PSI - Issue 64

406 Zhikang Deng et al. / Procedia Structural Integrity 64 (2024) 400–408 Zhikang Deng / Structural Integrity Procedia 00 (2019) 000 – 000 7 and - 108 μm/m for measurement s at 50 °C and 80 °C, respectively. The true measured strain ( ) was calculated by the strain gauge values ( ) minus the correction factors and using (2). _ ( ) ( ) f glass strain gauge test ref T T    = −  − (1) = m SG s f     − − (2) The strain gauge values and the calculated strains after temperature compensation are shown in Fig. 5(b). The calculated average strain after compensation was estimated to be around 132.9 μm/m at 50 °C and 302.0 μm/m at 80 °C. This indicated a pre-stress loss due to elevated temperatures.

Fig. 5 . (a) Temperature vs. time and (b) strain vs. time when service temperature increased from room temperature to 80 °C.

4.4. Total pre-strain and pre-stress calculation Since the strain gauges were zeroed before step 1, step 2 and steps 3, the total mechanical strain ( ) was calculated using Eq.(3). It contained the effects from all previous steps. ∗ was corrected strain gauge value measured in the previous step before zeroeing. was the strain gauge value at room temperature or that after temperature compensation at elevated temperature. It only reflected the strain change measured in each single step. (3) The stress of glass was determined based on the corrected strain values and the Young’s modulus of glass glass with Eq.(4). The stress from each single step was calcuated with the strain , while the total stress including effects from previous steps was calculated based on .The values for , , ,and for each of the two specimens are shown Table 3. = glass E    (4) It can be concluded that the generated pre-stress of glass was on average -5.4 MPa and -7.6 MPa when the activated temperature was 160 °C and 200 °C, respectively. The pre-tress was completely lost when the specimens were heated to a temperature of 50 °C. A tension stress of 13.6 MPa resulted in the glass when the specimens were heated further to a temperature of 80 °C. However, a part of the pre-stress is expected to be recovered once the specimens cool back to room temperature. Nevertheless, the loss of pre-sress when exposed to elevated temperatures needs to be accounted total m     = +