The authors are involved from several years ago in the disc-shape hardwood specimens’ testing. In this sense, they
conceived and realized several stands, destined for Photo-elastic Thin Layer Technique, for Moiré-Fringe
Technique, respectively for Holographic Interferometry, too. Some of these published experimental results are
mentioned in the References.
It is well-known fact that for anisotropic and orthotropic materials (how is also the hardwood), on can apply from the
Photo-Elasticity, only the so-called reflection method. In this sense, the thin layer technique, was developed and patented
by Vishay Company, and named PhotoStress. Using this reflection method, we are able to analyze specimens made of
different kinds of materials. This method became very useful especially in the case of materials with anisotropic or
orthotropic behaviours, like wood and wood-based materials. So, the point-by-point analysis of such specimens’ stressstate
offers within the limitations of the accuracy of the method, certain useful information. In this case, on apply a
relatively thin layer (2 to 3 mm) of photo-elastic material (by gluing it using some special reflecting material), and using a
special kind of Polariscope, on perform experiments similar to the transmission Photo-elasticity. Some very interesting
and useful results were obtained at the University of West-Hungary, Sopron, co-ordinated by Szalai (Borbás et al. 2001,
Hantos 2007, Szalai 1995, 2001a, 2001b). Amongst others, the dependence of the stress-state of disc-shaped wood
specimens on the fibre orientation was analysed using PhotoStress.