Robert M. Caddell

The macroscopic yield behaviour of polymers

A yield criterion, not previously compared with the actual macroscopic behaviour of polymers, is herein compared with the pressure-modified octahedral shear stress criterion earlier suggested by others. This new relation, which is a version of the von Mises criterion, accommodates differences in tensile and compressive yield strengths and accounts for any dependence of yielding on the hydrostatic component of the applied stress state.With the use of thin-wall tubes accounting for the majority of experimental points, the yield behaviour of polycarbonate and polyvinylchloride was investigated. Besides these findings, results previously reported by others have also been utilized in this paper. Since these various studies employed quite different polymers, the excellent overall correlation of experiment with prediction should merit the serious attention of persons interested in the macroscopic yield behaviour of polymers.Comparisons between this new criterion and the modified octahedral shear stress are also made in regard to the effect of pressure on subsequent yield behaviour. Although not fully verified, it is suggested that the predictions which result using this new criterion, seem a little more reasonable.

Pressure dependent yield criteria for polymers

Summary Y M Different criteria have been proposed to include the influence of pressure (or mean normal stress) on the yield behavior of polymers. It is difficult to distinguish among them using the type of experiments that produce data used in two-dimensional plots of yield loci. This is due to the fact that the maximum range of values of mean normal stress is relatively small in such experiments. Marked differences between these criteria do occur however as the hydrostatic pressure or mean stress is altered substantially. Experiments that show the effect of applied pressure on tensile and/or compressive yield strength provide one means for describing such differences. This paper considers two forms of a pressure modified yon Mises criterion and shows a comparison with available experimental information.

A yield criterion for anisotropic and pressure dependent solids such as oriented polymers

The anisotropic yield criterion first posed by Hill has been modified to account for differences in tensile and compressive yield strengths in a given direction; additionally, the influence of hydrostatic pressure on yielding is also considered. Predictions using this new criterion are compared with published experimental results involving oriented polymers and excellent agreement is found. It is suggested that this criterion is more correct on fundamental grounds than those put forth in earlier publications.

A macroscopic yield criterion for crystalline polymers

Abstract Yield studies including uniaxial tension, uniaxial compression and biaxial stress states (developed with internally pressurized thin wall tubes) were conducted with high density polyethylene. The experimental results are compared with a predicted yield locus based upon a pressure-modified von Mises criterion. Agreement was quite reasonable although a slight degree of anisotropy was noted in the test material. Since this same yield criterion has earlier been shown to provide excellent agreement with glassy amorphous polymers it appears unnecessary to employ different criteria for different polymers if one is concerned with macroscopic yielding.

Time-temperature dependent fracture toughness of PMMA: Part 1

A toughness-biased Ree-Eyring relationship gives a good description of fracture toughness data of PMMA over a range of temperatures (283 to 353 K) and crack velocities (10−5 to 1 m sec−1). Fracture toughness was measured by Gurneys sector method. The activation energy associated with the equation supports earlier work which suggests that, in the same temperature and velocity range, cracking in PMMA is controlled by craze growth, which is governed by secondary (β) molecular processes. Unstable cracking at moderate velocities (10−2 to 1 m sec−1) seems to be produced by an isothermal/adiabatic transformation; an analysis for the onset of instability is given. At temperatures below 283 K, changes in toughness behaviour are seen, and below 243 K no stable cracking at all was obtained. A discussion is given of various methods of characterizing resistance to cracking, and methods of transforming R(à, T) and K(à, T) data are compared.

Time-temperature dependent fracture toughness of PMMA

Some observations are made on the fractography of surfaces obtained by cracking “compact tension” profile testpieces of PMMA over a range of temperatures and crack speeds, both stably and unstably. To a first approximation, it was possible to group and “shift” (as in visco-elastic transformations) characteristic surface markings at various fracture toughness/temperature/crack velocity combinations, particularly in the range where a toughness-biased Ree-Eyring relationship described the experimental toughness data.

The laws of similitude and crack propagation

Abstract The mechanics of cracking follow the laws of similitude in an odd sense. As a result, crack load-external displacement-crack extension data are not usually non-dimensionalized. It follows that a new “group” should be used (analogous to the Reynolds, Froude or Cauchy numbers) when scaling ship ice-breaking resistance from tests of models in ice-towing-tanks.

Macroscopic yielding of oriented polymers

Uniaxial and biaxial stress states were employed to compare the measured macroscopic yield behaviour of oriented polycarbonate, polyethylene, and polypropylene with a proposed anisotropic and pressure-dependent yield criterion. A good correlation between theory and experiment was found for each material.

Yield locus studies of oriented polycarbonate An anisotropic and pressure-dependent solid

Abstract Uniaxial and biaxial stress states were employed so as to investigate the yield behavior of oriented polycarbonate. These experimental results are compared with a theoretical yield locus based upon a yield criterion proposed for solids that are both anisotropic and pressure dependent in regard to macroscopic yield behavior. A good correlation between theory and experiment was found.

Influence of hydrostatic pressure on the yield strength of anisotropic polycarbonate

Abstract Tension and compression specimens of anisotropic (oriented) polycarbonate were subjected to hydrostatic pressures up to 552 MPa (80 ksi). The resulting yield behavior was compared with a yield criterion for use with pressure dependent and anisotropic solids. A promising correlation was found and attention is called to certain problems related to the measurement of tensile true stress-true strain behavior during typical hydrostatic pressure testing.