under the influence of external loads,temperature and other factors,which seriously affects the reliability of the components.Ultrasonic testing is a non-destructive testing method that uses the acoustic response caused by the interaction between ultrasonic wave and propagation medium to characterize the material defects and degradation,It is applicable to the detection of various metallic and non-metallic materials.Compared with the traditional linear ultrasonic testing,the nonlinear ultrasonic detection method can realize the detection of micron or even nanoscale defects by the generation of high order harmonics or subharmonics,so the nonlinear ultrasonic detection method has obvious advantages in the detection of microscopic defects or the evaluation of early material performance degradation.The main research content of this paper is to combine non-linear ultrasonic guided wave detection technique with the tomographic imaging method for the quantitative evaluation of material surface microscale crack.Firstly,the 1060 aluminum alloy plate was taken as the research object,and crack defect was introduced on the surface of the specimen.The specimen was detected by using the traditional ultrasonic detection method,the second-order harmonic technique and the nonlinear ultrasonic phase-reversal technology,and the detection area of the specimen was reconstructed by combining with the tomographic imaging algorithm.By comparing the imaging results,it can be founded that the detection of micro cracks on material surface cannot be solved by traditional guided wave tomographic method.When the imaging results using guided wave tomography based on the harmonic method and nonlinear ultrasonic phase-reversal technique are compared with the actual defects,there is a good consistency in crack size,location and shape.Then,based on the linear and nonlinear acoustic response mechanism of the energetic material,the surface crack of PBX was quantitatively evaluated by the method of tomographic imaging with ultrasonic guided wave technology.Two kinds of surface crack defects with different shapes were introduced on the surface of the PBX specimen,and the ultrasonic guided wave transducers were designed and manufactured according to the defects and the characteristics of the specimen.Different transducer array methods were used to image the crack defects in simple shapes,the results show that the transducer array has a significant influence on the imaging results.At the same time,two different transducer arrays were used to image the surface cracks of PBX material with complex-shaped,and good imaging results were also obtained.The nonlinear ultrasonic guided wave tomographic technique was applied to the detection of PBX materials.The crack imaging effect is in good agreement with the actual defect.