Impact testing of polymeric foam using Hopkinson bars and digital image analysis |
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Institution: | 1. Structural Impact Laboratory (SIMLab), Department of Structural Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway;2. Centre for Advanced Structural Analysis (CASA), NTNU, NO-7491 Trondheim, Norway;3. Department of Civil Engineering and Energy Technology, Oslo Metropolitan University, Post box 4, St. Olavs plass, NO-0130 Oslo, Norway |
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Abstract: | The present investigation aims at testing polymeric foam under impact loading using large diameter nylon Hopkinson bars and optical field measurements. Accurate average stress-strain relations can be obtained when soft large diameter polymeric pressure bars and the appropriate data processing are used. However, as there are generally no homogeneous strain and stress fields for polymeric foams, an optical field observation is needed. In contrast to quasi-static tests where the digital image correlation (DIC) measurement is commonly used, technical difficulties still remain for the reliable use of DIC under impact conditions. In this paper, an accurate synchronization method based on the displacement measurement of the end of pressure bars (calculated by a robust DIC algorithm) is preferred to conventional MCDL box time synchronization. Also, the bar end displacement measurement offers a complementary calibration method for the tension/strain conversion coefficient. Strain fields are obtained for tests on foam sample at impact velocities up to 20 m/s. The localized strain fields permit better understanding of the observed stress plateau from SHPB results. The relevance of the present method for establishing mechanical response of polymeric foam is then demonstrated. |
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Keywords: | Polymeric foam Impact testing Hopkinson bars High-speed imaging Digital image correlation |
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