Residual stress measurement on coated surfaces by means of holography

The new method enables a temporally and spatially highly resolved determination of residual stresses in coated surfaces or in layered composites, which is performed optically and contactless using digital holography.

• Optical, contactless determination of residual stresses
• Even strongly curved free-form surfaces can be examined
• Even small objects (of a few mm²) can be examined
• Examination possible during the coating process (quasi-real-time method

For all coated surfaces subjected to residual stresses.

Decorative or functional surface properties are often realized through coatings. Coatings protect against corrosion and wear, for example, but they also complement special thermophysical and electrophysical properties. Coatings deposited by high-energy processes are often subject to manufacturing-related residual stresses, which can affect the coating behavior. For example, this may result in flaking or cracking. Therefore, it is important to identify these residual stresses and their effects on the layer composite.

Currently, various techniques are used to identify and measure residual stresses, such as the micro-circular milling or hole-drilling method and X-ray diffractometry. Special methods that can only be used for measurements on conductive or magnetic samples, such as the eddy current method, inductive measuring techniques and Barkhausen noise, are also applied. In general, the measuring systems developed so far do not allow temporally and spatially resolved examinations during the coating process. In addition, the hole-drilling method is limited to flat and relatively smooth surfaces, and X-ray diffraction has little depth resolution and is very time-consuming and costly.

The new method developed at the University of Stuttgart enables a temporally and spatially highly resolved determination of residual stresses in coated surfaces or in layered composites, which is performed optically and contactless.

The coated surface is exposed using a pulse laser, which locally removes or heats part of the layer to achieve deformation of the same. The new shape of the surface is then measured using digital holography. Thus, the residual stresses in the layer can be determined numerically. In addition, continuum mechanics calculations are performed, using finite element models to specify defined residual stress states. The optical method also allows strongly curved objects and component surfaces to be examined.