123 The results demonstrated that for a 4500 N initial bolt preload the hot melt adhesive-based bolt tension monitor exhibited the creep in the embedding thermoplastic itself, which is not desirable for long-term measurements. However, if a single-parameter approximation (see Eq. 13.1) is used and the relaxation parameter is assumed to be extensive property, the relaxation parameter of the studied configuration can be obtained by subtraction of the embedding thermoplastic relaxation parameter from the measured one. It was found that for the PLA-based bolt tension monitor, the PLA relaxation parameter slightly increased for higher initial preload. An important finding is that the PLA relaxation parameter was considerably lower than that of hot melt adhesive. This allowed more precise evaluation of the relaxation parameters for steel-GFRP and GFRP-GFRP bolted joint configurations. It was demonstrated that for both the 4500 N and the 8000 N initial preload cases, the relaxation parameter for the bolted joint with two composite panels was nearly twice as large as the relaxation parameter compared with a bolted joint with a single composite plate. This observation can serve as an indication that the relaxation parameter is an extensive property. Although the average values showed reduction of the relaxation parameter with increased preload, more experiments with higher preloads and longer monitoring times are required to confirm this trend. Finally, it was demonstrated that the presence of the thru-hole load cell in the assembly tends to reduce the measured relaxation parameter of the GFRP composite. This can be explained if the thru-hole load cell acts as a spring washer, which partially offsets the reduction of the preload, hence artificially decreasing the actual relaxation parameter. The results of these tests demonstrate that the single-parameter approximation used in the study provided good fitting of the experimental data and allowed simple case-by-case comparison, provided the relaxation parameter is proven to be an extensive property. The comparison of relaxation parameters for various configurations further confirms that the chosen approximation was valid. The next step is further validation of the proposed hypothesis for multi-material configurations under various conditions, and more detailed research on the limits of the chosen approximation. Overall, this study aims to assist in the development of a single procedure for simple, robust and cost-efficient characterization of preload relaxation that is caused by creep in bolted joints. Future work includes the use of distributed fiber optic sensors and long-term strain measurements, which can be accomplished if temperature and strain effects are discriminated. 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