Response Of High Density Polyethylene Research Articles

Response of High Density Polyethylene to Impact Loading

Response of High Density Polyethylene to Impact Loading

DC electrorheological response of polyethylene/organically modified layered silicate nanocomposites

The present work reports the electrorheological (ER) response of high-density polyethylene (HDPE)/organically modified silicate layers nanocomposites based on four commercially available HDPE matrices. Two single-site catalyzed bimodal resins, one single-site catalyzed unimodal resin and one Ziegler–Natta catalyzed unimodal resin are studied. It is revealed that the distinct separation of the two modes of the bimodal HDPE resins significantly enhances the ER response. It is proposed that the slower structural relaxation modes introduced by higher molecular weight species in the bimodal HDPE matrices enhance the ER response of the nanocomposites. This is ascribed to the longer induction time for leaking current density, which is an indicator of mobility and release of immobilized cationic surfactants at the silicate layers surface induced by field exposure. It is found that the screening effect of prematurely released cationic surfactants leads to a weaker ER response in nanocomposites whose matrices have faster relaxation modes. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017

Anisotropic mechanical behavior of semi‐crystalline polymers: Characterization and modeling of non‐monotonic loading including damage

ABSTRACTIn this work, the mechanical response of high density polyethylene (HDPE) to complex uniaxial tensile loadings is firstly characterized experimentally, taking into account the damage occurring in large deformation and the initial anisotropy induced by the forming process. Anisotropic effects are characterized through tensile tests using several complex loading paths involving large deformation, and for different orientation with respect to the extrusion direction. A mechanical model is then developed, based on a non‐equilibrium thermodynamic approach of irreversible processes, resulting in a new thermodynamic potential describing both the elasto‐viscoelastic–viscoplastic behavior and the volume variation due to damage. Results show that transverse strains and volume strain of HDPE highly depend on specimen orientation, whereas the apparent Young's modulus is not affected by this orientation. The developed model is validated for HDPE, and satisfyingly predicts the complex response of HDPE to complex loadings paths. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44468.

Mechanical response of high density polyethylene to gamma radiation from a Cobalt-60 irradiator

The response of High Density Polyethylene (HDPE) to gamma irradiation is important for a variety of applications, from the durability of hip replacements after gamma sterilization to the degradation of power cable insulation that guides the licensing and regulation of nuclear power plants. HDPE samples are irradiated with up to 58.8 kGy from a Cobalt-60 gamma irradiator, and mechanical properties are examined using models assuming exponential behavior. Increasing the radiation dose led to increases to the ultimate strength and the Rockwell hardness with a corresponding reduction in the maximum elongation at ultimate strength, supporting the hypothesis that the samples increased their strength and brittleness.

Modeling of viscoplastic cyclic loading behavior of polymers

A new theoretical approach, analyzed in previous works, is employed for the description of the nonlinear viscoelastic/viscoplastic response of high density polyethylene under tensile cyclic loading, experimentally studied elsewhere. The proposed analysis, developed for a 3-D problem, is applied for a uniaxial cyclic deformation, in a strain-controlled program, where tensile loading up to maximum strain is followed by unloading to zero stress. This procedure is repeated for ten cycles. The same model is also applied for the simulation of a stress-controlled program, where cyclic loading takes place between a \(\sigma_{\max}\) and \(\sigma_{\min}\) engineering stress. The hysteresis loops of both programs could be adequately captured, with a number of model parameters, related to both, nonlinear viscoelasticity and viscoplasticity. The simulated ratcheting strain as well as its evolution with number of cycles is a very good approximation of the experimental one. A systematic study of the values of the adjustable parameters has been performed in order to monitor the effect of every specific internal variable, responsible for either the nonlinear viscoelastic or viscoplastic path in the simulations. It was found that in the proposed analysis a rather low number of model parameters are required, compared to the works existing in the literature.

Melt-state elongational rheometry of mineral-filled polyethylene

A free-surface elongational rheometer has been further developed to study the melt state deformation and rupture behaviour of high-density polyethylene (HDPE) polymers, and corresponding elasticity characteristics. An upgraded, PC-interfaced data collection system has been introduced, appropriately calibrated to provide rapid and accurate data collection when tests are carried out at high extensional strain rates; good agreement has been obtained between experimental and theoretical displacements, over a range of strain rates (0.1–6.0 s−1). Deformation (extensional stress–strain) response of HDPE is shown to be sensitive to molecular weight, with low molar mass grades suffering melt rupture at Hencky strains between 2.0 and 3.0. Incorporation of inorganic calcium carbonate filler increases transient-state extensional viscosity and rupture stress, but reduces the extensional strain at failure. The degree of elastic recovery is dependent upon the total elongational strain developed during the test; calcium carbonate-filled HDPE melts exhibit lower recovery at a given strain, but elasticity is enhanced when fatty-acid coatings are used to assist filler dispersion in the compound.

Nonlinear Predictions for HDPE Pipe Response under Parallel Plate Loading

Polymer sewer pipe load carrying ability when deflected to 20% pipe deformation under parallel plate loading is used in industry as an index of pipe quality. New developments in limit states design also require the assessment of pipe response beyond the small deflection limits currently specified (between 5 and 7.5% change in pipe diameter). New material models recently developed to predict the nonlinear time-dependent response of high density polyethylene (HDPE) are used in a materially and geometrically nonlinear finite-element analysis to predict pipe response during the flattening test. Comparisons are made with test results for two different plain HDPE pipes. The new analysis permits good quality predictions of pipe response to be made at large deformations (20% vertical diameter decrease) where materially nonlinear polymer response occurs at crown, invert, and springline. Modeling permits pipe response to be evaluated under a constant rate of deformation, stress relaxation, and other load paths.

Nonlinear mechanical response of high density polyethylene. Part I: Experimental investigation and model evaluation

AbstractThe nonlinear behavior of high density polyethylene (HDPE) is investigated for samples cut from thick‐walled HDPE pipe. Extensive experimental work has been performed to characterize the non‐linear time‐dependent response of the material tested under uniaxial compression. Tests were conducted under conditions of constant strain rate, creep, stress relaxation, constant loading rate, abrupt change of strain rate, creep‐recovery, cyclic strain rate, and various combinations of these loading conditions. Creep and stress relaxation response after strain reversal and the effect of the transient response on the following stress‐strain behavior is examined. Permanent strains for the test specimens and their dependence on loading histories are investigated. Specimens cut at various orientations from the pipe are used to quantify the small amounts of local anisotropy in the pipe specimen. The experimental work has been used to develop both nonlinear viscoelastic (NVE) and viscoplastic (VP) constitutive models in a companion paper. Both the test results and the corresponding model predictions are reported in this paper. It is found that the VP model reproduces the nonlinear viscoelastic‐viscoplastic behavior of HDPE very well provided that the current strain is not below the maximum strain imposed (there is no strain reversal). The NVE model predicts the material behavior reasonably well for some loading conditions, but inadequately for others.

Nonlinear mechanical response of high density polyethylene. Part II: Uniaxial constitutive modeling

AbstractBased on the experimental data presented in Part I, two uniaxial constitutive models are constructed. The first, a nonlinear viscoelastic (NVE) model, is formulated using the mechanical analogy consisting of one independent spring and six Kelvin elements in series. Creep data are used to determine the model parameters. The second model, a viscoplastic (VP) formulation, is developed using the viscoplastic theory proposed by Bodner to characterize the uniaxial viscoplastic behavior of metals. Inelastic strain rate is introduced into the state variable in addition to inelastic work to depict the strong rate dependent behavior of HDPE. Experimental data from constant strain rate tests are employed to construct the material functions of the model. Limitations in the application of each model are discussed in conjunction with possibilities for future work.

A study of degradation of high density polyethylene in a corotating intermeshing twin screw extruder

AbstractA study of the effect of extrusion conditions on the degradation of high density polyethylene was carried out. The extent of degradation was quantified by dynamic viscometry. The effect of barrel temperature profile, screw speed, and feed rate on the degradation of high density polyethylene was determined. The viscosity response of high density polyethylene is complex with respect to extrusion conditions. Residence time studies were carried out to correlate the viscosity data. Simulations of simple theoretical models give an insight into the flow behavior of the polymer in a twin screw extruder.