13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Thermal shock residual strength of ultra-high temperature ceramics with the consideration of temperature Weiguo Li1,*, Dingyu Li1, Ruzhuan Wang1, Daining Fang2 1 College of Resource and Environment Science, Chongqing University, 400030, PR China 2 LTCS and College of Engineering, Peking University, 100871, PR China * Corresponding author: wgli@cqu.edu.cn Abstract When suffered from severe thermal shock ultra-high temperature ceramics for high-temperature applications will present to strength reduction or fracture due to microcrack and macrocrack propagation induced by the shocks. And the thermal shock residual strength is one of the most important indexes that evaluate the ceramic material for further use after thermal shock. This paper established a temperature dependent thermo-fracture mechanics model to predict the residual strength of ultra-temperature ceramics after thermal shock. And the critical thermal shock temperature that causes the material strength drop is determined and show differences from the one without temperature-dependent material properties. Also these studies demonstrate the significance of incorporating temperature-dependent material properties on the prediction of thermal shock residual strength of ultra-high temperature ceramic materials. Keywords Ultra-high temperature ceramics, Thermal shock, Fracture, Residual strength. 1. Introduction Ultra-high temperature ceramics (UHTCs) are a family of ceramic based composites mainly consisted of transition metal compounds, particularly refractory borides and carbides composites of Zr, Hf and Ta, such as ZrB2, TaC, HfN and HfB2, which have melting points higher than 3000 oC. Essentially, these UHTCs possess an excellent and unique set of bulk properties including unusually high melting points, high thermal conductivity, high elastic modulus and retain strength at high temperatures. And they can be potentially used at temperatures above 2000oC in an oxidizing environment [1-2]. This combination of properties make these materials potential candidates for a variety of high-temperature structural applications, including engines, thermal protection system such as leading edges and nose-cones for a new generation of hypersonic vehicles, plasma arc electrodes, furnace elements, and high temperature shielding [3-5]. In these applications, UHTCs usually experience severe thermal shock. And it is known that cracking and other forms of damages are induced in UHTCs or ceramic composites when subjected to severe thermal shocks. As a result, the strength of thermally shocked UHTCs can be significantly degraded. Therefore, it is necessary to investigate thermal shock fracture resistance behavior of UHTCs of these advanced materials. Significant progress has been made in the understanding of thermal shock fracture behavior of ceramic materials with great efforts of theories and experiments done by many researchers. At present, the research of thermal shock resistance mostly focuses on the effects of surface defects, temperature, indentation crack length [7,8,9], particle reinforced [10,11] or whisker reinforced [12] on thermal shock resistance performance to explain the mechanisms of thermal shock failure in experimental way. Theoretical researches of the effect of surface heat transfer coefficient on thermal shock resistance had been made [13], and several evaluation theories of thermal shock resistance had been reported [6,14,15,16]. In many experiment research works of thermal shock fracture behavior, residual strength is usually used to evaluate thermal shock resistance behavior of UHTCs [5,8,9,15]. Because the microcracks exist in ceramics, when a ceramic specimen is subjected to sufficiently severe thermal shocks, some of the pre-existing micro-cracks will initiate and grow to form macrocracks. Crack propagation in thermally shocked ceramics may be arrested depending on the severity of thermal shock, thermal stress field characteristics and material properties. What is more, the thermal shock
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