DETERMINATION OF RESISTANCE SPOT WELD QUALITY IN REAL TIME USING REFLECTED ACOUSTIC WAVES. COMPARISON WITH THROUGH-TRANSMISSION MODE
The determination of quality parameters of resistance spot weld in real time is of great importance in today’s industry. Acoustic waves can solve this problem in the sense of measuring certain features directly related to the weld quality – degree of heating of the weld zone, moment of beginning of melting and some others. The ultrasonic transducer incorporated into an electrode sends short acoustic pulses into the weld area and receives the signals reflected from different interfaces in heat affected zone during welding process. The specific image and signal processing algorithms applied to the received signals enable us to localize weak reflections from internal interfaces, recognize their nature and use this information to characterize the weld quality. Introduction: The inspection of resistance spot weld quality in real time is quite a challenging and at the same time promising field of NDT. It enables one to monitor the dynamics of the spot weld properties right at the moment of the weld manufacturing. It is extremely important on the industrial floor as it gives the operator information about the quality of the product and thus enables him to correct the mistake before it is too late. Quality inspection of “hot” welds is relatively new field of NDT studies which is potentially capable of providing 100% quality output of the production line. It becomes especially important as long as implementation of the in-line quality inspection techniques will help save a lot of time by elimination or, at least, reduction of costs of after-production inspection and fixing bad parts. Our approach to the above mentioned problem is using the acoustic waves which pass through the spot weld structure during welding. On their way through the setup the waves are modified by the media and thus carry some information about the processes taking place inside the system. The setup consists of a set of one or two ultrasonic transducers incorporated into the electrodes of the spot welder. The ultrasonic transducers are driven by pulser-receiver; the last is controlled by the computer. The schematic view of the setup is presented in Figure 1. Figure 1. Schematic view of the experimental setup. 1 – incident wave, 2 – reflected wave, 3 – transmitted wave. The detailed description of the operational process of the setup can be found in [1, 2]. In this article we will present the most important results obtained previously in the transmission mode and compare them with the reflection mode. The comparison of the two methods and advantages of application of either method will be discussed in this paper. Results: In transmission mode a series of pulses 1 is sent through the weld zone (see Figure 1) and is picked up by the receiving transducer as waves 3. As the welding current heats and melts the media, the material properties at different moments of welding change much. For this reason the waves passing through the weld at different moments of welding differ much from each other. The comparison of the wave characteristics provides useful information about the processes inside the stack-up. The finite-difference model of spot weld formation described in [2] shows the dynamics of the temperature and some material properties inside the weld zone during the spot weld formation. Figure 2 shows the temperature distributions in the spot weld at different phases of welding procedure. The calculations have shown very high inhomogeneity of the temperature distributions along the vertical direction in the weld setup. The temperature gradients in the middle of the structure reach 700-800 K/mm. The longitudinal velocity gradients at the same region are in the order of 1000 m/s per mm. It means that in the center of the liquid (white) area the velocity of sound propagation is around 3800 m/s while at the steel-copper interface the velocity is close to 5000 m/s. The same high gradients are for the other material properties such as stiffness, thermal expansion coefficient, density, etc. The acoustic impedance mismatch at all interfaces is also dependent on the temperature distribution at the given moment. Acoustic reflectivity and acoustic transparency of the structure also fluctuate during welding, thus affecting the amplitude and the magnitude spectrum of the signal. All these factors give rise to the modifications the wave experiences on its way through the weld zone. Previous analysis [1] has shown that among half a dozen of different parameters of the signal the time of flight is the most reliable one. There was found high correlation of throughtransmitted signal delay with the nugget size of the spot weld. Figure 3 shows the signature of the weld obtained in transmission mode. Such an image is sometimes called M-scan; here each vertical line represents one A-scan obtained consequently during 500 ms. In the given picture there are 200 waveforms put together to form an image.
Comparison between in-line ultrasonic monitoring of the spot weld quality and conventional NDT methods applied in a real production environment
Real-time spot weld characterization using ultrasound is a reality in today’s automotive industry. The main goal of such testing systems is to reduc e operational time and provide reliable means of quality inspection. Currently, several R&D groups are working to develop real-time ultrasonic system for spot weld quality monitoring, beca use potentially it allows one to eliminate expensive destructive tests, reduce the amount of off-line ul trasonic inspections and ensure the inspection quality. The purpose of this paper is to analyze preliminary results of a single in-line ultrasonic station, installed at a real body-in-white plant condi tion, and perform a comparison of these results with the conventional quality assurance methods cur rently used at the plant, such as ultrasonic inspections (manual equipment strategically pl aced in the welding line), destructive (tensile tests) and metallographic tests. The results o btained from this comparison can show us if it is possible to apply this new technique in a real production environment, with the same reliability that we are used to experience in our current inspection structure. It is believed that the obtained results will provide mo re flexibility for the plants to work with available ultrasonic inspection equipment, will incr ease the reliability and the control in the most critical regions of the bodies and will also dec rease the inspection time during production.
Monte Carlo Study of the Effect of Backscatter Materail Thickness on 99 mTc Source Response in Single Photon Emission Computed Tomography
Introduction SPECT projections are contaminated by scatter radiation, resulting in reduced image contrast and quantitative errors. Backscatter constitutes a major part of the scatter contamination in lower energy windows. The current study is an evaluation of the effect of backscatter material on FWHM and image quality investigated by Monte Carlo simulation. Materials and Methods SIMIND program was used for simulation of a Siemen’s dual-head variable angle scintillation gamma camera. Planar and SPECT scanning of a 99m Tc source and a Jaszczak phantom for varying thicknesses of Perspex slabs, as a backscatter media, were analyzed using the photopeak and scatter windows. Simulated planar images and reconstructed tomographic images were evaluated qualitatively, by two nuclear medicine specialists, and quantitatively, by Structural Similarity (SSIM) Index. Results In the 99m Tc photopeak window, no significant change in total counts due to backscatter material was measured. In the scatter windows, scattering was overestimated compared with a simulated backscatter free SPECT system. For instance, at a thickness of 10 cm, total counts of a 99m Tc source detected in the 72 keV windows eventually doubled with increasing backscatter material, compared with the situation without backscatter material. The backscatter contribution plateaued when more than 7 cm of scatter material was placed but there were optimized results for a backscatter thickness of 4.5 cm. Better image quality for the thickness was confirmed by the results of eye interpretation and also by SSIM algorithm. Conclusion Backscatter should be taken into account, particularly in model-based scatter correction methods in SPECT for an accurate simulation system optimization.