Real-Time Monitoring of Acoustic Linear and Nonlinear Behavior of Titanium Alloys During Cyclic Loading
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*** Former Director of the Center for Materials Diagnostics, University of Dayton ABSTRACT Variation in acoustic nonlinearity has been monitored in real time during fatigue, on four dogbone specimens of Ti-6A1-4V, under low cycle fatigue conditions, from the virgin state all the way to fracture. The results of these experiments show that the acoustic nonlinearity undergoes large changes during the fatigue and follows a similar trend for the material under given fatigue test conditions. Transmission electron microscopic (TEM) examination of the samples with similar composition fatigued to different stages indicates a gradual change in the microstructure and dislocation density, which correlates with the changes in acoustic nonlinearity. INTRODUCTION Ultrasonic attenuation and sound velocity measurements in a material undergoing fatigue show very small changes (less than 2%) and are not sensitive enough to characterize fatigue damage. It has been observed in ultrasonic wave propagation experiments, that a measurement of acoustic nonlinearity (P3)of the material shows large changes due to fatigue [1, 2, 3]. This methodology seems to be a promising tool to monitor the fatigue damage in materials. In these experiments, a fundamental frequency (f) ultrasonic wave is propagated through the material and the amplitude (A2) of the second harmonic frequency (2f) signal is measured. The acoustic nonlinearity is determined using equation 1 [4]. 8 (A 2 fk2 (_2~
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Where k is the ultrasonic wave number and A, is the amplitude of the fimdamental signal after propagating through the length x of the sample. This paper presents results of the measurement of sound velocity, ultrasonic attenuation, and acoustic nonlinear property, in dog-bone shaped samples of Ti-6AI-4V, acquired during fatigue. The parameters have been measured continuously from the virgin state of the material, all the way to its fracture. Measurements have been performed on several samples under similar fatigue conditions to observe repeatable and consistent changes in the ultrasonic parameters. Variations in the nonlinear acoustic parameter have been observed to show the same trend in all the samples tested.
79 Mat. Res. Soc. Symp. Proc. Vol. 5910 2000 Materials Research Society
EXPERIMENT An in-situ technique developed for characterization and early detection of fatigue damage in aerospace materials has been described in detail elsewhere [5]. In short the principle of measurement is based on propagating a fundamental frequency ultrasonic signal through a dogbone sample and detecting the second harmonic signal at the other end. Several improvements have been incorporated into the instrumentation for real-time measurement of acoustic properties. A newly designed transducer holder and special grips have greatly improved the reliability and reproducibility of the measurements. A block diagram for the experimental setup is shown in Figure 1. This technique requires a tone burst signal generator and a power amplifier to launch longitudinal sound waves into the sp
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