The grey scale method

Having worked with contact-free measurement procedures over many years, we have attained a high competence, particularly using the grey scale method developed by the Association for optical measurement. Optical measurement techniques present a progressive and attractive alternative to customary material test methods since

• continual increase of computer capacity leads to new or extended application possibilities,

• the procedures are contact-free and non-invasive, 

• optical measurement techniques cover the entire surface of the part measured and provide spatially localised deformations.

The following figures show a part made of long-glass-fibre reinforced PP with reinforcing ribs at an angle of 45°. The 3D CAD-model rendering is shown on the left. The part is shown with a stochastic grey scale pattern on the right.

Three-dimensional displacement and strain fields

During load measurements with the grey scale correlation procedure, the part to be measured is recorded with one or more CCD cameras. The optically contrasting structure on the surface, which follows the local deformations, is recorded under diferent loads and the deformation is then assessed by digital graphic processing. This provides:

• three-dimensional shear and extension fields under static or dynamic load in the range 0.05% to several 100% strain,

• the local strain peaks in the part,

• help in the construction, shape optimisation and failure analysis of parts.

High local spatial resolution in shear measurements is an important criterion for the precision and success of solutions to problems during parameter identification. The following two figures show the setup for 3D measurements for the identification of displacement fields during a pull-out test on the direct screw coupling of a part. The recordings identify both the deformation between and to the left of the ribs and also the deformation on top of the rib (right). Illumination is provided by two high power cold light spots.

The following figures show the steps in a typical analysis of results. The figure on the left shows the part of the sample seen by the left camera. The green region in the middle figure indicates the displacement field thus also indirectly the strain field to be assessed. The figure on the right shows the result of analysis as major axis surface strain.

Versatile analysis

With regard to the surface shear fields, the grey scale correlation procedure permits the same relative accuracy, independent of sample size. The smaller the sample, the smaller the absolute error. However, very small sample surfaces require greater effort in the preparation of the experimental setup. Calibration of the camera requires considerable experience.

From these results one can generate a large number of interesting diagrams. The operator can produce the evolution of a given variable in time, e.g. the displacement or the major axis strain at one or more mesh points, along a section, or as a film. The following animation shows the evolution of the vertical displacement and the major axis strain from two perspectives.