PSI - Issue 60

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Vaibhav Gangwar et al. / Procedia Structural Integrity 60 (2024) 123–135 Vaibhav Gangwar / StructuralIntegrityProcedia00(2024)000 – 000

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3.1 Procedure for strain gauge calibration For testing at higher strain rates, the selection of strain gauge is based on material elongation at lower strain rate (10 -1 s -1 ) which is usually less than 20%. The percentage of elongation of armor steel goes on increasing with strain rate as shown in Fig. (1). Therefore, high elongation strain gauges are used (to measure more than 20% strain) along with the high elongation adhesive. The strain gauge is pasted directly over the gauge length of specimen. The wire of the strain gauge is connected to the data acquisition system. The raw data is giving in the form of voltage. This voltage signal is converted into strain by suitable calibration of the strain gauge with the material under study. This require the calibration of the strain gauge using an extensometer at a quasi-static strain rate to verify the relationship exist between voltage output and extensometer displacement (P. K.C. Wood et al. 2007b). Different metals, alloys, and gauges will introduce different levels of steady strain rate into the specimen gauge length for a given test speed. Although it is desirable to minimize such strain rate variations as a function of specimen resistance, there are practical limitations to what can be achieved. Since the strain evolving stain rate can be calculated for each test using the specimen gauge length, it is crucial to calibrate both strain measuring devices(P.K.C. Wood and Schley 2009). The specimen is first calibrated up to the yield point at a quasi-static strain rate for this the strain gauge and extensometer are attached in the sample gauge length, as shown in Fig. (2). The DAQ gives signal in form of voltage vs time Fig. (3a) displays the filtered voltage vs. time curve along with the extensometer displacement vs. time plot. To determine the exact straining in the gauge section of specimen the signal is filtered without altering its original characteristics. The filtered voltage vs. extensometer displacement is then plotted, and a calibration constant 0.087 is obtained from the curve fitting shown in Fig. (4). The fitting is linear because the data obtained appears to be linear at both ends and also for convenience the potential difference versus extensometer data are fitted as linear.

Fig. 1. Elongation of armor steel with strain rate

Fig.2. Armour material tensile sample with strain gauge and extensometer for calibration of strain gauge