The laser-optical measuring principle video holography, also known as Electronic Speckle Interferometry (ESPI), allows a non-contact determination of surface deformations. The maximum resolution of this measuring technique is about 20 nm of 1µm/m.

The specimen is illuminated with laser light and observed by a CCD camera which transfers the video signal to the computer. Like in all interferometric measuring techniques, any modification refers to a defined initial state. Changes in the image of the illuminated specimen due to the deformation are displayed by interferometric fringes. By a mathematical derivation of these images, the strains can be determined and displayed as a whole field distribution (isolines of equal strains).

Depending on the measuring equipment, two in-plane deformation components and one out-of-plane deformation can be measured.
The specimens, samples of MDF and chipboard plates that have be cut according to DIN 52377, are tested in a Zwick testing machine (Type 1474). The investigations have been carried out with an ESPI system of GOM mbH, that observed the specimen from an angle of 45°. Only the deformation in tensile direction is measured because we do not expect any deformations in the other directions. The results for a chipboard plate are shown in Fig. 1. The measurement field, size about 8 x 4 cm² (Fig. 1, left), is located in the center of the specimen where we expect the largest deformation and, finally, the fracture.

Results

During the tensile test of the chipboard plate, between the initial state and shortly before fracture, deformations of about 120 µm (Fig. 1, center) or corresponding strains of about 3 mStrain (Fig. 1, right) occurred. The test clearly showed two areas of increased deformation. At the beginning, only the inner area of the chipboard stretched for a rather long period. Shortly before fracture, the essential strains occurred mainly at the front edge between the surface and inner area. That is also the area where the crack appeared. All the tested specimens had a strong strain concentration at the front edge in common.

By courtesy of Fraunhofer-Wilhelm-Klauditz-Institut, Holzforschung