Characterization of local strain fields in cross-ply composites under transverse loading

Transverse matrix cracks often occur in fiber-reinforced cross-ply polymer matrix composites subjected to tensile loads. Experimental visualizations of strain fields are necessary to calibrate and validate computational models that predict strain development and transverse crack initiation. In this study, fluorescent digital image correlation (DIC) measurements are used to study the evolution of highly localized displacements and strains under transverse tension. Composite coupons and Aluminum 6061-T6 control samples, both with dimensions of 50 mm in length, 2 mm in width, and 1.25 mm in height, were prepared with one polished surface spincast with a solution of fluorescent nanoparticles to produce a DIC speckle pattern. Specimens were tested in a miniature load frame in an optical microscope. Using a long working distance objective lens and a monochrome camera with a 668.4 by 534.7 μm resolution, fluorescent images of the polished surface were captured periodically to track the developing strains in the specimen. Strains were successfully measured using DIC for composite specimens. Bands of strain were observed to form perpendicular to 0° plies and often traced along pockets of resin. Additionally, strain concentrations formed between tow borders and pockets of resin and were nearly twice the value of the average strain for the area of interest. These results were consistent qualitatively with a photoelastic study shown in literature