A Procedure for Accurate One-Dimensional Strain Measurement Using the Grid Method

Abstract

This paper deals with the accurate calculation of strain using the grid method. The strain field is first directly deduced from the fringe pattern without calculating the displacement field. This procedure is validated with two numerical examples. Two types of experiment are then carried out: a translation and a tensile test. It is observed that some additional fictitious strains appear in both cases. They are due to two main reasons which interact with each other: the grid defects and the displacement of the grid lines during testing. A suitable procedure is proposed to cancel out these fictitious strains. This procedure is successfully applied in two cases of fringe patterns.

Experimental and Numerical Fracture Analysis of a Pre-cracked Aluminum Specimen Using the Grid Method

“…These quantities are directly proportional to the in-plane displacement and strain components, respectively [11]. It has been shown that the metrological performance of this technique could be significantly improved (especially for strain measurements) by getting rid of most of the grid marking defects which unavoidably occur when grids are printed on their support [12,13]. This leads to a good compromise between resolution and spatial resolution of the measurements.…”

“…Full details on small strain calculation can be found in Refs. [12,13] for unidirectional and crossed grids, respectively.…”

Implementation of the direct evaluation of strains using a phase analysis code for random patterns

“…[14] proposed a new derivation procedure for grid techniques. The technique is twofold: first, it consists in deriving the wavelet used for the phase extraction.…”

“…We noted here the following specific "phase culture" items: camera distortion can be evaluated with a single grid image [12], shape and shape variation can be detected in very good conditions using a video-projector and a camera [13]. Last, derivation can be derived analytically and the grid technique becomes sensitive to strain rather than displacements [14]. This item should bring to such method a breakthrough compared to DIC techniques; it will be developed in the following.…”

Study of Phase Transformation Intermittency in S.M.A. Using the Grid Method

“…A comparison between DIC and digital speckle pattern correlation to measure displacement fields in a Cu-AlBe SMA was presented in Reference [10]. The grid method [11] was used on a monocrystalline Cu-Al-Be [12], including a specific procedure to limit some negative effects of unavoidable grid defects [13,14]. This procedure limited the use of spatial filtering to reduce noise in strain maps, leading to a good compromise between strain resolution and spatial resolution in strain, and thus making possible the detection of very localized phenomena such as appearance and development of thin martensite needles.…”