PSI - Issue 64

Mao Ye et al. / Procedia Structural Integrity 64 (2024) 1824–1831 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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the lap-shear strength. A similar phenomenon seems to occur in most structural adhesives; therefore, a conservatively lower activation temperature should be more favorable for the adhesive bond. Considering strengthening application scenarios and recommendations from the supplier, this study investigated an activation temperature ranging from 120℃ to 300℃. To investigate the influence of prestrain level and activation temperature on the stress recovery behavior, activation tests were carried out (test set-up No.2, Fig. 2(b)). The prestrained specimens (introduced in section 2.2) were subjected to thermal activation (heating and cooling process). An overview of the 18 specimens is summarized in Table 2 and will be detailed subsequently. A few tests were conducted to evaluate the influence of heating rate and initial preload. As shown in Fig. 2(b), the tests were conducted using an LSI uniaxial testing machine (load capacity 100 kN) with a furnace. The specimen was connected to the extension rods of the testing machine through pins. To monitor the surface temperature of each specimen, two PT100 thermal resistance sensors were attached to the middle and upper part of the gauge length, with Teflon tapes. A special extensometer comprising two ceramic rods, with a gauge length of 25 mm, was used to measure strain in the parallel section of specimens. Feedback from the extensometer was used for control of the specimen strain. Before the activation process (heating and cooling), specimens were tensioned to approximately 300 N, corresponding to 20 MPa, to prevent slacking and buckling due to the thermal expansion at low temperature. During activation, the testing machine was switched to constant strain mode. The furnace was manually controlled to heat the specimens with the heating rate of 12~24 ℃ per minute. Once the target activation temperature was reached, the furnace was opened to cool specimens down to RT (room temperature). Three specimens experienced a re-activation (2 nd activation) at an equal or increased activation temperature, to investigate the influence of a subsequent re activation on the recovery stress, possibly for the application of on-site recovery stress adjustment. Based on preliminary tests, verified with a 12-hour monitoring result, a waiting period (started at the beginning of cooling) of about 30 minutes was chosen, which should suffice to accurately obtain the recovery stress of specimens at RT. 3. Results and discussion 3.1. Stress-strain behavior (test set-up No.1) Test results of a tensile failure test specimen are presented in Fig. 3(a), while Table 1 listed the basic mechanical properties of both specimens. Stress-strain curves of typical prestraining specimens (of different prestrain level) are plotted in Fig. 3(b), while the results of prestraining are summarized in Table 2. As can be seen in Fig. 3, a clear pattern is that after the elastic stage, all specimens showed a stress plateau (similar to the yielding of steel). This stress plateau is generally located between the stress level of 560~600 MPa, which statistically started when the strain reaches about 1.3% and ends at around 7%. It can be speculated that plastic strain may be accumulated during the stress plateau. After the stress plateau, stress would increase much slower than the elastic stage, which is similar to hardening. The maximum tension stress showed much distinction for different prestrain level. With the same prestrain level, e.g. 10%, the maximum tension stress had a deviation of about 40 MPa (close to the deviation of the stress level of stress plateau). Although from the same batch, this could be because specimens were cut from different locations of a long plate, leading to variance.

Table 1. Test results of tensile failure tests.

Specimen No. Elastic modulus E 0 (GPa) Elongation at break ε u (%) Ultimate tensile stress σ u (MPa) T-1 69.1 37.3 896.81 T-2 69.9 37.0 925.64 Average 69.5 37.2 911.23