This paper presents a modified Split Hopkinson Pressure Bar (SHPB) technique to obtain compressive stress strain data for NBR rubber materials. An experimental technique with a modified the conventional SHPB has been developed for measuring the compressive stress strain responses of materials with low mechanical impedance and low compressive strengths, such as the rubber and the polymeric material. This paper uses an aluminum pressure bar to achieve a closer impedance match between the pressure bar and the specimen materials. In addition, a pulse shaper is utilized to lengthen the rising time of the incident pulse to ensure dynamic stress equilibrium and homogeneous deformation of NBR rubber materials. It is found that the modified technique can determine the dynamic deformation behavior of rubbers more accurately.
The conventional split Hopkinson pressure bar (C-SHPB) technique with a special experimental apparatus is used to obtain a dynamic deformation material behavior under a high strain rate loading condition. An experimental modification is introduced to reduce the non-equilibrium on the dynamic material response during a short test period for two polymeric materials. The proposed method uses aluminum pressure bars to achieve a closer impedance match between the pressure bars and the specimen materials such as hot temperature degraded POM (Poly Oxy Methylene) and PP (Poly Propylene) to obtain more distinguishable experimental signals. In addition, a pulse shaper technique is used for increasing the rise time of the incident pulse to ensure the dynamic stress equilibrium and the homogeneous deformation in the specimen under dynamic compression loading condition. The details on the dynamic stress equilibrium and the duration of uniform strain rate during the dynamic deformation of the specimen are experimentally investigated. The effects of degradation at a few different hot temperatures on the maximum compressive stresses are also experimentally studied under varying impulsive loading conditions.
Cracks play an important role in evaluating the strength and failure behavior of engineering rock mass. In order to increase the understanding of strength and failure mechanism of precracked rock, crack propagation and coalescence from preexisting cracks under true triaxial compression are investigated using true triaxial compression tests and Cellular Automata Software for engineering Rockmass fracturing process (CASRock). Three types of specimens were studied experimentally and numerically. Experimental and numerical results show that both the preferential angle and areal intensity of preexisting cracks can affect the compressive strength and failure behavior of the specimens. The peak strength firstly decreases and then increases with increase of the preferential angle. Also, the peak strength nonlinearly decreases with the increase of cracks’ areal intensity. The numerical results show that the crack initiation and coalescence are observed and characterized from the inner and outer tips of preexisting cracks in specimens containing single crack and multiple parallel cracks. The main shear failure in the specimen containing multiple unparallel preexisting cracks initiate and propagate from one of the macroscopic preexisting cracks, and other preexisting cracks do not initiate, propagate, and coalesce until reaching the peak strength.
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