I am doing ENF test and I am interested in tracking the crack propagation in the adhesive layer between two beams. No matter how hard I try, I cannot see any trace of the crack tip or the crack itself in the adhesive layer. ( I use a relatively high resolution camera and the main reason I am not able to see the crack is because the translucent epoxy adhesive looks dark in the black&white camera).

I was wondering what are the best tricks you use to monitor the track propagation (using a light source behind the specimen? spraying paint? etc) and also, how do you measure it (using a printable ruler, using image analysis software, etc)

Your help and suggestions will be very much appreciated.