Optimization of Digital Image Correlation for High-Resolution Strain Mapping of Ceramic Composites

Abstract

Digital image correlation (DIC) is assessed as a tool for measuring strains with high spatial resolution in woven-fiber ceramic matrix composites. Using results of mechanical tests on aluminum alloy specimens in various geometric configurations, guidelines are provided for selecting DIC test parameters to maximize the extent of correlation and to minimize errors in displacements and strains. The latter error is shown to be exacerbated by the presence of strain gradients. In a case study, the resulting guidelines are applied to the measurement of strain fields in a SiC/SiC composite comprising 2-D woven fiber. Sub-fiber tow resolution of strain and low strain error are achieved. The fiber weave architecture is seen to exert a significant influence over strain heterogeneity within the composite. Moreover, strain concentrations at tow crossovers lead to the formation of macroscopic cracks in adjacent longitudinal tows. Such cracks initially grow stably, subject to increasing app lied stress, but ultimately lead to composite rupture. Once cracking is evident, the composite response is couched in terms of displacements, since the computed strains lack physical meaning in the vicinity of cracks. DIC is used to identify the locations of these cracks (via displacement discontinuities) and to measure the crack opening displacement profiles as a function of applied stress.

Degradation of a SiC‐SiC composite in water vapor environments

“…Baseline tensile properties were also measured at ambient temperature with periodic unloading‐reloading excursions to measure hysteresis. Displacement and strain fields were measured by digital image correlation following procedures described in detail elsewhere . These tests were used to ascertain stresses associated with cracking of the outer SiC overcoat.…”

Resistive embedded heating for homogeneous curing of adhesively bonded joints

“…After drying, black paint was sprayed quickly past the surface in a sweeping motion using an air brush, creating a high-contrast speckled pattern. Full image correlation requires that the speckle size be small enough in the deformed image such that, each subset contains sufficient unique information [22]. Speckle size was controlled by modifying the nozzle, air pressure, and throttling the spray.…”

An elastic–plastic constitutive model for ceramic composite laminates

“…First, there is measurement error associated with digital image correlation. For typical values of displacement noise and the filter length employed in the strain calculation (h f ¼ 1 mm), the strain error in regions of relatively uniform strain should be on the order of a few hundred microstrain [37]. This value is evidently insufficient to explain all of the strain inhomogeneity.…”

“…Finally, there are also non-stochastic effects associated with the woven fiber architecture. [37] observed that, in 2-D woven composites, local strain enhancement occurs at tow 'crossovers': locations where undulating warp tows dive beneath flat weft tows. Similar effects may be responsible for the strain 'hotspots' observed in Fig.…”