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High strain-rate compressive strain of welded 300W asteel joints
Author(s)
Magoda, Cletus Matthew
Date Issued
2011
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
The split Hopkinson pressure bar (SHPB) test is the most commonly used method for
determining material properties at high rates of strain. The theory governing the specifics of
Hopkinson bar testing has been around for decades; however, it has only been for the last
decade or so that significant data processing advancements have been made. It is the intent of
this thesis to offer the insight of application of SHPB to determine the compressive dynamic
behaviour for welded low carbon steel (mild steel). It also focuses on the tensile behaviour for
unheat-treated and heat-treated welded carbon steel.
The split Hopkinson Pressure bar apparatus consists of two long slender bars that sandwich a
short cylindrical specimen between them. By striking the end of a bar, a compressive stress
wave is generated that immediately begins to traverse towards the specimen. Upon arrival at
the specimen, the wave partially reflects back towards the impact end. The remainder of the
wave transmits through the specimen and into the second bar, causing irreversible plastic
deformation in the specimen. It is shown that the reflected and transmitted waves are
proportional to the specimen's strain rate and stress, respectively. Specimen strain can be
determined by integrating the strain rate. By monitoring the strains in the two bars and the
specimen's material, stress-strain properties can be calculated.
Several factors influence the accuracy of the results, including the size and type of the data
logger, impedance mismatch of the bars with the specimens, the utilization of the appropriate
strain gauges and the strain amplifier properties, among others. A particular area of
advancement is a new technique to determine the wave's velocity in the specimen with respect
to change in medium and mechanical properties, and hence increasing the range of application
of SHPB. It is shown that by choosing specimen dimensions based on their impedance, the
transmitted stress signal-to-noise ratio can be improved. An in depth discussion of realistic
expectations of strain gages is presented, along with closed form solutions validating any
claims.
The thesis concludes with an analysis of experimental and predicted results. Several
recommendations and conclusions are made with regard to the results obtained and areas of
improvement are suggested in order to achieve accurate and more meaningful results.
determining material properties at high rates of strain. The theory governing the specifics of
Hopkinson bar testing has been around for decades; however, it has only been for the last
decade or so that significant data processing advancements have been made. It is the intent of
this thesis to offer the insight of application of SHPB to determine the compressive dynamic
behaviour for welded low carbon steel (mild steel). It also focuses on the tensile behaviour for
unheat-treated and heat-treated welded carbon steel.
The split Hopkinson Pressure bar apparatus consists of two long slender bars that sandwich a
short cylindrical specimen between them. By striking the end of a bar, a compressive stress
wave is generated that immediately begins to traverse towards the specimen. Upon arrival at
the specimen, the wave partially reflects back towards the impact end. The remainder of the
wave transmits through the specimen and into the second bar, causing irreversible plastic
deformation in the specimen. It is shown that the reflected and transmitted waves are
proportional to the specimen's strain rate and stress, respectively. Specimen strain can be
determined by integrating the strain rate. By monitoring the strains in the two bars and the
specimen's material, stress-strain properties can be calculated.
Several factors influence the accuracy of the results, including the size and type of the data
logger, impedance mismatch of the bars with the specimens, the utilization of the appropriate
strain gauges and the strain amplifier properties, among others. A particular area of
advancement is a new technique to determine the wave's velocity in the specimen with respect
to change in medium and mechanical properties, and hence increasing the range of application
of SHPB. It is shown that by choosing specimen dimensions based on their impedance, the
transmitted stress signal-to-noise ratio can be improved. An in depth discussion of realistic
expectations of strain gages is presented, along with closed form solutions validating any
claims.
The thesis concludes with an analysis of experimental and predicted results. Several
recommendations and conclusions are made with regard to the results obtained and areas of
improvement are suggested in order to achieve accurate and more meaningful results.
Additional information
Thesis (MTech (Mechanical Engineering))--Cape Peninsula University of Technology, 2011
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Thesis
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