Tham khảo tài liệu 'friction, lubrication, and wear technology (1997) part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | where đ r2 and In is the ultrasonic intensity irradiated on the defect f is the backscattered amplitude is the attenuation coefficient in the material AF is the area of the transducer r is the depth of the defect and d is the solid angle subtended by the probe as seen from the defect. Because ũía2f in the Rayleigh regime Isc In Via6 is the ultrasonic frequency This means that very efficient transducers with a center frequency as high as possible must be employed in order to obtain a sufficiently high SNR for a given excitation voltage of the transducer. For other defect shapes expressions similar to Eq 2 also hold true Ref 20 . Therefore the electronic systems and probes in a HAIM system must be designed such that the highest SNR is obtained and losses are absolutely minimized. In the setup currently used by the authors the detection limit for defects is 30 m for inclusions of a few millimeters depth provided the ultrasonic attenuation in the material examined is less than 1 dB cm at 50 MHz. Figure 8 shows a block diagram of a typical HAIM system. This system also comprises a scanning system to obtain B-scan and C-scan images with a step-resolution of 10 m Ref 21 . A-scans generate ultrasonic data in which the amplitude is recorded as a function of time. In a B-scan the amplitude is recorded in varying shades of gray or a color scale as a function of time and one coordinate. C-scans generate amplitude data as a function of two coordinates. In general the maximum amplitude in C-scans is recorded for the image built up within a preset gate having a time delay that defines the time-of-flight of the signal and hence the depth of its origin within the sample. After rectification the portion of interest of an A-scan is cut out by a gate and the signal strength within this gate is used to build up the image. The authors system has been used to detect and evaluate defects lack of adhesion between two different materials homogeneity and surface damage of components. Details
