13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 3D effects increasingly drew attention in recent years from material experiments in laboratories to the real structure [6-8]. Experiments have shown that some damages or failure characteristic quantities, which are regarded as material constants, indeed are related to the structural geometry [5]. A large number of theoretical analysis, experimental investigations, and numerical computations reveal significant 3D effects such, as the thickness effect [9-14]. 3D fracture theory, founded in consideration both of in-plane and out-plane constraints, has made important progress [15-18]. The research on mixed-mode cracks gradually progresses from the 2D hypothesis to the 3D assumption [19-21]. Comparatively, research mainly focuses on the cracks of ideal mode I, 2D problems, theoretical analysis, and numerical computations. Experimental investigations, especially observations and measurements of real 3D fracture of mixed-mode cracks under high temperature, are very limited. There is no complete analytic solution for 3D fracture problems due to mathematical difficulties. Research on 3D fracture problem of mixed-mode cracks under high temperature still needs to be done. The existing theoretical fracture models have many weaknesses and are not verified by experiment and practical use [5]. In a conceptual paper on the past and future development of fracture mechanics, Erdogan [6] identifies 3D effects, high-temperature behaviors, computational methods, and experimental methods as the areas where further intensive research is needed. A new fracture experimental technique based on the moiré interferometry is introduced in this study, by which I-II mixed-mode fracture phenomenon could be surveyed and recorded when temperature is high (say at least several hundreds of degrees in Celsius). Finite through-thickness cracked specimens are employed in tests under the coupling of thermal environment and mixed-mode loading. Displacement field moiré fringes around the crack are recorded. This experimental method was successfully used in investigation of fracture properties of high-temperature alloys. Several specific thickness effects at high temperatures are revealed based on the experimental results. 2. High-temperature, mixed-mode fracture test 2.1. Experimental setup and procedures High-temperature moiré interferometry system has been successfully used in investigation of crack growth of pure mode I [22-24]. The same is employed in this present mixed-mode fracture research. The tensile-shearing specimen dimensions and mixed-mode loading fixture are given in Ref. [25]. A special orthogonal diffraction grating with high temperature resistance is chosen and fabricated on a highly polished surface of a specimen by photoetching and chemical etching [26]. Two sets of grid lines of the orthogonal grating are parallel to and perpendicular to the crack, respectively. A rotatable U-V mirror set [27] is invented for the purpose of the mixed-mode fracture experiment. The mirror set is designed to rotate by the same angle when angle β existed between loading direction and crack perpendicular, as shown in Figure 1. Consequently, the displacements normal to the crack (Mode I) and parallel to the crack (Mode II) were effectively separated, ensuring that moiré fringes corresponding to the u-displacement and v-displacement always are parallel and perpendicular to the crack at any I-II mixed-mode loading. The fracture testing procedure based on moiré interferometry at elevated temperature, including preparations of specimens, fabrication of specimen grating, heating, maintenance of temperature, and loading until fracture, is detailed in Ref.
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