Application of digital speckle photography for local strain analysis

Detailed characterizations of inhomogeneous material compounds, e.g. welds or composite materials, require extended knowledge about local material properties. However, steep property gradients in the sample under investigation hinder the application of conventional measurement techniques. Adapted methods with high lateral resolution must be applied, which ideally record strain maps in a wide measurement range. The Digital Speckle Photography (DSP) is an optical non- contact measurement technique that fulfills these requirements. This method bases on the well-known Speckle Photography and provides fieldwise in-plane displacements and strains, respectively. DSP can be applied to a large spectrum of measurements in the field of solid and fluid mechanics. Two typical applications of this method are presented in this paper. First, the properties of laser welded steel samples are investigated. Local stress-strain diagrams of each zone of the laser weld are experimentally determined. Typically, these zones have narrow dimensions and steep property gradients. In a second application the local Young's modulus and thermal expansion coefficient of graded materials are measured. These graded materials are produced by means of a laser beam cladding process and consist of a metal matrix with varying percentages of a hard phase. Dependent on the region of the sample different quantities of the material properties are provided, owing to the varied hard phase content.