Full Field Strain Measurement in Compression and Tensile Split Hopkinson Bar Experiments |
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Authors: | A Gilat T E Schmidt A L Walker |
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Institution: | (1) Department of Mechanical Engineering, The Ohio State University, Columbus, OH 43210, USA;(2) Trilion Quality Systems, West Conshohocken, PA 19428, USA |
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Abstract: | The 3D image correlation technique is used for full field measurement of strain (and strain rate) in compression and tensile
split Hopkinson bar experiments using commercial image correlation software and two digital high-speed cameras that provide
a synchronized stereo view of the specimen. Using an array of 128 × 80 (compression tests) and 258 × 48 (tensile tests) pixels,
the cameras record about 110,000 frames per second. A random dot pattern is applied to the surface of the specimens. The image
correlation algorithm uses the dot pattern to define a field of overlapping virtual gage boxes, and the 3-D coordinates of
the center of each gage box are determined at each frame. The coordinates are then used for calculating the strains throughout
the surface of the specimen. The strains determined with the image correlation method are compared with those determined from
analyzing the elastic waves in the bars, and with strains measured with strain gages placed on the specimens. The system is
used to study the response of OFE C10100 copper. In compression tests, the image correlation shows a nearly uniform deformation
which agrees with the average strain that is determined from the waves in the bars and the strains measured with strain gages
that are placed directly on the specimen. In tensile tests, the specimen geometry and properties affect the outcome from the
experiment. The full field strain measurement provides means for examining the validity and accuracy of the tests. In tests
where the deforming section of the specimen is well defined and the deformation is uniform, the strains measured with the
image correlation technique agree with the average strain that is determined from the split Hopkinson bar wave records. If
significant deformation is taking place outside the gage section, and when necking develops, the strains determined from the
waves are not valid, but the image correlation method provides the accurate full field strain history. |
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Keywords: | Split Hopkinson bar Digital image correlation High strain rate Copper |
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