Low Strain Rate Testing Based on Weight Drop Impact Tester

Transcription

1 Proceedings of the SEM Annual Conference June 1-4, 2009 Albuquerque New Mexico USA 2009 Society for Experimental Mechanics Inc. Low Strain Rate Testing Based on Weight Drop Impact Tester Guojing Li and Dahsin Liu Dept. of Mechanical Engineering, Michigan State University, East Lansing, MI ABSTRACT Material properties are fundamentally important to mechanical analysis and simulation. Material properties under quasi-static loading conditions can be easily identified with the use of a servo-hydraulic testing machine. Material properties at high strain rates, such as 10 2 /s to 10 5 /s, can be characterized with the use of the well-established split Hopkinson s pressure bars. For material properties at strain rates higher than 10 5 /s, plate impact and shock tube based techniques have also been explored. However, there seems to be lack of a standard and even practical technique for identifying material properties with low strain rates between quasi-static loading and 500/s. The low strain rate testing has become more and more important for characterizing low stiffness materials, such as plastics and their composites, as they are used for vehicle applications. The objective of this project is to identify a feasible testing technique for low strain rate testing. As a first approach, the project is focused on evaluating the commonly used weight drop impact testers. The advantages are certainly due to its availability and simplicity. However, since the simple weight drop impact testers are of open-loop type and has no feed back system, there is a need to correct, or control, the testing speed. In this project, a simple technique was identified to be feasible for testing some soft materials. The applications of the modified drop weight impact tester for many other low-stiffness materials may also be achieved.. 1. INTRODUCTION Based on a closed-loop operation, the conventional servo-hydraulic testing machine is limited to quasi-static testing although it can provide constant strain rate testing. On the contrary, a drop weight impact testing machine can provide higher testing speeds than the conventional servo-hydraulic testing machine. However, because it is operated under an open-loop condition, it cannot offer constant strain rate testing. In order to modify a servo-hydraulic testing machine for higher strain rate testing, one possible way is to modify it for an open-loop operation. Similarly, in order to modify a drop weight impact testing machine for constant strain rate testing, one possible way is to modify it for a closed-loop operation [1]. Modifying a machine from a closed-loop operation to an open-loop operation is relatively simpler than that from an open-loop operation to a closed-loop operation because it does not involve a sophisticated feedback system. However, a compensating technique is still needed to maintain the constant strain rate testing capability. The shaper technique used in split Hopkinson s pressure bars (SHPB), which is also based on an open-loop operation,

2 seems to offer an alternative solution. When a material is impacted by the striker bar of an SHPB, the strain rate during the test may not undergo a constant condition. This is due to the fact that the random combination of the striking force and the stiffness of the material do not necessarily warrant a constant strain rate testing condition. In fact, a delicate balance between the force and the stiffness is required to reach such an ideal condition. A shaper technique [2,3] has been used for promoting constant strain rate conditions in SHPB, at least approximately, along with other advantages. A shaper is an additional material installed in between the striker bar and the incident bar of an SHPB. The stiffness and thickness of the shaper material is carefully selected so a more uniform strain rate condition with a longer duration can be achieved during the test. Although the design of an effective shaper requires many iterative experiments or may be predicted by some analysis, it is much simpler than modifying an SHPB into a closed-loop testing system which requires a feedback mechanism involving sophisticated mechanical and electrical designs. The iterative experiments involved in designing a shaper to achieve a constant strain rate condition, at least approximately, in SHPB testing can be applied to modify an open-loop servo-hydraulic testing machine for achieving more uniform strain rate testing. However, instead of using a shaper in the servo-hydraulic testing machine to modify the stiffness involved in the testing, the testing experience, i.e. the force-deformation history, in a test can be used to modify the loading speed control in the subsequent test to improve the strain rate history to be more uniform. Repeating this process a few cycles, i.e. updating the loading speed control in a test based on the previous testing experience, a satisfactory constant strain rate testing condition can be reached. The high-speed servo-hydraulic system available in the market is based on this design concept. Based on the features listed above, the high-speed servo-hydraulic testing system can provide a range of constant strain rates from 10-3 /s to above 10 2 /s. On the low end, the strain rate widely overlaps with that of a conventional servo-hydraulic testing machine, which covers the range between 10-4 /s and 1/s, while on the high end, it narrowly overlaps with that of an SHPB, which functions between 10 2 /s to 10 5 /s. Figure 1 [4] shows three testing examples based on polymeric materials PE, PMMMA and PMMA. The peak stresses are dependent on the strain rates, which were obtained from a conventional servo-hydraulic testing machine, a high-speed servo-hydraulic testing system, a drop weight impact testing machine and an SHPB. It should be pointed out that the drop weight impact testing machine is based on an open-loop operation and usually does not offer constant strain rate testing as it does not include a compensating technique. Accordingly, there is a clear need to use a high-speed servo-hydraulic testing system or to modify the drop weight impact testing machine to identify the material response, at least, between 1/s and 5x10 2 /s. The objective of this project is to explore the feasibility of modifying a drop weight impact testing machine for near constant low strain rate uses. 2. EXPERIMENTAL SETUP An existing weight drop impact testing machine was modified to provide near constant testing rates. It included an addition of a mass of 300kg to the loading head and the installation of a shaper between the loading head and the specimen as shown in Figure 2. As the mass of the loading head increased, the rate of deformation of the specimen became more constant. The selection of the material type of the shaper and its thickness, however, was empirical although the experience learned in SHPB might be useful. Figure 2 also shows measurement techniques for displacement and forces.

3 Figure 1 Rate dependence of peak stresses covered by various testing machines. Shaper Specimen LVDT Load Cell Figure 2 Experimental Setup.

4 3. EXPERIMENTAL RESULTS Figure 3 shows the history of the measurements of the velocity of the loading head, the displacements from the LVDT sensors and the load from the load cell. The strain rates seem to be close to constant. Correlating the force and the displacement, the stress-strain curves for the specimen at three different strain rates were established and shown in Figure 4. Figure 3 History of velocity, displacement and load Stress (MPa) Static Loading Strain Rate: 6 1/s Strain Rate: 50 1/s Strain Figure 4 Stress-strain relations at various strain rates.

5 4. SUMMARY This project presented a technique for characterizing soft materials at low strain rates to bridge the quasi-static loading and high strain rates, such as between 1/s and 50/s. The primary concern in this project was to achieve constant strain rates, at least near constant strain rates. The large mass added to the loading head seemed to be feasible. ACKNOWLEDGEMENTS The authors wish to express their sincere thanks to the U.S. Army TACOM for financial support and the project monitor Dr. B.B. Raju of TARDEC, Warren, Michigan. REFERENCES [1] G. Li, Development of instrumented wireless projectile for low-velocity impact, Michigan State University Ph.D. Thesis, December, [2] W. Chen, B. Zhang and M.J. Forestal, A split Hopkinson s pressure bar technique for low-impedeance materials, Experimenal Mechanics, 39, 81-85, [3] B. Song and W. Chen, Dynamic stress equilibrium in split Hopkinson s pressure bar tests on soft materials, Experimental Mechanics, rr(3), , [4] X. Xiao, Dynamic tensile testing of plastic materials, Polymer Testing, 27, , S.A. Silling, Reformulation of elasticity theory for discontinuities and long-range force, Journal of the Mechanics and Physics of Solids, 48, , 2000.