PSI - Issue 64

7

Author name / Structural Integrity Procedia 00 (2019) 000 – 000

M. Pedram et al. / Procedia Structural Integrity 64 (2024) 621–628

627

area under the temporal variation of heat flux until the specified time was calculated using a numerical integration method, trapezoidal method in this study. Fig. 4 demonstrates variation of total energy (heat) per unit area for minimum detectable thermal contrast according to two criteria. The power function can represent the trend with a high coefficient of determination. This diagram shows that 0.5°C thermal contrast on D5 and D25 slabs (slabs with 5 and 25mm void cover) require 1.8 and 297.9 kJ/m 2 , respectively. Moreover, 1°C thermal contrast on D5 and D25 slabs requires 7.0 and 537.0 kJ/m 2 , respectively. This indicates a considerable difference in the amount of energy required for detection of shallow defects, 5mm deep, and deep defects, 25mm deep defect in this study.

0 100 200 300 400 500 600

(kJ/m^2)

5

10

15

20

25

Total energy per unit area

Defect Depth (mm) Total energy per unit area (kJ/m^2) to reach 0.5 (°C) thermal contrast Total energy per unit area (kJ/m^2) to reach 1 (°C) thermal contrast

Power (E= 0.0113D^3.1575; R² = 0.9876) Power (E= 0.1137D^2.6436; R² = 0.9881)

Fig. 4- Variation of total heat flux to create minimum detectable thermal contrast

4. Conclusions In this study, FEA was used to estimate the energy required for the detection of defects at various depths from 5 to 25mm depth at 5mm intervals. Variation of the total energy per unit area required for 0.5°C, and 1°C thermal contrast with depth of defect, follows a power function with an exponent of 3.16, and 2.64, respectively. This leads to an estimated total energy per unit area of 4651.5, and 5708 kJ/m 2 , respectively, to detect a defect at 60mm depth. This is a significant difference compared to estimated values of 73.8, and 171.8 kJ/m 2 , respectively, for detection of a defect at 15mm depth. Hence, this quantitative measure shows the reason for hardships in the early-stage detection of deep defects at rebar depth using IRT. This quantitative measure provides a basis for designing an appropriate thermography scenario. That is, such estimations form a basis for deciding on the appropriate timeframe for thermography using ambient thermal excitation, or selection of thermal excitation mechanism and waveform in an active thermography scenario. Future research will evaluate the practicality, accuracy level and reliability of subsurface defect detection by IRT in the timeframe (for excitation by ambient natural excitation such as solar irradiance) or thermal excitation mechanisms selected based on FEA estimations. This could be elaborated more by considering the uncertainties in the concrete as material, geometrical variables of defects, and ambient environment as the excitation mechanism.