PSI - Issue 64

6

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

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

626

the maximum thermal contrast follow the trend of IRT measurements with acceptable accuracy. The trend is well represented with a power function. Both heat up and cool down cases show almost similar absolute thermal contrast values as the ranges of cooling down or heating up are almost about 40°C.

0 2 4 6 8 10

Δ Tmax ( ° C)

5

10

15

20

25

Void cover (mm)

EXP: IT=-20°C

FEA: IT=-20°C

Power ( Δ T = 38.534*D^-0.92; R² = 0.9686)

Power ( Δ T = 29.92*D^-0.719; R² = 0.9817)

a)

Heat up from -20°C

0 2 4 6 8 10

| Δ Tmax| ( ° C)

5

10

15

20

25

Void cover (mm)

EXP: IT=60°C

FEA: IT=60°C

Power ( Δ T = 22.627*D^-0.692; R² = 0.9824)

Power ( Δ T = 20.408*D^-0.533; R² = 0.9717)

b)

Cool down from 60°C

Fig. 3- Variation of maximum thermal contrast with void cover

3.3. Estimation of required heat for detection of subsurface defect To make a subsurface defect observable to the IR camera, for example, the B335 which was used in the experimental programme, it had to exhibit a safe detectable thermal contrast of between 10 times NETD (0.5°C) and 20 times NETD (1°C) (Hiasa et al., 2017). ASTM D4788-03 standard recommends 0.5°C thermal contrast (ASTM D4788-03) for the detectability of subsurface defects in concrete bridge decks that matches the 10 times NETD (0.5°C) of the camera used in the current experimental programme. To estimate the total energy required for detectable thermal contrast on the surface, initially the times to reach the target thermal contrast was found. Then the