Interface Defects Ultrasonic Testing Technology And Its Application For Bonding Structur

More and more bonding structure have been widely applied in industry of chemical, aviation, aerospace and food for their superior performance. Disastrous accidents happen in process of manufacture and working at times for weak bonding or air hole of layered media. So it is urgently needed to make accurate detection for interface quality and insure the quality of products and working safety.Ultrasonic echo method is widely applied in non-destructive detection for its advantage of accurate defect locating, wide application, convenient operation and so on. When the interface properties of multi-layer materials is studied using pulse-echo method, the interface echo is not easy to be distinguished because of multiple reflections and transmission for ultrasound in layers of multi-layer materials. It is usually assessed by the echo of the overall energy, especially for a thin layered media, even thickness with a few hundred micrometers. In the current research, a novel ultrasonic imaging method, based on backscattered echo envelope integral (BEEI) imaging method is proposed and applied for the defect detection of bonding structure. Two kinds of interface defects are detected, one is the channel defects and inclusion embedded in bonded 2-sheet polyethylene film, the other is inclusion defect and air hole embedded in the adhesive structure. It has been demonstrated that the interface defects in bonding structure can be effectively detected using ultrasonic backscattered echo envelope integral imaging method. The main research works are given as follows.(1) System design for ultrasonic detection of defects in bonding structureUltrasonic system is developed for detecting interface defects embedded in bonding structure. The software system is designed based on the modularization, realizes experimental operation automatically controlled by computer, real time data acquiring, data on-line processing and storage, high efficiency compared with off-line processing. The results show that the software system has strong real time capability, high efficiency and reliability, and the echo with high signal-noise can be obtained to satisfy the need of signal analysis and processing.(2) Interface defects ultrasonic qualitative detection in bonding structureThe interface defects in bonding structure are detected using BEEI imaging method. The images are formed with an immersion spot focused 22.66-MHz ultrasound transducer which the -6dB pulse-echo beam diameter at the focus is 315μm. At the same time, the Coefficient of variation (CV) values of BEEI-matrix for each sample is studied for the channel defects and inclusion defects in bonded 2-sheet polyethylene film. The variation of the BEEI values in the direction vertical to the edge of the heat seal and in the direction parallel to the edge of the heat seal are compared for the channel defects. The results show that the interface defects in layered media can be effectively detected using ultrasonic BEEI imaging method, and the size of defects on the ultrasonic image is larger than its actual size which is correlated with the lateral resolution of the transducer. The CV values are lower than four percent for channel and inclusion defect, which indicate the variation of the BEEI values in matrix is small. The mean BEEI values on the defect region are about greater between thirty percent and forty percent than that on the background region.(3) Quantitative analysis for interface defects ultrasonic detection in bonding structureThe scanning size issue of the BEEI imaging method is investigated. The study of relationship between varying scanning step size and image quality for the BEEI imaging method is presented for the seal defects embedded in bonded 2-sheet polyethylene film. It show that the image quality degraded as the scanning step size increased in the direction parallel to the edge of the heat seal, and the reasonable scanning step size is small than -6dB focal beam diameter. Theoretical analysis for the influence of the transducer lateral resolution on ultrasonic result are made while scanning step size is less than -6dB lateral resolution of the transducer, and the relation between ultrasonic result and defects actual size is given in the different scope of size. The experimental results are in good agreement with the theoretical analysis.(4) Study of defects detection mechanism for bonding structureDefects detection mechanism for bonding structure is studied for the seal defects embedded in bonded 2-sheet polyethylene film from three aspects of reconstructed acoustic impedance, finite element heat conduction analysis and experiment. The BEEI imaging method is able to reliably inspect defects as small as 52-μm in diameter. The main reason is an annular impedance transition region surrounding the defect. The annular region is produced by material compounding during the heat sealing process. Because of the material compounding, this region with an impedance value higher than that of its surrounding medium. The impedance difference between the annular region and the defect region causes reflection. The reflection strength from annular region is strong compared with that of from defect.