Showing papers by "Eric Brown published in 2017"

Structure of the system-spanning dynamically jammed region in response to impact of cornstarch and water suspensions

Abstract: We experimentally study the impact response of concentrated suspensions consisting of cornstarch and water to identify the structure of the dynamically jammed region that appears and propagates ahead of the impactor once it spans from the impactor to a solid boundary. Using particle tracking at the boundary opposite the impactor, we observed a dead zone about the same size as the impactor cross-section with no particle flow in the central part of the dynamically jammed region. In the outer part of the dynamically jammed region at the bottom boundary, we observed particle flow with a net transverse displacement of up to about 5\% of the impactor displacement, which indicates shear. Direct imaging to the surface of the outer part of the dynamically jammed region reveals dilation as in a dense granular flow, and cracks like a solid. This shear and dilation are in contrast to the dynamically jammed region as it propagates through the bulk, where it is argued to exhibit no shear or decrease in packing fraction. The particle flow at the boundary occurs with a delay after impact, at about the same time as the the strong stress response, confirming that the strong stress response is a consequence of this dynamically jammed structure spanning between the impactor and a solid boundary. These observations suggest the dynamically jammed structure can temporarily support stress like a soil or dense granular material along a network of frictional contacts between the impactor and solid boundary.

On compression and damage evolution in two thermoplastics

Abstract: The well-known Taylor cylinder impact test, which follows the impact of a flat-ended cylindrical rod onto a rigid stationary anvil, is conducted over a range of impact speeds for two polymers, polytetrafluoroethylene (PTFE) and polyetheretherketone (PEEK). In previous work, experiments and a model were developed to capture the deformation behaviour of the cylinder after impact. These works showed a region in which spatial and temporal variation of both longitudinal and radial deformation provided evidence of changes in phase within the material. In this further series of experiments, this region is imaged in a range of impacted targets at the Diamond synchrotron. Further techniques were fielded to resolve compressed regions within the recovered polymer cylinders that showed a fracture zone in the impact region. The combination of macroscopic high-speed photography and three-dimensional X-ray imaging has identified the development of failure with these polymers and shown that there is no abrupt transition in behaviours but rather a continuous range of responses to competing operating mechanisms. The behaviours noted in PEEK in these polymers show critical gaps in understanding of polymer high strain-rate response.

On compression and damage evolution in PTFE and PEEK

Abstract: The well-known Taylor cylinder impact test, that follows the impact of a flat-ended cylindrical rod onto a rigid stationary anvil, is conducted over a range of impact speeds for two polymers, PTFE and PEEK. In previous work experiments and a model were developed to capture the deformation behaviour of the rod after impact. A distinctive feature of these works was that a region in which both spatial and temporal variation of both longitudinal and radial deformation showed evidence of changes in phase within the material. This region is X-ray imaged in a range of impacted targets at the I13 Imaging and Coherence beam line at the Diamond synchrotron. Further techniques were fielded to resolve compressed regions within the recovered polymer cylinders that showed a fracture zone in the impact region. This shows the transit of damage from ductile to brittle failure results from previously undetected internal failure.

Constitutive Modeling of the Dynamic-Tensile-Extrusion Test of PTFE

Abstract: Use of polymers in defense, aerospace and industrial applications under extreme loading conditions makes prediction of the behavior of these materials very important. Crucial to this is knowledge of the physical damage response in association with phase transformations during loading and the ability to predict this via multi-phase simulation accounting for thermodynamical non-equilibrium and strain rate sensitivity. The current work analyzes Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) experiments on polytetrafluoroethylene (PTFE). In particular, the phase transition during loading and subsequent tension are analyzed using a two-phase rate sensitive material model implemented in the CTH hydrocode. The calculations are compared with experimental high-speed photography. Deformation patterns and their link with changing loading modes are analyzed numerically and correlated to the test observations. It is concluded that the phase transformation is not as critical to the response of PTFE under Dyn-Ten-Ext loading as it is during the Taylor rod impact testing.

Back stress in modeling the response of PEEK and PC

Abstract: With the development of new methods for the characterization of equilibrium stress through cyclic loading, it is now possible to follow the evolution of back stress during the nonlinear deformation of polymers. Experiments on PEEK and PC below the glass-transition temperature indicate a back stress that may evolve with plastic deformation, and which is substantially different from that seen during the response in the rubbery range. In particular, the back stress during the response of PC shows the characteristic post-yield softening, possibly indicating that the observed post-yield softening in the response comes from the back stress. This is not seen in PEEK, which also shows no substantial post-yield softening. The equilibrium stress plays a central role in modeling both the quasi-static and dynamic response of PEEK.