Material processing by welding or cutting bears the risk of introducing stress or strain into the surrounding material. They reduce the value of the component considerably since they lead to expensive reworking. To measure and control such deformations on the micro- and nanometer scale, Fraunhofer IPM develops fast and robust techniques suitable for material testing as well as for measuring machine elements or electronic circuits under operation.
Measurement of axial deformation by Electronic Speckle Pattern Interferometry (ESPI)
A micro-deformation measurement system from Fraunhofer IPM detects minimal changes to a component’s topography very quickly, two-dimensionally and down to the nanometer range. This allows even slight changes or deformation of the component surface to be measured precisely, even at high production speeds. Such deformations may occur when the object is subjected to mechanical or thermal loads. The system is based on the principle of electronic speckle interferometry (ESPI). The great advantage of this measurement technique is its accuracy in the nm range in axial direction.
Strain measurement by digital image correlation (DIC)
For material testing, Fraunhofer IPM together with Fraunhofer IWM has developed a GPU-based real-time digital image correlation system (DIC) that combines the advantages of both, optical and mechanical extensometers.
In strain-control mode, it enables low-cycle fatigue experiments (LCF) in a contactless, marker-free, and path-independent manner because it correlates the microstructure of metallic surfaces with strain measurement rates well above 1 kHz. Therefore, no sample preparation is required, e.g. with markers or speckle paint, slipping is avoided and no shear strains are introduced to the specimen at the measurement points of the extensometer. This is in particular advantageous at high temperature experiments up to 1000 °C.
In full-field mode, the GPU-based algorithms evaluate 74,000 strain and displacement measurement points per second. This allows for real-time full-field evaluations of 10 Hz under mechanical or thermal stress – like the measurement of crack growth rates or crack flank displacements in crack growth experiments. The combination of both, strain-control and full-field evaluation, in a single sensor simplifies crack growth experiments enormously – in uniaxial and biaxial experiments.