13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Strain Rate Concentration Factor for Double-Edge-Notched Specimens Subjected to High Speed Tensile Loads Nao-Aki NODA 1,* , Yoshikazu SANO1, Makoto ANDO1, Yoshihito KUROSHIMA1,Takahiro SHINOZAKI1, Hayato OHTSUKA1 and Wenhai GUAN1 1 Department of Mechanical and Control Engineering, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu-city, Fukuoka, 804-8550, Japan * Corresponding author: noda@mech.kyutech.ac.jp Abstract Engineering plastics provide superior performance to ordinary plastics for wide range of the use. For polymer materials, dynamic stress and strain rate may be major factors to be considered when the strength is evaluated. Recently, high speed tensile test is being recognized as a standard testing method to confirm the strength under dynamic loads. In this study, therefore, high speed tensile test is analyzed by the finite element method; then, the maximum dynamic stress and strain rate are discussed with varying the tensile speed and maximum forced displacement. The strain rate concentration factor found to be constant independent of tensile speed, which is defined tK ε as the maximum strain rate appeared at the notch root over the average nominal strain rate at the minimum section. The maximum strain rate is controlled by the tensile speed alone independent of the magnitude of the forced displacement. It is found that the difference between static and dynamic maximum stress concentration (σmax-σst) at the notch root is proportional to the tensile speed when u/t≦5000mm/s. Keywords Stress Concentration, Notch, Dynamic Stress, Strain Rate, Finite Element Method 1. Introduction Engineering plastics are widely used in everyday products. Typically, a suitable engineering plastic is chosen for its range of enhanced physical properties. It is know that polycarbonate has superior impact and perforation resistance compared with other polymers, or indeed compared with some structural metals [1]. Most thermoplastics far below their glass transition temperature Tg give a brittle fracture when deformed in uniaxial tension. However, polycarbonate is an exception and deformed in a ductile manner. However, Izod impact studies of notched specimens show that the fracture mode changes from ductile to brittle below Tg. To investigate the brittle-ductile transition, which is affected by temperature and loading speed [2, 3], a high-speed tensile test is being recognized as a standard testing method in recent years. Generally, bluntly notched specimens failed in a fully ductile manner, and sharply notched specimens failed in brittle manner depending on the strain rate at the notch root. It should be noted that Izod and Charpy impact tests are not suitable for evaluating the impact strength of real products because the impact speeds do not correspond to the real failure. In the high-speed tensile test, it is necessary to obtain the strain rate correctly to understand the impact strength of the polymer specimen. For smoothing specimens, the strain rate can be determined as u tl ε = from the specimen lengthl and the tensile speed u t . On the other hand, for notched specimens, it is necessary to measure the strain at the notch root by strain gauge measurement, for example. However, because only the average value of the strain concerning the gauge width can be measured. It is not easy to measure the strain at the notch root. In the previous studies for dynamic stress concentration, circular holes [4] and elliptical holes [5] were investigated under step load [6, 7] and pulse load [7, 8]. In addition, several review papers for
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