Performance of Strain Gauge in Strain Measurement and Brittle Coating Technique

Introduction

Engineering structures are subjected to various loading conditions that results in the development of stress and strain. In the same way, the structures which are in the path of flowing fluid is more susceptible to distortion as a result of the high fluid current, which further causes the concentration of stress and strain on the surface of component (Karthik & Dhas, 2015, 2016, 2018). The stress evaluation is essential for the mechanical components to validate the design and the vibration and thermal effects can also cause stress in the materials and possess challeges to the mathematical methods (Balaji & Yadava, 2013; Balaji et al., 2015; Leblouba et al., 2015; Balaji et al., 2016a; 2016b;). Hence the strain gauge is considered more relialbe techniuque in many of the practical applications. The strain gauge system consists of a metallic foil supported in a carrier and bonded to the specimen by a suitable adhesive. Previous chapters discussed the construction, configuration, and material of the strain gauge. The strain gauge has advantages over the other methods that a strain gauge can give directly the strain value as output however in optical methods, and it is required to interpret the results. It is also required to be aware that the strain gauge technology is majorly used, and it can also be easily wrongly used. Hence it is required to obtain the proper knowledge of the strain gauge to get the full benefit of the technology (Karl., 2000). This chapter covers the majorly on the performance of the strain gauge, its temperature effects, and strain selection, and special gauges for various applications. Further this chapter also covers the brittle coating technique which is used to decide the position of the strain gauge in the applications.

Temperature Compensation in Strain Gauge

In the strain gauge instrumentation, it is required to exhibit ample care right from the selection of the strain gauge until the measurement of strain to obtain the results that can be useful (Hannah, 1992). Any error in the process can easily result in an error in strain value from the strain gauge, and they may not represent the real specimen strain condition. One of the main factors that affect strain reading is the temperature effect. The temperature effect has to be compensated in the strain reading or by bridge circuit connection methodology. This section covers the techniques to handle the temperature effects properly. The strain gauge resistance change due to temperature is given as,

(1) where S_T is gauge sensitivity to temperature, α_s is the thermal coefficient of expansion of specimen material and α_t is the thermal coefficient of expansion of gauge material. Eq.(1) shows how the change in resistance is influenced by temperature. The first term is the differential thermal expansion coefficient between gauge material and specimen. The second term incorporates the change in resistance from the temperature change. There are two methods in which the temperature effect is compensated. Firstly, by adjusting the gauge parameters, so the terms in Eq.(1) cancels each other, and Eq.(1) becomes zero. For this, certain design changes in material or gauge parameters are made in the strain gauge so that the temperature is compensated. The compensation using this method is called Self Temperature Compensation. The strain gauges are generally tailored made for an application so that the temperature change is adjusted by itself without manual involvement. Secondly is by cancelling the influence of the temperature by means of signal conditioner. Wheatstone bridge circuit can be configured suitably so that the overall temperature change is nullified from the resistance ratio.