Pull-Out Response of a Steel Post Inserted in a Pre-Drilled HDPE Cylinder: Analytical and Finite Element Analyses Using Pressure-Dependent Friction

Abstract

For the aim of fixing and stabilizing the Total Knee Arthroplasty (TKA) tibial component, pull-out strength of the post is one of the most important factors to be considered. The material properties of bone, coupled with the principal dimensions of bone/post assembly such as diameter, interference fit and implantation length, may affect the pull-out strength of the post fixation. In this study, a cylindrical stainless steel post inserted in a pre-drilled High Density Polyethylene (HDPE) cylinder with an initial interference fit was taken as a model to assess the contribution of post fixation to the initial stability of TKA tibial component. Pull-out experiments were carried out for different initial interference fits and implantation lengths. Under pull-out loading, the micro-slip initiation and propagation at the post/cylinder interface was found to be progressive and was modeled using Coulomb friction at the interface. In order to examine the experimental ultimate pull-out force results, an analytical model was developed. The analytical model was fitted to the experimental results by adjusting the friction coefficient for the considered ranges of initial interference fit and implantation length. It was found that friction coefficient depends on initial interference fit (Δr) as well as on the initial contact pressure (σ0). For the considered interference fit values Δr = 01 mm, Δr = 02 mm and Δr = 03 mm (σ₀ = 1362 MPa, σ₀ = 2724 MPa and σ₀ = 4086 MPa), the values of adjusted friction coefficient are 0.091, 0.067 and 0.058, respectively. Finite Element simulation was also carried out for the pull-out test using the ABAQUS program. It was found that numerical, analytical and experimental results are all in good agreement.     

Effect of pressure on the precipitation and sol-phase aqueous polymerization of methyl methacrylate

Effect of pressure (atmospheric to 120 kg/cm²) on the K₂S₂O₈–Na₂S₂O₄-initiated aqueous polymerization of methyl methacrylate has been studied at 25°C. When the concentrations of the redox initiator are so adjusted as to obtain the separating polymer phase as a coarse coagulum, the conversion, rate, and molecular weight of polymerization tend to rise initially with increase of pressure up to a certain value and fall subsequently to a limiting value. However, these parameters fall monotonously with an increase in pressure when the polymer phase separates out as a fine colloid at a lower concentration of the initiator. The initial rise in rate is consistent with an increase in k_p and or a decrease in k_t under high pressure; the ultimate fall in rate may be due to a decrease in the diffusion of monomer from the aqueous phase to the growing polymer radical site. The fall in the molecular weight with pressure is explained on the basis of enhanced monomer transfer. In the colloidal range the pressure dependence trend is related to the stability of the colloidal phase. The rate is proportional to the square root of the product of K₂S₂O₈–Na₂S₂O₄ and varies linearly as the first power of the monomer concentration as also observed under normal pressure conditions. The MWD values of the polymers are ca. 2.5 and do not change with applied pressure.     

The redox aqueous polymerization of acrylonitrile under pressure

Effect of pressure (atmospheric to 20 X 10⁶ Pa) on the K₂S₂O₈–Na₂S₂O₄ initiated aqueous polymerization of acrylonitrile has been studied at 25°C. The conversion and rate of the polymerization tend to rise initially with increase of pressure and fall subsequently to a limiting value. The initial rise in the rate is consistent with an increase of k_p or a decrease in k_t due to high pressure. The ultimate tendency of the rate to fall is possibly due to a decrease in the diffusion rate of the monomer from the aqueous phase to the growing polymer radical site. The molecular weight shows a more or less similar trend except that the fall in the molecular weight begins at a lower pressure range than the same in the rate or conversion. This is explained on the basis of enhanced monomer transfer reaction. These observed kinetic characteristics are not sensitive to the appearance of heterogeneity in the system due to insoluble polymer phase in as much as homogeneous reaction systems in DMF or DMSO essentially exhibit the same features. The rate is proportional to the square root of the product of [K₂S₂O₈] and [Na₂S₂O₄] and varies linearly as the first power of the monomer concentration.     

Fatigue mechanism of fiber-reinforced polypropylene swollen by oil

Fatigue mechanism was investigated for the fiber-reinforced polypropylene (FRPP) swollen by a small amount of oil. The curves of applied cyclic stress (S) vs. logarithm of cycles to failure (N) shifted into smaller values of S and N, respectively, by adding oil into the polymer and with increasing a test temperature. The endurance limiting stress (σ_e) for the swollen FRPP, defined as the stress at a long lifetime, became much the same as the one for the swollen unfilled polypropylene, though the former was considerably larger than the latter for the breaking strength measured in standard bending test. The activation energy and the activated volume, which have been determined by the Eyring's model from the S–N curves at higher stress levels, suggest that the introduction of a small amount of oil into the polymer lowers a motional unit associated with a fatigue process from 30 to several repeat units. The major decrease in σ_e by swelling can be explained in terms of the crack–nucleation theory. It is indicated that this decrease yields from the change in the stress–concentration factor.     

New criterion for craze initiation

Existing criteria for craze initiation are reviewed, and their limitations are discussed. The most obvious problem is that they are formulated simply in terms of principal stresses, making no provision for the known effects of small inclusions and surface imperfections. To solve this problem, a new criterion is proposed, which is based on linear elastic fracture mechanics. Craze initiation is treated as a frustrated fracture process rather than a yield mechanism. Calculations show that the strain energy release rate, G_I(nasc), required to generate a typical 20 nm thick nascent craze, is less than 1 J m⁻². This explains why flaws less than 1 μm in length are capable of nucleating crazes at stresses of 20-30 MPa. Subsequent craze propagation is dependent upon two flow rates, one relating to fibril drawing at the craze wall and the other to shear yielding at the craze tip. Under biaxial stress, the second principal stress σ₂ affects craze tip shear yielding but not fibril drawing. This model is used in conjunction with the von Mises yield criterion to derive a new expression for the crazing stress σ₁(craze), which provides a good fit to data on visible crazes obtained by Sternstein, Ongchin and Myers in biaxial tests on cast PMMA [Sternstein SS, Ongchin L, Silverman A. Appl Polym Symp 1968;7:175; Sternstein SS, Ongchin L. Polym Prepr Am Chem Soc Div Polym Chem 1969;10:1117; Sternstein SS, Myers FA. J Macromol Sci Phys 1973;B8:539].     

Development of crystalline structure during tubular film blowing of low-density polyethylene

Development of crystalline structure during the tubular film blowing of low-density polyethylene was investigated, using wide-angle X-ray diffraction technique, low-angle light scattering, and scanning electron microscopy. In the study, commercial grades of both high-pressure low-density polyethylene (HP-LDPE) and low-pressure low-density polyethylene (LP-LDPE) (also, commonly referred to as linear low-density polyethylene, LLDPE) were used. The applied stresses at the freeze line were determined using theoretical expressions derived in an earlier publication [C. D. Han and T. H. Kwack, J. Appl. Polym. Sci., 28 , 3399 (1983)]. The applied stresses, S_11F and S_33F, at and above the freeze line in the machine and transverse directions were expressed in terms of the tension at the take-up device, take-up ratio, blow-up ratio, and the pressure difference across the film of the bubble. These applied stresses were used to interpret the crystalline axes' orientation in the tubular blown films. It was found that the magnitude of S_11F is an important process parameter for the crystalline axes' orientation and that the biaxial stress ratio (S_11F/S_33F) appears to be a determining factor in the distribution of fibrillous nuclei and crystalline texture, as well as film anisotropy.     

Control of molecular orientation

I. Amorphous polymers . The mechanical performance of a glassy amorphous polymer is strongly dependent upon molecular orientation. The pattern of molecular orientation is governed by the kinematics (and temperature) of mechanical forming operations. Three types of controllable orientation are: (a) uniaxial, (b) biaxial, and (c) “crossed.” The optimum pattern of orientation in a part is one which is appropriate for the mechanical stresses encountered in service. For a fiber subjected to tensile and bending loads, uniaxial orientation is appropriate. A shell structure, subjected to multiaxial stresses, requires either biaxial or crossed orientation for maximum performance. As a rule, the maximum achievable multidirectional strength in such a structure is less than the maximum strength of a uniaxially oriented fiber. II. Crystalline polymers . Oriented crystalline polymer structures can be created in two distinct ways. An isotropic polycrystalline polymer can be deformed below the melting point, with extensive reorganization of the crystal morphology, or an oriented amorphous melt can undergo crystallization to yield oriented crystalline polymer. Performance of an oriented semicrystalline polymer depends upon orientation of the amorphous portion as well as orientation of the crystallites. As with amorphous polymers, orientation can be uniaxial, biaxial, or crossed. “Orientation” usually denotes c-axis orientation only, but drawing followed by rolling can result in double orientation—orientation of a-axis, b-axis, and c-axis.     

Yielding of polyethylene through propagation of chain twist defects: Temperature, stem length and strain-rate dependence

The temperature, the stem length and the strain-rate dependence of the yield stress of polyethylene (PE) is investigated via a modified crystal plasticity approach. Yielding is considered in terms of nucleation and propagation of [001] screw dislocations with Burgers vector c/2 due to migration of 180° chain twist defects. The stress-induced twist motion within the dislocation cores is modeled as an Eyring activated rate process. This gives an inelastic contribution to the dislocation core energy depending on the stem length and the strain rate and results in improved predictions of the crystal plasticity approach. The model is compared to available experimental data as well as to the predictions of the modified crystal plasticity approach proposed by Brooks and Mukhtar.     

Study on soluble polyaniline by positron annihilation technique

The solubility, electrical conductivity, and other properties of polyaniline (PANI) are highly dependent on its oxidation state. In this work, polyaniline (PANI1) prepared by peroxodisulphate induced polymerization of aniline in acidic aqueous medium in presence of benzenediazonium chloride salt was found to exist in lower oxidation state than emeraldine form of PANI and was highly soluble in common organic solvents. This polymer was subjected to positron annihilation spectroscopic study to investigate the correlation between the oxidation state of the polymer and defect sites generated by different degrees of protonation that in turn affect its electrical conductivity. The positron annihilation lifetime data were resolved to yield a three‐component fit for PANI1 subjected to different levels of protonation. The variation of positron annihilation parameters (τ_1,I₂) and Doppler broadening parameters (R, S) as a function of protonation level of the polymer indicate the dopant sites increase initially on protonation and reach a saturation value after a certain level of acidification. The lower value of electrical conductivity and the intensity of intermediate lifetime component (I₂) for PANI1 compared to PANI in emeraldine oxidation state indicate the presence of lesser number of quinoid–imine moieties that could undergo protonation and thus yield highly enriched trapping centers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Polymerization of styrene in an electrodeless glow discharge

The polymerization (polymer deposition) rate of styrene in an electrodeless glow discharge from styrene vapor and a mixture of styrene vapor and gas (H₂, He, A, and N₂) was investigated. The rate of polymerization, R, was found to be independent of the discharge power. The rate of polymerization of the pure monomer was found to be proportional to the square of monomer pressure p_M. The addition of gas increased the rate of polymerization depending upon the partial pressure of the gas, p_x, and R can be generally expressed by R = a[p_M]²{1 + b[p_x]}. The value of b is dependent of the type of gas and follows the order of N₂, > A > He > H₂. The distribution of polymer deposition was found to be nearly independent of the partial pressure of the gas and of the discharge power with N₂ and H₂ as plasma gas; however, with He and A, the distribution is highly dependent on the partial pressure of the gas and on the discharge power. The study strongly suggests that polymerization occurs in the vapor phase and that the growing polymer radicals deposit on the surface of the discharge vessel, yielding highly crosslinked polymer deposition.     

Dielectric dispersion and AC conductivity of acrylonitrile butadiene rubber‐poly(vinyl chloride)/graphite composite

Dielectric properties and ac electrical conductivity of Acrylonitrile Butadiene Rubber‐poly(vinyl chloride)/Graphite Composite were studied at different frequencies (10²?10⁶ Hz) in the temperature range (298–423 K). The results show that the dielectric constant (ε′), dielectric loss (ε″), ac electrical conductivity (σ_ac) and, the electric modulus are strongly dependent on the frequency and temperature. The dielectric constant ε′ increases with temperature and decreases with frequency, whereas the dielectric loss ε″ displays a broad maximum peak whose position shifts with temperature to a higher frequency region. Cole–Cole diagrams have been used to investigate the frequency dependence of the complex impedance at different temperature and graphite loading. Interfacial or Maxwell‐Wagner‐Sillars relaxation process was revealed in the frequency range and temperature interval of the measurements, which was found to follow the Havriliak–Negami approach for the distribution of relaxation times. At constant temperature, the frequency dependence of ac conductivity was found to fit with the established equation σ_ac(ω) = Aω^s quite well. The values of S for the investigated samples lie between 0.88 and 0.11. The conduction mechanism of ac conduction was discussed by comparing the behavior of the frequency exponent S(T) with different theoretical models. It was found that the correlated barrier hopping (C.B.H.) is the dominant conduction mechanism. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

SANS studies on catalyst ink of fuel cell

The structure of solid polymer electrode and catalyst ink of fuel cell has been investigated by focusing‐ (FSANS) and contrast‐variation small‐angle neutron scattering (CV‐SANS). The solid polymer electrode, consisting of carbon (C), platinum, and ionomer (polymer, P), exhibited a power‐law function with two asymptotes, i.e., from I(q) ～ q^?1 to I(q) ～ q^?4 with a crossover around q ≈ 0.005 Å^?1. The scattering functions of the catalyst ink, i.e., the polymer electrodes dispersed in water, were successfully decomposed to the corresponding partial structure factors, S_CC(q), S_PP(q), S_CP(q), exclusively representing C‐C, P‐P, and C‐P correlations. S_CC(q) was a monotonic decreasing function of q, dominating in the scattering from carbon clusters. On the other hand, S_PP(q) exhibited a scattering maximum characteristic of polyelectrolyte solutions. This suggests that ionic clusters in polyelectrolyte solutions are formed in catalyst ink. The cross term, S_CP(q), indicated that the carbon scattering is dominant and significant amount of ionomer is adsorbed on the carbon agglomerates. It is concluded that the catalyst ink consists of carbon agglomerates surrounded by ionomers and the presence of ionic‐cluster path plays a key role in the performance of the solid polymer electrodes in polymer electrolyte fuel cells. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39842.     

Theoretical evaluation of the conformational entropy of fusion and the melting temperature of polymers

A method is presented to theoretically estimate the conformational entropy ΔS_c of fusion of polymer chains of any stereochemical constitution. The effect of the type of crystalline conformation and the conformational energy differences on ΔS_c is discussed with examples. The dependence of ΔS_c on the configurational partition function is traced. A method for theoretically evaluating the melting temperature of a polymer from its conformational characteristics is presented. The application of the method to a total of 11 polymers leads to good agreement between the calculated and experimental values of the melting temperatures.     

Wall slip and compressibility like effects in capillary rheometer tests on complex polymer systems

Abstract

Using an appropriate set of capillary dies, the applicability of the Mooney wall slip method has been investigated with several filled compounds. Inconsistent results were obtained, for example, ‘negative’ slip velocity. With respect to data measured in practical capillary rheometry, a model was developed for treating (barrel) pressure data versus die length/diameter ratio at fixed applied apparent shear rate, i.e. the so called Bagley plots. A very simple equation was obtained

P _meas = 2P _ends - 1/β ln [exp (P _ends β) - 4σ_w0 β L/D ]

which yields the wall shear stress at zero pressure σ_w0 , the ends pressure loss P _ends , and a factor β when fitted to experimental data by non-linear regression. Experimental results show that the factor β accounts for both wall slip and compressibility like effects. Negative β values indicate dominating wall slip effects, while positive values demonstrate compressibility like effects. Slip velocity is thus pressure dependent and consequently the combination of wall slip and a pressure dependent viscosity can mask the expected gap dependence in the analysis by Mooney.     

Kinetics of drug release from drug carrier of polymer/TiO2 nanotubes composite—pH dependent study

This study was to investigate the kinetics of drug release from polymer/TiO₂ nanotubes composite. Lidocaine and carprofen were selected as model drugs to represent weak base and weak acid drugs, respectively. Mathematical models used to fit the in vitro drug release experimental data indicate that at higher pH, the drug release was first order diffusion controlled. At lower pH, the release of the two drugs exhibits two staged controlled release mechanism. The first phase is due to drug diffusion and the second stage is a result of poly(lactic‐co‐glycolic acid) (PLGA) polymer degradation. The rate of drug release from polymer/TiO₂ nanotubes drug carrier was mainly controlled by three pH dependent factors: the solubility of the drug, the degree of polymer swelling/degradation, and the electrostatic force between polymer and drug. This study suggests that controlled release could be achieved for polymer/TiO₂ nanotubes drug carrier via the modulation of pK_a values of polymers and drug solubility. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41570.     

Dislocation loop evolution in Kr-irradiated ThO2

The early stage of microstructural evolution of ThO₂, under krypton irradiation at 600, 800, and 1000°C, was investigated using in situ transmission electron microscopy (TEM). Dislocation loops grew faster, whereas their number density decreased with increasing irradiation temperature. Loop density was found to decrease with ion dose. Interstitial dislocation loops, including Frank loops with Burgers vector of a/3〈111〉 and perfect loops with Burgers vector of a/2〈110〉, were determined by traditional TEM and atomic resolution–scanning TEM techniques. Atomistic and mesoscale level modeling are performed to interpret experimental observations. The migration energy barriers of defects in ThO₂ were calculated using density-functional theory. The energetics of different dislocation loop types were studied using molecular dynamics simulations. Loop density and diameter were analyzed using a kinetic rate theory model that considers stoichiometric loop evolution. This analysis reveals that loop growth is governed by the mobility of cation interstitials, whereas loop nucleation is determined by the mobility of anion defects. Lastly, a rate theory model was used to extract the diffusion coefficients of thorium interstitials, oxygen interstitials, and vacancies.     

Permeation and sorption of CO2 through amine-contained polyurethane and poly(urea–urethane) membranes

As an absorbent of CO₂, tetraethylenepentamine (TEPA) and/or N-methyldiethanolamine (MDEA) was introduced into the hard segment as chain extenders of the series urethane polymer (PU), urea–urethane polymer (PUU), and segmented urethane/urea–urethane copolymer (SPU) based on 4,4′-diphenylmethane diisocyanate (MDI) and either poly(ethylene glycol (PEG)-400 or -600. The obtained polymers thus contained a nearly equal weight composition of both soft and hard segments and were prepared as polymer membranes for permeation and sorption of CO₂. The properties of the polymer membranes were characterized using a Fourier transform IR spectrophotometer, thermal gravimetric analysis, and rheometric measurement. The permeation and sorption isotherms as a function of temperature and pressure as well as the activation energy of permeability and diffusivity and the enthalpy change of the solution were measured. The test temperatures were carried out above and below the T_gs of soft-segment domains or the T_gh of hard-segment domains. The steady-state permeability (P) and diffusion coefficients (D) obtained ranged from 1.01 to 12.9 barrer and 1.04 to 4.04 cm²/s, respectively, and the solubility coefficients (S), from 0.529 to 3.43 cm³(STP)/cm³ polymer-atm at 10 atm and 35°C. The TEPA-containing polymer membranes showed a smaller P and D but a larger S than did MDEA-containing ones. The membranes comprising PEG-600 exhibit the values of P, D, and S at about 11.5-, 2.5-, and 4.5-fold the PEG-400 ones, respectively. For SPU membranes, above T_gs or T_gs, the pressure dependencies of P followed the modified free-volume model. On the other hand, below T_gs, they exhibited a minimum permeability at a certain pressure caused by the plasticization action of sorbed CO₂. The sorption isotherms of CO₂ indicated that the membranes comprising PEG-400 can be described by a dual-mode sorption model below T_gs. Also, the SPU polymer membranes obied the Henry's law above T_gs as well as T_gh. The characteristic constants of the sorption model were also determined and are discussed. © 1996 John Wiley & Sons, Inc.     

Preparation and Electric Properties of Bi0.5Na0.5TiO3–Bi(Al0.5Ga0.5)O3 Lead‐Free Piezoceramics

A new lead‐free BNT‐based piezoelectric ceramics of (1 ? x)Bi_0.5Na_0.5TiO₃–xBi(Al_0.5Ga_0.5)O₃ (x = 0, 0.02, 0.03, 0.04, and 0.05) were synthesized using a conventional ceramic fabrication method. Their structures and electrical properties were investigated. All the samples show a typical ferroelectric P(E) loops and S(E) curves at room temperature. The optimal properties are obtained at the composition of the x = 0.03. The substitution of Bi(Al_0.5Ga_0.5)O₃ enhances piezoelectric constant and increases Curie temperature from 58 pC/N and 310°C of pure BNT to 93 pC/N and 325°C of the x = 0.03. The temperature‐dependent P(E) loops and S(E) curves of 0.97BNT–0.03BAG indicate that phase transition from ferroelectric to antiferroelectric takes place over a very wide temperature region from 80°C to 180°C. The results show that the introduction of BAG improves the electrical properties of BNT.     

Failure analysis of adhesively bonded tubular joints of laminated FRP composites subjected to combined internal pressure and torsional loading

Abstract

Finite element analysis has been carried in the present research to study individual and combined effect of internal pressure and torsional loading on stress and failure characteristics in case of an adhesively bonded Tubular Single Lap Joints (TSLJ) made of laminated Fiber reinforced polymer (FRP) composite materials. Effect of changing torsional load magnitude on an internally pressurised adhesively bonded TSLJ on interlaminar stresses and onset of different joint fracture modes (adhesion and cohesion failures) has also been studied in the present analysis. Three dimensional stress analysis of the adhesively bonded TSLJ has been carried out through suitable ANSYS Parametric Design Language (APDL) of ANSYS 14.0. Tsai-Wu coupled stress criterion has been used for predicting the onset of joint failures in the TSLJ. It has been observed that stresses (σ_r, σ_θ, σ_z, τ_rz) induced within the joint region under pure internal pressure loading are least affected through introduction of a torsional loading in the TSLJ. However, the stresses (τ_rθ and τ_θz) which are considered to be significant under pure torsional loading get tremendously enhanced due to the varying torsional loading. The interface between the outer tube and adhesive of the TSLJ has been observed to be the most critical bondline interface which is prone to undergo adhesion failure towards the free edges under pure internal loading conditions. However, under pure torsional loading conditions it tends to fracture through adhesion failure towards the clamped edge of the TSLJ. Under combined torsional and internal pressure loading the joint fails towards the clamped edge of the along the critical path which happens to be within the bondline interface, indicating predominance of torsional loading over the pure internal pressure loading. A comparative study based on the magnitude of failure index revealed that torsional loading marginally affects the joint failure as the internal pressure loading improves the compactness of the bonded joint hence improving the resistance of the TSLJ against initiation of joint fractures.