Measuring size effects in the strain to failure of cast adhesive samples

The problem

Peak strains in structural adhesive bonds are highly localised. In order to develop adequate methods for the prediction of failure in bonded joints the effect of changing the volume of stressed material on the stress and strain at failure must be found. To do this three sample sizes were taken and a stress / strain had to be measured for each one.

The smallest samples were far too small for strain gauges or extensometers (0.75mm wide, 3mm gauge length, 0.5mm thick). Strain gauges also give over-stiff results as the gauge stiffness is much higher than the adhesive�s. Strain gauges and extensometers also give premature failure due to stress concentrations.

The solution

The use of the Video Gauge allowed the size effect in strain to failure to be established (and also allowed Poisson�s ratio to be measured at each size out to high strain values). Adhesives are strain rate sensitive materials so the measurements had to be performed in real time without stopping the test at any point.

Advantages of the Video Gauge

This is a case in which the characteristics of the Video Gauge, (scale insensitive, non-contact, real time, multipoint strain measurements) uniquely matched the requirements of the task to be undertaken with minimal effort being required to achieve excellent results and demonstrate previously unrecognised aspects of materials� performance.