Measurement of the tensile and compressive properties of small diameter (<0.7mm) carbon fibre rods

The Problem

It was necessary to measure the tensile and compressive stress/strain curves to failure, and Poisson's ratio, of a novel type of reinforcing rod, composed of carbon fibres in an epoxy matrix. The rods were only 0.7mm in diameter, and the test gauge length in compression was less than 2mm. Strain gauges or extensometers could not be fitted to such small samples.

The Solution

Rod samples were bonded into steel carriers and mounted in a test machine either in a die set for compression or into the machine jaws for tension. Four targets were used to pick up Poisson�s ratio as well as longitudinal strains.

Targets were white lines applied to specimen edges on a thin foil, appearing as dots viewed from the front. Tension and compression data were used to construct the complete stress / strain curve shown here

Advantages of the Video Gauge

The use of the Video Gauge allowed the measurement of both tensile and compressive properties and Poisson�s ratio to allow a master curve to be generated for the carbon rods. Conventional extensometry would not have been able to cope with such small samples, especially in compression and for the measurement of Poisson�s ratio. The compressive samples were rather inaccessible being held between steel carriers in a die set in a test machine, but so long as a video image could be extracted with a telephoto lens good quality measurements could be achieved.