Rapid crack propagation in tough polymers; A note on the crack tip temperature field

Abstract

The High Speed Double Torsion (HSDT) test has recently been used to determine the dynamic fracture resistance, Gt,, of tough pipe grade polyethylenes and other engineering thermoplastics [1]. These materials, which exhibit very high resistance to slow tearing, tend to become relatively brittle at high crack speeds. Fracture resistance is not normally measured directly, it is inferred from the crack driving force or energy release rate provided by the structure. However, extracting consistent Gt, data for non-linear elastic materials requires careful energy accounting when deriving the crack driving force [2]. The problems involved in this approach could be avoided if a method of measuring the energy consumed locally at the crack tip were available.Previous research on brittle polymers has shown that much of the energy consumed by the irreversible processes occuring close to the crack tip is converted to heat. Fuller et al. [3] reported that the heat generated in polymethylmethacrylate (PMMA) increased with crack speed in a similar manner to that in which GD was thought to vary. DUll [4] compared the amount of heat generated in PMMA to the dynamic energy release rate determined from the remotely applied stress. He concluded that approximately 60 percent of the energy released was converted to thermal energy. He also reported that as the molecular weight of the PMMA increased more heat was produced at a given crack velocity. Zimmermann et al. [5] determined the dynamic energy release rate from the response of strain gauges placed close to the crack path in PMMA samples and concluded that all of the energy released appeared as heat. These observations on amorphous polymers suggest that the transition to brittle rapid crack propagation in tough semi crystalline polymers may be caused by breakdown of the reinforcing crystalline phase, due to the heat generated adiabatically within the process zone. Direct measurements of heat generated near a fast running crack tip could therefore serve two objectives. Firstly, it could provide a direct, straightforward measurement of the intrinsic fracture resistance of the material, unaffected by any uncertainties about bulk elastic properties that affect the crack driving force. Secondly, it could provide an insight into the mechanism by which increasing crack speed adversely influences the separation process, and may suggest material development strategies to counteract this influence.

Impact and dynamic fracture of tough polymers by thermal decohesion in a Dugdale zone

“…Within this time, heat generated by plastic work at the cohesive zone surface is conducted to a depth characterised, at the separation point, by h = (~;/~y)l/Z, ~; being the thermal diffusivity. Wheel [4] has measured this conducted heat flux directly, much further away from the crack plane -and therefore, of course, later in time.…”

“…However, if the load is applied rapidly enough at a low enough temperature, it somehow overwhelms the crack blunting mechanism, and the crack propagates with very little apparent ductility. Such effects have been studied using SENB [1,2], edge-notched tensile [3,4] and compact tension specimens [5]. Despite substantial work by Ward [6] and others, there is still no quantitative model to predict the transition in terms of temperature, thickness and impact speed.…”