The creep of thin beams under small bending moments

2006 ◽  
Vol 462 (2073) ◽  
pp. 2863-2875 ◽  
Author(s):  
V Srivastava ◽  
H Jones ◽  
G.W Greenwood
Keyword(s):  
Elevated Temperatures ◽  
High Stress ◽  
Diffusional Creep ◽  
Beam Deflection ◽  
Creep Process ◽  
Polycrystalline Materials ◽  
Bending Moments ◽  
Beam Geometry ◽  
Cantilevered Beams ◽  
Thin Beams

When polycrystalline materials deform at elevated temperatures under low applied stresses by the stress directed migration of vacancies, specific features need consideration in the bending of thin beams since a relatively high stress variation may arise across individual planar grain boundaries in addition to the variation that exists between boundaries of differing inclination. The features depend on grain and beam geometry and expressions are derived for their effect on the rate of deflection of cantilevered beams. Experiments were carried out on beams of high purity copper 100 and 250 μm thick. The cantilever profiles supported the theoretical approach and showed creep rates linearly dependent on stress at rates in accord with predictions based on a diffusional creep process. A further indication of this process was the associated strain localization that resulted in fracture of 100 nm thick alumina coatings applied to some of the beams. The analysis shows how relationships change as the beam thickness approaches the grain size and permits an evaluation of the rate of beam deflection under small bending moments in terms of grain and beam dimensions.

Investigation of Smooth Pipe Bends Under the Effect of In-Plane Bending

2017 ◽  
Author(s):  
Diana Abdulhameed ◽  
Michael Martens ◽  
J. J. Roger Cheng ◽  
Samer Adeeb
Keyword(s):  
High Stress ◽  
Circular Cross Section ◽  
Bending Moment ◽  
Bend Angle ◽  
Bending Moments ◽  
Bend Radius ◽  
Pipe Bend ◽  
Cross Sectional ◽  
Smooth Pipe ◽  
Plane Bending

Pipe bends are frequently used to change the direction in pipeline systems and they are considered one of the critical components as well. Bending moments acting on the pipe bends result from the surrounding environment, such as thermal expansions, soil deformations, and external loads. As a result of these bending moments, the initially circular cross-section of the pipe bend deforms into an oval shape. This consequently changes the pipe bend’s flexibility leading to higher stresses compared to straight pipes. Past studies considered the case of a closing in-plane bending moment on 90-degree pipe bends and proposed factors that account for the increased flexibility and high-stress levels. These factors are currently presented in the design codes and known as the flexibility and stress intensification factors (SIF). This paper covers the behaviour of an initially circular cross-sectional smooth pipe bend of uniform thickness subjected to in-plane opening/closing bending moment. ABAQUS FEA software is used in this study to model pipe bends with different nominal pipe sizes, bend angles, and various bend radius to cross-sectional pipe radius ratios. A comparison between the CSA-Z662 code and the FEA results is conducted to investigate the applicability of the currently used SIF factor presented in the design code for different loading cases. The study showed that the in-plane bending moment direction acting on the pipe has a significant effect on the stress distribution and the flexibility of the pipe bend. The variation of bend angle and bend radius showed that it affects the maximum stress drastically and should be considered as a parameter in the flexibility and SIF factors. Moreover, the CSA results are found to be un-conservative in some cases depending on the bend angle and direction of the applied bending moment.

Tunability and Sub- and Superharmonic Entrainment of Limit Cycles in CW Laser Driven MEMS

2012 ◽  
Author(s):  
David B. Blocher ◽  
Alan T. Zehnder
Keyword(s):  
Thermal Stress ◽  
Limit Cycles ◽  
Limit Cycle Oscillation ◽  
Nonlinear Feedback ◽  
Beam Geometry ◽  
Cw Laser ◽  
Reflected Light ◽  
Mechanical Oscillators ◽  
Cycle Oscillation ◽  
Thin Beams

The nonlinear dynamics of nanoscale mechanical oscillators driven both inertially and by CW laser light are explored experimentally. The oscillators are singly and doubly-supported beams, 200 nm thick with lengths up to 40 microns. The optically thin beams, suspended over a Si substrate, form a Fabry-Pérot interferometer. The net effect is that the fractions of absorbed and reflected light are periodic functions of the gap. Thus, monitoring the reflected signal allows the motion to be measured. In addition, motion of the device through the interference field modulates the temperature and hence thermal stress of the oscillator. The thermal stress provides a thermo-mechanical drive to the beam, resulting in nonlinear feedback that can drive the beam into limit cycle oscillation. The laser power needed for the onset of limit cycles is studied as a function of beam geometry, and laser placement. The oscillators show both hardening and softening behaviors, sub- and superharmonic entrainment and wide frequency tunability.

Process Development of Silicon-Silicon Carbide Hybrid Micro-Engine Structures

2001 ◽  
Vol 687 ◽  
Author(s):  
Dongwon Choi ◽  
Robert J. Shinavski ◽  
Wayne S. Steffier ◽  
Skip Hoyt ◽  
S.Mark Spearing
Keyword(s):  
Silicon Carbide ◽  
Residual Stresses ◽  
Elevated Temperatures ◽  
Process Development ◽  
High Stress ◽  
Development Program ◽  
High Strength ◽  
Operating Conditions ◽  
Source Power ◽  
Silicon Silicon

AbstractA MEMS-based gas turbine engine is being developed for use as a button-sized portable power generator or micro-aircraft propulsion source. Power densities expected for the micro- engine require high combustor exit temperatures (1300-1700K) and very high rotor peripheral speeds (300-600m/s). These harsh operating conditions induce high stress levels in the engine structure, and thus require refractory materials with high strength. Silicon carbide has been chosen as the most promising material for use in the near future due to its high strength and chemical inertness at elevated temperatures. However, techniques for microfabricating single- crystal silicon carbide to the level of high precision needed for the micro-engine are not currently available. To circumvent this limitation and to take advantage of the well-established precise silicon microfabrication technologies, silicon-silicon carbide (SiC) hybrid turbine structures are being developed using chemical vapor deposition of poly-SiC on silicon wafers and wafer bonding processes. Residual stress control of SiC coatings is of critical importance to all the silicon-silicon carbide hybrid structure fabrication steps since a high level of residual stresses causes wafer cracking during the planarization, as well as excessive wafer bow, which is detrimental to the subsequent planarization and bonding processes. The origins of the residual stresses in CVD SiC layers have been studied. SiC layers (as thick as 30µm) with low residual stresses (on the order of several tens of MPa) have been produced by controlling CVD process parameters such as temperature and gas ratio. Wafer-level SiC planarization has been accomplished by mechanical polishing using diamond grit and bonding processes are currently under development using interlayer materials such as silicon dioxide or poly-silicon. These process development efforts will be reviewed in the context of the overall micro-engine development program.

Strength of Elastomers. A Perspective

1978 ◽  
Vol 51 (2) ◽  
pp. 225-252 ◽  
Author(s):  
Thor L. Smith
Keyword(s):  
High Speed ◽  
Elevated Temperatures ◽  
High Stress ◽  
High Strength ◽  
Extension Rate ◽  
Stress Concentrations ◽  
Chain Mobility ◽  
Growing Crack ◽  
High Elongation ◽  
Progressive Growth

Abstract The strength and extensibility of an elastomer depend on its overall viscoelastic properties, as reflected in the time and temperature dependence of stress-strain curves, and also on those discrete processes, including crack formation and growth, that culminate in high-speed crack propagation. The discrete processes determine the lifetime of a specimen; the viscoelastic characteristics affect the dependence of stress on deformation. The interplay between these effects causes strength and extensibility to depend strongly on test conditions. An elastomeric network composed solely of highly mobile chains is very weak indeed and fractures at a low elongation. This characteristic differs diametrically from that expected of an idealized network of mobile chains. If such a network were stretched, stress concentrations and unbalanced forces at the molecular level, which can result from short chains, entanglements, and network imperfections, would be vitiated rapidly by stress-biased segmental diffusion, especially at the elevated temperature. Therefore the network should be able to withstand a high elongation and thus a high stress. Hence, the low strength always exhibited by a single-phase non-crystallizable elastomer at elevated temperatures is incompatible with the characteristics ascribed to a network in the molecular theory of rubber elasticity. A network of mobile chains is weak for two reasons. First, microcracks develop readily in a stretched specimen. Their formation is usually attributed to stress concentrations near heterogeneties either within or on the surface of a specimen. Second, and most importantly, a microcrack—once it forms—encounters little resistance to growth because the chains are highly mobile. High strength results not because microcracks do not develop but because their growth is impeded. Unless processes that impede growth come into play, a microcrack enlarges rapidly and catastrophic propagation soon follows. When chain mobility is relatively low, the dissipation of energy through viscoelastic processes near the tip of a slowly growing crack retards its progressive growth. But this source of strength is rather ineffective except within narrow ranges of temperature and extension rate, or time scale more generally. Thus, high strength and toughness result from other mechanisms that impede crack growth. Effective mechanisms usually come into play and impart toughness if colloidal particulate fillers or plastic domains are present, except at low concentration.

Electrical Resistivity as a Characterization Tool for Nanocrystalline Metals

1999 ◽  
Vol 581 ◽  
Author(s):  
J.L. McCrea ◽  
K.T. Aust ◽  
G. Palumbo ◽  
U. Erb
Keyword(s):  
Thermal Stability ◽  
Grain Size ◽  
Electrical Resistivity ◽  
Grain Boundary ◽  
Nanocrystalline Materials ◽  
Elevated Temperatures ◽  
Polycrystalline Materials ◽  
Grain Growth Kinetics ◽  
Insight Into ◽  
Grain Boundary Resistivity

ABSTRACTThe electrical resistivity as a function of temperature (4K to 673K) of several electrodeposited nanocrystalline materials (Ni, Ni-Fe, Co) has been examined. The contribution of the grain boundaries to the electrical resistivity was quantified in terms of a specific grain boundary resistivity, which was found to be similar to previously reported values of specific grain boundary resistivity for copper and aluminum obtained from studies involving polycrystalline materials. In the high temperature range, the resistivity of the nanocrystalline samples was monitored as a function of time. The observed time dependence of the resistivity at elevated temperatures was correlated to microstructural changes in the material. The study has shown that electrical resistivity is an excellent characterization tool for nanocrystalline materials giving useful information regarding grain size and degree of thermal stability, as well as some insight into the grain growth kinetics at various temperatures.

scholarly journals Influence of Mo Segregation at Grain Boundaries on the High Temperature Creep Behavior of Ni-Mo Alloys: An Atomistic Study

Materials ◽  
2021 ◽  
Vol 14 (22) ◽  
pp. 6966
Author(s):  
Qian Li ◽  
Jiayong Zhang ◽  
Huayuan Tang ◽  
Hongwu Zhang ◽  
Hongfei Ye ◽  
...  
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Strain Rate ◽  
Grain Boundaries ◽  
Creep Strain ◽  
High Stress ◽  
External Stress ◽  
Creep Strain Rate ◽  
Creep Process ◽  
Boundary Slip

Based on molecular dynamics simulations, the creep behaviors of nanocrystalline Ni before and after the segregation of Mo atoms at grain boundaries are comparatively investigated with the influences of external stress, grain size, temperature, and the concentration of Mo atoms taken into consideration. The results show that the creep strain rate of nanocrystalline Ni decreases significantly after the segregation of Mo atoms at grain boundaries due to the increase of the activation energy. The creep mechanisms corresponding to low, medium, and high stress states are respectively diffusion, grain boundary slip and dislocation activities based on the analysis of stress exponent and grain size exponent for both pure Ni and segregated Ni-Mo samples. Importantly, the influence of external stress and grain size on the creep strain rate of segregated Ni-Mo samples agrees well with the classical Bird-Dorn-Mukherjee model. The results also show that segregation has little effect on the creep process dominated by lattice diffusion. However, it can effectively reduce the strain rate of the creep deformation dominated by grain boundary behaviors and dislocation activities, where the creep rate decreases when increasing the concentration of Mo atoms at grain boundaries within a certain range.

scholarly journals Viscoelastic Damage Characteristics of Asphalt Mixtures Using Fractional Rheology

Materials ◽  
2021 ◽  
Vol 14 (19) ◽  
pp. 5892
Author(s):  
Qipeng Zhang ◽  
Xingyu Gu ◽  
Zilu Yu ◽  
Jia Liang ◽  
Qiao Dong
Keyword(s):  
Damage Evolution ◽  
High Stress ◽  
Creep Damage ◽  
Asphalt Mixtures ◽  
Creep Process ◽  
Model Parameters ◽  
Creep Tests ◽  
Steady Stage ◽  
Stress Levels ◽  
Different Temperatures

The mechanical behavior of asphalt mixtures at high stress levels are characterized by non-linear viscoelasticity and damage evolution. A nonlinear damage constitutive model considering the existence of creep hardening and creep damage mechanisms in the entire creep process is proposed in this study by adopting the fractional rheology theory to characterize the three-stage creep process of mixtures. A series of uniaxial compressive creep tests under various stresses were conducted at different temperatures to verify the model. The results indicated that the model predictions were in good agreement with the creep tests. The relationship between the model parameters and applied stresses was established, and the stress range in which the mixture exhibited only creep consolidation was obtained. The damage to the asphalt mixture was initiated in the steady stage; however, it developed in the tertiary stage. A two-parameter Weibull distribution function was used to describe the evolution between the damage values and damage strains at different stress levels and temperatures. The correlation coefficients were greater than 0.99 at different temperatures, indicating that a unified damage evolution model could be established. Thus, the parameters of the unified model were related to material properties and temperature, independent of the stress levels applied to the mixtures.

scholarly journals Characterisation of beam-to-column composite joints beyond current Eurocode provisions

2018 ◽  
Author(s):  
Jean-François Demonceau
Keyword(s):  
Composite Structures ◽  
Elevated Temperatures ◽  
Technical Committee ◽  
European Convention ◽  
Composite Joints ◽  
Bending Moments ◽  
Axial Loads ◽  
Rotational Stiffness ◽  
Design Rules ◽  
Recent Developments

In EN 1994-1, design rules are given for the evaluation of the mechanical properties of structural steel-concrete composite joints (rotational stiffness, resistance and ductility) based on the component method offered in EN 1993-1-8 and adding specific components for composite joints. These rules cover only the situations for the joints subjected to shear forces and hogging moments. However, during the last decades, researches have been conducted on the behaviour of composite joints subjected to different kind of actions such as sagging bending moments, cyclic loadings, combined bending moments and axial loads, elevated temperatures etc. with the objective of improving/extending the rules presently proposed in the Eurocodes design rules. As an outcome of the Technical Committee 11 of the European Convention of Constructional Steelwork (ECCS) dedicated to the behaviour of composite structures, a publication summarising these recent developments and their main outcomes is under finalisation. Within the present paper, it is proposed to highlight these main outcomes which could be seen as proposals for future improvements of the beam-to-column provisions in Eurocodes in general and of Eurocode 4 in particular.

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