Development of Photoelastic Methods Towards Study of Mechanical Aging of 2-Dimensional Granular Assemblies.

Abstract

Phototelasticity is a well-known method for experimental stress analysis of solid transparent materials. It has commonly been used to measure stresses from the analysis of photoelastic fringes. In this study, a test apparatus was built for geotechnical engineering research in which both the specimen and sensors were of photoelastic material. Digital image processing of light color intensities was used to analyze stress in the specimens. Glass particles were used to model the sand grains. The shapes of the particles included cylinders and angular prisms.

Glass plates with a central hole were manufactured and used as sensors for determining horizontal stresses in 2D assemblies of glass particles. The horizontal stresses in the 2D granular assemblies were measured during loading and unloading, and with elapsed time under constant vertical stress. The horizontal stress was determined through changes in relative red intensity of the sensors. Experimental results show potential uses of the relative red intensity for measuring horizontal stress, and estimating Ko of particulate assemblies. Results showed that the values of Ko ranged from 0.30 to 0.84. Variations of the Ko values were attributed to unbalanced loading during the tests and swinging of a load frame. The use of the relative red intensity in glass plate sensors may be adapted to other engineering applications and scientific uses.

The calibrated sensors were used to monitor time-dependent increases in horizontal stress in 2D granular assemblies. Time-dependent surface settlement as a result of particle rearrangement in granular assemblies may be a critical factor in mechanical aging. Particle rearrangement results in time-dependent increases in horizontal stress in granular assemblies possibly via the formation of particle arches, through which forces are transferred horizontally.

Time-dependent changes under constant vertical stress depended on vertical stress, particle shape and particle surface roughness. In 2D granular assemblies, individual particles move as a response to forces transferred from the specimen surface. The angular prisms used in this study were observed to pack very efficiently. Particle surface roughness inhibits particle motions, but micro-interlocking in the granular assemblies may be developed with time.