Showing papers by "Eric Brown published in 2011"

Use of the tapered double-cantilever beam geometry for fracture toughness measurements and its application to the quantification of self-healing:

Abstract: The successful invention of self-healing polymer composites a decade ago necessitated a methodology to quantify the ability of the material to heal and recover structural properties following damage. Healing efficiency was defined as the ratio of healed to virgin fracture toughness, η = KIChealed/KICvirgin. Early work took advantage of the crack length independence offered by a tapered double-cantilever beam (TDCB) fracture geometry to simplify calculation of healing efficiency to the ratio of healed to virgin critical loads, η = PChealed/PCvirgin. The current work investigates the application of the TDCB geometry and three common geometries utilized in the broader fracture literature (the compact tension (CT), single-edge notch bend (SENB), and single-edge notch tension (SENT) geometries) to the measurement of healing efficiency. While the TDCB geometry simplifies the calculation of healing efficiency because the crack lengths do not need to be accounted for, it is shown that if the virgin and healed cra...

Constitutive modeling of shock response of polytetrafluoroethylene

Abstract: Polytetrafluoroethylene (PTFE) is a polymer with a simple atomic structure that shows complex behavior under pressure and demonstrates a highly variable metastable phase structure in shock waves with amorphous and crystalline components In turn, the crystalline component has four known phases with the high-pressure transition of the crystalline domain from crystalline phase IV at ambient through phase II to III At the same time, as has been recently studied using spectrometry, the crystalline region nucleates from the amorphous one with load Stress and velocity shock-wave profiles acquired recently with embedded gauges demonstrate features that may be related to the impedance mismatch between the phase domains subjected to such transitions resulting in variations of mechanical and thermophysical characteristics We consider the inter-phase non-equilibrium and the amorphous-to-crystalline and inter-crystalline transitions that are associated with the high pressure and temperature transformations under shock wave loading as possible candidates for the analysis The present work utilizes a multi-phase constitutive model that considers strength effects to describe the observed response under shock loading of the PTFE material Experimental plate impact shock-wave histories are compared with calculated profiles using kinetics describing the transitions The study demonstrates that the inter-phase pressure non-equilibrium of the state parameters plays the key role in the delay of the shock wave attenuation At the same time, the forward transition associated with the crystallization might be responsible for the velocity spike in the experimental velocity profiles at high impact velocity and the modulus variation at low impact velocity On the other hand, an accelerated attenuation of the velocity in the rarefaction wave is associated with another transition resulting in the residual crystallinity change during unloading

Dynamic-tensile-extrusion response of polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE)

Abstract: Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) experiments have been utilized to probe the dynamic tensile responses of polytetrafluoroethylene (PTFE) and polychlorotrifluoroethylene (PCTFE). These fluoropolymers exhibit more irregular deformation and stochastic-based damage and failure mechanisms than the stable plastic elongation and shear instabilities observed in metals. The technique elucidates a number of tensile mechanisms that are consistent with quasi-static, SHPB, and Taylor Impact results. Similar to the observed ductile-to-brittle transition for Taylor Impact loading, PCTFE failure occurs at a peak velocity greater than for PTFE. However, for the Dyn- Ten-Ext PCTFE exhibits even greater resistance to failure due to the tensile stress-state. While PTFE generates a large number of small fragments when extruded through the die, PCTFE draws out a smaller number of larger particles that dynamically evolve during the extrusion process through a combination of local necking mechanisms and bulk relaxation. Under Dyn-Ten-Ext loading, the propensity of PTFE to fail along normal planes is observed without indication of any localization, while the PCTFE clearly forms necks during the initial extrusion process that continue to evolve.

Dynamic-Tensile-Extrusion of Polyurea

Abstract: Polyurea was investigated under Dynamic-Tensile-Extrusion (Dyn-Ten-Ext) loading where spherical projectiles were propelled at 440 to 509 ms-1 through a conical extrusion die with an area reduction of 87%. Momentum of the leading edge imposes a rapid tensile deformation on the extruded jet of material. Polyurea is an elastomer with outstanding high-rate tensile performance of interest in the shock regime. Previous Dyn-Ten-Ext work on semi-crystalline fluoropolymers (PTFE, PCTFE) elucidated irregular deformation and profuse stochastic-based damage and failure mechanisms, but with limited insight into damage inception or progression in those polymers. The polyurea behaved very differently; the polymer first extruded a jet of apparently intact material, which then broke down via void coalescence, followed by fibrillation and tearing of the material. Most of the material in the jet elastically retracted back into the die, and only a few unique fragments were formed. The surface texture of all failed surfaces was found to be tortuous and covered with drawn hair-like filaments, implying a considerable amount of energy was absorbed during damage progression.

Rate dependent response and failure of a ductile epoxy and carbon fiber reinforced epoxy composite

Abstract: An extensive characterization suite has been performed on the response and failure of a ductile epoxy 55A and uniaxial carbon fiber reinforced epoxy composite of IM7 fibers in 55A resin from the quasistatic to shock regime. The quasistatic and intermediate strain rate response, including elastic modulus, yield and failure have are characterized by quasistatic, SHPB, and DMA measurements as a function of fiber orientation and temperature. The high strain rate shock effect of fiber orientation in the composite and response of the pure resin are presented for plate impact experiments. It has previously been shown that at lower impact velocities the shock velocity is strongly dependent on fiber orientation but at higher impact velocity the in-plane and through thickness Hugoniots converge. The current results are compared with previous studies of the shock response of carbon fiber composites with more conventional brittle epoxy matrices. The spall response of the composite is measured and compared with quasistatic fracture toughness measurements.