Strain Measurement

The strain measurement along with stress measurement is an essential component of the measurements during the testing. The stress-strain variation depicts the behaviour of the composite material under test. It gives the information about the linear and non-linear behaviour. This information is very essential when one is measuring the other properties like moduli, Poisson’s ratios and other engineering properties. When one is deriving the engineering properties from these behaviours then the initial linear portion of the behaviour must be used. This is because in this initial linear behaviour it is assumed that there are no damages due to the loading and plasticity effects, if any, introduced.
 There are three commonly used methods to measure the strains. The strains can be measured using extensometers, strain gages and optical methods.

1. Extensometers: They provide average strain over a finite length, typically of the order 1 in (25.4 mm).

These are used primarily for measurement of axial strains. They can also be used to measure transverse strains.

2. Strain gages: The strain gages can be uniaxial, bi-directional and rosettes. They measure strain in one, two and three directions, respectively.
3. Rosettes: They provide a complete description of the average strain over the region of measurement through strain transformation equations.

The strain gages are available in a range of sizes with smallest of the order 1.59 mm bidirection and rosettes can be stacked or adjacent

The length of the gauge may be specified by the relevant standards, but should always be significantly shorter than the gauge length of the specimen. Composites can cause particular difficulties not encountered with metals. The issues that must be addressed are as follows:

1. High gauge resistances are desirable because high voltages (2–4 V) with low current can then be used; this improves hysteresis effects and zero load stability.
2. If possible, use gauges with lead wires attached, or solder wires to the gauge before installation; this should avoid soldering damage to the composite.
3. Ideally the pattern of the autoclave scrim cloth should be removed before gauge installation; this is particularly important if contact adhesives are used.
4. Corrections may be necessary to gauge transverse sensitivity effects; errors of over 100% between actual and measured strains can be obtained.
5. Gauges must be precisely aligned; errors of 15% can result from a  misalignment. There is no universally acceptable way of ensuring alignment. The scrim cloth pattern can be misleading. Sometimes C-scan after installation can be useful or checking with failure surfaces after fracture.
6. Dummy gauges are the preferred method for temperature compensation but, again, precise alignment is needed. It is necessary to mount the dummy gauges on an ‘identical’ piece of laminate, with the same orientation relative to the fibres as used for the active gauges.