Digital Image Correlation Evaluation of Kink Bands under 3-point Bending of Polymer Matrix Composites
Polymer matrix composites (PMC) are attractive materials in structural applications where strengths and stiffness to weight ratio are the important factors. Due to its demanding and critical applications in aerospace and defense industries, it is essential to understand complex failure/deformation mechanisms under compression. One of the key deformation mechanisms in PMCs is recognized as plastic microbuckling with strains large enough to induce nonlinear matrix deformation. Plastic microbuckling causes kink bands that develop on the compression side and propagate towards the tension side of the bending specimens and span the sample thickness. This study was initiated in early 60’s in an effort to predict compressive strength of UDCs. In last four decades, a large body of research has been published in the literature that addresses kink band morphology using theoretical models and experimental techniques. However, reports on experimental measurements of the strain fields leading to and developing inside these kink bands are scarce and need to be addressed to have a full understanding of the phenomenon. Hence, three-point bend tests have been conducted on dyneema HB80 and Spectra shield specimens to study kink band formation and evolution. Digital Image Correlation (DIC) has been carried out to quantify strain fields during kink band formation, particularly inside the bands. Images have been captured at regular intervals during 3 point bending test, and strain fields were measured with the help of DIC software (ARAMISTM). Moreover, the microstructure of a deformed dyneema HB80 sample in the region of the kink band was observed using transmission optical microscopy. Figure 1 (left image) shows the deformation produced by 3-point bending and the developing kink band.
Figure 1: (a) Dyneema HB80 sample after 3-point bend test. (b) kink band development, axial splitting of fibers
Analysis of these data shows additional details on strain localization in kink bands under load, which present significant amounts of normal and shear strain. Failure was found to be due to axial splitting of plies, with some indications of ply delamination, all driven by plastic microbuckling, as has been observed in composites having a high fiber volume fraction. The resolution of the DIC measurements, however, was not ideal given the length scale of the kink bands, so a new procedure to increase it is described and will be implemented in the near future.
The study of plastic kinking, particularly quantification of large strains, from nucleation to evolution becomes extremely important as it can provide better insight into the state of stress and how to improve material’s capacity to withstand high compressive loads. Furthermore, the study of buckling of elastic-plastic and elastic-viscoplastic material with finite strains is of paramount importance to develop efficient computational models that account for the statistical nature of this phenomenon.
Hence, this is a good topic of research and needs to be addressed in experimental findings. Digital image correlation (DIC) is an effective tool in determination of displacements and strains measurements. Implementing design of experiments (DOE) with micro-scale speckle pattern on small-scale samples and variations in magnification techniques could lead to a significant contribution in capturing strain fields within the localized area, in particular kink bands.