Control of peak stresses in tensile testing machine components subjected to shock loading Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4f16c5899

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  • The mechanical properties of materials at cryogenic temperatures have been the subject of increasing interest during the past few years, although very little actual materials testing has been done at extremely low temperatures because of the high cost and low heat of vaporaization of liquid helium. Recently, the Department of Mechanical and Industrial Engineering at Oregon State University initiated a research program of materials testing at liquid helium temperatures, 4.2° K. A specially designed cryogenic tensile testing machine has been built employing a variable rate of loading and an electronic chart drive recorder. A Baldwin U-1 load cell was placed in the loading column of the testing machine to measure the magnitude of the applied load. Previous experience has found that damage to the load cell can occur because of the shock environment present upon fracture of a tensile specimen. The object of this thesis is to analyze the effects of shock loading on the cryogenic tensile testing machine and to develop a means of reducing any shock environment seen by the load cell, After determining that 92.6 percent of the strain energy in the loading column was stored in the tensile rod between the load cell and tensile specimen, a theoretical analysis of the response of the tensile rod and load cell to specimen fracture was made using the mathematical model of a long, slender rod with fixed-free end conditions and a suddenly removed axial load. The theoretical fundamental frequencies of the tensile rod and load cell were found to be 1570 and 5940 cycles per second, respectively. In order to verify the problem of a shock environment imposed on the load cell and to check the theoretical results, an experimental study of the actual system was made. Strain gages on the tensile rod, on the adapter above the load cell, and in the load cell measured the motion response of these components to specimen fracture at room temperature. An oscilloscope and camera were used to record the results. The actual fundamental frequencies of the tensile rod and load cell were 1080 and 3250 cycles per second, respectively, and the assumed problem of shock environment on the load cell was verified. After studying many methods of absorbing and damping the shock loading, it was decided to design and fabricate a shock absorber having a piston and cylinder arrangement with a shock absorbing medium placed between the piston and cylinder head. Polyethylene and polyester urethane foams were chosen as possible shock absorbing mediums because of their excellent energy dissipation properties in compression. After the shock absorber was fabricated and placed in the testing machine, its effectiveness was determined by following the experimental procedures previously used. Results indicated that the initial load release was adequately damped with polyethylene foam. Less damping was noticed with the polyester foam. In both cases, however, a delayed peak compressive force on the load cell occurred three milliseconds after specimen fracture. When it was determined that this force was due to the motion of the top plate of the testing machine, the jack on the top plate was spring mounted. Test results of this system showed that the maximum peak compressive force following specimen fracture was reduced to 15 percent of the tensile force before fracture when polyethylene foam was used as the shock absorbing medium; however, when the polyester foam was used, the test results indicated that the foam bottomed out. Polyethylene foam thus proved to be a substantially superior solution to the problem under consideration.
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