摘要:

The free-volume model for interpreting the initial yielding behavior of glassy polymers is extended to include kinetic energy influences. Molecular mobility is assumed to be determined by the product of the probabilities of attaining sufficient local free volume to allow molecular rearrangement, and kinetic energy to overcome restraining forces. Yielding then is initiated at the point where the free-volume increase resulting from the dilational component of the applied stress is sufficient to bring the local molecular mobility to that characteristic of the unstressed polymer at Tg.

ISSN:0022-2348    

DOI:10.1080/00222347008229378

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The strain rate of polymethyl methacrylate (PMMA) in yielding under a constant load varies by a factor of approximately 50 with the thermal history. PMMA cooled rapidly from above to below the glass transition temperature yields at a high rate; isothermal innealing of the quenched PMMA below Tgreduces the yield rate. The rate of gain of flow resistance is maximal near 95°C annealing temperature (Tg= 110° C). We expect straining, like quenching, to perturb the glass structure. Accordingly the acceleration of the strain rate with increasing strain (strain-softening) observed in yielding under a constant load is attributable to the perturbation of the annealed glass by strain. Quenching and small plastic strains as well reduce the density of PMMA by 10−4to 10−3g/cm3.

ISSN:0022-2348    

DOI:10.1080/00222347008229379

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

The object of this investigation was to determine the origin of deformation bands in one-way oriented polyethylene terephthalate (PET) and their relation to the upper yield point. The tensile yield point was measured at room temperature as a function of the angle () between the original draw direction and the tensile axis. The effect of strain rate was observed. The growth and geometry of deformation bands were studied. The results showed that the formation of a deformation band in PET cannot be interpreted as the strength-limiting yield mechanism for all and that the viscoelastic behavior plays the primary part in determining the yield point of this semicrystalline polymer.

ISSN:0022-2348    

DOI:10.1080/00222347008229380

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

Electron paramagnetic resonance (EPR) provides a sensitive tool by which microscopic bond rupture can be monitored simultaneously with observations of macroscopic deformation and failure. Past techniques for studying fracture in semicrystalline polymers have been extended to investigate degradation of unfilled ruber in the presence of ozone. It was found that the rate of free radical production was linearly proportional to stretch ratio and ozone concentration and that stress relaxation and creep were not directly proportional to this production rate. The latter behavior was attributed to the particular dependence of crack density and growth on stress. It was concluded that at low strains, comparatively few surface cracks form; however, at higher strains, many more crack centers become active. Although many more surface cracks are present, they do not progress into the material as rapidly. Therefore, although more bonds were broken at higher strains and stresses, the stress relaxation rate and creep rates were not significantly increased.

ISSN:0022-2348    

DOI:10.1080/00222347008229381

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

Stamping of a flat sheet into a hollow, seamless item involves tensile, frictional, bending, and compressive effects. A quantitative analysis of stresses developed in the sheet during such a “deep drawing” operation is presented indicating that the maximum depth of draw depends on the ratio of tensile stress and compressive yield stress in the plane of the sheet. Several materials are examined and their performance in deep drawing analyzed in terms of proposed criteria for deep drawing. Techniques are discussed which can be used to increase the depth of draw of polymeric material.

ISSN:0022-2348    

DOI:10.1080/00222347008229382

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

摘要:

This investigation introduces quantitative morphological criteria for defining the structural state of a polymer sample and develops a general structural state deformation model for characterizing mechanical behavior. This general model discards the present fragmented view of the autonomy of observed mechanical processes (yielding, fracture, recovery, etc.) and brings the fabrication process, the mechanical tests, and even the construction of the sample (fiber or film) into focus as simply different aspects of a single process of deformation. The fabrication, fracture, yielding, and recovery behavior of two series of uniaxially drawn isotactic polypropylene films and two series of drawn isotactic polypropylene fibers, totaling thirty different structural states, have been deter-mined. The structural state of each of these samples is known quantitatively. The measurements were made over a range of strain rates from 1 to 1,000,000%/min and temperatures from 23° to -196°C. This study has resulted in new quantitative relationships between mechanical properties and morphological structure and has developed a deeper insight into the underlying deformation mechanisms that are occurring.

ISSN:0022-2348    

DOI:10.1080/00222347008229383

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor

ISSN:0022-2348    

DOI:10.1080/00222347008229367

出版商:Taylor & Francis Group    

年代:1970

数据来源: Taylor