Three-parameter approach for elastic–plastic fracture of the semi-elliptical surface crack under tension

Systematic three-dimensional elastic–plastic finite element analyses are carried out for a semi-elliptical surface crack in plates under tension. Various aspect ratios (a/c) of three-dimensional fields are analyzed near the semi-elliptical surface crack front. It is shown that the developed J–Q annulus can effectively describe the influence of the in-plane stress parameters as the radial distances (r/(J/σ₀)) are relatively small, while the approach can hardly characterize it very well with the increase of r/(J/σ₀) and strain hardening exponent n. In order to characterize the important stress parameters well, such as the equivalent stress σ_e, the hydrostatic stress σ_m and the stress triaxiality R_σ, the three-parameter J–Q_T–T_z approach is proposed based on the numerical analysis as well as a critical discussion on the previous studies. By introducing the out-of-plane stress constraint factor T_z and the Q_T term, which is determined by matching the finite element analysis results, the J–Q_T–T_z solution can predict the corresponding three-dimensional stress state parameters and the equivalent strain effectively in the whole plastic zone. Furthermore, it is exciting to find that the values of J-integral are independent of n under small-scale yielding condition when the stress-free boundary conditions at the side and back surfaces of the plate have negligible effect on the stress state along the crack front, and the normalized J tends to a same value when φ equals about 31.5° for different a/c and n. Finally, the empirical formula of T_z and the stress components are provided to predict the stress state parameters effectively.     

Anisotropic plasticity with application to sheet metals

Hill’s 1948 anisotropic theory of plasticity is extended to include the concept of isotropic–kinematic hardening. The “anomalous” effect can be accounted for by kinematic hardening. It is shown that the quadratic yield function can be used for sheet metals irrespective of its plastic strain ratio R. It is further shown that effects of thickness reduction due to further rolling may be accounted for by kinematic hardening.     

Structure and mechanical properties of NiAl and Ni3Al-based alloys

The investigation of Ni–Al–Fe–Ti–B alloys was carried out to determine the influence of iron and small titanium and boron additions on the phase composition, microstructure and mechanical characteristic, particularly with respect to high-temperature deformation conditions. These alloys, containing Al 35.8 at% and Fe 3.6–8.6–17.6 at% were prepared from high-purity components and Al master alloy containing Ti₂B particles. The influence of alloying additions of chromium and iron on the mechanical properties of directionally solidified Ni–Al–Cr–Fe alloy was investigated. Additions of both Cr 8 at% and Fe 2 at% result in higher strength than exhibited by unalloyed Ni₃Al. However, the ductility is reduced by the formation of the β′ phase. The typical, lamellar structure of Ni–20Al–8Cr–2Fe alloy undergoes coagulation during a high-temperature deformation process. The sequence of structural changes of NiAl and Ni₃Al-based alloys has been correlated with mechanical characteristics of high-temperature deformation process, determined in uni-axial compression tests. Two ranges of work hardening have been identified on the stress–strain curves of these alloys. It has been found that the first range of the deformation of Ni–Al–Fe–Ti–B alloys corresponds to the intergranular slip system operating within individual grains, while the second one is connected with transgranular slip. In the directionally solidified Ni–20Al–8Cr–2Fe alloy similar work hardening curves were observed in relation to the microstructural evolution from the lamellae shape, through elliptical shape into circular shape.     

Characterization of nanoscale recording mark on Ge2Sb2Te5 film

We have fabricated nanoscale recording marks on Ge₂Sb₂Te₅ (GST) films with conductive atomic force microscope (AFM). GST films were deposited on glass or polyimide film with thickness of 150–200 nm by the rf–sputtering method. Through current–voltage (I–V) spectroscopy, good cantilevers for fabrication and characterization of nanoscale marks on GST were selected. A fresh and highly conductive tip showed voltage-switching characteristic in the I–V curve, where the threshold voltage was 1.6 V. Nanoscale dot and wire arrays of crystalline phases were successfully obtained by varying sample bias voltage from −10 to 10 V. With highly conductive tips, nanowires having full-width at half-maximum of 20 nm could be fabricated, whereas nanowires could not be fabricated with bias voltage below −2 V. The width of the nanoscale mark was increased by repetition of AFM lithography even with same applied voltage and lithography speed. For a thicker nanowire, the width measured in current-image (C-image) was observed to be 2 times of that measured in topography-image (T-image). This result supports that current sensing provides an image of phase-changed GST area with higher resolution than topography sensing.     

Kinematic hardening analysis of ratchet strain in the “pulley test”

The classical Bree problem—which represents an uniaxial model of a thin tube subjected to combined internal pressure and cyclic thermal stress across its wall—can be simulated by means of the pulley test in which a wire or strip specimen is subjected to combined steady tensile stress and cyclic bending stress. In this paper, accumulation of ratchet strain in the pulley test is investigated using a linear kinematic hardening material model from which perfect plasticity can be generated as a special case. The results of the investigation show that asymptotic ratchet strains are linearly related to the excess in mean stress σ_D above its value σ*_D at the ratchetting limit regardless of the thermal stress amplitude. Comparisons with test results on copper wire specimens—which exhibit non-linear hardening rate—confirm the qualitative validity of this simple relation. Deviations between theory and experiment are attributed to metallic cyclic creep. Further, perfect plasticity results are shown to be well predicted by a linearized lower bound estimate.     

A study of the thermal behaviour of a specimen during fatigue

A steel specimen (XC 10), submitted to fatigue by alternating symmetrical torsion, gives rise to heat dissipation, which is a function of the amplitude of the applied forces.The relationship between the amplitude of the applied forces and the quantity of heat dissipated was measured while a specimen underwent fatigue testing (E_n curves).The results appear to show that this relation is not identical for similar specimens at the beginning of the tests, initial state which is assumed to correspond to something like a “virgin state”, but that there is a convergence of the E_n curves as rupture is approached.     

High-temperature tribological properties of spark-plasma-sintered Al2O3 composites containing barite-type structure sulfates

Al₂O₃–50BaSO₄–20Ag, Al₂O₃–50BaSO₄–10SiO₂, Al₂O₃–50(mass%)SrSO₄, Al₂O₃–50PbSO₄–5SiO₂, Al₂O₃–50BaSO₄ and Al₂O₃–50BaCrO₄ composites (mass%) were prepared by spark plasma sintering and their microstructure and high-temperature tribological properties were evaluated. Al₂O₃–50BaSO₄–20Ag composites (mass%) showed the lowest friction coefficients at the temperature ranging from 473 to 1073 K. Thin Ag film was observed on the wear tracks of the composites above 473 K. In addition, the friction coefficients of Al₂O₃ composites containing SrSO₄ and PbSO₄ were as low as those of Al₂O₃–BaSO₄ and Al₂O₃–BaCrO₄ composites at the temperatures up to 1073 K. The thin films formed on the wear tracks of the Al₂O₃–SrSO₄ composites were composed of Al₂O₃ and SrSO₄ phases, while the films formed on the wear tracks of the Al₂O₃–PbSO₄–SiO₂ composites consisted of Al₂O₃, PbSO₄ and SiO₂ phases.     

Identification of yield stress and plastic hardening parameters from a spherical indentation test

Assuming plastic hardening of metals are specified by the stress–strain curve in the form , the material parameters σ₀, k and m are identified from spherical indentation tests by measuring compliance moduli in loading and unloading of the load–penetration curve. The curve P(h_p) is analytically described by a two term expression, each with different exponents. Here, ε_p and h_p denote the plastic strain and permanent penetration. The proposed identification method is illustrated by specific examples including numerical and physical identification tests.     

Predictive model to estimate the stress–strain curves of bulk metals using nanoindentation

Many studies have shown that finite element modeling (FEM) can be used to fit experimental load–displacement data from nanoindentation tests. Most of the experimental data are obtained with sharp indenters. Compared to the spherical case, sharp tips do not directly allow the behavior of tested materials to be deduced because these produce a nominally-constant plastic strain impression. The aim of this work is to construct with FEM an equivalent stress–strain response of a material from a nanoindentation test, done with a pyramidal indenter. The procedure is based on two equations which link the parameters extracted from the experimental load–displacement curve with material parameters, such as Young's modulus E, yield stress Y₀ and tangent modulus E_T. We have already tested successfully the relations on well-known pure metallic surfaces. However, the load–displacement curve obtained using conical or pyramidal indenters cannot uniquely determine the stress–strain relationship of the indented material. The non-uniqueness of the solution is due to the existence of a characteristic point (ε_c, σ_c); for a given elastic modulus, all bilinear stress–strain curves that exhibit the same true stress σ_c at the specific true strain ε_C lead to the same loading and unloading indentation curve. We show that the true strain ε_c is constant for all tested materials (Fe, Zn, Cu, Ni), with an average value of 4.7% for a conical indenter with a half-included angle θ=70.3°. The ratio σ_c/ε_c is directly related to the elastic modulus of the indented material and the tip geometry.     

Measurement of springback

Springback, the elastically-driven change of shape of a part after forming, has been measured under carefully-controlled laboratory conditions corresponding to those found in press-forming operations. Constitutive equations emphasizing low-strain behavior were generated for three automotive body alloys: drawing-quality silicon-killed steel; high-strength low-alloy steel; and 6022-T4 aluminum. Strip draw-bend tests were then conducted using a range of die radii (3<R/t<17), friction coefficients (0<μ<0.20), and controlled tensile forces (0.5<F_b/F_y<1.5). Springback angles and curvatures were measured for bend and bend–unbend areas of the specimen, the latter corresponding to the “sidewall curl” region, which dominates the geometric change and the dependence on process variables. Friction coefficient and R/t (die-radius-to-sheet-thickness) greater than 5 have modest but measurable effects over the ranges tested. As expected, strip tension dominates the springback sensitivity, with higher forces reducing springback. For 6022-T4, springback is dramatically reduced as the tensile stress approaches the yield stress, corresponding to the appearance of a persistent anticlastic curvature. The presence of this curvature, orthogonal to the principal curvature, violates the simple two-dimensional models of springback reported in the literature. The measured springback angles and curvatures are reported both in graphical summary and tabular form for use in assessing analytical models of springback.     

Effect of the functional groups in ionic liquid molecules on the friction and wear behavior of aluminum alloy in lubricated aluminum-on-steel contact

Four imidazolium-based room temperature ionic liquids containing phosphonyl functional groups, i.e. 1-(3′-O,O-diethylphosphonyl-n-propyl)-3-alkylimidazolium tetrafluoroborates and hexafluorophosphates, were synthesized. The physical properties of the resulting synthetic products were evaluated, and their tribological behaviors as the lubricants for an aluminum-on-steel sliding system were evaluated on an oscillating friction and wear tester, with the emphasis being placed on the effect of the O,O-diethylphosphonyl groups in the ionic liquid molecules on the tribological behaviors. Thus the friction and tests were conducted at a frequency of 25 Hz, a sliding amplitude of 1 mm, and for a duration of 30 min. The worn aluminum surface was analyzed by means of scanning electron microscopy, energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy. It was found that the synthesized ionic liquids had better friction-reducing and anti-wear ability for the aluminum-on-steel system than their nonfunctionalized courterparts (1-ethyl-3-hexylimidazolium tetrafluoroborate, coded as L206, and 1-propyl-3-octylimidazolium hexafluorophosphate, coded as LP308). Especially, they had much better load-carrying capacity than L206 and LP308. The tribological behaviors of the synthetic lubricants were dependent on both the anions and the side-substituted alkyl chains attached to the imidazolium cations. Moreover, physical adsorption and complicated tribochemical reactions were involved during the sliding process of the Al-on-steel system under the lubrication of the synthetic functionalized ionic liquids, which led to the generation of physically adsorbing and chemically reacting films composed of five-member-ring complex compounds, metal fluorides, nitrogen oxide, and FePO₄ on the rubbed Al surface. Those physically adsorbing and chemically reacting films contributed to effectively decrease the friction and wear of the aluminum sliding against steel.     

TEM studies of anti-wear films/wear particles generated under boundary conditions lubrication

Friction and wear performance of engine oil were studied in presence of Zinc-dialkyldithiophosphate (ZDDP) and ZDDP–iron fluoride (FeF₃) combination using a ball-on-ring wear testing device under boundary conditions. Friction and wear performance of engine oil improves in presence of ZDDP–FeF₃ combination. In order to understand the wear mechanisms the microstructure and the chemical composition of wear debris generated during wear process were investigated using TEM together with EDX analyzes. Novel observations on the wear debris generated at different testing loads are presented. Independent of normal loads, amorphous debris containing P, O, Fe and Zn elements and crystalline debris of Fe₂O₃ are formed. No trace of S is present in amorphous debris under low load (2.32 GPa) conditions while S is a dominating element under high loaded (3.68 GPa) conditions. On the other hand, at lower loads a few iron oxide is formed while at higher loads larger sizes of iron oxides are formed resulting in larger friction and wear.     

An efficient approach to characterize pseudo-merohedral twins by precession electron diffraction: Application to the LaGaO3 perovskite

Pseudo-merohedral twins are frequently observed in crystals displaying pseudo-symmetry. In these crystals, many [u v w] zone axis electron diffraction patterns are very close and can only be distinguished from intensity considerations. On conventional diffraction patterns (selected-area electron diffraction or microdiffraction), a strong dynamical behaviour averages the diffracted intensities so that only the positions of the reflections on a pattern can be considered. On precession electron diffraction patterns, the diffracted beams display an integrated intensity and a “few-beam” or “systematic row” behaviour prevails which strongly reduces the dynamical interactions. Therefore the diffracted intensity can be taken into account. A procedure based on observation of the weak extra-reflections connected with the pseudo-symmetry is given to identify without ambiguity any zone axis. It is successfully applied to the identification and characterization of {1 2 1} reflection twins present in the LaGaO₃ perovskite.     

Cohesive and continuum damage models applied to fracture characterization of bonded joints

In this work, two different methods for simulating damage propagation are presented and applied to fracture characterization of bonded joints in pure modes I and II. The cohesive damage model is based on a special developed interface finite element including a linear softening damage process. In the continuum damage model the softening process is performed by including a characteristic length associated with a given Gauss point. The models were applied to the simulation of “double cantilever beam” (DCB) and “end notched flexure” (ENF) tests used to obtain the critical strain release rates in mode I and II of bonded joints. In mode I it was observed, under certain conditions, a good agreement between the results obtained by the two models with the reference value of critical strain energy release rate in mode I (G_Ic), which is an inputted parameter. However, in mode II some discrepancies on the obtained G_IIc values were observed between the two models. These inaccuracies can be explained by the simplifying assumptions inherent to the cohesive model. Better results were achieved considering the crack equivalent concept.     

On the scratch behaviour of self-lubricating WSe2 films

Transition metal dichalcogenides films are well known for their self-lubricating properties. These films are having lamellar structure – whereby weak “van der Walls” forces act between the layers – commonly believed to be responsible for their excellent self-lubricating properties. Among these films, diselenoids have shown less sensitivity to humidity and they are more oxidation resistant in humid environment than sulphides. In view of the above, a comprehensive work is undertaken to study friction properties in macro- and micro-scale. The present work deals with preliminary study and critical examination of friction due to scratching of WSe₂ film. WSe₂ film is deposited using sputtering technique. The composition of the film is determined by means of energy dispersive spectrometry (EDS) and X-ray photoelectron spectrometry (XPS). The micro-structural features, topography and mechanical properties of the film are evaluated using transmission electron microscopy (TEM), atomic force microscopy (AFM) and nano-indenter. The film is scratched at different constant loads and also with increasing load using a scratch tester with a spherical indenter in macro-loading regime. A 3D confocal microscope is used to study the scratched portion. In micro-loading regime the film is scratched with an angular indenter. The results show that even though self-lubricating effect comes into play in macro-loading regime, this effect cannot be seen in micro-loading regime. Further coefficient of friction in different loading regime is independent of applied load.     

Closed-form solution for wedge cutting force through thin metal sheets

A simple kinematic model is developed which describes the main features of the process of the cutting of a plate by a rigid wedge. It is assumed in this model that the plate material curls up into two inclined cylinders as the wedge advances into the plate. This results in membrane stretching up to fracture of the material near the wedge tip, while the “flaps” in the wake of the cut undergo cylindrical bending. Self-consistent, single-term formulas for the indentation force and the energy absorption are arrived at by relating the “far-field” and “near-tip” deformation events through a single geometric parameter, the instantaneous rolling radius. Further analysis of this solution reveals a weak dependence on the wedge angle and a strong dependence on friction coefficient. The final equation for the approximate cutting force over a range of wedge semiangles 10° ≤ θ ≤ 30° and friction coefficients 0.1 ≤ μ ≤ 0.4 is: F = 3.28σ₀(δ_t)^0.2l^0.4t^1.6μ^0.4, which is identical in form and characteristics to the empirical results recently reported by Lu and Calladine [Int. J. Mech. Sci.32, 295–313 (1990)].This analysis is believed to resolve a controversy recently developed in the literature over the interpretation of plate cutting experiments.     

Broad-Band Measurements of Electromagnetic Properties of Film-Shaped Materials Using Striplines

Two broad-band techniques for measuring the electromagnetic properties of isotropic film-shaped materials are presented. The electromagnetic properties are computed from S-parameter measurements of coplanar and microstrip lines. These lines can be used as cells, propagating the quasi-TEM mode. The measurements are easy to be implemented. They are carried out with a network analyzer and on-wafer systems covering 0.05–40 GHz. The quasi-TEM dispersion is very low for a coplanar cell shape such as h>W+2S. Thus, an extraction method of the coplanar substrate properties ( _r , _r ) has been developed from analytical relationships. It is faster than the microstrip extraction method, which requires a numerical analysis method of the propagation in order to take into account the quasi-TEM dispersion. Moreover, a theoretical study of the electromagnetic propagation through these cells is presented. It has allowed us to determinate the best cell to be used according to the material properties to be characterized. Measured _r and _r data for several materials are presented in the 0.05–40 GHz frequency range. These techniques show good agreement between measured and predicted values. However, the loss tangent measurements of low-loss materials are not possible with these techniques.     

Erosion–corrosion of engineering steels—Can it be managed by use of chemicals?

One significant contributory factor in the degradation of both pipelines and downhole tubulars in the oil and gas industry is erosion–corrosion. An erosion–corrosion investigation was carried out with three different steels—carbon steel, martensitic stainless steel and superduplex stainless steel. The materials were chosen to represent “active” and “passive” corrosion materials and are the same materials used in completions. Tests were carried out under three different regimes spanning a range of fluid velocities to simulate the severity of the mechanical erosion effect. A commercial corrosion inhibitor was used to investigate the inhibitor ability to reduce damage due to erosion–corrosion. In each of the conditions, pure corrosion and combined erosion–corrosion were studied by electrochemical and gravimetric techniques. The experiments were conducted using a jet impingement rig capable of producing jet velocities up to 20 m/s in a CO₂-saturated environment with sand. Erosion–corrosion mechanisms were determined from microstructural studies by SEM and inhibitor adsorption tests. The paper shows that the inhibitor effectively reduced erosion–corrosion damage for carbon steel; it was only in severe erosion–corrosion conditions that inhibitor has any noticeable effect for martensitic stainless steel and there were no conditions where the inhibitor offered a benefit for the superduplex stainless steel.     

The Adhesion of Monomolecular Hydroxyl-terminated Perfluoropolyether Liquid Films on the Sputtered Silicon Nitride Surface as a Function of End Group Acidity and Mobility

The intermolecular interactions at the interface between monomolecular hydroxyl-terminated perfluoropolyether (PFPE) liquids (Zdol, Zdol-TX, Z-Tetraol, Zdiac) and a sputtered amorphous silicon nitride film (SiN_x) are investigated using contact angle goniometry, Fourier transform infrared spectroscopy, and ab initio computational chemistry. The results demonstrate that the adhesion between the PFPE liquids and the SiN_x surface occur via the polar interactions between the PFPE end groups (-OH, -COOH) and the polar sites on the SiN_x surface (e.g., SiOH). The attractive interactions lead to a lowering of the polar surface energy with increasing PFPE coverage up to a monolayer. The binding energy is computed to be approximately −4 to −9 kcal/mol, depending upon the polarity of the PFPE end group. Adsorbed water is shown to compete with PFPE for surface bonding sites on SiN_x (−4.4 kcal/mol) that can lead to a significantly reduced level of adhesion for some of the hydroxyl-terminated PFPEs. A higher level of adhesion between the PFPEs and SiN_x can be attained by increasing the strength of the hydrogen bond and/or increasing the configurational entropy of the PFPE end group to facilitate the hydrogen bonding reaction.     

Stress analysis of the strain rate hardening materials in axially symmetric field

This paper presents an analysis of a circular hollow cylinder composed of strain rate hardening plastic material subjected to a sudden internal pressure loading. Materials satisfying the constitutive relation, and Levy-Mises equation are considered in the analysis. Dynamic and static analyses of axially symmetric material loaded under plane strain condition are described. The paper discusses the effect of strain rate hardening exponent, m, on the dynamic and static plastic response in the axially symmetric medium. A method is presented for the determination of the strain rate hardening exponent by measuring the hoop stress on the outer surface of a thin cylindrical specimen using the static solution.