Test Technologies Contributing to Electrification - 6. X-ray CT Nondestructive Measurement Technology Supporting Vehicle Body Weight Reduction TechnologyFig. 2 Reconstruction process of X-ray CT3. Internal structure visualization technology using X-ray CT3.1 Overview of X-ray CT3.2 Challenges to implementing X-ray CT for vehicle body weight reductionFig. 3-1 Illustration of X-ray energy transmissionFig. 4 Deviation of transmission rate as a function of Fig. 3-2 Illustration of hardening transmission lengthand a higher absorption is observed on the low-energy side, which allows only the high-energy X-rays to be transmitted. This is called “hardening” (beam hardening) because the apparent energy distribution shifts to the higher side. The illustrations of X-ray energy transmission and hardening are presented in Figs. 3-1 and 3-2, respectively.Hardening causes the relationship between the transmission length and transmission rate (irradiation intensity/transmission intensity) to deviate from the Lambert–Beer law, as indicated in Fig. 4. This deviation creates noise (metal artifacts) in the CT image that is not actually present as a structure during the reconstruction process.In general, metallic filters can be used to suppress hardening. Noise can be reduced by placing a metal plate as a filter in the X-ray irradiation section. The following steps are required to obtain CT images with reduced noise: the installation of a metal filter, trial imaging, and 62must be analyzed and evaluated under different conditions to understand the mechanism of the phenomena and identify factors that affect the processing results. However, conventional analysis and evaluation techniques for internal structures rely on the observation of fractured cross-sections obtained after mechanical processing. Moreover, acquiring information about a specific cross-section is time-consuming. Fatigue strength under repeated loading must be considered when designing vehicle body parts. Identifying internal structural changes over time using the same sample is an effective approach to understanding the fatigue mechanism. Therefore, a technology to efficiently analyze the internal structure in a nondestructive manner is expected to help realize competitive advantages in the development of vehicle body weight reduction technology. A technology for imaging three-dimensional internal structures developed in-house using X-ray CT is described in this special feature.X-rays can be generated using the following procedure: An electric current is passed through a filament inside the source, which heats the filament to generate thermal electrons. Thermal electrons are then accelerated using a high voltage. These collide with the target to generate X-rays, which are irradiated from all directions (360■) onto an object, and the X-ray transmission intensity is recorded as a transmission image by a detector located on the opposite side. The data from each direction are called projection data, and the three-dimensional image created by reconstructing all projection data from all directions (360■) is called a CT image. The sequence of this process is illustrated in Fig. 2.The energy of X-rays irradiated from commercially available industrial X-ray CT systems indicates a continuous distribution, with a peak on the low-energy side and a gradual decrease toward the high-energy side. X-rays are easily absorbed by parts made of dense iron,
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