An examination of techniques based on two strain gages for the determination of mode I stress intensity factor in orthotropic composite materials

In this work, we have examined the single strain gage technique proposed by Chakraborty et al. (2014) (CMC1) for the accurate determination of mode I stress intensity factor (SIF) in orthotropic composite materials. Numerical simulations carried out on single edge crack specimen with different values of the height to width ratio ($h/b$) and of the crack length to width ratio ($a/b$) have shown the limits of this technique and that the use of a single strain gage is not always reliable to provide precise measurements of SIF, $K_I$. To avoid such a situation, we have proposed two new techniques based on two strain gages named CMC2 and M_CMC2. The CMC2 technique is a natural extension of the CMC1 technique and the M_CMC2 technique is a modification of the CMC2 technique. Accordingly, general finite element approaches are developed to estimate the extent of the valid region of these two techniques. The results of the numerical simulations show that the two techniques CMC2 and the M_CMC2 can give a very accurate value of $K_I$ when the two strain gages are placed within valid region. Also, these results show that these two techniques provide good solutions to ensure an accurate measurement of $K_I$ when the technique of CMC1 fails to provide the desired precision and accuracy of $K_I$. In addition, the results derived from experimental data (Chakraborty et al., 2017) proved the effectiveness of the CMC2 technique.