Damage in large scale solid state deformation of thermoplastic polymers at elevated temperatures and varying strain rates

Abstract

A key factor which limits the production speed of the polymer die drawing process is the premature fracture of the material on exit from the die. In this paper, the growth of damage in the material during the die drawing process has been studied using a combination of thermoplastic finite element analysis and structural characterisation by means of scanning electron microscopy and small angle X-ray scattering for the specific case of die drawing of polyoxymethylene. It is demonstrated that special profiled dies offer a more beneficial strain rate distribution than the conventional conical dies and allow higher production speeds to be obtained. Voids grow in the material as a result of the tensile stresses pertaining near the die exit and then, crazes appear from within the material at a critical stress level leading ultimately to final fracture. The results suggest that although the crazes initiate at a critical stress, the extent of crazing at the maximum draw ratio obtained (～13) is independent of the type of die and hence the stress level. Fracture of the drawn product occurs at different stresses for different die profiles but always at the maximum draw ratio of 13, suggesting that this relates to the limiting extensibility of a molecular network.     

Rheological properties of carbon mixes at very low shear rates using a capillary rheometer—III

A capillary rheometer has been used to determine the rheological properties of carbon mixes consisting of petroleum coke as filler and coal tar pitch as binder. It is shown that carbon mixes behave as Bingham materials with definite yield stresses. The yield stress was found to be a general property of all carbon mixes and its value is independent of the size of the capillary die. It was also seen that extrusion at low shear rates through big diameter capillaries results in slipflow. The slipflow occurs when the applied stress is somewhat below the yield stress, the material then extrudes without the formation of a core which otherwise would cause cracks in the product after baking. This explains why big diameter rods are generally extruded at very low speeds in usual manufacturing process. The modified Buckingham-Reiner equation has been applied to the case of slipflow and the values of yield stress and plastic viscosity calculated. These values agree well with those obtained from the consistency curve.     

Prediction of residual stress distribution in plastic pipe extrusion

In thermoplastic pipe extrusion, the extrudate emerging from the die is typically sized and cooled from outside in a quench tank. This process causes quick solidification of the external layer, while the inner mass of molten material cools only gradually. The slow cooling and crystallization, and the associated shrinkage of this material, can lead to build-up of severe stresses in the final part that can affect the long term service performance. In this paper, a simple theoretical analysis of this process of residual stress build-up is presented. The pipe undergoing quenching is modeled as an annular cylinder of molten polymer being cooled at a controlled rate from outside. The overall stresses are derived numerically by adding up the stress contributions due to incremental advance of the solidification boundary. The results of the analysis are found to be in qualitative accord with the experimentally measured stress profiles in thermoplastic pipe.     

Drawing of polymers through a conical die

A new process is described in which ultra-oriented polymers are produced by drawing a billet of initially isotropic polymer through a converging die. The process, called die-drawing, has been used to make oriented polypropylene rods with room temperature Young's moduli up to 20.6 GPa. The considerable advantages of this process compared with conventional tensile drawing and hydrostatic or ram extrusion are discussed.     

Simulation of PTFE sintering: Thermal stresses and deformation behavior

A finite element model has been used to study the sintering process of polytetrafluoroethylene (PTFE) cylinders in order to predict residual thermal stresses; both solid (rods) and hollow (billets) blocks were studied. The simulation of the process was performed considering three separate stages: thermal, deformation, and stress analysis. For each stage, relevant material properties were determined experimentally. In particular, the deformation behavior of PTFE was thoroughly investigated by means of thermo‐mechanical analysis (TMA). It is shown that experimental results can be explained considering deformation recovery and orientation effects. Predictions of the model are compared with experimental measurements performed on real PTFE‐sintered cylinders. Temperature and deformation distributions determined with the model agree well with experimental data. Fair agreement between predicted and experimentally measured residual stresses is obtained, and the influence of cylinder size and applied cooling rate on residual stresses is correctly predicted. Polym. Eng. Sci. 44:1368–1378, 2004. © 2004 Society of Plastics Engineers.     

Die drawing: Solid phase drawing of polymers through a converging die

Die drawing, a new process for the production of oriented thermoplastic rods with an ultrahigh modulus of elasticity, involves drawing an isotropic workpiece through a converging die at temperatures below the polymer melting point. Die drawing of polypropylene is described, but the process is applicable to any polymer which can be drawn by conventional means. The considerable advantages of die drawing compared with conventional tensile drawing and hydrostatic or ram extrusion are discussed.     

Finite Element Simulation of Thermal Residual Stresses in Joining Ceramics with Thin Metal Interlayers

Finite element analysis was used to estimate thermal residual stresses developed in silicon nitride bodies bonded by metallic interlayers. Stresses were calculated for various characteristic metals, namely, Ni, Al, and Si, assuming elastic and elastic-plastic behavior. The relative importance of the metal properties, such as thermal expansion coefficient, stiffness and ductility, has been evaluated. Two different joint geometries, butt and lap, have been used in stress calculations, and special care was taken in the mesh generation, to obtain comparable results. The yield stress of the interlayer material rather than thermal expansion mismatch is the critical factor in thermal residual tensile stress buildup inside ceramic adherents.     

Effects of orientation on mechanical properties of uniaxially oriented polystyrene films

Oriented samples of polystyrene have been produced by drawing at temperatures just above the glass transition range. Birefringence measurements have been used to characterise the degree of orientation. Mechanical measurements of elastic modulus and tensile yield stress have been made in the direction of drawing and it has been established that the birefringence value does not uniquely determine the mechanical properties—samples drawn to a high draw ratio at high temperatures have a higher modulus and yield stress than samples drawn at lower temperatures and lower draw-ratios to the same birefringence. The results are explained qualitatively by the convective constraint release theory of McLeish et al.     

The effect of physical ageing on the properties of poly(ethylene terephthalate)

Physical ageing rates of poly(ethylene terephthalate) have been measured, and ageing is interpreted to be associated with the conventional glass formation process, which occurs at a more rapid rate at higher temperatures. Ageing is accompanied by a marked change in mechanical properties, increased tensile yield stress and drawing stress, more localized yielding of the polymer and a marked decrease in impact strength. The fracture results have been attributed to the increased yield stress and a change in contribution of plane stress and plane strain conditions in the samples. Fracture surfaces show evidence of mixed modes of fracture.     

Experimental approach to mechanical property variability through a high‐density polyethylene gas pipe wall

An experimental investigation was designed to establish the distribution of mechanical properties throughout a high‐density polyethylene (HDPE) gas pipe wall. The proposed approach used a continuous and uniform filament that was automatically machined from the pipe on a precision lathe at a very low cutting speed and an optimal depth of cut to minimize heating and structural disturbances. Typical engineering stress–strain curves, in every layer, were obtained on a testing machine especially designed for polymers, and they were statistically analyzed. The stress–strain behavior of HDPE pipe material could basically be divided into three distinctive zones, the second of which remained important. The average stress level illustrating cold drawing for a given layer was almost constant throughout the pipe wall. The measured stresses and moduli correlated very well with the pipe thickness, and they increased from the outer layers toward the inner layers. This was explained by the crystallinity evolution because the pipe production process was based on a convective water‐cooling system with a temperature gradient, which generated residual stresses. Computed statistical stress–strain correlations at yielding, the onset of cold drawing, and fracture points revealed acceptable linear relations for an error level of p ≤ 0.05. On the other hand, an increasing linear correlation characterized the relationship of the yield stress and elastic modulus. This result was confirmed by literature for standard specimens, prepared by compression molding, that did not represent an actual pipe structure with respect to an extrusion thermomechanical history. Such an approach to mechanical property variability within an HDPE pipe wall highlighted the complexity of the hierarchical structure behavior in terms of stress–strain and long‐term brittle failure. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 272–281, 2005     

Calculated internal stress distributions in melt-spun fibers

Although melt spinning is a basic process in the synthetic fiber industry, theoretical understanding of heat transfer and stress development in a melt-spun fiber is limited. In this work, the finite-element method is first applied to the melt-spinning process to determine radial and axial temperature distributions in a solidfying fiber. A thermal stress analysis is then made, again by the finite-element method. Calculated stresses are found to reach maximum values shortly after the fiber solidifies. Because material properties are reduced at these elevated temperatures, this is a location of potential mechanical failure. Anisotropy due to drawing may add to this problem. Analysis of the effects of spinning parameters shows that ambient air temperature is the most critical variable in controlling the internal stresses. Mass flow rate and take-up speed have smaller effects.     

Rheological analysis of stabilizing forces in wire-coating dies

A rheological analysis of a wire-coating die is presented. The rheological forces which might play a role in the stability of the wire are estimated. In particular, consideration is given to lateral forces related to the secondary normal stress function acting on the wire which is in an eccentric position, and the hydrodynamic force related to the viscosity function acting on the wire which moves at an angle to the die axis. For the former a simple, yet general, expression was derived by solving the flow problem (without axial pressure gradient) with the Ericksen equation in bipolar coordinates. Results indicate that normal stresses stabilize the wire, i.e., tend to restore it to the central location, provided the secondary normal stress function is negative. The hydrodynamic effect tends to reduce the angle between wire and die axes, thus drawing attention to the need of perfect mechanical centering of the guider tip, since in this case this effect also reduces eccentricity. The need is stressed for further work, in particular, experimental measurement of the secondary normal stress function.     

Finite element analysis of wire coating

A general-purpose finite element program has been used to simulate the flow of polymers through wire-coating dies. The analysis includes Newtonian and power-law fluids. The effect of normal stresses was examined through a simple viscoelastic constitutive equation, Nonisothermal wire coating was studied to obtain the temperature field within the melt. The effect of a slip condition at the solid boundaries was also examined. The determination of the coating melt free surface was carried out through an iterative procedure. The finite element solution provides details about the existence and extent of recirculation regions, about hot spots due to viscous dissipation, and also captures the stress singularities present at the impact of the melt with the wire and at the exit from the die. Pressure distribution, maximum temperature rise, haul-off wire tension, maximum wire tension, and stresses at the wire surface and die wall are also presented.     

高取向聚乙烯的功能收缩特性

作者在较宽的温度范围内对高密度聚乙烯口模拉伸材料的收缩特性进行了研究,表明该材料可作为一种具有记忆效应和开关特性的功能收缩材料予以开发。文章还定义了两个参数,用于定量描述材料的功能收缩特性。     

Measurement of the rheological properties of polymer melts with annular rheometer

An annular die has been designed having a very thin gap distance between two coaxial cylinders. The die was then used to measure wall normal stresses along the longitudinal direction of polymer melts flowing through the thin annulus. The materials investigated were high-density polyethylene, low-density polyethylene, polypropylene, and polystyrene. Also investigated were blends of polystyrene and polypropylene, and blends of polystyrene and high-density polyethylene The measurements of wall normal stresses were used to determine the rheological properties of the melts, namely, the melt viscosity from the slope of axial wall normal stress profiles and the melt elasticity from exit pressures. The interpretation of the experimental data was made possible by the fact that the narrow-gap annular die can be considered as a substitute for a thin slit die. It has been found that the results obtained in the present study are consistent with those reported earlier by the author, who at that time used both capillary and slit dies.     

Process-induced residual stresses in compression molded UHMWPE

Non-isothermal cooling during processing causes the development of residual stresses, which are analyzed for compression molded UHMWPE, and affects the dimensional stability. The development of thermal residual stresses was predicted using an incremental stress analysis that included temperature-dependent material properties. Strain gauges were used to measure the residual stresses as layers were removed from a molded disk using a Process Simulated Laminate (PSL) approach. The PSL technique has not previously been applied to a compression molded neat polymer. For initial surface cooling rates of ～ 11°C/min, the model predicted a compressive stress at the bottom surface of 14 MPa and a tensile stress near the center of 2.5 MPa and matched the experimental distribution well. Because the compressive residual stress was 70% of the yield strength (～20 MPa), a lower cooling rate was also tested (2.6°C/min). The maximum tensile and compressive stresses for this cooling rate were, 0.91 MPa and 2.5 MPa, respectively. The model demonstrated its use for predicting thermal residual stresses in compression molded parts, instead of trial-and-error experimentation. UHMWPE is shown to develop residual stresses continually from ～ 120°C to 23°C.     

Distribution of stress in a craze of the tip of a uniformly extending crack

A model of a craze at the tip of a uniformly extending crack is developed which permits the calculation of the stress distribution in the craze. In accord with experimental observations by Kramer¹¹ the craze is modelled as a collection of independent fibrils that draw from the substrate by a process akin to the drawing of textile fibres with necking. Except at the very tip of the craze where complex yielding type phenomena occur, the stress in the craze is taken to correspond to the drawing stress. The craze stress is treated as the cohesive crack closing stresses in the Barenblatt treatment of crack tips. The principal fact used in the development is that the drawing stress depends upon the rate of draw and hence upon the slope of the craze displacement. This leads to a non-linear integral equation for the craze stress. Using an empirical relation between drawing stresses and rate of draw, this equation is solved for the stress distribution in the craze by numerical methods. The distribution shows peaks at the craze tip and at the crack tip as observed in some experiments. The magnitude of the peaks depends upon the materials parameters used. For certain values of these parameters, the constant stress Dugdale model yields a good approximation to the displacement profile.     

口模拉伸过程PP/CaCO3复合材料结构演变及机理

利用口模拉伸法制备了轻质、高强、高模的聚丙烯（PP）/碳酸钙（CaCO₃）复合材料，分析了口模拉伸对材料微观结构、晶体结构、热性能和密度的影响，揭示了口模拉伸过程中材料结构演变及其机理。结果表明，随着拉伸过程的进行，材料内的原始球晶向微纤状晶体转变，同时发生晶粒细化现象，结晶度增大表明存在拉伸诱导结晶现象。微孔尺寸的显著增大及材料密度的显著降低主要发生在材料离开收敛流道壁面后的自由拉伸过程中。     

Development of continuous die drawing production process for engineered polymer cores for wire ropes

Abstract

The process of continuous die drawing has been adapted as the means of production of polymer cores for wire ropes. The cores have a precisely engineered cross-sectional shape, which is produced by the die drawing process. In order to control this shape accurately, the drawing is carried out in two stages, first drawing in a heated circular conical die and secondly in a cooled fluted conical die. A production line has been developed, which produces core at commercially acceptable rates and with precisely controlled dimensional and mechanical properties. The polymer core serves as a replacement for the three strand cores made from natural fibres.