Optical Strain Field Analysis
Conventional physical test methods show averaged characteristic values over the specimen cross-section, e.g. the elongation at break during tensile testing in %. However, it is not possible to carry out tests with regard to local property fluctuations with this method - however, they are of particular interest for many further processing processes, e.g. the creasing of folding boxboard. In order to exclude an influence of the additional measuring system on the material properties, the so-called optical strain field analysis (digital image correlation) is applied.
Any two-dimensional test is carried out in a universal testing machine and accompanied by an image recording during the test sequence. For this purpose a camera system is installed behind the testing machine (optionally also with integrated climatic chamber), which ensures the exact alignment of the camera to the specimen and establishes the connection between recorded measurement images of the camera and recorded measurement data of the universal testing machine via an implemented triggering. The resulting image sequence is digitally provided with measuring points and their displacement is calculated by means of correlation analyses. The results of this technique range from general displacements to the representation as high-resolution local strains. The camera technology allows 1,628 px to be displayed over a width of 3.6 mm. However, macroscopic tests can also be recorded by a suitable choice of camera and lens. Recording speeds of 60 frames per second also allow the evaluation of faster experiments.
In addition to the advantages of high-resolution local evaluation of material properties, the non-contact system also offers another advantage: Measurements are taken directly on the material, i.e. all disturbing influences - such as displacement of the testing device or an adhesive used - are not included in the calculation. This also makes it possible to examine existing measuring methods for their accuracy and, if necessary, to optimize them.
A small selection of material investigations which are carried out or supported by means of optical strain field analysis is listed below:
- Global displacement / movement
- Local strain distribution
- Speed or acceleration characteristic values
- Poisson's ratio
- Out-of-Plane thrust values
In general, almost any two-dimensional load case can be supported by optical strain field analysis - due to the mobility of the system, this can even be done at your company.