Co-authors​ :

 Ahker Gunes DILBER (TURKEY), Basak OZEROGLU (TURKEY) 

Composite materials are used commonly in aerospace industry because they provide remarkable mechanical characteristics compared to conventional materials like steel alloys. The major interest on the mechanical behaviour of the composites is generally on the stiffness because of the importance of the structural integrity of the designs, and the material stiffness can be evaluated through quasi-static tensile tests at coupon level. The stress-strain response of the tensile specimens is observed from tensile testing machine and strain measurement device. The tensile force data is consistently gathered from the tensile testing machine and the stress values are calculated via the force data, however, the number of the instruments on the strain measurement of tensile test coupons is significantly high. The strain measurement methods vary in different approaches such as optical techniques, contact extensometers, and electrical strain gauges. The suitable strain measurement method should be chosen because accurate data is needed to determine the material stiffness for structural integrity assessments. In this study, digital image correlation, advanced video extensometer and contact type biaxial extensometer devices are used to evaluate the performance of the different strain measurement techniques by admitting the strain gauge as the reference measurement method thanks to its versatile and reliable features. Tensile tests were conducted according to ASTM D3039, and the values were gathered from the specimens simultaneously for each evaluated technique and strain gauge. Two different material orientations which are fibre and matrix directions were evaluated. Three different mechanical properties that Chord Modulus, Elastic Modulus and Poisson’s Ratio were calculated for each technique and material orientation. The values obtained from each technique were compared with the strain gauge data. The results provided significant outcomes about the performance of the measurement methods with respect to the strain gauges. Initially, it was found that DIC technique is the best approach among the techniques to catch the transverse deformation of the tensile test specimens according to the variation of the Poisson’s Ratio values with respect to the strain gauge results. Moreover, all studied technique provides sufficient Elastic and Chord Modulus response, however, the biaxial contact extensometer shows the highest variance for the matrix direction modulus values while the variance of the modulus values gathered from the video extensometer is the highest for the fibre direction. In conclusion, the measurement techniques studied in this work supply sufficient axial strain measurement, but the transverse strain measurement ability of both extensometers is not observed as sufficient enough. In addition to quasi-static tensile tests, the fatigue testing can be studied for the future works including different material orientations, ply stacking, and material types.