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 shortand 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.

Micromechanical behavior of cortical bone as inferred from laser speckle data

“…The laser speckle strain rate measurement technique employed here was described earlier, [3][4][5] and therefore will be only summarized here. The concept is based upon observing translating laser speckle with a linear array charge-coupled device (CCD) camera (i2S, IDC161-BC-8, 10.7-m pitch) normal to the specimen as the specimen is illuminated sequentially at equal but opposite, off-angle directions of ± s with respect to the sample normal.…”

“…In addition, if the rigid body terms of Equation 2 are large with respect to the strain term, this may also present a limitation to the technique. As indicated by Duncan et al, 4 the rigid body terms need to be kept at least to the same order as the strain term. The magnitude of the rigid body terms can be investigated by adding the slopes of the data in the frequency domain rather than subtracting them, as was done above.…”

“…A number of these noise sources have previously been identified and accounted for. 4 These noise sources included temperature transients in the laser, causing laser wavelength drift, temperature transients in the camera structure, barometric pressure fluctuations, and deviations from symmetry in the experimental setup. The effects of these noise sources can be mitigated by allowing sufficient warmup time for the equipment, employing a thermoelectric cooler for the laser and through care in the optical setup.…”

“…The effects of these noise sources can be mitigated by allowing sufficient warmup time for the equipment, employing a thermoelectric cooler for the laser and through care in the optical setup. Duncan et al 4 demonstrated that the effects of slight deviations from symmetry are benign. Care must be taken, however, to limit these deviations.…”

“…Variations of this technique have been applied previously to stainless steel, 3 titanium reinforcement fibers, 4 orthodontic wires, 5 and restorative dental materials. 6 In these applications, the technique was shown to be both accurate and precise.…”