PSI - Issue 64

Ali Jafarabadi et al. / Procedia Structural Integrity 64 (2024) 2059–2066 A. Jafarabadi et al./ Structural Integrity Procedia 00 (2019) 000 – 000

2063

5

epoxy resin adhesive with temperature limit of -270 – 260°C was utilized for installation of the strain-gauges. The adhesive was cured in two steps. The first curing involved heating the installed strain-gauge up to 200°C for half an hour while applying pressure on the workpiece by means of a formwork. A subsequent curing was applied for two hours at 230°C in order to subject the strain-gauge to higher temperature than the operating temperature, ensuring precision measurement. Moreover, during the experimentation procedure, in order to further minimize the thermal induced strains the steel tubes were thermally loaded in the absence of a mechanical trigger (Fe-SMA tube). Afterwards, the measured strain values observed in the dummy sample were deducted from the strains measured at the setup. The activation was conducted through a controlled heating and cooling process. The samples were initially heated up to 25°C to reach the start temperature and held for 5 minutes followed by heating with a rate of 2°C/min up to a maximum temperature of 200°C. The oven temperature was kept at 200°C for 20 minutes to introduce a homogenous and stable thermal state in the oven. Afterwards, through a nitrogen assisted cooling process, the samples were cooled down to 25°C. Extra thermocouple sensors were placed on the samples to monitor the actual sample temperature in addition to the furnace sensor. Hence, the actual samples temperature was employed as the temperature input during strain measurements. The free recovery tests were conducted in similar activation condition except for the presence of the restraining substance. Thus, the FID of Fe-SMA tubes shifts to the recovered inner diameter (RID) freely as a result of the SME. The RID was measured after activation via an internal-micrometer device. Ultimately, the recovery ratio for both heat treated and as-received cases can be expressed as the ratio of recovered inner diameter over the expanded diameter which reads: = −− = (2) where and are the absolute recovery and expansion of the Fe-SMA tubes ’ inner diameter , respectively. 3. Results and discussions After application of the T-rosette strain-gauges on the steel tubes, followed by a two-step curing process, the tubes were placed inside the furnace chamber. In order to monitor the performance of the strain-gauges and to further

Figure 2. Measured axial and circumferential principal strains at the inner diameter of the steel tube during activation of Fe-SMA tubes (bottom row), quantified interface contact pressure as a result of SME (top row)