Measurement of strain rates in reinforcement fibres

Abstract

We have developed a highly sensitive method for measuring thermal expansion, mechanical strain and creep rates. We use the well-known technique of observing later speckle with a linear array detector, but employ a novel data processing approach based on a two-dimensional spectral transform of the speckle history as recorded by the detector. This technique can effect large gauge sizes, which are important in the assessment of the spatial statistics of creep. Furthermore, although the measurement approach uses objective (non-imaged) speckle, the algorithm provides simultaneous global estimates of the strain rates at both small- and large-scale sizes. Such estimates may be of value in investigating materials with different short- and long-range orders. General advantages of our technique are compact design, modest resolution requirements, insensitivity to slow surface microstructure changes (as seen with oxidation) and insensitivity to zero-mean-noise processes such as turbulence and vibration. Herein we describe the theory of our processing algorithm, present results of measurements of strain in titanium wires and discuss the resolution limits of the measurement technique and subsequent data processing.