The mark tracking method is a simple and efficient technique for a local strain measurement. We show the possibilities of the mark tracking for field measurement. We present a procedure which allows us to experimentally characterise this technique depending on the sensor and the marks' dimensions. The method presented is insensitive to illumination variation and to in-plane rotation of the specimen. The strain values are obtained by analysis of four marks. In order to obtain a displacement field, we analyse a series of small groups of spots. With the help of an average of the parameters (using a series of initial images), we suppress the systematic error that we can observe when we use a method comparing an initial state to a deformed one. So the strain uncertainty becomes less that 0.0002 for a local study and 0.002 for a whole field study (measurement base 20×20 pixel2).

Development of the marks tracking technique for strain field and volume variation measurements

Moire interferometry has been a valuable experimental technique for the understanding of the mechanical behavior of materials and structures. Over the last decade less emphasis has been placed on the development of the technique and more towards applications. This paper is a review article on recent applications using moire interferometry in the fields of microelectronics devices, material characterization, micromechanics, residual stress, composite materials, fracture mechanics, and biomechanics. The general principles of moire interferometry and advancement of techniques will not be discussed in this text, but references will be provided.

Recent Applications of Moiré Interferometry

Optical methods have been developed to measure large strains. Two alternatives are presented: the first one uses a cross grating, obtained by an innovative manufacturing process and the second one uses a mark-tracking technique. The production of the grating is based on the deposit of a thin film through a master grid under vacuum. This creates small studs independent of each other, on the specimen surface. For the grid interrogation two techniques are discussed, one using the analysis of the optical diffraction figure and the other one the Fast Fourier Transform algorithm. Markers are simply deposited on the studied surface with a pen and strain determination is then obtained by mark-tracking. The experimental validation consists of several simple tensile tests. Experimental results for polymers and latex materials confirm the quality of measurement. Strains are measured for values up to 1,200% without loss of information, with a strain accuracy close to 5times10-4 at minimum gauge lengths of 0.125 mm.

Three Optical Procedures for Local Large‐Strain Measurement

This paper describes three different optical techniques for the measurement of in-plane displacements close to the crack tip which can then be used to determine the presence of crack closure in propagating fatigue cracks. Conventional temporal phase shifting moire interferometry is described first, followed by an improvement to this technique, load sequence phase stepping, which allows data to be collected much more frequently during a load cycle. Finally, more recent measurements using digital image correlation are reported. Results from all three techniques are compared and the advantages and disadvantages of each approach are discussed.

Optical methods for measurement of fatigue crack closure: moiré interferometry and digital image correlation

The primary output from several full-field deformation measurement techniques, e.g., Digital Image Correlation (DIC), is the displacement vector at a dense grid of points covering the area of interest. Since such displacement data inherently contain noise, they are usually smoothed first and then differentiated to obtain strains. Another common approach is to use finite-element shape functions for the strains and compute them by treating the measured displacements as nodal displacements. In this paper, we propose a novel method for strain calculation from full-field data, based on the multivariate analysis technique of Principal Component Analysis (PCA) using which we first obtain the singular values and singular vectors for each component of the displacement field. By choosing only the dominant singular values and their corresponding singular vectors, we show that the dimensionality of the displacement data is sharply reduced and a significant portion of the noise is eliminated. Moreover, the shapes of the dominant singular vectors offer physical insight into dominant deformation patterns. We demonstrate the accuracy of the proposed technique by applying it to two cases each of homogeneous and inhomogeneous strain fields and show that in all cases the proposed method yields excellent results.

Computation of Full-field Strains Using Principal Component Analysis

The moire fringe method is described and reviewed with particular reference to its application in strain analysis. The theory is briefly dealt with, followed by a discussion of the common m...

J McKelvie

Papers

Moiré strain analysis : an introduction, review and critique, including related techniques and future potential