Ultrasonic Measurement of Axial Stress

Abstract

Abstract Several applications of ultrasonic techniques are gaining acceptance for accurate preload determination of critical fasteners. In this presentation, one class of techniques based on continuous wave ultrasonics is examined in detail, with benefits and limitations discussed as they apply to accurate determination of fastener preload. The theory of acoustic propagation in strained media is reviewed as it applies to bolt geometries. Propagation measurements in these geometrics require an understanding of guided-wave effects which can influence stress determination. However, studies of the acoustic spectra of rods identify diffraction and resulting mode conversion as the main artifact for such a measurement. The acoustic spectra, obtained with a novel tone burst spectroscopy method, clearly show mode conversion effects at low frequencies to be absent at high frequencies. Changes in the acoustic spectra are shown to accompany applied stress and suggest an instrument technique for measuring changes in fastener strain. The method, called a Reflection Oscillator Ultrasonic Spectrometer (ROUS), is discussed with an improved technique called Pseudo Continuous Wave Method (PCW) described. Data presented are obtained with the PCW for strain in several engineering materials. Bolt geometry is shown to have an influence on the stress measurements. In particular, percent fastener length under load is evaluated for its effect on material strain measurements. A computer model predicting a linear dependence on the percent of fastener under load is verified using the PCW technique. Other aspects of bolt geometry are discussed including poor geometry bolts, bolts with transverse holes, and bolt-loading anisotropy. Experimental techniques for performing measurements on fasteners with poor geometry are discussed.