Use of laser spot thermography for the non-destructive imaging of thermal fatigue microcracking of a coated ceramic matrix composite

ABSTRACT

In this paper, an environmental barrier coating (EBC), which has undergone thermal cycling under spatiotemporal gradients, is inspected using active thermography. The technique considered herein is known as laser spot thermography. This study deals with the detection of a very thin crack network of unknown orientation. A numerical analysis is presented to investigate the sensitivity of experimental parameters on the detection. Image processing using a Laplacian operator combined with histogram equalisation to mitigate optical defects is then proposed to construct a cartography that is similar to scanning electron microscopy (SEM) observations despite the very adverse conditions.

KEYWORDS:

• Laser spot thermography
• crack network imaging
• non-destructive evaluation

Nomenclature

$\alpha$	=	thermal diffusivity ${\mathrm{m}}^2.{\mathrm{s}}^{-1}$
$c$	=	specific heat $\mathrm{J}.\mathrm{k}{\mathrm{g}}^{-1}.{\mathrm{K}}^{-1}$
$e$	=	crack opening in $\mathrm{m}\mathrm{m}$
$k$	=	thermal conductivity $\mathrm{W}.{\mathrm{m}}^{-1}.{\mathrm{K}}^{-1}$
$P_0$	=	laser beam power $\mathrm{W}$
$r_s$	=	laser beam radius at 1/e2 from the source $\mathrm{m}$
$t$	=	time $\mathrm{s}$
$x,y,z$	=	spatial coordinates in $\mathrm{m}\mathrm{m}$
$T$	=	temperature $\mathrm{K}$
$V$	=	speed of scanning $\mathrm{m}.{\mathrm{s}}^{-1}$
$\rho$	=	mass density $\mathrm{k}\mathrm{g}.{\mathrm{m}}^{-3}$
$\phi$	=	laser power in $\mathrm{W}$
$Pe=\frac{V{r}_s}{\alpha }$	=	Péclet dimensionless number

Acknowledgments

The authors would like to thank Dr. Jean-Claude Krapez and Dr. François Hild for their help and advice on this work, which was supported under PRC MECACOMP, French research project co-funded by DGAC and SAFRAN Group, managed by SAFRAN Group and involving SAFRAN Group, ONERA and CNRS.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Thibaut Archer

Thibaut Archer PhD student working on ceramic materials and thermomechanical behavior of coatings at high temperature.

Pierre Beauchêne

Pierre Beauchêne Senior physicist working on thermal characterization and processing of materials.

Bruno Passilly

Bruno Passilly Experimenter specialised in nano-indentation at high temperature.

Jean-Michel Roche

Jean-Michel Roche Research engineer in thermal characterization and non destructive evaluation techniques.