Comparison of shear-yielding modes in polycarbonate and a thermoplastic polyurethane

A thermoplastic polyurethane (PU) introduced by The Dow Chemical Company under the tradename Isoplast® possesses certain engineering properties that resemble those of polycarbonate (PC), including optical clarity, a glass transition of 140 °C, and comparable modulus and ductility. In this study, the shear-yielding behaviour at a circular hole was compared. Strain birefringence was used to analyse deformation in the linear elastic region. At higher stresses, optical microscope techniques were used to characterize the flow lines through the thickness of the plastically deformed region. Differences in shear-yielding behaviour were attributed to differences in the true stress-true strain relationship of the two materials. The true stress-true strain curve of PC exhibited a yield instability marked by a drop in the true stress. Shear yielding was described in terms of idealized elastic-plastic behaviour, and large-scale flow that accompanied yielding was responsible for the strong thickness dependence of shear yielding. The true stress-true strain curve of PU did not exhibit a true stress drop at the yield point. The primary features of shear yielding were consistent with elastic-plastic behaviour with work hardening; small local plastic strains resulted in more diffuse yielding that was not strongly dependent on thickness.     

Instabilities in yielding

This paper addresses itself to instabilities observed during tensile testing, and complements the papers of Rodriguez and Ananthakrishna presented at this Meeting. The work of Cagliotiet al on the elastic to plastic transformation is first reviewed. The work of Kubinet al on the serrated (repeated) yielding observed at liquid helium temperatures is then discussed in brief. Finally, our own work relying on electronic simulation is described. We conclude with some brief remarks on a few important questions that merit attention in the future.     

Annealing polycarbonate sheets

An annealing cycle for polycarbonate sheets is proposed in this paper. Tests carried out on 0.75–3.00 mm thick sheets show the good reliability of the technique. Further confirmation can be found in practical applications reported in other papers.     

On yielding of oriented solids

Summary Yielding and failure of oriented solids is studied within the framework of tensor representations. Transversely isotropic materials are considered. Using the information available regarding the tensor generators and the set of independent stress and mixed stress-material orientation invariants the general form of constitutive relation for incipient plastic motion of transversely isotropic solid is given and the yield condition is discussed. Specific forms of yield criteria for cohesive materials as well as for materials with internal friction are developed and compared with experimental information. A novel approach to flow and failure of oriented solid is thus explained on example of stratified material, starting with experimental motivation, passing through theoretical development and terminating on comparisons with experiments.

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Zum Fließen von orientierten Festkörpern

Zusammenfassung Das Fließen und Versagen von orientierten Festkörpern wird mittels der Tensorfunktionsdarstellung untersucht. Transversal isotrope Materialien werden betrachtet. Mit Hilfe der verfügbaren Information, die die Tensorerzeugenden und die Menge der unabhängigen Spannungs- und gemischten Spannungs-Material-Orientierungsinvarianten betrifft, wird die allgemeine Form der Materialbeziehungen für beginnende plastische Bewegung von transversal isotropen Festkörpern angegeben und die Fließbedingung diskutiert. Bestimmte Formen von Fließkriterien für kohäsive Materialien, sowie für Materialien mit innerer Reibung werden entwickelt und mit experimentellen Ergebnissen verglichen. Eine neuartige Näherung für das Fließen und Versagen von orientierten Festkörpern wird am Beispiel eines geschichteten Materials erklärt. Ausgehend von der experimentellen Motivation wird die theoretische Entwicklung dargelegt und abschließend werden Vergleiche mit Versuchen angegeben.

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With 11 Figures

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Institute of Fundamental Technological Research, Warsaw, Poland     

Plane-stress fracture of polycarbonate films

The use of the specific essential work of fracture, W _e, to characterize fracture of polycarbonate films is described. It is shown that the plane-stress specific essential work of fracture for polycarbonate film can be obtained from single-edge-notched-tension specimens, by extrapolating the straight-line relationship between the total work of fracture, W _f, and ligament length, L, to zero ligament length. From the data, it seems that, for a given film thickness, W _e is almost independent of the specimen width but increases with increasing thickness. The non-essential work of fracture as obtained from the slope of a W _f versus L plot showed no significant width dependence, and for the majority of thicknesses it was almost invariant with thickness, indicating that the shape of the outer plastic zone surrounding the fracture process zone is almost invariant with the dimensions of the test specimen.     

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.     

聚碳酸酯和2,2'-双(4-羟苯基)丙烷聚苯二甲酸酯的相容原理研究

通过熔融共混法和溶液共混法制备了双酚A型聚碳酸酯(PC)和2,2'-双(4-羟苯基)丙烷聚苯二甲酸酯(PAR)的共混物.采用溶解度参数比较、透明性测试、DSC、DMA、SEM及扭矩流变测试等多种方法,全面研究了PC与PAR的相容性.双螺杆挤出机制备的熔融共混物具有良好的透明性、单一玻璃化温度和均相SEM特征,但扭矩流变测试和对溶液共混物热处理后的DSC分析结果表明,PC和PAR是部分相客体系,在熔融温度下酯交换反应得以进行,生成的嵌段共聚物起到了相容剂的作用,使得共混物成为均相体系.     

Biaxial impact fatigue of polycarbonate

An impact fatigue test has been designed for studying the repeated biaxial impact load response of polymers. In the test geometry, a thin plate-like sample is rigidly held by an annular clamp and is repeatedly impacted by a hemispherically-tipped plunger. Results have been obtained for polycarbonate. Under impact fatigue, cracks initiate on the bottom surface where high biaxial tensile stresses exist. These cracks propagate along radial lines on the sample surface and also into the sample normal to the surface. The load-bearing capability and the residual impact strength decrease as the cracks propagate. The development of impact fatigue cracks, the lifetime behaviour and the change in mechanical properties after partial impact fatigue are discussed.     

Computer simulation of serrated yielding

A model for serrated yielding based on the negative resistance characteristics of materials is discussed. An analog computer based on this model is described. The simulated curves show oscillations which are regular and uniform in amplitude. To simulate more realistic tensile test curves, a refined model which includes the effects of fluctuations in dislocation density and velocity is described. Some simulated curves using this refined model are presented. These results are close to observed tensile test curves.     

Creep and recovery of polycarbonate

The creep behaviour of polycarbonate was investigated at high stresses in tension and compression. The creep curve was separated into a recoverable strain and persistent strain. The equation describing the recoverable strain was directly related to the kinetics of the recovery curve. The concept of an internal stress was useful in describing the recoverable part of the creep and recovery. The persistent strain-rate was related to the failure of the material by accelerated creep. Measurements of the temperature during creep showed that heating was not responsible for the occurrence of the material softening which lead to accelerated creep. It was found that the strain behaviour during loading and unloading was asymmetrical in that the instantaneous loading strain was less than the instantaneous unloading strain.     

The impact strength of polycarbonate

Measurements of the Charpy notched impact strength of polydiancarbonate indicate that the polymer can exist in one of two modifications A and B. There is a temperature (below room temperature) above which the A form is unusually tough; the B modification is comparatively brittle at all temperatures. The A form can be converted to the B form by an annealing process which proceeds rapidly above 80°C. We have not been able to determine the precise morphological difference between the A and B forms but most of the data can be explained by making a hypothesis about the yield behaviour of the A form.     

Steel disc reinforced polycarbonate

Stainless steel platelets have been embedded in polycarbonate under carefully controlled conditions, so that the strength of the components and the interface were known with confidence. It was observed that the strength of the composite depended on the volume fraction of platelets according to a modified rule of mixtures expression. No dependence on aspect ratio of the platelets was observed for aspect ratios in the range 100 to 800, and round and square platelets gave substantially the same results. Although strength improvement is biaxial in the case of platelets (while it is uniaxial for aligned fibres) the gain in strength along one direction is at the expense of strength along the other.     

Nonlinear creep behavior of viscoelastic polycarbonate

A study of the tension behavior of polycarbonate (PC) under room temperature and various sustained loads is presented. Time-dependent axial elongations and transverse contractions of the specimen were simultaneously measured at nine different stress levels, from 15.89 to 59.4 MPa, and modeled according to a time-stress superposition principle. The test duration was only one hour. It was shown that creep compliance vs. log time curves at different stresses can be horizontally shifted to form a smooth master curve for one year at a reference stress of 30.97 MPa. Moreover, the stress shift factors for axial extension creep curves and transverse contraction creep curves are found to be identical for the stress levels considered, and this is verified using the Poisson's ratio measurements.     

聚碳酸酯合金板的性能研究

研究了新开发的铁路客车用聚碳酸酯合金板 (商品名称Polyshet)的光老化性能、热老化性能、阻燃性能、冲击性能及表面硬度 ,并与PC/ABS合金板作了性能对比。     

Dip-coated thin-film polycarbonate optical waveguides

We report on our systematic and detailed study of fabrication and characterization of polycarbonate thin-film optical waveguides. Various waveguide properties, such as refractive index, optical loss, waveguide thickness, dispersion as a function of temperature of waveguides and substrates, and the effect of thickness gradient on optical guiding losses, have been reported. A comparative study of the properties of waveguides fabricated with dioxane and tetrahydrofuran as solvents is also presented.     

Cooperative cavitation in rubber-toughened polycarbonate

Particle cavitation in the stress-whitened zone ahead of a semicircular notch in polycarbonate blended with a core-shell rubber was characterized by transmission electron microscopy. Cavitation of rubber particles at five locations in the stress-whitened zone was correlated with the local stress and strain history. It was found that cavitation initiated some distance ahead of the notch when a mean stress condition was met. Initially, only a fraction of the particles cavitated and these were randomly distributed. Single cavitated particles grew into cavitated domains by cooperative cavitation of nearby particles until cavitation was arrested when shear yielding of the matrix provided an alternative mechanism for relief of strain energy. Far from the notch, where the stress state approached uniaxial tension, cavitated domains grew into linear arrays of cavitated particles. A mechanism of cooperative crazing in microlayer composites of polycarbonate and styrene/acrylonitrile copolymer was adapted to cooperative cavitation of core-shell rubber particles. It was proposed that cooperative cavitation of nearby particles occurred by impingement of a small plastic zone that formed at the equator of a cavitated particle.     

The environmental stress crazing of polycarbonate

The environmental crazing from a central hole in extruded polycarbonate sheets, using ethanol as a crazing liquid, was studied. A stress analysis was made to analyse the criteria for craze initiation and growth. The craze length was found to change linearly with the square root of time and the rate of crazing was found to vary exponentially with the applied stress (above a certain stress level). It was concluded that the principal strain is the controlling parameter for environmental craze initiation and growth.     

聚碳酸酯的市场与技术动向

主要介绍了PC的生产、需求及技术动向 ,比较了两种不同的PC制造方法 ,并介绍了几个PC生产商的相关技术及一些新的特殊PC品级产品。