Optimization of Digital Image Correlation for High-Resolution Strain Mapping of Ceramic Composites
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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.
Keywords3-D digital image correlation Woven-fiber ceramic-matrix composites Spatial resolution Strain error Crack opening displacement
This work was supported by the Pratt & Whitney Center of Excellence at the University of California, Santa Barbara (monitored by Douglas Berczik), and the US AFOSR (Ali Sayir) and NASA (Anthony Calomino) under the National Hypersonics Science Center for Materials and Structures (AFOSR Prime Contract No. FA9550-09-1-0477 to Teledyne Scientific and Sub-contract No. B9U538772 to UCSB). The authors gratefully acknowledge the assistance of Renaud Rinaldi with the finite element analysis.
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