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 5 Fatigue testing that employs sample resonance at ultrasonic frequencies is a relatively underutilized technique that is attracting increased interest as a tool to rapidly assess the suitability of alloys for 3D printing. This testing method is especially useful for applications in aerospace, automotive, medical device, and other industries that require high reliability and extremely long performance life. Compared with conventional fatigue tests, ultrasonic testing provides both faster test speeds and the unique ability to test to extremely high cycle numbers in a practical time frame. Very high cycle fatigue (VHCF) testing can reveal material failure modes that would remain undetected under lower cycle fatigue test conditions. Gigacycle fatigue tests, which would take years using traditional test methods, can be performed in about six days with ultrasonic testing, as shown in Fig. 1. This article provides a simple introduction to ultrasonic fatigue testing along with examples of its utility in the characterization of metals including 3D-printed alloys. TESTING BASICS Fatigue testing involves the periodic stressing of materials below their yield strength to produce plots of applied stress versus the number of test cycles to failure. These S-N plots are used to estimate the expected lifetime of materials for intended design applications. With traditional fati- gue instrumentation, sam- ples are stressed by cyclic external loads generated by a servo-hydraulic or electromechanical mechanism and easily measured with a load cell. Uniaxial fatigue testing methods are well established and thoroughly documented in ASTM publications such as STP 566-EB Handbook of Fatigue Testing, as well as ASTM standards E466-21 and E606, which describe force-controlled and strain-controlled procedures, respectively. ASTM standard E467 describes procedures for dynamic load verification. Conventional fatigue machines can generate maximum cycle frequencies of about 100 Hz. Consequently, fatigue tests are typically restricted to a ULTRASONIC FATIGUE TESTING FOR ADDITIVELY MANUFACTURED METAL ALLOYS Compared with traditional fatigue testing, the ultrasonic method achieves much speedier results along with the ability to run extremely high test cycles in a reasonable time frame. TECHNICAL SPOTLIGHT Fig. 1 — Gigacycle fatigue tests that require years of testing using conventional techniques only take about six days with ultrasonic testing. Fig. 2 — Shimadzu USF-2000A ultrasonic fatigue testing system.
RkJQdWJsaXNoZXIy MTYyMzk3NQ==