Scanning Acoustic Microscopy and X-ray Diffraction Investigation of Near Crack Tip Stresses
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ABSTRACT Scanning Acoustic Microscopy has been used to measure and map the Rayleigh wave velocity and the Surface Skimming Longitudinal wave velocities near a crack tip in a sample of Ti-6AI-4V. X-ray diffraction measurements have been performed to map the stress in the same region of the sample. The differences in the contrast between the two acoustic velocity images and their sensitivity to stress are examined. Similarities between x-ray stress images and acoustic velocity images are discussed.
INTRODUCTION A plastically deformed zone is formed ahead of a crack tip in metallic materials. An understanding of the size, stress distribution, deformation of the material in the plastic zone just ahead of the crack tip is important for predicting the nature of the growth of the crack. Many experimental techniques have been utilized to acquire information about the plastically deformed zone [1, 2, 3, 4,5]. Nondestructive measurement techniques of x-ray diffraction and ultrasonic velocity measurements have been used to determine the size of the plastically deformed zone, the stress distribution and the length of the crack. In the x-ray diffraction measurements, the x-ray spot size limits the resolution. Ultrasonic techniques have a similar resolution restriction, and also they measure an average over the distance of propagation. More over most of the ultrasonic techniques that have been used to perform stress distribution around a crack tip have been through transmission techniques. In this case the stress determination is averaged over the thickness of the sample and over the diameter of the transducer. Use of focused acoustic beams can decrease the spot size limitation in acoustic measurements and improve the resolution. Such acoustic microscopes have been used for measurement of stress variations [6,7]. But these have been limited to measurement in two orthogonal directions using line focus beam lens [6,7]. Although high precision measurements of stress variations have been obtained with the technique it has never been used for measuring stress distribution. An acoustic microscope with a special lens capable producing both longitudinal and shear waves simultaneously and a double pulse electronic instrumentation has been utilized to measure stress distribution in microelectronic components [8]. A spatial resolution in the range of 5 -10 gmm and a stress resolution of 0.3 MPa have been achieved with such a system. This paper presents the use of an acoustic microscope technique based on impulse excitation for measuring and making acoustic velocity images from both the Rayleigh Surface Wave (RSW) and the Longitudinal Surface Skimming Wave (LSSW) velocity in the region ahead of a crack tip in a sample of Ti-6AI-4V. The variation of the velocity is compared with a two-dimensional x-ray diffraction stress image over the same region.
55 Mat. Res. Soc. Symp. Proc. Vol. 591 02000 Materials Research Society
EXPERIMENTAL TECHNIQUES Scanning Acoustic Microscopy A small change in the velocity of sound is observed when a stress i
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