13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Multiscale strain investigation of stressed carbon fiber reinforced composites based on data of strain gauging, digital image correlation, acoustic emission Sergey Panin1,2,*, Mikhail Burkov1, Anton Byakov1, Pavel Lyubutin1, Vladimir Titkov1, Alexander Eremin2 1 Institute of Strength Physics and Materials Science SB RAS, Tomsk, Russia 2 National Research Tomsk Polytechnic University, Russia * Corresponding author: svp@ispms.tsc.ru Abstract. A combined method for investigation of localized deformation processes in carbon fiber reinforced composite specimens is applied in order to reveal characteristic stages of strain and fracture. Stress concentrators have the shape of a circular hole and edge crack of various sizes. Use of simultaneous strain data registration has allowed us to register and compare parameters under analysis during entire time of the experiments. The reasons of similarity and difference of the results are shown and discussed. It is offered to apply the obtained results for the aims of non-destructive testing of structural materials being based on revealing characteristic stages of strain development and a particularly prefracture stage. Keywords: strain gauging; digital image correlation; acoustic emission; carbon fiber composite, non-destructive testing, health monitoring of structures. 1. Introduction In compliance with literary data and results of our previous investigations [1, 2] the stage character of deformation development can be registered with various methods of studying and estimation of strain. Each method due to operation principle is sensitive to particular scale level. In situ application of 3 methods: strain gauging, acoustic emission (AE) and digital image correlation (DIC) allows one to obtain complete pattern of deformation and fracture of stressed structural materials. Application of stain gauging and AE is described in papers [3, 4] while use of DIC method is explained in [5, 6]. Thus depending on physical principle of a method used it is possible with high sensitivity to register and characterize quantitatively the deformation processes at each scale. Strain gauging characterizes macroscale level or response of a specimen as a whole on applied stress. DIC (Meso II) describes level which is comparable with macrolevel, because in DIC the area of observation considers with whole width of a specimen under testing. AE (Meso I) complies level generally of debonding on matrix-fiber interface. AE characterizes changes inside material, while DIC method is revealing processes in outer ply of a specimen, that is critical to composite materials with different fiber layers orientation. For numerical analysis of data information parameters for 3 methods were chosen. For the AE method count rate dNAE/dt was calculated; for DIC - γ shear strain intensity, for strain gauging - dσ/dt (time derivative of the external applied stress). 2. Materials and investigation technique CFRC is pseudo-isotropic composite made of unidirectional carbon fiber layers [0°, 45°, -45°, 90°] sintered in carbon matrix. Dimensions of specimens were taken according to ASTM D5766/5766M. Strain localization degree is varied by different dimensions of stress concentrators (central hole diameter 7; 10; 13 mm; depth of edge crack 14,5; 18; 21,5 mm). Dimensions of the specimens are presented on Fig.1.
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