A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 2 2 7 The technique is limited to materials that can resonate at 20 kHz without excessive heating. Consequently, this method has been primarily used to test metal samples, although recent studies have successfully employed the technique to test carbon fiber reinforced plastics[1]. To prevent excessive heating of samples, the system is configured with a forced-air cooling mechanism. In addition, the computer control system allows the oscillations to be regularly halted during the experiment to let the sample cool during the test. During testing, sample temperature is monitored using a radiative temperature measuring device. A more thorough description of the basics of ultrasonic fatigue testing may be found in the ASM Handbook published in 2000[2]. In 2017, The Japan Welding Engineering Society published a standard test method for ultrasonic fatigue testing of metals (WES 1112:2017), which describes the theory and testing procedures in detail[3]. EXAMPLES To demonstrate the technique, the staff of the Global Applications Development Laboratory of Shimadzu Corporation in Kyoto, Japan, tested SNCM439 steel according to WES 1112:2017 using a Shimadzu USF2000A system[4,5]. Intermittent operation and forced-air cooling were employed to maintain the sample temperature of 30°C or less as mandated by the standard. Figure 6 shows how fatigue failure occurred at 108 to 109 cycles for low-stress magnitudes. Figure 7 shows an optical micrograph of the fracture surface of an SNCM439 sample, indicating that failure initiated at the site of an inclusion. These data show the importance of fatigue testing at 107 cycles and beyond for materials intended for long-lifetime, high-reliability applications. Such testing can reveal failure mechanisms due to internal defects that may go undetected with lower cycle testing. Therefore, ultrasonic fatigue testing can be a powerful technique to characterize alloys intended for additive manufacturing (AM) applications where the effects of AM process parameters on internal microstructure and material performance are not fully understood. An example of how AM processing parameters can affect VHCF performance is shown in Fig. 8. AlSi12 alloy samples were manufactured using selective laser melting with and without heating of the base[6]. Both samples failed beyond 107 cycles. Base plate heating (Batch II) resulted in significantly higher fatigue strength due to a reduction in gas pore size. Additional examples of ultrasonic fatigue testing of AM materials may be found in the review prepared by Andrea Tridello and Davide Paolino[7]. A comprehensive AM VHCF review article was authored recently by Maryam Avateffazeli and Meysam Haghshenas[8]. ASTM Committee E08 has established a group to discuss the nuances of this technique and develop a best practices guide[9]. ~AM&P For more information: Christopher J. Macey, business development group leader, Shimadzu Scientific Instruments, 7102 Riverwood Drive, Columbia, MD 21046, 410.381.1227, cjmacey@ shimadzu.com, www.ssi.shimadzu.com. References 1. T. Miyakoshi, et al., Evaluation of Transverse Crack Initiation of Cross-ply CFRP Laminates by using Ultrasonic Testing, Eighth International Conference on Very High Cycle Fatigue, July 5, 2021, online. Fig. 6 — S-N curve for SNCM439 steel sample. Fig. 7 — Fracture surface of SNCM439 sample. Fig. 8 — Two batches of AlSi12 alloy samples tested to failure. Batch II has base plate heating, while Batch I does not.
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