(Received 6 August 2009; accepted 22 March 2010)
Published Online: 2010
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An examination of the fracture surfaces found on residuals originating external to the specimen contacting indenter of a ASTM D2512 tested isotropic polymeric material has been conducted for the first time. The repeated appearance and interrelationship of a select series of distinct features was identified. Cracks which plunge from the surface into the bulk of the tested specimen were routinely observed. They are developed by the independent mechanism of sliding Hertzian fracture. Radial cracks, propagating through the entire thickness of the tested specimen, from the most central portion of the original specimen through to its back edge were always observed. They were developed by the independent mechanism of tensile hoop failure. This tensile hoop mechanism also caused the point loaded backside impact damage feature occasionally observed. Shield shaped cracks and indications of secondary point loaded impacts were also routinely observed. Extrinsically generated, finely crushed granular and/or flake powder was commonly found tenaciously adhering to the fractured and top edges of many of these residuals. This powder feature always exhibited indications of having flowed onto the residual and frequently indications of having experienced intense heating. The forced flow of the powder caused the powder’s surface to be heated by the mechanism of kinetic friction heating. While kinetic friction heating did generate features indicative of exposure to intense heat, there were effectively no lasting indications that ignition had originated on these external residuals.
Air Force Research Laboratory, USAF, Fairborn, OH
Stock #: JAI102681