Measuring the shear component in three point bend testing of composite beams

The Problem

The beams were made by embedding carbon fibre composite rods in a soft, polyurethane elastomer, matrix. The aim was to produce a beam that was very compliant against twisting but very stiff in bending and tension. Beams with those properties could be used in helicopter rotor hubs. In order to understand how the samples deformed it was necessary to measure the shear deformation at the ends of the beam, as well as the overall bending of the beam.

The Solution

The bend test set-up was as shown here, the information needed was the extent to which the ends of the sample deformed during the test.

Three targets (green dots) were used to allow the angles A and B to be monitored by the video gauge during a 3point bend test. Pure bending would lead to angle B remaining unchanged as angle A changes. Pure shear would lead to the change in angle the change in angle B being equal to that in angle A.

The measured relationship between angles A and B is shown here, over four cycles of loading and unloading.

It can be shown that about 25% of the deformation is in shear, and this can be done in real time as the experiment progresses.

Advantages of the Video Gauge

In this case there is no simple alternative to the use of the Video Gauge. A strain gauge could be used to measure the shear strains and this then would have to be combined with angle information calculated from sample deflection data to achieve the required results. The Video Gauge gave the required results, directly and with minimal sample preparation.