Laser Speckle Metrology Research Articles

Simultaneous acquisition of 3D shape and deformation by combination of interferometric and correlation-based laser speckle metrology.

A metrology system combining three laser speckle measurement techniques for simultaneous determination of 3D shape and micro- and macroscopic deformations is presented. While microscopic deformations are determined by a combination of Digital Holographic Interferometry (DHI) and Digital Speckle Photography (DSP), macroscopic 3D shape, position and deformation are retrieved by photogrammetry based on digital image correlation of a projected laser speckle pattern. The photogrammetrically obtained data extend the measurement range of the DHI-DSP system and also increase the accuracy of the calculation of the sensitivity vector. Furthermore, a precise assignment of microscopic displacements to the object's macroscopic shape for enhanced visualization is achieved. The approach allows for fast measurements with a simple setup. Key parameters of the system are optimized, and its precision and measurement range are demonstrated. As application examples, the deformation of a mandible model and the shrinkage of dental impression material are measured.

Experimental evaluation of hinge phenomenon in notched three point bend bars using laser speckle metrology

In elastic-plastic fracture, material behavior is often characterized by the J-integral or crack tip opening displacement (CTOD) parameters. In order to evaluate these parameters accurately, the location of the plastic hinge point, and subsequently the value of the plastic rotational factor, r p , must be determined. Traditionally, hinge point location and r p have been inferred through crack opening displacement (COD) measurements. However, this work indicates that laser speckle metrology can be used to analyse directly the hinging phenomenon. Using notched three point bend bars fabricated from a high stretch low alloy steel, a full-field map of in-plane displacements was generated over the course of each test. The results indicate the existance of a hinge region, rather than an explicit hinge point. The hinge region appears to contain the computed hinge point location using the range of cited r p values. This indicates that it may be appropriate to use the centroid of the hinge region in subsequent CTOD and J-integral calculations.

Experimental Analysis of Fracture in High Strength Cementitious Composites

AbstractCharacterization of cementitious composites by their fracture properties has been difficult due to controversial results reported in the technical literature. Existing studies on the fracture behavior of plain concrete reveal some fracture characteristics that differ from those normally observed in metallic materials. Among these characteristics is the existence of a micro-cracking zone or process zone at the tip of an advancing crack. The determination of the fracture process zone in concrete is a difficult experimental problem, because the resulting deformation is strongly localized.In the present study, in-plane displacements in front of notched high strength concrete have been monitored. Laser speckle metrology, which is a special technique utilizing the speckle patterns of laser light for measurement of in-plane displacements, is employed. Experimental results indicate that a precise description of the fracture process zone is possible by speckle metrology.

Digital Imaging Techniques In Experimental Stress Analysis

Digital imaging techniques are utilized as a measure of surface displacement components in laser speckle metrology. An image scanner which is interfaced to a computer records and stores in memory the laser speckle patterns of an object in a reference and deformed configuration. Subsets of the deformed images are numerically correlated with the references as a measure of surface displacements. Discrete values are determined around a closed contour for plane problems which then become input into a boundary integral equation method in order to calculate surface traction in the contour. Stresses are then calculated within this boundary. The solution procedure is illustrated by a numerical example of a case of uniform tension.