In-situ measurement of shear strains in bonded joints

The Problem

To understand the structural response of adhesive bonded joints requires that the shear strains in the adhesive to be measured at various points along the bond line. Specialised extensometry can be used to measure shear strains in adhesive joints, but measurement of these strains in real joints is difficult to achieve reliably. The adhesive is strain-rate sensitive so it is important to be able to capture the data in real time without having to stop the test and permit stress relaxation.

The Solution

Small target spots were painted on the adherends at various positions along the length of the joint. To maximise sensitivity the relative movement between the inner and outer adherends was measured at each position at a higher magnification than is shown here.

The data shown has been corrected for shear strains in the adherends. As expected the shear strain in the middle of the joint is lower than at the ends, but increases towards the end of the test as the adhesive yields throughout the whole of the joint.

Advantages of the Video Gauge

Whilst it is possible to purchase special extensometers to measure shear strains in adhesive joints they are limited to that specific application, are relatively costly to acquire and can be delicate. The Video Gauge permitted the same results to be obtained from a piece of general purpose equipment with minimal specimen preparation and allowed measurements to be taken to the point of failure without any concerns for damage to the measurement equipment. It also enabled results to be obtained at elevated temperature.