surface beyond the glass transition temperature, Tg, of the FRP matrix (Kharbari et al
2003).
Pulse phase IRT
The experimental setup and data acquisition procedure used for pulse phase IRT
(PPT) is similar to the pulse thermography procedure described above. After the series of
thermal images is collected, a discrete Fourier transform operation is performed on each
pixel of the images in the time domain. This operation results in a series of images in the
frequency domain with each pixel consisting of an imaginary number. Phase images are
obtained for each frequency by computing the inverse tangent of the imaginary part
divided by the real part.
/f dulated Heat Source
Note: Max
SIdealized temp response above A)defect for #1
Sdeect free region #1 occurs at lower
S\/ A"" n f\-, than #2
/~~ ~~~ / \ -
-- --Adefect #2
;\ / \ / Idealized temp response above P
subsurface defect Phase Image
Figure 3-7. Defect detection with lock-in thermography
The advantage of this method is that the resulting phase images are relatively
independent of non-uniform heating. There is also a strong relationship between the
frequency at which a defect first appears in the phase images and the depth of the defect.
Defects that are closer to the surface appear in higher frequency phase images while
deeper defects only appear at lower frequencies. A major disadvantage of this method is
I \
- I