Material testing

The Video Gauge is well suited to:

• Batch testing
• High temperature testing
• Harsh environments (chemical/radioactive)
• High strain > 100%
• High speed (impact) testing using high-speed cameras
• Very small (<1mm) or delicate samples
• Biological samples

The Video Gauge is able to measure all of the following standard material properties:

• Strain
• Shear strain
• Poisson’s ratio
• Stress – Strain curve
• Modulus
• Proof stress, ultimate stress, ultimate failure strain

We offer a number of lens options that are specifically for material testing. They offer outstanding performance with resolutions better than 5με and gauge lengths between 5mm and 20mm. Our standard lenses are also suitable for material testing, offering resolutions to 20με and gauge lengths from less than 1mm to over 50mm.

Multiple measurements can be made simultaneously and in real-time at 15Hz. By using multiple cameras, measurements can be made on several faces of the sample.

The voltage input/output module can be used to feed load or displacement outputs from the test machine into the Video Gauge where they are logged along side the Video Gauge’s measurements. Similarly, strain measurements made by the Video Gauge can be fed via the voltage outputs to your existing data-logging equipment or into your test machine for feedback control.

Case studies

	Residual stress measurement at the micro-scale The aim of this project was to measure residual stresses within metal samples at the micro-scale. This required measurement of surface deformations in the order of a few nanometers. Images produced with a Focused Ion Beam (similar to a scanning electron microscope) were analysed by the Video Gauge in order to make these measurements. (Image is Copyright 2008 Nicholas Daynes, Graeme Horne, Dept. of Mech. Eng., University of Bristol. Reproduced by permission.)
	Accuracy of Video Gauge for strain measurement Strain gauges are the standard way to generate high quality and reliable strain measurements in both sample testing and full scale structural tests. In this straightforward experiment Video Gauge strain measurements are compared directly to strain gauge readings in order to validate the Video Gauge as a technique for accurately measuring strain.
	Extracting strain data from high speed video When carrying out tests at high speed it is very difficult to extract reliable data on strains and displacements. Conventional solutions such as strain gauges tend to become debonded before the test has proceeded very far and a large number of expensive tests may be needed before a valid result can be obtained, making it extremely difficult to obtain statistically reliable data.
	Measuring the shear component in three point bend testing of composite beams 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.
	Measurement of the tensile and compressive properties of small diameter (<0.7mm) carbon fibre rods 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.
	Comparison of strain gauge, clip-on extensometer and Video Gauge outputs in the tensile testing of steel samples To generate confidence in any new measurement technique it is necessary to carry out trials unambiguously comparing the new technique with established and accepted standards.
	Measuring size effects in the strain to failure of cast adhesive samples 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.
	Measurement of the consolidation and shrinkage processes in composites manufacture It is very important in controlling the processing of advanced composites to develop an understanding of the consolidation and resin shrinkage processes throughout the cure cycle. For typical aerospace epoxies this requires the application of pressure up to a few Bar and temperatures up to about 180°C to a prescribed cycle. Equipment is available to do this such as Thermo Mechanical Analysers (TMA), but their sample sizes are very small and interpreting their outputs can be problematical.
	In-situ measurement of shear strains in bonded joints 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.