Showing papers by "Eric Brown published in 2007"

Influence of Molecular Conformation on the Constitutive Response of Polyethylene: A Comparison of HDPE, UHMWPE, and PEX

Abstract: The current work presents the characterization and comparison of the mechanical response of three different industrial forms of polyethylene. Specifically, high-density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), and cross-linked polyethylene (PEX) were tested in compression as a function of temperature (−75 to 100°C) and strain-rate (10−4 to 2,600 s−1). The responses of UHMWPE and PEX are very similar, whereas HDPE exhibits some differences. The HDPE samples display a significantly higher yield stress followed by a flat flow behavior. Conversely UHMWPE and PEX both exhibit significant strain hardening after yield. The temperature and strain-rate dependence are captured by simple linear and logarithmic fits over the full range of conditions investigated. The yield behavior is presented in terms of an empirical mapping function that is extended to analytically solve for the mapping constant. The power-law dependence on strain-rate observed in some polymers is explained using this mapping function.

Soft recovery of polytetrafluoroethylene shocked through the crystalline phase II-III transition

Abstract: Polymers are increasingly being utilized as monolithic materials and composite matrices for structural applications historically reserved for metals. High strain-rate applications in aerospace, defense, and the automotive industries have lead to interest in the shock response of polytetrafluoroethylene (PTFE) and the ensuing changes in polymer structure due to shock prestraining. We present an experimental study of crystalline structure evolution due to pressure-induced phase transitions in a semicrystalline polymer using soft-recovery, shock loading techniques coupled with mechanical and chemical postshock analyses. Gas-launched, plate impact experiments have been performed on pedigreed PTFE 7C, mounted in momentum trapped, shock assemblies, with impact pressures above and below the phase II to phase III crystalline transition. Below the phase transition only subtle changes were observed in the crystallinity, microstructure, and mechanical response of PTFE. Shock loading of PTFE 7C above the phase II-III...

Effect of halogenation on the shock properties of semicrystalline thermoplastics

Abstract: The high-strain rate response of polymers is a subject that has gathered interest over recent years due to their increasing engineering importance, particularly in the transport industries This work explores the effect of fluorination upon semicrystalline thermoplastics Polyethylene, with its simple hydrocarbon chain, is considered; with increasing fluorination of the carbon backbone polyvinylidene difluoride, and then polytetrafluorethylene (PTFE) is also compared All three show a nonlinear equation of state behavior, which may be related to partial crystallinity PTFE is shown to have anomalous release speed, which reflects its transformation from phase II to phase III transition at 07 GPa Strength effects show a different ordering of behavior between the three materials The first results are accountable purely using van der Waals forces between the chains However, strength effects show the effect of further attractive forces that indicate some tacticity in the material associated with the state behind the shock

Mixed-mode-I/II fracture of polytetrafluoroethylene

Abstract: The current work deals with mixed-mode-I/II fracture in polytetrafluoroethylene (PTFE). Mode-I fracture in PTFE 7C has previously been shown to exhibit strong phase dependence with a brittle-to-ductile transition associated with the room temperature phase transitions. Increases in fracture toughness around room temperature and above result from the onset of stable fibril formation bridging the crack plane and increased plastic energy dissipation. Mixed-mode-I/II loading conditions are achieved using a modified compact tension specimen with Arcan type fixtures. The fracture toughness δVC of PTFE 7C exhibits a 42% decrease from pure mode-I to near mode-II loading due to a marked decrease in fibril formation.

Influence of polyethylene molecular conformation on Taylor impact measurements: a comparison of HDPE, UHMWPE, and PEX

Abstract: The current work presents the comparison of the Taylor impact response of three different industrial forms of polyethylene. Specifically, high‐density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), and cross‐linked polyethylene (PEX) were tested. From quasi‐static and intermediate strain‐rate compression measurements as a function of temperature (−75 to 100 °C) and strain‐rate (10−4 to 2600 s−1) the responses of UHMWPE and PEX are very similar, whereas HDPE exhibits some differences. The HDPE samples display a significantly higher yield stress followed by a flat flow behavior. Conversely UHMWPE and PEX both exhibit significant strain hardening after yield. Taylor impact experiments are presented as a function of velocity to probe the dynamic yield behavior and ductile‐to‐brittle response of these polymers.

In situ measurement of crystalline lattice strains in phase IV polytetrafluoroethylene

Abstract: Measurements of crystalline lattice strain development during deformation of polytetrafluoroethylene (PTFE) have been performed utilizing neutron diffraction measurements at the SMARTS diffractomet...

An investigation of surface velocimetry of shocked polyethylene using HetV

Abstract: The velocity history of a shocked free surface has traditionally been measured using established techniques such as VISAR or Fabry‐Perot. In recent years a third type of velocimetry has been developed by LLNL which uses Heterodyne techniques, Photon Doppler Velocimetry (PDV). This technique generates a Doppler beat frequency between light incident on the surface and light internally reflected within the system. Unlike the other two techniques PDV does not use an interferometer, instead it relies upon having the ability to directly record the high beat frequency. The setting up and fielding of PDV is therefore much simpler. A low power (Class 1 laser) system using this principal, locally known as Heterodyne Velocimetry (HetV) has been developed and assembled. A series of experiments has been carried out to investigate the Hugoniot of polyethylene using HetV and embedded stress gauges. The results obtained with HetV have been directly compared with the embedded gauge data from the same experiment.