The manufacture of ultra-high modulus polyethylenes by drawing through a conical die

Various grades of linear polyethylene have been drawn through a heated conical die at 100° C. It was found that after a suitable start-up procedure, continuous drawing was possible in all cases with a stable neck region extending beyond the die exit. The degree of deformation attainable was found to depend strongly on the draw velocity. Very high deformation ratios could be obtained and the Young's moduli of the die-drawn products were comparable to those of similar products obtained by solid state extrusion and tensile drawing, reaching values as high as 60 GPa. The effects of molecular weight and co-polymerization are substantial, but not exactly analogous to those previously observed in hydrostatic extrusion and tensile drawing. This is probably due to the non-isothermal nature of the final stage of deformation in the case of die drawing.     

The creep behaviour of ultra-high modulus polypropylene

The creep and recovery behaviour of highly drawn polypropylene monofilaments has been studied over the temperature range 20 to 50° C. A range of samples was examined to identify the influence of draw ratio and molecular weight. It is concluded that the permanent flow creep arises from the presence of two thermally activated processes, one of which relates to the -relaxation process and is associated with the crystalline regions of the polymer, and the second with the molecular network.     

Creep and stress-relaxation in ultra-high modulus linear polyethylene

It has been shown that the creep and stress-relaxation behaviour of oriented linear polyethylene can be satisfactorily described by a model where two thermally activated processes are acting in parallel. The creep behaviour of an oriented sample which does not reach a constant strain-rate can be described by the addition of a further plastic flow term where the viscosity increases linearly with increasing plastic strain.     

Effect of annealing on the wrinkling behaviour of the commercial pure aluminium grades when drawn through a conical die

Experiments have been carried out to evaluate the onset of wrinkling behaviour in three grades of annealed commercial aluminium sheets namely ISS 19000, ISS 19600 and ISS 19660 having thickness 2.00 mm, while drawing them through a conical die. The deep drawing of circular blanks of different diameters of the aforesaid sheet metals was carried out using flat bottom punch. The analysis of the experimental data has revealed that the onset of wrinkling takes place when the ratio of the plastic strain increments (dε_r/dε_θ) reached a critical value. It was further found that the measured strains followed non-linear paths in the wrinkling limit diagrams. Further, it was observed that the aluminium grade having high strain hardening index value, low yield stress and high normalized hardening rate shows better resistance against wrinkling. Furnace cooled aluminium grades showed better resistance against wrinkling compared with air cooled blanks.     

Anisotropy in oriented fibres from synthetic polymers

On the assumption that synthetic fibres at low strains can be considered as transversely isotropic elastic bodies the five independent elastic compliances have been measured for filaments of low and high density polyethylene, nylon 6.6, polyethylene terephthalate and polypropylene, all at room temperature. Experimental values are compared with those predicted using an aggregate theory which assumes that the partially oriented fibre can be considered as an aggregate of elastic units which are aligned by the drawing process, and whose properties are those of the fully oriented fibre.The applicability of this aggregate theory is discussed, and possible explanations are advanced in those cases where agreement between theory and experiment is unsatisfactory.     

Die drawn wood polymer composites. II. Micromechanical modelling of tensile modulus

The Cox–Krenchel micromechanical model was applied to give predictions for the tensile moduli of isotropic and oriented wood polymer composites (WPC). The oriented WPC were produced by the Leeds die-drawing process using polypropylene filled with softwood and hardwood powders. The wood particles were extracted from the composites to determine their density and aspect ratio by dissolving in hot decalin. To measure particle shape and size, image analysis was employed. These experimental parameters were then introduced to the Cox–Krenchel model which was found to give prediction of tensile modulus in very good agreement with the experimental values.     

Pressure dependence of the shear modulus of various polymers

A torsion pendulum has been used to measure the shear modulus of a range of polymers as a function of applied hydrostatic pressure at 20° C. The pressure medium was usually nitrogen gas and the maximum pressure 20000 psi. The results show that the shear modulus of each polymer is increased by the application of pressure, and the magnitude of the increase is greatest for experiments carried out at temperatures just above an atmospheric relaxation temperature. The increase in shear modulus takes a finite time, of the order of minutes, to be achieved, the equilibrium value being reached in a shorter time at higher temperature.     

A local modulus analysis of rapid crack propagation in polymers

This work is concerned with the analysis of rapid crack propagation (RCP) in Polymethylmethacrylate (PMMA), Polycarbonate (PC) and two-layer PMMA/PC systems. Remarkably constant crack speeds were observed, and higher crack speeds corresponded to the higher preloads. Uniform fracture surfaces were associated with these constant speed RCPs. An indirect method was used to characterise dynamic fracture properties of the materials. The method relies on the recorded crack length histories and boundary conditions which are incorporated in a dynamic Finite Element (FE) code to generate the crack resistance (G _ID). The numerical simulation of the constant speed RCPs generated highly scattered G _ID data. Very large variations of the computed G _ID with the crack length did not correspond to fracture surface appearances. Geometry dependent and multivalued crack resistance results with respect to the crack speed cast doubt on the uniqueness of G _ID. In this work, attempts were made to overcome these difficulties by exploring the concept that the anomalies arise from large local strains around the rapidly moving crack tip, resulting in the crack seeing a low local modulus. It is demonstrated that the critical source of error on the analysis of RCP, is the improper linear elastic representation of the material behaviour around the propagating crack tip. Since the parameters describing the behaviour of the materials near the propagating crack tip were unknown, local non-linear effects were approximated by a local low modulus strip along the prospective crack path. The choice of the local modulus was justified by measurements of the strain histories along the crack path during RCP. The local strip low modulus model generated a larger amount of the kinetic energy in the sample and the crack resistance was reduced compared to results from the single constant modulus approach. Most importantly, G _ID data were nearly independent of the crack length, crack speed and the specimen size. This local modulus concept was also successfully applied to the analysis of RCP in the duplex specimen configuration.     

Thermal expansion behaviour of ultra-high modulus carbon fibre reinforced magnesium composite during thermal cycling

The thermally induced strain response of unidirectional P100S/AZ91D carbon fibre-reinforced magnesium composite was studied over five cycles in the ±100 °C temperature range. A temperature-dependent one-dimensional model was employed to predict the anticipated response to the cycling thermal environment. Strain hysteresis was observed during cycling and attributed to matrix yielding. First cycle residual plastic strains were modelled with reasonable agreement. Experimental results deviated from predictions during subsequent cycles with continued thermal ratcheting shifting the hysteresis loops to higher strains with increasing cycles. This was thought to be associated with interfacial debonding and frictional sliding at fibre/matrix interfaces. The effect of thermal treatment on composite expansion behaviour was investigated and the results discussed in terms of minimising thermally induced deformations during anticipated service conditions. Treatments were found to affect the first cycle behaviour, reducing in particular residual plastic strain generation. Matrix yield strength was exceeded over the thermal cycle due to a lack of sufficient hardening, and since interfacial conditions were unaltered, interfacial sliding and thermal ratcheting could not be eliminated. The potential for improvement of C/Mg composite thermal strain response was explored in the light of the current findings.     

A study of the effect of molecular weight on the tensile strength of ultra-high modulus polyethylenes

In this paper the influence of molecular weight and molecular weight distribution on the tensile strength (tenacity) of melt spun and drawn linear polyethylene are investigated with the aim of outlining the requirements for a high strength fibre. The tenacity was investigated over the molecular weight average ¯M _w range 60×10³ to 330×10³ with polydispersities ¯M _w/¯M _n ranging from 1.1 to 13.3. It was found that both molecular weight and its distribution affected tensile strength. The drawing conditions were also found to be important, a high draw temperature and a high draw ratio being needed for a high strength, high modulus fibre. By using a polymer of high ¯M _w and low polydispersity, and drawing at the optimum conditions, strengths of 1.65 GPa and moduli of 85 GPa have been achieved for test temperatures of –55° C.     

A novel method to measure the elastic modulus of polymers as a function of tensile deformation

A method to measure the elastic modulus of semicrystalline polyethylene during an instantaneous strain rate change is described. The elimination of machine transients during the strain rate change permits the observation of the stress reduction which must occur to maintain the abrupt change in non-elastic strain rate. This change can be implemented fast enough to precisely determine the incremental changes in stress and strain before relaxation processes take place. The results show that at 23.2° C the unrelaxed modulus decreases with strain whereas at 51° C it is constant. Furthermore, at room temperature the strain dependence of modulus change decreases with decrease in the degree of crystallinity. Such observations are in accordance with the thermal-mechanical data which indicate that both crystalline and viscous flow takes place near ambient but at elevated temperatures or slower strain rates, flow primarily occurs in the amorphous regions.     

Analysis and modelling of the die drawing of polymers

An analysis of the mechanics of the die-drawing process for polymer orientation has been undertaken on the basis of the lower bound theory by Hoffman and Sachs for metals, with several important modifications. First it is accepted that the flow stress in polymers is strongly dependent on plastic strain, strain rate and temperature. Secondly, thermal effects during die drawing due to heat generated by plastic deformation are included in the analysis. Finally, the temperature distribution in the die is estimated, taking into account both heat condition and axial translation of the polymer during the drawing process. Good agreement was obtained between theoretically predicted drawing stresses and those determined experimentally for a range of drawing speeds.     

Hydro and thermal stability of die drawn wood polymer composites in comparison to solid wood

Both isotropic and oriented wood polymer composites (WPC) based on 40% w/w of a softwood powder/hardwood powder and polypropylene (PP), together with solid pieces of wood, were subjected to water immersion and thermal expansion tests. Although generally die drawing increased the amount of water absorbed by the WPC by about 2-fold when compare to isotropic WPC, the oriented WPC exhibited extremely high hydro-dimensional stability. The values of the longitudinal and transverse swelling/shrinkage of the WPC oscillated only between 0 and −2.3% compared to values of between 4 and 14% for the solid woods. Incorporation of soft/hard wood powders into PP also substantially decreased its thermal expansion coefficient α in both the isotropic and the oriented states. This extremely positive effect was enhanced by increasing the draw ratio. In the longitudinal direction, α decreased from about 80 × 10⁻⁶ °C⁻¹ (for the isotropic PP) to 5 × 10⁻⁶ °C⁻¹ for the highly drawn PP filled with softwood.     

Advantages of a 3-parameter reduced constitutive model for the measurement of polymers elastic modulus using tensile tests

Exact measurements of the rheological parameters of time-dependent materials are crucial to improve our understanding of their intimate relation to the internal bulk microstructure. Concerning solid polymers and the apparently simple determination of Young’s modulus in tensile tests, international standards rely on basic protocols that are known to lead to erroneous values. This paper describes an approach allowing a correct measurement of the instantaneous elastic modulus of polymers by a tensile test. It is based on the use of an appropriate reduced model to describe the behavior of the material up to great strains, together with well-established principles of parameter estimation in engineering science. These principles are objective tools that are used to determine which parameters of a model can be correctly identified according to the informational content of a given data set. The assessment of the methodology and of the measurements is accomplished by comparing the results with those obtained from two other physical experiments, probing the material response at small temporal and length scales, namely, ultrasound measurements with excitation at 5 MHz and modulated nanoindentation tests over a few nanometers of amplitude.     

An analytical characterization of the anisotropy of the elastic modulus of misaligned short-fiber-reinforced polymers

Short-fiber-reinforced polymers (SFRP) are very attractive because of their ease of fabrication, relatively low cost and mechanical properties which are superior to those of relevant polymer resins. Owing to the partial orientation distribution of the fibers in final components, SFRP composites show direction-dependence, namely anisotropy in their mechanical properties. The fiber-length distribution (FLD) and the fiber-orientation distribution (FOD) in SFRP composites play an important role in determining the composite mechanical properties. In the present paper, the FLD and the FOD are modelled with suitable probability density functions and the laminate analogy approach is used to derive the expression of the elastic modulus of SFRP as a function of any given direction, the FLD and the FOD. The direction-dependence, i.e. the anisotropy, of the elastic modulus of SFRP has been studied in detail by taking into consideration the effects of the FLD and the FOD. The present theory is applied to existing experimental results, and the agreement is found to be very satisfactory.     

In situ measurement of Young's modulus of carbon nanotubes inside a TEM through a hybrid nanorobotic manipulation system

A hybrid nanorobotic manipulation system inside a scanning electron microscope (SEM) and a transmission electron microscope (TEM) is presented. The SEM manipulators have been constructed with 8 degrees of freedom (DOFs) with three units for effective TEM sample preparation. The TEM manipulator consists of a 3-DOF manipulator actuated with four multilayer piezoelectric actuators and a 3-DOF passively driven sample stage. High resolution and transmission image of TEM is readily used for measurement and evaluation of samples. The stage is premanipulated by the SEM manipulator for sample preparations inside the SEM. This methodology is called the hybrid nanorobotic manipulation so as to differentiate it from those with only an exchangeable specimen holder. To show the effectiveness of the system, the Young's modulus of a carbon nanotube (CNT) was measured to be 1.23 TPa inside a TEM after being premanipulated inside the SEM. With this system, we can measure the inner diameter of a CNT and improve the accuracy in measuring the Young's modulus of a CNT.