In this paper, the effects of tapering and introducing axisymmetric indentations on the crash performances of thin-walled tubes are investigated. The crash performances of the tubes are evaluated ...using two metrics: the crush force efficiency (CFE, the ratio of the average crushing load to the peak load), and the specific energy absorption (SEA, absorbed energy per unit mass). The optimum values of the number of the axisymmetric indentations, the radius of the indentations, the taper angle and the tube thickness are sought for maximum CFE and maximum SEA using surrogate based optimization. In addition, multi-objective optimization of the tubes is performed by maximizing a composite objective function that provides a compromise between CFE and SEA. The CFE and SEA values at the training points of surrogate models (metamodels) are computed using the finite element analysis code LS-DYNA. Polynomial response surfaces, radial basis functions, and Kriging are the different surrogate models used in this study. Surrogate based optimization of the tubes showed that the tubes with indentations have better crush performance than tubes without indentations. It is found that maximum CFE requires large number of indentations with high radius, small thickness, and medium taper angle, while maximum SEA requires small number of indentations with low radius, large thickness and small taper angle. It is also found that the globally most accurate surrogate model does not necessarily lead to the optimum.
The contact problem for two deformable solids with FGM coatings is considered. The problem is considered under the assumption of plane strain, Coulomb friction and linear nonhomogeneous elasticity. ...The variation of the elastic modulus in the FGM coatings are exponential. It is assumed that the contact area is small compared to radii of curvatures of the cylinders. Thus the standard Hertzian assumption may be used. After solving the half space problem the Green’s functions necessary for deriving the integral equations of the contact problem are obtained. The resulting integral equations are solved to obtain a series of analytical benchmark results for examining the influence of such factors as material inhomogeneity constants, the coefficient of friction, and various length parameters on the critical stresses that may have a bearing on the fatigue and fracture of the components with FGM coatings. The results regarding the contact stresses, the in-plane component of the surface stress and the contact length versus the applied load relationship are presented.
This paper focuses on the study of a frictional sliding contact problem between a homogeneous magneto-electro-elastic material (MEEM) and a perfectly conducting rigid flat punch subjected to ...magneto-electro-mechanical loads. The problem is formulated under plane strain conditions. Using Fourier transform, the resulting plane magneto-electro-elasticity equations are converted analytically into three coupled singular integral equations in which the main unknowns are the normal contact stress, the electric displacement and the magnetic induction. An analytical closed-form solution is obtained for the normal contact stress, electric displacement and magnetic induction distributions. The main objective of this paper is to study the effect of the friction coefficient and the elastic, electric and magnetic coefficients on the surface contact pressure, electric displacement and magnetic induction distributions for the case of flat stamp profile.
Contact mechanics of graded coatings Guler, M.A.; Erdogan, F.
International journal of solids and structures,
07/2004, Letnik:
41, Številka:
14
Journal Article
Recenzirano
The main interest of this study is in the fracture initiation in graded coatings under sliding contact loading. The structural component under consideration is a metallic substrate bonded to a ...metal/ceramic coating with continuously varying thermo-mechanical properties. The coating is 100% ceramic at the free surface and 100% metal at the interface. It is assumed that the thickness variation of the shear modulus of the coating is exponential and the Poisson's ratio is constant. The loading is provided by a sliding rigid stamp subjected to constant normal and tangential forces and the underlying elasticity problem is two-dimensional. On the contact area, it is also assumed that the conditions of Coulomb friction prevail. The objective of the study is to obtain a series of analytical benchmark solutions for examining the influence of such factors as material inhomogeneity constants, the coefficient of friction and various length parameters on the critical stresses that may have a bearing on the fatigue and fracture of the coating.
Graded materials, also known as functionally graded materials (FGMs), are generally two-phase composites with continuously varying volume fractions. Used as coatings and interfacial zones they can ...reduce thermally and mechanically induced stresses resulting from material property mismatch, increase the bonding strength and provide protection against adverse environments. In this paper, the contact problems of parabolic and cylindrical stamps on graded coatings are considered. The objective of this study is to obtain a series of analytical benchmark solutions for examining the influence of such factors as material inhomogeneity constants the coefficient of friction and various length parameters on the critical stresses that may have a bearing on the fatigue and fracture of the components with graded coatings.
In this study, the plane contact problem for a rigid cylindrical punch and a functionally graded bilayer is considered. The layers have different thicknesses and elastic constants. The normal and ...tangential forces are applied to the upper layer with a rigid cylindrical punch, and the lower layer is fully bonded to a rigid substrate. Poisson’s ratios are taken as constant, and elasticity moduli are assumed to vary exponentially through the thickness of the layers. With the use of Fourier integral transform, the plane contact problem is reduced to a singular integral equation in which the unknowns are the contact pressure and the contact width. The singular integral equation is solved numerically using Gauss–Jacobi integration formula. The effect of several geometrical and physical parameters such as the material inhomogeneity, the friction coefficient, the layers’ height, the mismatch in the material properties at the interface, and the contact width on the contact stress and in-plane stress are investigated in detail.
Aim: To determine efficacy and tolerability of dutogliptin, a dipeptidyl peptidase 4 (DPP4) inhibitor, in patients with type 2 diabetes mellitus.
Methods: This was a 12‐week, multicentre, randomized, ...double‐blind, placebo‐controlled trial in 423 patients with type 2 diabetes with suboptimal metabolic control. Following a 2‐week single‐blind placebo run‐in, patients aged 18–75 years with a body mass index of 25–48 kg/m2 and baseline HbA1c of 7.3–11.0% were randomized 2:2:1 to receive once‐daily oral therapy with either dutogliptin (400 or 200 mg) or placebo on a background medication of either metformin alone, a thiazolidinedione (TZD) alone or a combination of metformin plus a TZD.
Results: Average HbA1c at baseline was 8.4%. Administration of dutogliptin 400 and 200 mg for 12 weeks decreased HbA1c by −0.52% (p < 0.001) and −0.35% (p = 0.006), respectively (placebo‐corrected values), with absolute changes in HbA1c for the 400 mg, 200 mg and placebo groups of −0.82, −0.64 and −0.3%, respectively. The proportion of patients achieving an HbA1c < 7% was 27, 21 and 12% at dutogliptin doses of 400 and 200 mg or placebo, respectively (p = 0.008 for comparison of 400 mg vs. placebo). Fasting plasma glucose (FPG) levels were significantly reduced in both active treatment groups compared to placebo: the placebo‐corrected difference was −1.00 mmol/l (p < 0.001) for the 400 mg group and −0.88 mmol/l (p = 0.003) for the 200 mg group. Dutogliptin caused significantly greater reductions in postprandial glucose AUC 0–2h in both the 400 and 200 mg groups (placebo corrected values −2.58 mmol/l/h, p < 0.001 and −1.63 mmol/l/h, p = 0.032, respectively). In general, patients tolerated the study drug well. There were minor, not clinically meaningful differences in adverse events (AEs) between dutogliptin‐treated patients and placebo controls, and 60% of all reported AEs were mild. Vital signs and body weight were stable, and routine safety laboratory parameters did not change compared with placebo. Trough ex vivo DPP4 inhibition at the end of the 12‐week treatment period was 80 and 70%, at the 400 and 200 mg doses of dutogliptin, respectively.
Conclusions: Dutogliptin treatment for 12 weeks improved glycaemic control in patients with type 2 diabetes who were on a background medication of metformin, a TZD or metformin plus a TZD. Tolerability was favourable for both doses tested. The 400 mg dose of dutogliptin resulted in larger changes of HbA1c and FPG and more subjects reached an HbA1c target of < 7% than the 200 mg dose.
The plane strain problem of a slender and weightless beam-plate loaded by a transversal point force in unilateral contact with a couple stress elastic foundation is investigated. The study aims to ...explore the consequences of the material internal lengthscale on the contact mechanics. In particular, compatibility between the beam and the foundation surface demands that both displacement and rotation match along the contact line. To this aim, couple tractions are exchanged besides the traditional contact pressure until separation between the beam and the foundation occurs. The problem is formulated making use of the Green's functions for a point force and a point couple acting atop of a couple stress elastic half-plane. A pair of coupled integral equations is thus derived, that governs the distribution of contact pressure and couple tractions, with one of them being immediately solved to provide an explicit relation between the two unknowns. In this sense, we retrieve the concept of a mechanically equivalent action, as it is the case of the Kirchhoff shear for plates. The remaining integral equation sets a cubic eigenvalue problem, whose linear term accounts for the microstructure. Its numerical solution is sought by expanding the equivalent contact pressure in series of Chebyshev polynomials vanishing at the contact region ends points, namely the lift-off points, and then applying a collocation strategy. The contact length, the distributions of contact pressure and couple tractions under the beam and the shearing force and bending moment along the beam are then obtained as a function of the material characteristic length. Results clearly indicate that accounting for the material internal lengthscale is mainly realized through exchange of the couple tractions, in the lack of which results much resemble those of the classical solution. Specifically, greater contact lengths and a stronger focusing effect about the loading point are encountered, which become very significant when the contact length approaches the internal lengthscale.
•A loaded beam in unbonded contact with a foundation with microstructure is considered.•Contact pressure and couple tractions are transmitted along the contact zone.•Compatibility of strain and rotation along the contact zone is imposed.•Size effects due to the foundation microstructure and generalized tractions are investigated.
This study focuses on the thermo-elastic rolling contact problem of a graded coating/substrate system. The problem is formulated under the plane thermoelasticity framework. Assuming an exponential ...variation of the shear modulus within the coating, the governing singular integral equations are extracted by means of the Fourier transform. The solution to problem is provided via the Gauss-Chebyshev integration method. The sensitivity of the contact stresses as well as the surface temperature rise to the stiffness ratio, the coating thickness and the non-dimensional speed is investigated. The results indicate that the thermal expansion ratio substantially affects the contact stresses. Also, the softening coatings will result in maximum surface temperature rise. The coating thickness can alter the surface temperature rise such that an increase of the coating by a factor of 1.6 may result in 50% reduction of the maximum surface temperature.
Objective
To describe effects of non‐ablative erbium‐doped:yttrium‐aluminium‐garnet (Er:YAG) laser on vaginal atrophy induced by iatrogenic menopause in the ewe.
Design
Animal experimental, ...randomised, sham and estrogen‐treatment controlled study with blinding for primary outcome.
Setting
KU Leuven, Belgium.
Sample
Twenty‐four ewes.
Methods
Menopause was surgically induced, after which the ewes were randomised to three groups receiving vaginal Er:YAG laser application three times, with a 1‐month interval; three sham manipulations with a 1‐month interval; or estrogen replacement and sham manipulations. At given intervals, ewes were clinically examined and vaginal wall biopsies were taken. Vaginal compliance was determined by passive biomechanical testing from explants taken at autopsy.
Main outcome measures
Vaginal epithelial thickness (primary), composition of the lamina propria (collagen, elastin, glycogen and vessel content), vaginal compliance, clinical signs.
Results
Animals exposed to Er:YAG laser application and sham manipulation, but not to estrogens, displayed a significant and comparable increase in vaginal epithelial thickness between baseline and 7 days after the third application (69% and 67%, respectively, both P < 0.0008). In laser‐treated ewes, temporary vaginal discharge and limited thermal injury were observed. Estrogen‐substituted ewes displayed a more prominent increase in epithelial thickness (202%; P < 0.0001) and higher vaginal compliance (P < 0.05). None of the interventions induced changes in the lamina propria.
Conclusions
Vaginal Er:YAG laser has comparable effect to sham manipulation in menopausal ewes.
Tweetable
Vaginal Er:YAG laser has comparable effect to sham manipulation in menopausal ewes #LASER #GSM #RCT.
Tweetable
Vaginal Er:YAG laser has comparable effect to sham manipulation in menopausal ewes #LASER #GSM #RCT.
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