Strain can be defined as the deformation of a material caused by an applied force and is measured by a change in length. It can be compressed or stretched. A strain gauge is a device that is capable of changing its resistance proportionally to its application, which can be caused by a number of external or internal influences, including temperature, pressure, or structural changes. The most commonly used instrument is a bonded metal strain gauge, which consists of a grid pattern of metal foils or filaments.
The grid is pasted onto the test sample and identifies the length change that occurs when a load is applied. This causes a change in the resistance measured by the circuit. Non-uniform materials can present various complexities in achieving accurate strain measurement.
Properties of heterogeneous materials
Until recently, most strain measurements were applied to metals such as aluminum and steel alloys. The current trend of replacing metals with composites and polymers has led to an increasing number of reinforced plastics with significantly different chemical, thermal and mechanical properties. A major difference between the physical properties of polymers and metals is the elastic modulus. Based on the volume of reinforcement and fiber materials, composites with plastic or polymer substrates can have more than twice the elastic modulus of metals.
As a result, the strain measurements of these heterogeneous materials are significantly greater than those of metals, often requiring special wiring techniques and strain-gauge bonding.
Compared with metals, the thermal conductivity of polymer-matrix composites can be as low as two orders of magnitude. When the measuring instrument and the test sample are deformed, the measuring resistance changes in response to the strain and produces a calibrated voltage shift. Strain gauges with higher resistance will result in less voltage heating generated. However, polymer-matrix composites do not conduct heat well and may allow heat to accumulate in the meter. Rising temperatures can lead to increased resistance, which in turn leads to strain measurement errors. In some cases, this may require temperature compensation.
Because some non-uniform materials are hygroscopic, they may contract or expand as their water content changes. These dimensional fluctuations are indistinguishable from the thermal output and lead to incorrect strain measurements. The hygroscopic properties of plastic composite matrix change significantly.
