13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Crack Propagation in PMMA Plates under Various Loading Conditions Ivan Smirnov*, Yuri Sudenkov St. Petersburg State University, 198504, Russia * Corresponding author: ivansmirnov@math.spbu.ru Abstract The experimental results of the dynamics of crack propagation in polymethylmethacrylate plates under quasi-static and dynamic loading are presented. Registration of the cracks was made by the method of slit-scanning of an image using a streak camera. Either a crack trajectory or caustic at the crack front was recorded in the experiments. It is shown that crack front extension has a stepwise character for any type of loading. However the average speed at the quasi-static loading increases smoothly up to the maximum value, and the average speed at pulsed loading takes the maximum value rather instantly. The value of the stress intensity factor at the moment of crack start under dynamic loading essentially exceeds the corresponding value for quasi-static loading. Furthermore, under dynamic loading, the crack speed depends on the thickness of the sample. Keywords Crack propagation, Dynamic loading, Quasi-static loading 1. Introduction Despite the fact that our knowledge constantly updates with experimental data about the process of fracture, questions about the conditions of initiation, propagation and stop of fracture remain actual. The problem is compounded by the fact that the data, which are used to develop the theoretical approaches, have been obtained at different scales, with different spatial and temporal resolution of recording equipment, under various load conditions and energy exchange of a sample and external environment. Crack propagation in quasi-brittle and brittle materials has been studied for many decades. However, there is no complete understanding of the crack development process. Experimental results on quasi-static tensile of plates with a notch [see e.g. 1] lead to the conclusion that the speed of a crack is a monotonic function of time, and the relationship between the speed of a crack and the stress intensity factor can be described by an L-shaped curve. Studies of cracks in plates under dynamic load [2] showed that the speed of a crack is constant, but the corresponding stress intensity factor can change. In papers [3, 4] showed that the speed of a crack is unstable and stepwise. Such divergence of results suggests an idea about need of carrying out "systematizing" experiments which will allow to look at the behavior of strength characteristics of a material in terms of the scale factors, structural features and characteristics of energy input into the material. In this work we have made an attempt to conduct the generalizing study of crack propagation process in brittle and quasi-brittle materials under quasi-static and dynamic loads. The loading schemes have been applied by analogy to the schemes in [1-4]. The quasi-static loading was carried out by slow uniaxial tension of plane samples with a starter notch. The dynamic load was carried out by means of electrical explosion of a wire between edges of a notch in a plane sample. Polymethyl methacrylate was chosen as the model material. PMMA shows quasi-brittle fracture and
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