PSI - Issue 64

Zhikang Deng et al. / Procedia Structural Integrity 64 (2024) 400–408 Zhikang Deng / Structural Integrity Procedia 00 (2019) 000 – 000

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to 40 °C might soften the adhesive in the anchorage zone, this effect was reversible once the temperature returned to room temperature, as reported in the lap-shear test with activation by Li et al. (2023a). According to the strain gauge measurements, the compression strain of the glass could be further increased by - 33.6 μm/m and - 30.1 μm/m for specimens AC01 and AC02, respectively. Since the strain gauge was zeroed before the measurement of this step, the compression strain in the glass plate resulting from the activation temperature of 160 º C needs to be added to obtain the total pre-strain.After superposing the values at 160 º C, the resulting compression strain in the glass plate was around -108 μm/m on average. No visible damages to the glass or the adhesive were observed during the activation process. The successful activation at 200 °C demonstrated the potential for utilizing the SME of Fe-SMA to achieve higher pre-stress levels without causing damage to the specimens.

Fig. 4. (a) Temperature vs. time and (b) strain vs. time during and after activation when the activation temperature was 200 °C.

4.3. Steps 3 and 4: Pre-strain loss due to elevated temperature When exposed to direct sunlight during summer, the glazing systems will be subject to elevated temperatures. The temperature of glazing system components in facades can reach up to 80 °C as defined by ETAG 002 (1999), which exceeds the glass transition temperature ( T g ) of SikaPower®-1277 (around 67 °C). Such high temperatures may potentially soften the adhesive. Additionally, the elastic modulus of SikaPower®-1277 exhibited a significant decrease from 1940 MPa to 138 MPa when the material temperature increased from 23 °C to 80 °C, as indicated in Table 1. This softening of the adhesive could result in pre-stress loss, making it crucial to investigate the bonding performance at elevated service temperatures. In steps 3 and 4, the entire specimens were placed in a climate chamber for 20 hours until a stable temperature of 50 °C (step 3) and 80 °C (step 4) was achieved. The temperature and strain measurements during this process are depicted in Fig. 5. It is evident from the data that there was a positive value recorded by the strain gauges when the temperature rose from room temperature to 50 °C and 80 °C. It should be noted, that the initial zero-strain indicates a new strain measurement after a nullification in the measurement system, rather than a strain free state. The positive strain values resulted from the higher thermal expansion coefficient of Fe-SMA compared with that of glass and from the pre-stress loss due to softening of the adhesive at elevated temperatures. According to the strain gauge measurements, the average measured strains were 57.6 μm/m at 50 °C and 111.6 μm/m at 80 °C, respectively. However, it is essential to consider the temperature compensation of the strain gauge to calculate the true resulting strain . For correction of the strain values, the thermal polynomial ( ) was calculated using the equation given by the datasheet of the strain gauges. They were -21.3 μm/m and -82.4 μm/m for measurement s at 50 °C and 80 °C, respectively. The deviating temperature coefficient of the substrate material was also considered, and the corresponding correction factor ( ) was calculated by Eq. (1) given by HBK. These correction factors were − 54 μm/m