In test and measurement applications, the mechanical properties of a material are vital for design studies, finite element, and modeling calculations. They are also crucial for providing an indication of the measurement uncertainty obtained through the application of the proposed experimental methods. Strain measurement is an essential experimental approach in the mechanical testing process. There are a huge variety of methods available to test strain both electrically and mechanically, including extensometers, electromechanical testing machines, and strain gauges. Due to their high accuracy and low-costs strain gauges have become quite popular in a great majority of applications.
Installing a strain gauge is a delicate task performed through a series of guidelines provided by the vendor. However, with so many design options available, selecting an appropriate strain gauge sensor is equally an important task as installing it. A careful and rational selection of gauge characteristics and parameters is very crucial in optimising gauge performance for specific environments and operating conditions, getting better accuracy in results, and minimising the overall cost and effort of installation.
In this article, we are going to discuss some factors that need to be considered while selecting a strain gauge to optimise the overall strain measurement process.
Strain Gauge Construction
There are a variety of strain gauge geometries available in terms of grid positioning, shape, number, and orientation. These design configurations are meant for measuring strain developed due to stress exerted in different directions. These designs include linear, double linear, T-rosette, shear, full-bridge, and many other variants of strain gauges applicable for a full range of gauge installation and strain measurement requirements.
A linear strain gauge is typically mounted in the direction of the main force, whereas the double-linear configuration is made of two linear strain gauges and is designed for double-sided mounting on a bending beam. A rosette configuration consists of two or more strain gauges aligned together at certain angles to measure strain developed from an unknown principle direction. It is preferred for strain measurement in tensile or compression bars. A shear strain gauge is used to measure torque in a rotating object or a shear beam and is designed with two grids at an angle of 45 degrees from the torsion shaft. Other configurations of strain gauges are designed based on further advanced geometries and configurations of rosette, linear and shear strain gauges to serve complex applications. Therefore, depending on the direction of strain and the structure of the measuring object, a strain gauge design should be selected.
Gauge Length
The gauge length of a strain gauge has a huge impact on strain measurement. It is the measure of the actual grid length in the sensitive direction. Strain gauges with small gauge lengths are generally employed for measuring stress peaks. Selecting a gauge with a shorter gauge length will be suitable for surfaces with small mounting space, where accuracy is not a critical aspect. It is ideal for measuring strains on a fillet, hole, or notch with a small diameter. On the other hand, a strain gauge with larger gauge lengths can be employed for strain measurement on inhomogeneous materials such as concrete. Generally, in these inhomogeneous structures, the measure of average strain within the measuring grid range is sought. With larger gauge lengths, it is possible to span several pieces of aggregate in order to measure the representative strain in the structure.
Electrical Resistance of Strain Gauge
The gauge resistance of a strain gauge is the measure of the electrical resistance of the gauge at room temperature under no external stress. The generally available gauge resistances are 60 Ω, 120 Ω, 350 Ω, and 1000 Ω. The strain gauges with electrical resistances 60 Ω, 120 Ω can be considered as low resistance strain gauges, the 350 Ω and 1000 Ω variants can be categorised as high resistance strain gauges.
The choice of resistance of a strain gauge is dependent on the overall size of the gauge grid, cost, and certain performance parameters. A higher resistance gauge is preferred when the requirement is to reduce the heat generation rate. This is due to the fact that the reduction in the current flow associated with the higher resistance reduces gauge self-heating. However, a strain gauge with low resistance is more likely to develop self-heating at a given excitation voltage for the Wheatstone bridge circuit as compared to the high resistance strain gauge. It also requires a higher power to operate, which is why it is advised to go for high resistance, preferably 350 Ω or 1000 Ω, if the Wheatstone bridge is battery operated. On the other hand, a low resistance strain gauge offers advantages including a lower influence of a change in the isolation resistance and a lower influence of electromagnetic interferences.
Temperature Compensation
Temperature changes can have huge impacts on the strain measurement of an object. This is mainly due to the fact that the material used for constructing the object expands as the temperature increases. For this reason, self-compensated strain gauges are employed for operations with inconsistent temperatures. These strain gauges are developed to compensate for the temperature behaviour of certain materials such as plastic, steel, aluminium or titanium by their own temperature behaviour. Therefore, it is important to select a strain gauge having a similar temperature response as the material.
Other Considerations
The adhesive creep using which a strain gauge is bonded to the measuring object also gets influenced by increased temperatures. The adhesives become soft, thereby limiting the transfer of strain from the object to the gauge. Hence, it is also important to observe the temperature limits of the adhesive and to choose them appropriately for installing strain gauges. Based on the type of application and operating environment, a strain gauge can be connected to the test object either by bonding or welding.
For specific applications, strain gauges are also custom-designed to suit certain measurement requirements. If you are still unsure on how to properly select the correct strain gauge that suits your requirements, please contact us.
