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Structural integrity plays an important role in any industrial activity, due to its capability of assessing complex systems against sudden and unpredicted failures. The work here presented investigates an unexpected new mechanism occurring in structures subjected to monotonic and cyclic loading at high temperature creep condition. An unexpected accumulation of plastic strain is observed to occur, within the high-temperature creep dwell. This phenomenon has been observed during several full inelastic finite element analyses. In order to understand which parameters make possible such behaviour, an extensive numerical study has been undertaken on two different notched bars. The notched bar has been selected due to its capability of representing a multiaxial stress state, which is a practical situation in real components. Two numerical examples consisting of an axisymmetric v-notch bar and a semi-circular notched bar are considered, in order to investigate different notches severity. Two material models have been considered for the plastic response, which is modelled by both Elastic-Perfectly Plastic and Armstrong-Frederick kinematic hardening material models. The high-temperature creep behaviour is introduced using the time hardening law. To study the problem several results are presented, as the effect of the material model on the plastic strain accumulation, the effect of the notch severity and the mesh element type and sensitivity. All the findings further confirm that the phenomenon observed is not an artefact but a real mechanism, which needs to be considered when assessing off-design condition. Moreover, it might be extremely dangerous if the cyclic loading condition occurs at such a high loading level.
Variation of stress across the length and thickness of a cantilever during creep allows obtaining multiple pairs of strain rates and stress under steady-state condition. This work applies digital image correlation (DIC) and conjugate analytical models to obtain several such “strain rate–stress” pairs during steady-state creep by testing a single cantilever at a constant applied load. Furthermore, these strain rate–stress pairs are used to accurately determine the stress exponent of the material (e.g., Al and Pb). In addition, an empirical observation of plotting strain rate as a function of stress at fixed strain during primary creep for estimating stress exponent is extended to bending creep, wherein strain rates of the points in the cantilever lying on an iso-strain contour were plotted against the moment at the point to determine stress exponent. This study, thereby, proves that the “bending creep–DIC” combination is a high throughput test methodology for studying steady-state creep.
The stress and hence strain fields in a cantilever deforming as per power-law creep vary across the length and thickness of the sample, which allow obtaining multiple stress–strain pairs from a single test. Here, a high-throughput method is described to quantify the primary-cum-steady-state creep response of materials by testing a single cantilever sample in bending and mapping strain fields using digital image correlation. The method is based on the existence of stress invariant points in a cantilever, where the value of stress does not change during creep. It is demonstrated that strain evolution throughout primary and steady-state stages at these points is identical to the creep response obtained under uniaxial tests. Furthermore, the gained insights were exploited to obtain various parameters of a power-law type primary-cum-steady-state creep equation by testing only one cantilever sample. The developed method allows obtaining uniaxial creep curves at multiple stresses by testing a single cantilever, thereby reducing the time and number of samples required to understand the creep behavior of a material. The method has been validated by performing bending tests on Al and comparing the results with those of corresponding uniaxial tests.
In young rabbit, digestive disorders are frequently observed around weaning. Stimulating the onset of feed intake in the suckling rabbit might be a way to promote gut health. The aim of this study was to determine the rabbit’s acceptability for different feed presentations and its preferences for flavours at an early stage of life. Two trials were conducted to evaluate the effects of physical form and flavouring on creep feed attractiveness. All the diets tested were provided in the nest from 3 to 17 days, and the daily intake per litter was recorded as of 8 days of age. In the first trial, five feed presentations were tested separately (n = 60 litters). Three dry presentations were chosen: commercial pellet (P), crumb from commercial pellet (cP) and crumb from beet pulp pellet (cBP). Hydrated feeds were also provided with either raw fodder beetroot (B) or a semi-solid feed in agar gel form produced with fodder beetroot juice and pulp (gB). In the second trial, double-choice tests were performed on four feed gels (n = 72 litters), leading to six comparison treatments. These agar gels were made of pellet mash without or with a sensory additive: one non-odorised control gel and three gels with 0.20% banana flavour, 0.06% red berry flavour and 0.10% vanilla flavour, respectively. In the first trial, kits ate more gB in fresh matter than other feed presentations (P < 0.001), with a total intake of 7.0 ± 1.8 g/rabbit from 8 to 17 days. In DM, the total consumption of pellets P (1.6 ± 0.4 g of DM/rabbit) was the highest together with the gB form (1.4 ± 0.4 g of DM/rabbit), whereas cBP was barely consumed (0.3 ± 0.1 g of DM/rabbit). Gel feed supplemented with vanilla was slightly more consumed than other flavoured and non-odorised gels (relative consumption of 57% when compared to control gel; P = 0.001). The gel feed intake was independent of the milk intake but was correlated with the litter weight at 3 days (r = 0.40, P < 0.001). In both trials, rabbit growth before and after weaning was not affected by the type of creep feed provided. Our results confirmed that providing creep feed promotes the solid intake of rabbits at early stages. Gel feed form motivated rabbits to eat and vanilla flavour supplementation increased the feed palatability. Those creep feed characteristics should be explored further for seeking effective stimulation of the onset of the feed intake in suckling rabbit.
This paper presents a novel method for quantifying the effect of ambient, environmental and operating conditions on the progression of degradation in aircraft gas turbines based on the measured engine and environmental parameters. The proposed equivalent operating time (EOT) model considers the degradation modes of fouling, erosion, and blade-tip wear due to creep strain, and expresses the actual degradation rate over the engine clock time relative to a pre-defined reference condition. In this work, the effects of changing environmental and engine operating conditions on the EOT for the core engine booster compressor and the high-pressure turbine were assessed by performance simulation with an engine model. The application to a single and multiple flight scenarios showed that, compared to the actual engine clock time, the EOT provides a clear description of component degradation, prediction of remaining useful life, and sufficient margin for maintenance action to be planned and performed before functional failure.
Mechanical fracture of electrodes will occur during lithiation caused by large volume changes, which leads to the capacity loss of the lithium-ion battery. Herein, we present a new analytical model to investigate the effect of creep deformation on stress relaxation and fracture of the lithiated tin (Sn) electrode under the galvanostatic and potentiostatic operation. Interestingly, it is found that the presence of creep can improve fracture resistance and toughness of the Sn electrode. In addition, the surface effect has the capacity to weaken the creep deformation effectively. And the different size of the Sn electrode shows different effects for creep deformation. This conclusion explains the difference in charging conditions, and the mechanism of stress change inside the electrode is also different. Deeply, the base on our model, the stress strength factor, and critical size of the electrode have been evaluated under galvanostatic and potentiostatic operation with creep deformation effects. Finally, the safety area during lithiation is established to determine the critical size of the Sn electrode. And the presence of creep deformation may significantly increase critical dimensions of the electrode. These results will provide a valuable basis to design the durable electrodes.
In the present work, Mo was added to an Al–Si–Mg foundry alloy to study its influence on the evolution of dispersoids during various heat treatments. The microhardness and the elevated-temperature tensile properties and creep resistance were measured to evaluate the contribution of dispersoids. Results showed that the addition of Mo greatly promoted the formation of α-dispersoids. During solution treatment, the formation of α-dispersoids started after 8 h at 500 °C. At high temperature (540 °C), the coarsening of dispersoids with increasing time became predominant. The optimum condition of dispersoids can be reached by 520 °C/12 h or 500 °C/4 h + 540 °C/2 h, leading to the highest differences in microhardness between the Mo-containing alloy and base alloy. The tensile strengths were improved at both room temperature and elevated temperatures, while the elongation at elevated temperature was greatly increased. The creep resistance at elevated temperature is further enhanced due to the Mo addition.
The behaviour of composite materials is often sensitive to changes in temperature. This arises for two main reasons. First, the response of the matrix to an applied load is often temperature-dependent; and second, changes in temperature can cause internal stresses to be set up as a result of differential thermal contraction and expansion of the two constituents. These stresses affect the thermal expansivity (expansion coefficient) of the composite. Furthermore, significant stresses are normally present in the material at ambient temperatures, since it has in most cases been cooled at the end of the fabrication process. Changes in internal stress state on altering the temperature can be substantial and may influence the response of the material to an applied load. Thermal cycling can thus have strong effects on, for example, creep characteristics. Finally, the thermal conductivity of composite materials is of interest, since many applications and processing procedures involve heat flow of some type. This property can be predicted from the conductivities of the constituents, although the situation may be complicated by poor thermal contact across the interfaces.
Bond coats are essential in gas turbine technology for oxidation protection. Freestanding MCrAlY (M = Ni, Co) bond coats were investigated with respect to their creep strength at elevated temperatures. Three types of MCrAlY, a Ni-based bond coat Amdry 386, a Co-based bond coat Amdry 9954 and Amdry 9954 + 2 wt% Al2O3 (ODS = oxide dispersion strengthened) produced by low pressure plasma spraying, were analyzed. The two phase microstructure of the bond coats consists of a fcc γ-Ni solid solution and a B2 β-NiAl phase. Constant load experiments were performed in a thermomechanical analyzer at temperatures between 900 and 950 °C. Microtensile test specimens with a diameter of 450 µm were produced by a high-precision grinding and polishing process. Creep rupture was mainly due to void nucleation along the β–γ interfaces and grain boundaries. The time to failure is larger in Ni-based Amdry 386 compared to that in Co-based Amdry 9954 due to a higher fraction of the high-strength β-NiAl phase at test temperatures. The addition of ODS-particles in the Co-based bond coat Amdry 9954 resulted in a better creep resistance but lower ductility in comparison to ODS-particle-free Amdry 9954.
This paper applies the concept of emergency powers to the crisis politics of international organizations (IOs). In the recent past, IOs like the UN Security Council, the WHO, and the EU have reacted to large-scale crises by resorting to assertive governance modes bending the limits of their competence and infringing on the rights of the rule-addressees. In contrast to rational and sociological institutionalist notions of mission creep, this paper submits that this practice constitutes ‘authority leaps’ which follow a distinct logic of exceptionalism: the expansion of executive discretion in both the horizontal (lowering of checks and balances) and the vertical (reduction of legal protection of subjects) dimension, justified by reference to political necessity. This ‘IO exceptionalism’, as argued here, represents a class of events which is observable across fundamentally different international institutions and issue areas. It is important not least because emergency politics tend to leave longer-term imprints on a polity’s authority structures. This article shows that the emergency powers of IOs have a tendency to normalize and become permanent features of the institution. Thus IO exceptionalism and its ratcheting up represent a mechanism of abrupt but sustainable authority expansion at the level of IOs.
A high-temperature nanoindentation system was used to examine the steady state indentation creep behavior of CsHSO4. This high proton conductivity solid-acid material is a candidate for use as a solid-state electrolyte in intermediate temperature fuel cells. Constant strain rate indentation creep tests yielded a stress exponent and a creep activation energy in close agreement with results obtained from previous uniaxial compression testing. The large penetration depths reached during creep testing necessitated validating an indenter area function well beyond depths measurable in fused silica. The developed methodology is material agnostic meaning it can be used for indentation creep measurements in other high creep rate materials. In addition, it is shown how an analysis developed by Bower et al. (Proc. Royal Soc. 441, 97–124, 1993) can be successfully used to convert the indentation creep parameters into the more common material parameters measured in uniaxial creep tests.
As high-entropy alloys (HEAs) are being actively explored for next-generation structural materials, gaining a comprehensive understanding of their creep, fatigue, and fracture behaviors is indispensable. These three aspects of mechanical properties are particularly important because (i) creep resistance dictates an alloy’s high-temperature applications; (ii) fatigue failure is the most frequently encountered failure mode in the service life of a material; (iii) fracture is the very last step that a material loses its load-carrying capability. In consideration of their importance in designing HEAs toward applicable structural materials, this article offers a comprehensive review on what has been accomplished so far in these three topics. The sub-topics covered include a comparison of different creep testing methods, creep-parameter extraction, creep mechanism, high-cycle fatigue S–N relation, fatigue-crack-growth behavior, fracture toughness, fracture under different loading conditions, and fractography. Directions for future efforts are suggested in the end.
The effective lifetimes of electronic packages are affected by various thermos-mechanical deformations. Creep is considered the most salient mechanism in the failure of solder joints. Many researchers have conducted reasonable studies to portray the behavior of creep deformation using numerical models and further extended their research scope to forecast the lifetimes of packages with the results obtained from creep models. Many studies have identified particular creep and lifetime models to be nominal based on experimental data.
In this study, the characteristics of familiar creep models were examined in detail, and their significance was made known. Lifetime prediction models that seem prominent among researchers were discussed in detail. Finite element analysis of a wafer level chip-scale package (WLCSP) used to figure out the engagement of different creep models and their capability of materializing creep deformation was investigated via simulation. The results from the simulation were applied to different lifetime prediction models, and their predictions were examined carefully. After considering the various factors that affected the reliability study of the solders, the Garofalo-Arrhenius creep model and modified strain energy density model seemed to be convincingly productive for studying the reliability of various electronic packages.
In the present study, assuming that the thermo-elastic creep response of the material is governed by Norton's law, an analytical solution has been developed for the purpose of time-dependent creep response for isotropic thick-walled cylindrical pressure vessels. To study the creep response, the first-order shear deformation theory (FSDT) is applied. To the best of the researchers’ knowledge, in the literature, there is no study carried out into FSDT for time-dependent creep response of cylindrical pressure vessels. The novelty of the present work is that it seeks to investigate creep life of the vessels made of 304L austenitic stainless steel (304L SS) using Larson-Miller Parameter (LMP) based on FSDT. Using this analytical solution, stress rates are calculated followed by an iterative method using initial thermo-elastic stresses at zero time. When the stress rates are known, the stresses at any time are obtained and then using LMP, creep life of the vessels are investigated. The Problem is also solved, using the finite element method (FEM), the result of which are compared with those of the analytical solution and good agreement was found. It is found that the temperature gradient distribution has significant influence on the creep life of the cylinder, so that the maximum creep life is located at the outer surface of the cylinder where the minimum value of temperature is located.
As the operating temperature of disk service was elevated from 650 °C to 700 °C, the creep properties urged to be paid attention. To investigate the creep properties of spray-formed low solvus, high refractory (LSHR) superalloy at about 700 °C, creep tests were conducted under seven different stress ranging from 690 MPa to 897 MPa. By means of creep curves and fracture microstructure observation, the creep behaviors and fracture mechanisms of spray-formed LSHR were analyzed. Stress exponent of the alloy was comparable to other disk superalloys such as Waspaloy and Inconel 718. It was interesting to find a transition in the creep behavior in two stress regimes. The contribution of grain boundary sliding in the low stress regime was greater than that in the higher stress. Under higher stress microcracks initiated along the intragranular slip bands because of strain concentration. The spray-forming LSHR exhibited a good creep resistance at low stress compared with other two superalloys by using Larson–Miller parameter, which was consistent with the transition of fracture behaviors.
In this paper, taking into account the external loading, growth strain, creep, and bending deformation during the metallic high-temperature oxidation, a residual stress evolution model is developed according to the force- and moment-equilibrium equations. In this model, oxidation kinetic relationship (the stress-dependent growth rate) is related to the stress in the oxide scale, not classical parabolic law. If and only if the stress in the scale or the activation volume is equal to zero, this relationship can reduce to the parabolic law. Then the stress-dependent oxidation kinetics is compared with the stress-independent one (the parabolic law). Finally, effects of the external loading on the stress distribution in the oxide scale, the curvature of the system and the scale thickness are discussed, and numerical results show that the tensile external loading decreases the oxidation stress and promotes the growth rate of the oxidation layer.
Acceleration-factor (AF) equations have been developed to rapidly predict product lifetime, and the most widely used equation is Norris-Landzberg (N-L) equation. In recent years, some researchers have found that the current AF equation does not accurately predict the experimental results for thermal cyclic loading at high ramp rates; indeed, it may yield the opposite results, due to the changing effect of the solder strain rate at different ramp rates. Modifying the current AF equation to better assess product reliability has thus become an important task for researchers.
In this study, a novel AF equation was developed from a wafer level chip-scale package (WLCSP) under different thermal cyclic loadings. The frequency term used in the N-L equation was replaced with a new term in the proposed AF equation to distinguish between the effects of ramp rate and dwell time under thermal cyclic loading conditions. Proposed AF equation showed a high level of correlation with the simulation and test results for various thermal cyclic loadings. In addition, the proposed AF equation was validated by confirming the consistency of its results with experimental data on a range of packages and thermal-cycling profiles reported in the literature.
A novel strain-rate jump method was developed for the plane-strain bulge test and used to investigate the time-dependent deformation behavior of gold thin films in the thickness range 100–400 nm. The experimental method is based on an abrupt variation of the pressurization rate. The evaluated strain-rate sensitivity was found to be five times higher for films in freestanding condition (m = 0.094) than for films tested on a SiNx substrate (m = 0.020). Bulge creep tests confirmed this increased time-dependence. The observation of the surface of the freestanding films after the creep tests provided evidence of apparent grain boundary sliding taking place next to intragranular plastic deformation. The out-of-plane deformation was presumably favored by the columnar microstructure of the samples, with grains extending between both free surfaces. In the case of SiNx-supported films, grain boundary sliding was prevented by the good adhesion of gold to the SiNx substrate.
The accurate and effective prediction of the failure for an inelastic structure, such as a solder joint in an electronic chip packaging, remains a current issue. Subjected to sub-critical cyclic loading, the solder can undergo fatigue cracks, leading to the failure of the whole system after a certain number of power cycles. In this paper, a model for describing the viscoplastic behavior of the solder material under power cycling is implemented in the finite element code Abaqus and a continuum damage procedure is used for lifetime prediction. Damage initiation criterion and damage evolution law, based both on the inelastic strain energy per stabilized cycle as proposed by Darveaux, are used in conjunction with the direct cyclic procedure available in Abaqus. This latter technique allows reducing the considerable computation time needed to obtain the stabilized states during the repetitive loading cycles.
From the biological/chemical perspective, interface concepts related to the cell surface/synthetic biomaterial interface and the extracellular matrix/biomolecule interface have wide applications in medical and biological technologies. Interfaces also play a significant role in determining structural integrity and mechanical creep and strength properties of biomaterials. Structural arrangement of interfaces combined with interfacial interactions between organic and inorganic phases significantly affects the mechanical properties of biological materials, allowing for a unique combination of seemingly inconsistent properties, such as fracture strength and tensile strength being both high—as opposed to traditional engineering materials, which have high fracture strength linked to low tensile strength and vice versa. While there has been a tremendous amount of work focused on the effects of structural arrangements on biomaterial properties, both experimental and computational studies of the strength, deformation, and viscosity of the interface itself are limited to just a few systems. Even in such studies, the actual interface stress is rarely analyzed, and correlated to the overall material strength or creep properties. This article provides a focused overview of such studies in hard biological materials, followed by a new vision of how the results of interfacial molecular studies could be consistently linked to multiscale, micromechanics-based perceptions of hierarchical biological materials.