Load Transfer Analysis at the Interface Between a Steel Post and Polyethylene Matrix Using Pull-Out Test: Experimental and Theoretical Parametric Study

In various kinds of orthopedic or dental prostheses, adequate fixation of prosthetic components to the host bone represents a great challenge. The short- and long-term fixation stabilities of the implants are often provided by the use of posts. In this work, a cylindrical steel post inserted in a predrilled High Density Polyethylene (HDPE) cylinder with an initial interference fit was used as a model to analyze the contribution of post fixation to the initial stability of Total Knee Arthroplasty (TKA) tibial component. To characterize the ability of the steel post/HDPE cylinder interface for transferring mechanical load, pull-out tests were carried out on the model post fixation for different initial interference fits and implantation lengths. Experimental ultimate pull-out force was found to increase in a quasi-linear fashion with increase in the interference fit or the implantation length. On the basis of Coulomb friction at the post/HDPE interface, an analytical model has been developed for the analysis of the experimental ultimate pull-out force results. For the ranges of initial interference fit and implantation length considered, the analytical model was fitted to the experimental results by adjusting the friction coefficient. The mean value of the friction coefficient obtained was 0.043. Finite element simulation was also carried out for the pull-out test using the ABAQUS program. It was found that the finite element simulation describes well the experimental ultimate pull-out forces than the analytical calculation.     

Effect of torque loading on the pull-out response of a steel post inserted in a polyethylene cylinder

Posts are used in various implant designs to contribute to the short- and long-term fixation stability of artificial joints. This study was undertaken to investigate the effect of torque loading on the pull-out response of a steel post inserted into high-density polyethylene (HDPE) material. An experimental apparatus was designed and fabricated to perform mechanical characterization of a steel post embedded in a polymer cylinder with initial interference fit under pull-out, torque and combined torque/pull-out loadings. To analyze the effect of preload applied torque to the load transfer at the post-fixation interface under pull-out loading, we have chosen HDPE material with uniform mechanical and tribological properties. Under pull-out loading, the micro-slip initiation and propagation at the post-HDPE interface was found to be progressive and assuming Coulomb friction at the interface, the friction coefficient was calculated from the measured pull-out force. In the torque loading condition it was found that the torque dropped suddenly from the maximum value to an initial dynamic sliding torque value. The interface behaves like a chemically bonded one, and static and dynamic friction coefficients were determined. It has been found under combined torque/pull-out conditions that in addition to the reduction of the maximum pull-out force, the preload applied torque generates two instabilities in the pull-out behavior. The first one happens once the maximum pull-out force is reached where the load falls to a level required for the post extraction from the HDPE cylinder. The second instability takes place during the extraction process for a residual (critical) implantation length which depends on the preload applied torque value. This latter instability was marked by a sudden rotation of the HDPE cylinder against the steel post.     

Finite element analysis of load transfer at a fibre–matrix interface during pull-out loading

Load transfer ability of the fibre–matrix interface is well known to mainly control the mechanical behaviour of fibre-reinforced materials. This load transfer phenomenon is of great importance in dentistry when a post is used for fixing a ceramic crown on the tooth. The pull-out test has been well accepted as the most important micromechanical test for evaluating the interaction properties between the fibre and matrix. In this study, a finite element model is developed to analyse the pull-out process of a steel fibre from an epoxy matrix. Based on the pull-out force–displacement curves, developed in our previous experimental work, specific load transfer laws at the fibre–matrix interface have been proposed for each stage of the pull-out process, i.e., before and after fibre–matrix debonding. Predicted initial extraction forces for different implantation lengths were fitted to experimental values and an initial interference fit of 4 μm was determined. An interfacial shear strength of 21 MPa was then determined by fitting the predicted debonding forces for different implantation lengths to the experimental values. According to the load transfer laws considered, analysis of the interfacial shear stress indicates that fibre–matrix debonding initiates simultaneously at both the lower and upper extremities of the interface.     

Effect of hydrostatic pressure on the deformation behavior of polyethylene and polycarbonate in tension and in compression

The stress-strain response of crystalline high density polyethylene and of amorphous polycarbonate has been determined in tension and in compression at superimposed pressures up to 1104 MPa(160 ksi). Strain softening occurred in the polycarbonate at low pressures but was inhibited by pressure. Tensile necking occurred in both materials, but was promoted by pressure in polyethylene and inhibited in polycarbonate. The initial modulus, E, and the flow stress, σ, at a given offset strain varied linearly with the mean pressure, P, with essentially the same pressure coefficient, α. Thus, E = (1+αP)E₀ and σ = (1+αP)σ₀, where E₀ and σ₀ are values at zero mean pressure. In polyethylene, the coefficient, σ₀, was the same in tension and compression, indicating that the strength differential between tension and compression was a simple manifestation of pressure-dependent yielding. In polycarbonate the coefficient, σ₀, was different in tension and in compression, implying an effect due to the third stress invariant or to anisotropy. The results suggest a constitutive model for polymers in which the flow stress is linearly dependent on mean pressure, but in which inelastic volume change is negligible. The results also suggest that the pressure dependence of flow stress in polymers is the same as that of the initial modulus.     

Thermodynamic study of a pressure‐temperature phase diagram for poly(N‐isopropylacrylamide) gels

The volume change, ΔV_h,_, accompanying the hydrophobic hydration associated with the volume phase transition in Poly(N‐isopropylacrylamide) gels was measured by a simple method. The hydration accompanies a negative ΔV_h′?2.5 cm³/mol. The P‐T phase diagram, the coexistence curve, for the gels was determined from the swelling ratio‐pressure curves up to 350 MPa for various constant temperatures. The contour of the coexistence curve is shaped like an ellipsoid on the P‐T plain, which is a feature peculiar to the reversible pressure‐temperature denaturation of a protein. The thermodynamic analysis of the Clausius–Clapeyron relation for the measured ΔV_h elucidates that the obtained coexistence curve represents the phase boundary between thermodynamic different phases like the two phases, native and denatured, of a protein and gives the transition enthalpy, ΔH, 5.2kJ/mol by estimate, which well coincides with the transition heat measured by a calorimetric method. Considering the volume‐dependent free energy, Δv_mi · P, for the mixing free energy of the gel, we can fit the calculated curve to the measured swelling ratio‐pressure curve of PNIPA gels. The value of Δv_mi changes the sign from negative to positive above around 100MPa. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 405–412, 2005     

Mechanical properties and superstructure of poly(ethylene terephthalate) fibers zone-drawn and zone-annealed by CO2 laser heating

A laser-heating zone-drawing and zone-annealing method using a continuous-wave carbon dioxide laser was applied to poly(ethylene terephthalate) (PET) fiber to improve its mechanical properties. The as-spun fiber was zone-drawn under a applied tension (σ_a) of 4.44 MPa at a laser power density (PD) of 6.08 W cm⁻², and then the laser-heated zone-drawn fiber was zone-annealed. The laser-heating zone-annealing was carried out in three steps: the first annealing was carried out under σ_a = 139 MPa at 4.83 W cm⁻²; the second annealing was carried out under σ_a = 283 MPa at 4.83 W cm⁻², and the third annealing was carried out under σ_a = 432 MPa at 3.45 W cm⁻². The surface temperature distribution of the fiber irradiated with the CO₂ laser was measured by using an infrared thermographic camera equipped with a magnifying lens. The relation between the laser power and the surface temperature of the fiber became clear in the laser-heating zone-drawing and the laser-heating zone-annealing. The fiber obtained finally had a birefringence of 0.239, a degree of crystallinity of 55%, a tensile modulus of 19.8 GPa, and a storage modulus of 25.7 GPa at 25°C. In FTIR measurements, a trans conformation increased with the processing, but a gauche one decreased. The laser-heating zone-drawing and zone-annealing method was found to be effective in producing the PET fiber with high modulus and high strength. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2775–2783, 2001     

The Softness Parameters of Ions. Prediction of the Occurrence of Miscibility Gaps in Molten Salt Mixtures

Softness parameters σ_M for cations and σ_X for anions, have been calculated as dimensionless quantities for approx. 90 cations and 18 anions. They are given by σ_M = [σ_A (M^m+) - σ_A(H⁺)]/σ_A(H⁺) and σ_X = [σ_B(X^a?) - σ_B(OH^?)]/σ_A(H⁺) where σ_A = [σI_i(M) + ΔH⁰_h(M^m+)]/m and σ_B = [-E_a(X) + ΔH⁰_h(X^a?)]/a are Ahrland's parameters. The new normalized and comparative (to the test ions H⁺ and OH^?) softness parameters are positive for soft ions and negative for hard ones. These parameters, obtained independently, are used with a four-coefficient equation to calculate coordinate bond energies for metal halides with acceptable accuracy. Considerations of the average coordination in reciprocal molten salt mixture lead to an expression for the metathesis energy change as proportional to the product of the differences in softness parameters of the two cations and the two anions. An empirical one-coefficient equation involving the softness parameters is proposed to deal with next-nearest-neighbor interactions in binary common-ion molten salt mixtures. These relationships are then used with Blander and Topol's equation to predict the occurrence of irascibility gaps in uni-univalent reciprocal salt mixtures. The gaps found in other systems are also discussed in terms of the softness of the constituent ions.     

Predictions of nucleation theory applied to ehrenfest thermodynamic transitions: II. The effects of pressure and stress

This paper is a sequel to an earlier one on the applicability of classical nucleation theory to second-order transitions in the Ehrenfest sense (1). In each case the approach was to obtain the critical size r_c and energy barrier ΔG_c for the growth of a nucleus of β-phase in an α-phase matrix by a Maclaurin series expansion of the free-energy-density g = (G_β ? G_α)/v_β as a function of θ (in BC-I) and of ΔP and Δσ in this paper where θ = (T ? T_t) is the degree of undercooling and ΔP and Δσ are analogous terms for the hydrostatic pressure shift and tensile stress shift away from the equilibrium transition. The expansion coefficients were determined by the use of thermodynamic relationships. For second-order transitions, r_c = 4γv_β T_t/ΔC_pθ², r_c = 4γ/Δβ(Δp)², and r_c = 4γ/Y_αY_β(Δσ)², respectively, for the three cases. The terms ΔC_p, Δβ, and ΔY denote the differences in heat capacity, compressibility, and Young's modulus, e.g., ΔY = Y_β ? Y_α. The interfacial energy γ_αβ is denoted by γ. The activation energy barriers for the cases developed in this paper were ΔG_c = (16π/3)γ³/(Δβ)² (Δp)⁴ and ΔG_c = (64π/3)γ³Y_α²Y_β²/(ΔY)²(Δσ)⁴. More complicated expressions are given in the paper for the r_c and ΔG_c for first-order transitions. In the long run, these expressions may prove more useful than the ones for second-order because of the modifications expressions for the kinetics of transformations.     

Compressional behavior of inked open-cell foams

The mechanical behavior of various classes of inked and non-inked (dry) open-cell foam rollers has been investigated from stress/strain measurements in compression. Hysteresis, creep, and recovery in dynamic loading were used to differentiate the rollers as to their utility in printing applications. Creep, ?_c(t), and recovery, ?_r(t), were determined in a compression mode from force measurements and strain decay, ?(t), i.e., Δ?_c(t) = ?₀ – ?(t) [?₀ = ?(t = 0)] or ?_c(t) = 2?₀ – ?(t). The creep function, Δ?_c(t), represents the plastic strain, ?_pl(t = t_h), and is uniquely defined from the recovery function, Δ?_c(t) = ?_r(t = t_h), where t_h is the hold or contact time. The recovery results for a polyurethane ester (PUE) and acrylonitrile/butadiene (AB) rollers (dry/ink), poly(vinylidene fluoride) film (PVF₂) (air/vapor) and low-density polyethylene film (LDPE) (air/vapor) were found to fit a master curve of the form F_r(θ) = EXP[-K_r(t_h)θ] = [?_r(t) – ?_∞(t_h)]/[?₀(t = O) – ?_∞(t_h)] at a reduced time of K_r(t_h)θ {θ = t/t_h and K_r(t_h) = k′_r(t_h)t_h = C₀/(t_h)^α?1 (where C₀ depends on the material's “dry” or “wet” state, α is a function of the type of material, and ?_∞ is the permanent set). These empirical results are consistent with the observed decreases in print intensity during transfer to a paper substrate and weight changes of the roller, i.e., creep and recovery are important in the printing characteristics of a given roller material. Other factors of importance in the overall transfer and print quality, but of longer-term considerations, are diffusion processes within the polymer and the nature of the polymer (e.g., porosity, chemical constitution, surface and interfacial tensions).     

Friction dynamics of human hair treated with water or cationic surfactant aqueous solution

Cationic surfactants are adsorbed on the surface of human hair and exhibit a lubricating effect. Here, we evaluated the friction when the hair treated with water or 1 wt% cationic surfactant (cetyltrimethylammonium bromide) aqueous solution was rubbed by a contact probe equipped in a sinusoidal motion friction evaluation system. Because of the rough structure of the cuticle on the human hair surface, an oscillatory phenomenon with a frequency of 50–70 Hz was observed when untreated hair was rubbed with a contact probe. On the other hand, the oscillatory phenomenon was not observed when the hair contained a large amount of water because stick–slip phenomena were inhibited on the soft, swollen hair surface. Furthermore, the change in kinetic friction coefficient and delay time, which is a normalized value of the time difference between the reaction of the friction force to accelerated motion, from untreated hair, Δμ_k and Δδ, was almost zero. However, we found a large difference in Δμ_k and Δδ for the hair treated with cationic surfactant aqueous solution. The treatment with cationic surfactant reduced both friction parameters, Δμ_k and Δδ, indicating that the treatment induced not only the frictional force but also the profile at the beginning of frictional sliding. The significant lubrication is due to cationic surfactant adsorption on the hair surface. These data imply that the smoothness of hair treated with a cationic surfactant is related to a reduction in friction coefficient and delay time δ.     

Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part I: local bond strength

The techniques aimed at adhesion strength measurement between reinforcing fibers and polymer matrices (the pull-out and microbond tests) involve the measurement of the force, F _max, required to pull out a fiber whose end is embedded in the matrix. Then, this maximum force value is used to calculate such interfacial parameters as the apparent bond strength, τ_app, and the local interfacial shear strength (IFSS), τ_d. However, it has been demonstrated that the F _max value is influenced by interfacial friction in already debonded regions, and, therefore, these parameters are not purely 'adhesional' but depend, in an intricate way, on interfacial adhesion and friction. In the last few years, several techniques for separate determination of adhesion and friction in micromechanical tests have been developed, but their experimental realization is rather complicated, because they require an accurate value of the external load at the moment of crack initiation. We have developed a new technique which uses the relationship between the maximum force and the embedded length ('scale factor') to separately measure fiber-matrix interfacial adhesion and friction. Using the equation for the current crack length as a function of the applied load, based on a stress criterion of interfacial debonding, we modeled the pull-out and microbond experiments and obtained the maximum force value versus the embedded length. By varying τ_d and interfacial friction, τ_f, to fit experimental plots, both interfacial parameters were estimated. The micromechanical tests were modeled for three types of specimen geometries (cylindrical specimens, spherical droplets, and matrix hemispheres in the pull-out test) with different levels of residual thermal stresses and interfacial friction. The effect of all these factors on the experimental results is discussed, and the importance of specimen geometry is demonstrated. One of the most interesting results is that the 'ultimate' IFSS (the limiting τ_app as the embedded length tends to zero) is not always equal to the 'local' bond strength.     

Viscoelastic and Ultimate Behaviour of Swollen Poly (2-hydroxyethyl methacrylate) Networks Filled with Poly (2-hydroxyethyl methacrylate) in the Rubbery Region

Abstract

Composite systems of the crosslinked and soluble poly(2-hydroxyethyl methacrylate) (PHEMA) matrix with various contents of crosslinked PHEMA particles containing 0.4, 1.0 and 20.0 wt. percent ethylenedimethacrylate as the crosslinking agent were prepared. Stress relaxations at the elongation λ = 1.1, stress-strain curves, Poisson's ratios, the degree of swelling and ultimate characteristics of samples swollen in water at 25°C were measured. With increasing filler content the time effects in the systems and the stress-at-break σ_b, and strain-at-break, σ _b , also increase; these results corroborate the viscoelastic concept of the ultimate behaviour. An agreement was found between the experimentally determined course of the initial modulus G ₀ and that predicted by Van der Poel as a function of the volume fraction of filler ν_f only for those systems in which the content of the crosslinking agent in the filler is smaller than in the matrix; systems with a higher content of the crosslinking agent in the filler than in the matrix require ν_f? = 3ν_f to fit the theory. Water present during the formation of filled systems preferentially swells the filler particles and reduces the polymerfiller interaction, which has a negative effect on the modulus G₀ and on σ_b and ?_b.     

Organotin polymers. IV. Binary and ternary copolymerizations of tributyltin acrylate and methacrylate with styrene,allyl methacrylate,butyl methacrylate,butyl acrylate,and acrylonitrile

The monomer reactivity ratios for the copolymerization of tributyltin acrylate with styrene and allyl methacrylate have been found to be r₁ = 0.213, r₂ = 1.910 and r₁ = 0.195, r₂ = 2.257, respectively. Also, the copolymerization parameters of tributyltin methacrylate with styrene and allyl methacrylate were as follows: r₁ = 0.256, r₂ = 1.104 and r₁ = 2.306, r₂ = 1.013. Copolymerization reactions were carried out in solution at 70°C using 1 mole % AIBN, and the copolymer compositions were determined by tin analysis. Ternary copolymerization of the three systems butyl methacrylate–tributyltin methacrylate–acrylonitrile, butyl acrylate–tributyltin methacrylate–acrylonitrile, and styrene–tributyltin acrylate–acrylonitrile have been studied, and the terpolymer composition of each system was determined through tin and nitrogen analyses. The variation of instantaneous and average terpolymer composition with conversion fit satisfactorily the experimental results over a wide range of conversion.     

PET microfiber prepared by carbon dioxide laser heating

In preliminary experiments to optimize the condition of a laser heating, zone drawing for poly(ethylene terephthalate) (PET) fiber, a microfiber was prepared by a continuous‐wave carbon dioxide (CW CO₂) laser heating. CW CO₂ laser heating was carried out at an extremely low applied tension (σ_a) at a higher laser power density (PD) as compared to the optimum condition for the laser heating, zone drawing of PET fiber reported previously. The microfibers were obtained by CO₂ laser heating carried out at a PD of 15.8 W cm^?2 and under a σ_a of 0.66 MPa or lower. The diameter of the fiber decreased with a decreasing σ_a and increasing PD. The smaller the diameter, the higher was its birefringence. The smallest diameter fiber obtained at σ_a = 0.17 MPa at PD = 21 W cm^?2 had a diameter of 4.5 μm and a birefringence of 0.112, and its draw ratio estimated from the diameter reached 3086 fold. Such a high draw ratio was not previously attained by any drawing method. In a wide‐angle X‐ray diffraction photograph of the smallest diameter fiber, indistinct reflections due to oriented crystallites were observed. An SEM micrograph of the smallest diameter fiber showed a smooth surface without any crack and was uniform in diameter. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3297–3283, 2003     

Thermolysis of glycol lignin in the presence of tetralin

An organosolv (ethylene glycol) hardwood lignin has been thermally treated in the presence of tetralin, a hydrogen donor solvent. The effects of reaction temperature and time on product distribution were studied as a function of a seventy index, X_c = ?n (R_ω with R_ω = Δt t × exp [(T_r - T_b)/ω] where Δt = time, T_r = reaction temperature, T_b = base temperature, and ω = characteristic parameter. Under the conditions used, up to 60% of the lignin can be converted to liquid and gaseous products. Syringols, guaiacols, aromatic aldehydes and ketones predominate at low treatment severities whilst phenol, catechol and their methyl and ethyl derivatives are the main monomers at high severities. Comparison of our data with parallel works is shown to be effectively done through the severity index, X_c.     

A comparison between the stress–relaxation behavior of molybdenum and polyethylene

The stress–relaxation behaviors of molybdenum (Mo) and polyethylene are compared, especially with regard to the role played by the internal stress level, in the relation F = 0.1(σ₀ − σ_i). Here F is the maximum slope in the inflexion region of stress vs. In time curves (σ vs. ln t), σ₀ is the initial stress, and σ_i is the internal (equilibrium) stress. Despite a significant difference in σ_i in the two materials, this relation was obeyed in both cases. The data for Mo are for room temperature and 90 K; those for low and high density polyethylene for room temperature only. Some of the data are reevaluated results of earlier measurements. The shape of the σ_i(ϵ) curves is reminiscent of the σ(ϵ) behavior for both Mo and polyethylene. The implications of the results for interpretation of the relaxation process in terms of current theoretical concepts is discussed.     

Diffraction Studies on Concentrated Aqueous Hydrochloric Acid Solutions

X-ray diffraction and neutron diffraction experiments with H/D isotopically substituted aqueous 21 mol% hydrochloric acid solutions were carried out in order to obtain detailed features concerning the intermolecular hydrogen-bonded structure in highly concentrated aqueous acidic solutions. Structure parameters, namely, intermolecular distance, root-mean-square amplitude, and coordination number, for the nearest neighbor H₃O⁺···H₂O interaction were determined from the least-squares-fitting analysis of the observed X-ray interference term. These were, respectively, r(H₃O⁺···H₂O) = 2.45(1) Å; l(H₃O⁺···H₂O) = 0.11(1) Å; and n(H₃O⁺···H₂O) = 1.8(1). The intermolecular nearest neighbor distances, r(H···H) = 2.02(5) Å and r(O···H) = 1.69(2) Å, were determined from the least-squares fit to partial structure factors, a_ij (Q), derived from the observed neutron intermolecular interference terms for sample solutions with different H/D isotopic ratios. The present values of intermolecular distances are significantly shorter than those for pure liquid water, implying that extremely strong hydrogen bonds exist in concentrated aqueous acidic solutions.     

Fabrication of dense ZrB2/B4C composites using pulsed electric current pressure sintering and evaluation of their high-temperature bending strength

ZrB₂/x·vol%B₄C (x = 30–90) composites were fabricated from ZrB₂ and amorphous B/C powders using pulsed electric current pressure sintering (PECPS) from 1600 °C to 1900 °C for 6.0 × 10² s (10 min) under 50 MPa in a vacuum, accompanied by self-propagating high-temperature synthesis (SHS). Since the B₄C phase was formed at 1600 °C, the relative density (D_r) was evaluated; the composites sintered at 1900 °C attained the highest D_r. Their D_r values increased gradually from 99.35% to 99.99% with increasing B₄C contents up to 60 vol% and showed a constant value above 60 vol%. At room temperature, the mechanical properties of Vickers hardness (H_v), fracture toughness (K_IC) and three-point bending strength (σ_b) were measured. H_v exhibited a monotonous increase from 20.3 to 32.7 GPa. On the other hand, K_IC and σ_b revealed the same behavior for each of the compositions; both exhibited the highest values, i.e., 10.2 MPa m^1/2 for K_IC and 870 MPa for σ_b, in the 60 vol%B₄C sample, and then the K_IC decreased gradually to 9.73 MPa m^1/2, and σ_b dropped suddenly from 850 MPa (70 vol%) to 340 MPa (80 vol%) and stayed as low σ_b in the 90 vol% B₄C sample. Next, the high-temperature σ_b values of the composites (40–70 vol%) were measured in Ar. The composites (40–60 vol%) revealed high σ_b (≥640 MPa) from R.T.~1600 °C; the maximum value of 803.5 MPa was observed for the 60 vol%B₄C composites at 1600 °C, and then the σ_b of all composites dropped to around 340 MPa at 1800 °C. From their stress-strain curves, elastic and plastic deformations were observed at 1600 °C and 1800 °C, respectively.     

Adsorptive removal of cationic dye from aqueous solutions by ZnO/ZnMn2O4 nanocomposite

The ZnO/ZnMn₂O₄ nanocomposite (ZnMn) was used as adsorbent for the removal of cationic dye Basic Yellow 28 (BY28) from aqueous solutions. The adsorbent was characterized by X-ray diffraction, scanning electron microscope, TEM, Fourier transform infrared ray, BET, particle size distribution and zeta potential measurements. The adsorption parameters, such as temperature, pH and initial dye concentration, were studied. Kinetic adsorption data were analyzed using the pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetic models. The Langmuir and Freundlich isotherm models were applied to fit the equilibrium data. The maximum adsorption capacity of BY28 was 48.8 mg g^?1. Various thermodynamic parameters, such as ΔG°, ΔH° and ΔS°, were calculated.     

J-integral studies of crack initiation and crack tip-blunting of phenolphthalein polyether ketone

The J-integral is applied to characterize the fracture initiation of phenolphthalein polyether ketone (PEK-C) for which the concepts of linear elastic fracture mechanics (LEFM) are inapplicable at high temperatures for reasonably-sized specimens due to extensive plasticity. The multiple-specimen resistance curve technique recommended by the ASTM is the basic method employed. The values of J_IC increase with increasing temperature. The crack tipblunting of PEK-C is observed. The parameters, such as crack opening displacement (COD), δ₀, and the stretching increment Δa_b1 are introduced to describe the blunting phenomenon. The δ₀ increases with increasing temperature, as does Δa_b1. This indicates that the blunting occurs easily as the temperature increases; i.e., as the material's yield stress, σ_y steadily falls. The relationships between δ₀, Δa_b1, and σ_y are also discussed. © 1994 John Wiley & Sons, Inc.