Research On The Evolution Of Dynamic Damage Of Concrete And Mortar

Concrete and mortar (concrete-like material) are most widely used materials in thenational economy construction and defense engineering. In practice, concrete and mortarare often in the dynamically loaded three-dimensional stress state. But for SHPB tests ofconfined concrete-like materials, the research on the dynamic mechanical properties anddamage evolution of every small range of strain is still seriously deficient. Ls-dynasoftware is introduced in this paper to investigate the effect of axial strain acceleration onenhanced strength in mortar SHPB test; According to specification for mix proportiondesign of mortar and concrete, the confined specimens made by water-drill rig are tested inthe74mm SHPB device with damage-freezing technology to explore the dynamicmechanical properties and damage evolution. Finally, results can be obtained as follows:(1) According to JGJ55-2011and JGJ/T98-2010, specification for mix proportiondesign of50MPa concrete and30MPa mortar are designed, and compared specimens madeby water-drill rig and by PVC pipe casting method, the samples are considered to be morehomogeneous and less porous, which can reflect properties of the concrete-like materialmuch better.(2) By the MTS testing machine, static compressive strength of50MPa concrete and30MPa mortar are47.5MPa and33.3MPa respectively. And the errors between the designedstrength and tested strength are permitted.(3) In order to ensure the specimens in a uniform stress state in SHPB tests on concreteand mortar, a larger diameter rod SHPB is needed; So as to maintain the accuracy of thetests, long bullet (50cm), shapers and universal joints are supposed to be introduced;Meanwhile, to ensure the safety, impact velocity bullet cannot be infinite.(4) With a new method in which both the damage "freezing in” technique and the“passive confining pressure load”technique are introduced, concrete and mortarspecimens made by water-drill rig are tested withφ74mm Split Hopkinson Pressure Barimpacted by a bullet at different velocities for many times. Finally, curves of axialsress-strain, tangent strain-axial stain and tangent stress-time have been achieved to analyzedifferent damage in every impact.(5) The axial enhanced strength caused by the acceleration of the axial strain is verysmall by simulation tests with Ls-dyna, which can be ignored. The strain rate of the sampleequals1.0x106s-1or so. While, the strain rate of axial units of the inner sample is much lessthan this dimension.