Significant Strain Gradients Research Articles

Deformation at the nanometer and micrometer length scales: Effects of strain gradients and dislocation starvation

Nanomechanical devices are certain to play an important role in future technologies. Already sensors and actuators based on MEMS technologies are common and new devices based on NEMS are just around the corner. These developments are part of a decade-long trend to build useful engineering devices and structures on a smaller and smaller scale. The creation of structures and devices calls for an understanding of the mechanical properties of materials at these small length scales. Here we examine some of the effects that arise when crystalline materials are mechanically deformed in small volumes. We show that indentation size effects at the micrometer scale can be understood in terms of the hardening associated with strain gradients and geometrically necessary dislocations, while indentation size effects at the nanometer scale involve the concepts of dislocation starvation and the nucleation of dislocations. We also describe uniaxial compression experiments on micrometer size pillars of single crystal gold and find surprisingly strong size effects, even though no significant strain gradients are present and the crystals are not initially dislocation free. We argue that these size effects are caused by dislocation starvation hardening, with dislocations leaving the crystal more quickly than they multiply and leading to the requirement of continual dislocation nucleation during the course of deformation. A new length scale for plasticity, the distance a dislocation travels before it creates another, arises naturally in this treatment. Hardening of crystals smaller than this characteristic size is expected to be dominated by dislocation starvation while crystals much larger than this size should exhibit conventional dislocation plasticity.

Modified Transfer Matrix Formulation for Bragg Grating Strain Sensors

This paper presents a formulation for the application of the transfer matrix method to Bragg grating strain sensors. A modified T-matrix representation is detailed for the sensor problem based on an effective period derived from the coupling coefficients. This modified T-matrix formulation is shown to converge to the coupled-mode equations solution for a large number of grating segments, even in the presence of significant strain gradients. Several numerical examples are presented to demonstrate the importance of inclusion of the strain gradient in the calculation. In addition, the current formulation is validated by application to previously published experimental data.

Experimental investigation into early age shrinkage of cement paste by using fibre Bragg gratings

A new technique based on fibre optics has been utilised for investigating early age shrinkage of cement paste. Fibre Bragg gratings are strain sensors within optical fibres which were embedded here in cement paste specimens. In this way, shrinkage strains could be measured starting from the beginning of the setting. Sufficient bond strength between the sensor fibre and the surrounding early age cement paste has been proved in pull-out tests. Influences of the w/c-ratio, the curing conditions as well as the specimen geometry were under investigation in the experiments. The significant influence of the drying conditions on the shrinkage strain during the first day after casting is demonstrated. Whereas the early age shrinkage appeared to result in a rather uniform deformation the subsequent drying shrinkage caused significant strain gradients in the specimens.

Local Strain in Corrugated Pipe: Experimental Measurements to Test a Numerical Model

Abstract Measurements of local strain in corrugated steel and high-density polyethylene (HDPE) pipe are used to test the effectiveness of a three-dimensional finite element analysis. Measurements were made at the horizontal diameter for the pipes subjected to parallel plate loading, a load condition that produces significant strain gradients around the pipe as well as through the corrugation. The test values are used to evaluate the performance of elastic analysis for both steel and polyethylene pipes, as well as viscoelastic analysis used for the polyethylene pipe alone. It is demonstrated that the theoretical model generally yields estimates of local strain within 5 to 15% of the measured values. The viscoelastic predictions of local strain through a load-unload cycle on the HDPE pipe were very good. The load-deflection response of HDPE is predicted well for monotonic loading, but improvements in the viscoelastic model for HDPE appear necessary to improve predictions through a full load unload cycle.