Mechanical Creep Research Articles

Reconstruction of Complicated Scalp Defect Via Skin Traction

Management of an open wound is a problem frequently faced, with skin defects that cannot be closed primarily. Functional and aesthetic outcome could be improved if primary approximation of skin in these large defects were possible. Primary closure may be assisted using the viscoelastic properties of the skin. The viscoelastic properties of mechanical creep and stress relaxation in the skin were described more than 40 years ago. If skin is stretched with a constant force, it will expand with time as long as it is kept under tension, a phenomenon known as mechanical creep. In contrast, if the skin is stretched to a constant distance, it will expand and lead to a decrease in the force or tension on the skin with time, a phenomenon known as stress relaxation. We have recently applied these stretching properties to close the scalp because of a defect, which previously would have undergone pericranial flap and split-thickness graft. Because of the partial necrosis of the pericranial flap, skin grafting failed. In the second stage, we inserted only the wrist part of elastic latex gloves to the wound edges in full thickness. The skin margins were advanced slowly and gently.

Influence of surface modification and long-term exposure on mechanical creep properties of γ-TiAl G4

An investigation of the corrosion processes were performed for coated and uncoated γ-TiAl G4, an alloy designed to work in the temperature range 750 – 800°C, where oxidation and corrosion phenomena occur. An aluminising pack cementation treatment was used to improve the oxidation resistance of this γ-TiAl G4 alloy. Cyclic corrosion tests were performed at 800°C in air for up to 800 1-hour cycles with a Na2SO4/NaCl mixture. The influence of both aluminisation and the corrosion phenomena on the creep behaviour was investigated. The cyclic corrosion resistance of the coated γ- TiAl G4 was shown to be improved by aluminising. The pack cementation treatment had no detrimental effect on the creep behaviour. Moreover, neither is creep affected by the corrosion of coated specimens. As corroded uncoated specimen exhibited good creep behaviour, it can be concluded that this alloy is suitable, even without coating, for turbine applications in hot corrosion atmospheres at least up to 800°C.

Development of oxides dispersion strengthened steels for high temperature nuclear reactor applications

By introducing a dispersion of nanosized yttrium oxides particles into a steel matrix, the upper temperature limit in mechanical creep strength can be enhanced in temperature by 100K at least. Production routes for the production of a new class of oxides dispersion strengthened (ODS) steels are investigated within this work. Preliminary results obtained when doping pure iron matrix phase with two types of yttrium oxides (Y2O3) nanoparticles (commercial as well as laboratory fabricated nanopowder) are presented. The twofold purpose of this work is firstly to obtain a comparative analysis between the commercial and the laboratory fabricated Y2O3 nanopowder used to produce the doped iron, and secondly to demonstrate the feasibility of new production route by observing the nanostructure of the first test batches with pure iron. Observations are carried out with transmission electron microscopy (TEM) to determine the size distribution of the particles in the powder, while glow discharge optical emission spectroscopy (GDOES) and high resolution-scanning electron microscopy (HR-SEM) are used to analyze the chemical composition and the homogeneity of the produced doped iron. It is demonstrated, that even with small size particles nanopowder fabricated in the laboratory, the distribution is fairly homogeneous compared to the one obtained with a relatively large particles commercial nanopowder, confirming the feasibility of the new production route.

Influência do dano na resposta de geogrelhas submetidas ao arrancamento

Em projetos de maciços reforçados com geossintéticos, a resistência à tração de projeto é determinada a partir da aplicação de um fator de redução global sobre a resistência característica do reforço. O fator de redução global é, geralmente, desmembrado em fatores parciais de redução, que consideram a perda de resistência decorrente de danos mecânicos, degradação biológica e deformações por fluência. Em maciços reforçados, o fator de redução por danos mecânicos prepondera sobre os demais, podendo reduzir, significativamente, as propriedades mecânicas do geossintético. O presente trabalho apresenta resultados de ensaios de arrancamento executados com 2 tipos de geogrelhas, previamente danificadas, imersas em solo arenoso, sob diferentes condições de densidade relativa. O dano mecânico foi imposto artificialmente pela ruptura física de elementos da malha. O objetivo desses ensaios consistiu no estudo da influência do dano mecânico no comportamento de geogrelhas. Os resultados indicaram que a influência do dano aumenta com a densidade relativa do solo e sugerem que o dano deve ser investigado a partir de ensaios confinados, considerando-se a interação da geogrelha com o solo envolvente.

Review of responses to a landfill modelling challenge

This paper reviews the responses from various international research groups to a challenge to model and predict the performance of a well-constrained laboratory experiment on the settlement and biodegradation of municipal solid waste (MSW). The paper outlines and compares the modelling approaches taken by the various groups and compares their predictions against actual long-term data from the experiment. The opportunity to compare models side-by-side against a common dataset has been extremely valuable in highlighting differences in approach, input parameters and identifying areas that need further investigation. One of the main differences identified related to hydrolysis, with three different algorithms used. Within the context of this modelling challenge it was not clear which was the appropriate formulation to use and thus the need for further work in this area was highlighted. This is also an example where the need to standardise on one pathway (or algorithm) could be investigated. There appears to be a clear imperative for settlement models to include a component for mechanical creep. An objective of the modelling challenge was to create a framework by which various models can be compared and act as a starting point for discussion and further collaboration in the future. This paper forms a starting point to this process.

Structural Anisotropy in Metallic Glasses Induced by Mechanical Deformation

We observed structural anisotropy in metallic glasses samples deformed by homogenous mechanical creep and by inhomogeneous compression using high energy X-ray diffraction. Pair distribution function analysis indicates bond anisotropy in the first atomic shell. This suggests that mechanical deformation involves rearrangements in a cluster of atoms by a bond reformation.

Effect of barley β-glucan molecular size and level on wheat dough rheological properties

The effect of adding low (10 5 Da) or high (2.03 × 10 5 Da) molecular weight barley β-glucans in two wheat flours of different breadmaking quality were studied. Mechanical spectra and creep–recovery analysis data within (low stress) and out (high stress) of the linear viscoelastic region were obtained. The results revealed that the rheological behavior of β-glucan-enriched doughs depend on concentration and molecular weight of the polysaccharide as well as on the flour type used. Addition of β-glucan increased the G′ values of the good breadmaking quality flour doughs, whereas decreased the G′ of the poor quality wheat cultivar. Supplementation with β-glucans increased the resistance to deformation, flowability and elasticity of the doughs under low stress. Significant correlations between frequency sweep and creep–recovery parameters of optimally developed doughs from both flours were found. The addition of β-glucan in the dough recipe of the poor breadmaking wheat flour may result in similar rheological responses to those obtained from a non-fortified good breadmaking quality wheat flour.

Water Uptake and Conductivity of Cross-linked SPEEK Membranes

Water uptake is an important attribute of all membrane electrolytes, as it is related to mechanical strength, creep, and dimensional stability. Thus some means of controlling water uptake by a hydrophilic sulfonic acid polymer is desirable. This is particularly important for SPEEK, which at high degrees of sulfonation (i.e., low equivalent weight) has demonstrated substantial water uptake, on order of 10's of water molecules per sulfonic acid unit. We have attempted to control water uptake in SPEEK membranes by performing cross-linking reactions on the finished polymer. Water-soluble SPEEK (85% degree of sulfonation, or 418 g/eq EW), 1,4-benzenedimethanol, and various transition metal chloride catalysts were cast together to form a membrane that was heated at 200 oC for 10 min. The resulting insoluble membrane was shown to have conductivity comparable to an uncross-linked SPEEK membrane over a range of relative humidities.

Elaboration of Nickel Aluminide Diffusion Coatings: Application to Oxidation Resistance

Aluminide diffusion coatings are widely used in aeronautic domain. Nickel based superalloys present good mechanical properties and creep resistance at high temperature but their corrosion resistance is somewhat poor. In order to enhance their lifetime at high temperature, aluminide coatings can be applied on alloy surface. Aluminium present on material surface can form, at high temperature, alumina scale which is a protective oxide scale. In the present study, nickel aluminide coatings were carried out by aluminisation in a single step pack cementation process on nickel based superalloys 600 and AM1. Ni-Al intermetallic was obtained in each case. Oxidation tests were carried out at 900°C in air under atmospheric pressure. At this temperature, transient alumina as γ-Al2O3 and θ-Al2O3 appear on surface and are less protective than the stable α- Al2O3. Reactive elements can enhance α-Al2O3 nucleation and change diffusion mechanism during oxidation leading to a better adherence of alumina scale. In the present work, yttria was introduced by Metal-Organic Chemical Vapour Deposition (MOCVD) prior to the aluminisation. Yttria doped and undoped sample behaviours were compared under oxidising atmospheres.

Mechanical properties of functionally graded 2-D cellular structures: A finite element simulation

Functionally graded cellular structures such as bio-inspired functionally graded materials for manufacturing implants or bone replacement, are a class of materials with low densities and novel physical, mechanical, thermal, electrical and acoustic properties. A gradual increase in cell size distribution, can impart many improved properties which may not be achieved by having a uniform cellular structure. The material properties of functionally graded cellular structures as a function of density gradient have not been previously addressed within the literature. In this study, the finite element method is used to investigate the compressive uniaxial and biaxial behavior of functionally graded Voronoi structures. Furthermore, the effect of missing cell walls on its overall mechanical (elastic, plastic, and creep) properties is investigated. The finite element analysis showed that the overall effective elastic modulus and yield strength of structures increased by increasing the density gradient. However, the overall elastic modulus of functionally graded structures was more sensitive to density gradient than the overall yield strength. The study also showed that the functionally graded structures with different density gradient had similar sensitivity to random missing cell walls. Creep analysis suggested that the structures with higher density gradient had lower steady-state creep rate compared to that of structures with lower density gradient.

Nanoindentation hardness, Young’s modulus, and creep behavior of organic–inorganic silica-based sol-gel thin films on copper

In this study, the mechanical properties and creep behavior of hybrid sol-gel silica-based coatings on copper substrates were investigated. Sol-gel processing was used to synthesize the organically modified silanes using mixtures of tetraethoxysilane and vinyltrimethoxysilane or glycidoxypropyltrimethoxysilane precursors. The mechanical and creep properties of the coatings were assessed using nanoindentation. The link between film structure and creep behavior from nanoindentation experiments was examined, and simple mechanical models were used to extract Young’s modulus and viscosity from fits to creep data. It is shown that the creep response of the coatings was influenced dramatically by the chain length and amount of organic substituent.

Experimental investigation of Daqing Oilfield mudstone’s creep characteristic under different water contents

In order to analyze mechanism of casing damage, the uniaxial compression experiment and creep experiment of interbedded mudstone samples from Sanan development area of Daqing Oilfield under different water contents were carried out. The changes of the mudstone’s mechanical parameters and creep characteristics with the increment of water saturation were studied. The results indicate that the rock strength and elastic modulus decrease rapidly with the increment of water content, at the same time, the creep strain and creep strain rate of steady state increase with the increment of water content, and also the steady state creep strain rate is enhanced with the increment of deviatoric stress. Through the creep characteristic curves, a non-linear creep constitutive equation of mudstone considering the change of water contents is established, which will be used in future numerical analysis.

The long-term settlement of landfill waste

The biodegradation of solid waste materials, the main cause of secondary settlement in landfills, has a significant impact on the post-closure performance of landfill capping systems. Excessive settlement may in turn lead to water ingress and enhanced leachate generation, significantly increasing the risk of harm to the environment. Large-scale long-term laboratory experiments were conducted to identify the rate and magnitude of waste settlement using three purpose-designed test reactors (consolidating anaerobic reactors (CARs)). This paper presents a detailed characterisation of the waste, its associated chemical and physical properties, and the overall volume changes associated with degradation over time. The total magnitudes of settlement after 919 days were 27·6% in CAR 1 (under 150 kPa pressure) and 25·0% in CAR 2 (50 kPa). Long-term secondary settlement was found to be dependent on waste depth—that is, stress dependent. Increased stress (150 kPa) led to a 20% increase in the rates of long-term secondary compression in comparison with a stress level of 50 kPa. Secondary settlement due to biodegradation was found to be of comparable magnitude to the component of secondary settlement caused by mechanical creep (values of 11·1–13·7%). If the settlement behaviour in the CARs approximates to full-scale observations, this data set could be used for validation of quantitative landfill settlement models.

Interferometric study of creep deformation and some structural properties of polypropylene fiber at three different temperatures

AbstractInfluence of temperature on creep deformation for polypropylene PP fiber under a constant load was studied interferometrically. The automated multiple‐beam Fizeau system in transmission was equipped with a mechanical creep device attached to a wedge interferometer. This system was used to determine the optical properties (n∥, n⟂, and Δn) of PP fiber during the creep process at constant loading with varying temperature. The creep compliance was drawn as a function of both time and temperature. An empirical formula was suggested to describe the creep compliance curves for PP fibers and the constants of this formula were determined. Two Kelvin elements combined in series were used to provide an accurate fit to the experimental compliance curves. The stress–strain curve via creep was studied to determine some mechanical parameter of PP fibers, Young's modulus E, yield stress σy, and yield strain εy. The optical orientation function f(θ), the dielectric constant d, the dielectric susceptibility χ, the surface reflectivity $ \overline R $, and the average work per chain W′ were also calculated. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Optimum consolidation of self-reinforced polypropylene composite and its time-dependent deformation behavior

A systematic investigation was carried out to determine the optimum consolidation process conditions of a co-extruded polypropylene self-reinforced composite (SRC) considering its structural characteristics and static and time-dependent deformation behavior. It was observed that very little change in the processing condition influenced significantly the mechanical behavior of consolidated SRCs, e.g., the peel strength can be improved by approximately 30% within 10° change in temperature, the reason for which was investigated with structural analysis such as DSC and XRD and mechanical tests, including T-peel, tensile, and creep tests. A creep potential with three orthotropic material parameters was utilized to describe the anisotropic and nonlinear time-dependent deformation behavior of the SRC. Combined with the creep potential, the creep test of an off-axis coupon specimen with the fiber orientation of 22.5° was found to represent the anisotropic and nonlinear creep behavior of other off-axis specimens within a reasonable error.

Invited Review Article: Accurate and fast nanopositioning with piezoelectric tube scanners: Emerging trends and future challenges

Piezoelectric tube scanners have emerged as the most widely used nanopositioning technology in modern scanning probe microscopes. Despite their impressive properties, their fast and accurate operations are hindered due to complications such as scan induced mechanical vibrations, hysteresis nonlinearity, creep, and thermal drift. This paper presents an overview of emerging innovative solutions inspired from recent advances in fields such as smart structures, feedback control, and advanced estimation aimed at maximizing positioning precision and bandwidth of piezoelectric tube scanners. The paper presents a thorough survey of the related literature and contains suggestions for future research prospects.

Magnesia grain size effect on in situ spinel refractory castables

Alumina–magnesia castables present an expansive behavior due to in situ spinel formation and the reaction is affected by the magnesia source and its grain size. In this study, castables were prepared with different magnesia grain sizes (<45 or <100 μm) and the samples were evaluated by an assisted sintering technique, mechanical strength, creep resistance and microstructural evaluation. The expansion, the resulting phases and properties scaled with the magnesia grain size. The use of coarse MgO grain (<100 μm) led to phases which were predicted by local equilibrium phase diagrams, such as forsterite and monticellite. Due to a completely different microstructure developed for the castable containing large grains, a large number of cracks were generated, worsening the mechanical strength and creep resistance. Magnesia grain size selection is thus a key issue for alumina–magnesia castable design, as it affects the material's performance during service. As the change of a single parameter affected the final castable microstructure and properties, this study is a typical example concerning the complexity of refractory ceramic systems.

Mechanical Properties and Creep Behaviors of Zr-Sn-Fe-Cr and Zr-Nb-Sn-Fe Alloy Cladding Tubes

Since the 1990s, the second generation of Zirconium alloys containing main alloy compositions of Nb, Sn and Fe have been used as a replacement of Zircaloy-4 (Zr-Sn-Fe-Cr), a first-generation Zirconium alloy, to meet severe and rigorous reactor operating conditions characterized by high-burn-up, high-power and high-pH operations. In this study, the mechanical properties and creep behaviors of Zr-Sn-Fe-Cr and Zr-Nb-Sn-Fe alloys were investigated in a temperature range of <TEX>$450{\sim}500^{\circ}C$</TEX> and in a stress range of <TEX>$80{\sim}150\;MPa$</TEX>. The mechanical testing results indicate that the yield and tensile strengths of the Zr-Nb-Sn-Fe alloy are slightly higher compared to those of Zr-Sn-Fe-Cr. This can be explained by the second phase strengthening of the <TEX>$\beta$</TEX>-Nb precipitates. The creep test results indicate that the stress exponent for the steady-state creep rate decreases with the increase in the applied stress. However, the stress exponent of the Zr-Sn-Fe-Cr alloy is lower than that of the Zr-Nb-Sn-Fe alloy in a relatively high stress range, whereas the creep activation energy of the former is slightly higher than that of the latter. This can be explained by the dynamic deformation aging effect caused by the interaction of dislocations with Sn substitutional atoms. A higher Sn content leads to a lower stress exponent value and higher creep activation energy.

Effect of rare earth on mechanical creep–fatigue property of SnAgCu solder joint

In this paper, the effects of rare earth (RE) element additions on the mechanical creep–fatigue property of the Sn3.8Ag0.7Cu solder joint have been investigated. The results show that adding a small amount of the RE elements can evidently increase the creep–fatigue rupture lifetime of the Sn3.8Ag0.7Cu solder joint. The increase in the creep–fatigue property can be attributed to the reduction of the creep–fatigue damages and the change of microcrack propagation site during the fracture process, which depends on the refining of intermetallic compounds due to the RE elements. In addition, it is also found that adding the RE elements decreases the strain amplitude of the solder joint under the constant stress amplitude.

The effect of proteolytic treatment on plastic deformation of porcine aortic tissue

The objective of this work was to assess whether selective proteolysis of elastin and/or collagen in a porcine aorta followed by mechanical creep loading would result in an aneurysm-like permanent tissue stretch. The underlying motivations were to (1) test the feasibility of developing an in vitro abdominal aortic aneurysm (AAA) model, and (2) understand what role, if any, that passive creep-induced stretching plays in aneurysmal dilation. Multiple circumferentially oriented flat specimen strips were cut from the porcine thoracic aorta of ten adult pigs. Specimens were subjected to one of six treatment protocols: Untreated controls ( U C ; N = 23 ) , complete elastin degradation ( E ; N = 10 ) , partial elastin degradation ( E p ; N = 10 ) , partial collagen degradation ( C p ; N = 22 ) , and partial degradation of both elastin and collagen ( E p + C p ; N = 3 ) . All specimens were then subjected to cyclic creep (10 min/cycle) with increasing load amplitude until failure. The zero-load strain prior to the creep cycle where failure occurred was defined as load-induced plastic strain. The plastic strain induced by treatment alone, creep loading alone and the total was determined for all specimens. The total plastic strain was significantly greater for E (mean ± SD = 48.2 ±17.6, p<0.005), E p ( 41.6 ± 11.1 , p < 0.0005 ) , but not for E p + C p ( 48.9 ± 21.6 , p = 0.17 ) and C p ( 22.2 ± 12.8 , p = 0.14 ) compared to UC (17.7±6.1). Of the total plastic strain, treatment-induced plastic strain was high for those specimens subjected to partial or total elastin degradation ( E , E p , E p + C p ) , but not for those where elastin was intact ( C p ) . However, load-induced plastic strain in the treated specimens was not different in any of the treated groups compared to controls. Maximum total plastic strain of 78.6% was induced in one porcine aortic tissue from the E group. Even this is far lower than what would be needed for creating a realistic in vitro AAA model. Our findings do not support the feasibility of developing an in vitro AAA by enzymolysis followed by passive stretching of the aorta. The findings also suggest that AAA formation is unlikely to be a passive creep-induced stretching of a proteolytically degraded aortic wall as conventional thinking may suggest, but rather may be predominantly due to growth and remodeling of the aortic wall.