## 4 Results and Discussion
It can be seen that the \(V_{e}\) increases as the response of increased \(V_{m}\) and vice versa. However, the \(V_{e}\) the response is not perfectly stable. This can occur due to the movement of fiber arrangements inside the plain-woven CFRP, which might not be a linear response as discussed in sub-Section 2.1. To investigate this nonlinearity further, \(V_{m}\) is then converted to \(\varepsilon_{m}\) by using Equation (10), whereas the \(V_{e}\) is converted to \(\varepsilon_{e}\) by using Equation (11). Figure 8: Electrical resistivity of plain-woven CFRP in thickness and wrap directions. Figure 5:0 Example of voltage signals obtained from the strain gauge and woven CFRP under loading and unloading conditions. Figure 5:1 (**a**) contact resistance values and (**b**) tensile strength of plain-woven CFRP for different polishing treatment. Figure 11a shows one example of data processing of voltage signals (\(V_{m}\) and \(V_{e}\)) to \(\varepsilon_{m}\) and \(\varepsilon_{e}\). It may be observed that the residual \(\varepsilon_{e}\) appears when \(\varepsilon_{m}\) equals 0 in the unloading process. The results of all seven data processing of voltage signals are then plotted as shown in Figure 11b,c. From the figure, it can be observed that \(\varepsilon_{e}\) is proportional to \(\varepsilon_{m}\). This proves that the positive piezoresistivity in the woven CFRP can be observed with Wheatstone bridge circuits, similar to unidirectional CFRP as reported by [PERSON] et al. (2018). However, for the plain-woven composite subjected by loading, the variance of \(\varepsilon_{e}\) becomes higher when the higher loading is applied. In particular, the variance is clearly seen when the \(\varepsilon_{m}\) reaches 0.0002. This may occur because when the loading is applied, the fibers having plain-woven pattern have more different possible arrangements which directly determine the \(R_{CFRP}\). Upon unloading, plain-woven composite only displays a small reduction of the variance. There is hysteresis appearing in the measurement. This result indicates the fibers do not return back to the initial position after unloading. Thus, residual \(\varepsilon_{e}\) of 0.0016 was observed when \(\varepsilon_{m}\) returned back to 0. For SHM development, \(G_{CFRP}\) is a parameter that needs to be investigated. The variance of the \(G_{CFRP}\) must be considered in the development. In this work, the linear least square method is used to analyze the data obtained because this method allows researchers to study how the variables are connected and related by placing the line's equation as close as possible to all data. In Figure 11, \(\varepsilon_{m}\) is the independent random variable or predictor and \(\varepsilon_{e}\) is the responsible variable. The linear least square equation shows how both variables are connected in a mathematical way. Here, confidence interval theory, usually expressed in percentage, is also used to ensure the processed data is statistically valid. For example, the commonly used interval, 95% confidence interval is a range of values that the researcher can be 95% certain it contains the true mean of the population or when the experiment is repeated over and over again, 95% of the results will match with the results from the population. Thus, by using this method, it will ensure the obtained and processed data is statistically valid. It is important to note that the calculation is divided into two cases, loading case and unloading case. This has to be done because there are different characteristics appearing which depends on the loading condition. Table 1 shows a summary of the gauge factor calculation. The gauge factor during loading conditions is higher than the unloading condition. Figure 5:2 Electrical and mechanical strains relationship for plain-woven composite: (**a**) example of one loading and unloading case, (**b**) seven test results of specimens subjected to loading and (**c**) unloading. The occurrence of electric strain variance must be considered in SHM development for plain-woven CFRP composite.