PSI - Issue 25

Daniel J. Cruz et al. / Procedia Structural Integrity 25 (2020) 316–323 Daniel J. Cruz et al. / Structural Integrity Procedia 00 (2019) 000–000

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the occurrence of buckling phenomena (3). Therefore, the solution for these specific applications is to reduce the dimensional scale of the specimens from a macro to a smaller scale. This process called miniaturisation is a simple and powerful technique in the mechanical characterisation of materials and can be done from a macro scale to a meso, micro or nanoscale - Fig. 1. In this work, only the mesoscale will be considered.

Fig. 1: Diferent scales of miniaturization according to NIST - National Institute of Standards and Technology ; adapted from (4)

Traditionally on the macro tensile test, extensometers attached to the sample are used to measure longitudinal and transversal strains. However, miniature-size specimens do not allow the use of standard extensometers for strain mea surement data. Also, the displacement data obtained from crosshead travel of the machine may give large scatter in data (10). Therefore, for mesoscale specimens, optical methods such as Digital Image Correlation (DIC), Electronic Speckle-Pattern Interferometry (ESPI), and photoelasticity proved to be more suitable. Owing to the rapid advance ment of the digital-imaging and data-processing hardware the optical digital-image techniques for strain measurement have attracted increasing attention (6). Compared to the other methods, DIC o ff ers the advantage of allowing for a less complex experimental setup (Fig. 2) and less thoroughly controlled experimental conditions. The principle of the Digital Image Correlation (DIC) method has been known since the 1970s and is based on the recognition of a change in the sequence of images. All that is needed for a simple 2D DIC test is a digital camera, a white light source and a speckle pattern achieved, for instance, by spray-painting. Under the load, the specimen is deformed and so is the applied stochastic pattern. Comparing the images recorded by digital camera, changes in the pattern are registered and displacements and strains are calculated. It is also versatile in the sense that it can be used in combination with a variety of digital image acquisition tools, such as microscopes (7).The main disadvantage is the lower accuracy of strain measurements when compared to interferometry-based techniques- Therefore, it is not recommended for small deformations (8).

Fig. 2: Typical optical image acquisition system for the 2D DIC method