A Tensile Split Hopkinson Bar for Testing Particul

Authors: A T Owens HV Tippur

Publish Date: 2008/11/26

Volume: 49, Issue: 6, Pages: 799-

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Abstract

A tensile split Hopkinson bar apparatus is developed for testing high strain rate behavior of glassfilled epoxy The apparatus uses a specimen gripping configuration which does not require fastening and/or gluing and can be readily used for castable materials Details of the experimental setup design of grips and specimen specimen preparation method benchmark experiments and tensile responses are reported Also the effects of filler volume fraction 0–30 and particle size 11–42 μm are examined under high rates of loading and the results are compared with the ones obtained from quasistatic loading conditions The results indicate that the increase in the loading rate contributes to a stiffer and brittle material response In the dynamic case lower ultimate stresses are seen with higher volume fractions of filler whereas in the corresponding quasistatic cases an opposite trend exists However the absorbed specific energy values show a decreasing trend in both situations The results are also evaluated relative to the existing micromechanical models The tensile response for different filler sizes at a constant volume fraction 10 is also reported Larger size filler particles cause a reduction in specimen failure stress and specific energy absorbed under elevated rates of loading In the quasistatic case however the ultimate stress is minimally affected by the filler sizeSpecimen Grip Design A dovetail shaped specimen gripping mechanism was chosen in an effort to minimize the attenuation that can occur in more complex attachment configurations such as specimens with threaded ends clamps with fasteners etc Finite element analysis was used to arrive at a specific shape that would allow the bars to be used repeatedly without damage The details of this process are outlined in the followingAn initial geometry was selected machined and tested in the actual setup For this initial specimen–bar interface geometry the dovetail in the incident bar end failed A closer look at this geometry with finite element analysis revealed that the stresses produced in the aluminum grips during the experiment due to the stress concentration were in excess of the failure stress for the 7075T6 aluminum being used Thus this geometry was used as a benchmark for refining the specimen–bar interface shape such that the tensile testing would not result in failure of the grip region of the incident barContact elements were used along the interface between the bar and specimen on the hatched surface shown in Fig 19c The contact was formulated for both the surface normal and tangential directions The constitutive law for contact elements included a linear stress–strain behavior in the direction normal to the surface with a stiffness of approximately ten times the stiffness of the aluminum grip The aluminum surface was chosen as the master surface In the direction tangential to the surface the friction between the aluminum grip master surface and specimen end slave surface was also accounted for using a stiffness penalty method In this method a certain amount of shear stress is carried across the interface between the master and slave surfaces The shear stress is directly proportional to the normal load between the surfaces This allowed for the estimation of the friction during the loading event The end of the bar was constrained from translation in the horizontal direction and a uniform pressure was applied on the end surface of the specimen as shown in Fig 19aThe plots in Fig 20c correspond to the normalized vonMises stresses along the line shown in Fig 20a The origin of the plot corresponds to the midplane of the cylindrical rod As can be seen iteration a exceeded the yield strength of the aluminum by 20 and iteration d of the dovetail design had significantly lower stresses

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