have been contributed by impact splitting-tensile test, together with impact direct tension test [4-7]. Recently an experimental method for dynamic tensile testing by spalling is reported [8,9]. However, it does not appear that there is a standard test method for measuring impact tensile strength of concrete materials [10]. This paper concerns with the measuring method for the impact tensile strength and strain rate sensitivity of concretes by means of the measuring method of reflected tensile stress waves. The experimental method has been proposed for brittle materials such as plaster, ceramics and concrete in our previous papers [11-15]. The impact tensile experiment is conducted by the Hopkinson bar technique and it is based on the superposition and concentration of tensile stress waves reflected both from the free ends of a striking bar and a specimen bar. The impact tensile experiment for concretes was carried out and the tensile strength of concretes under impact loadings was discussed as well as the effect of strain rates. This study focused on the estimation and measurement of strain rates using crack gages. Because the tensile stress region varies with time in this measuring method, it is necessary to find the beginning time when a specimen bar is broken after starting of tensile stress waves superposition, in other words, the gage length that the tensile stress waves have reached until the initiation of tensile break. Consequently, the impact tensile strength of concrete at the strain rate of 100 sec-1 was found to be approximately twice of the static tensile strength, and it was remarkably influenced by strain rates ranging from 100 to 101sec-1. STRESS WAVE PROPAGATION IN A CONCRETE BAR The measuring theory had already been presented in the previous papers [11,13], but it is briefly explained here by means of a numerical simulation. Figure 1 (a) shows a concrete bar specimen with the length of l and an incident compressive stress σ0 into the impact end of the bar. T= t/t0 is the dimensionless time and t0= l/c0 is the time of the wavefront reaching at the free end of the bar. c0 is the velocity of stress wave in the concrete specimen bar. Assuming that the pulse length of an incident compressive stress has the same length with a concrete specimen bar, the numerical simulation was carried out under the condition of two-dimensional axisymmetric model using a FEM code, MARC. (b) -7/6 T=0.5 T=1.0 T=1.2 T=1.3 T=1.45 T=1.5 T=1.51 T=1.55 T=1.7 T=1.8 Compressive d l -5/6 -3/6 -1/6 Incident compressive stress 1/6 1 3/6 5/6 0 T = 1.0 Nondimensional time 7/6 Tensile (a) Figure 1: (a) Concrete specimen bar and an incident compressive stress in a numerical analysis. (b) Propagation of incident compressive stress wave and development of tensile stress region produced by reflected tensile stress waves.
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