Showing papers on "Creep published in 1988"

Creep in metallic materials

Abstract: 1. Deformation and Creep. Deformation. Definition of creep. Time dependence of creep strain. Creep Curve. Mechanisms of plastic deformation: A general survey. Mechanical equation of state. Creep test compared with tensile test at constant strain rate and constant loading rate. Creep tests at constant load and constant stress. 2. Motion of Dislocations. Dynamic Recovery. Motion of dislocations. Free, mobile and moving dislocations. Dynamic recovery. 3. Temperature Dependence of Creep Rate. Activation energy of creep. Methods of determination of activation energy of creep. Correction of experimentally determined activation energy of creep for temperature dependence of elastic modulus. Activation energy or creep and activation enthalpy of diffusion. 4. Applied Stress Dependence of Creep Rate. Initial creep rate. Steady-state creep. Transient creep. 5. Influence of Grain Size and Stacking Fault Energy. Grain size. Stacking fault energy. 6. Orowan and Bailey-Orowan Equations. Orowan equation. Bailey-Orowan equation. Relation between Orowan equation and Bailey-Orowan equations. A consequence of the equivalence of Orowan and Bailey-Orowan equations. Experimental verification of Bailey-Orowan equation. Experimental determination of quantities r and h. Incubation period and ``Frictional'' stress. 7. Back Stress. Internal, threshold and frictional stress. Internal and effective stress. Concept of internal and effective stress and the mechanical equation of state. Definitions of experimental parameters. Interpretation of experimental parameters. 8. Dislocation Structure. Development of dislocation structure during creep. Basic quantitative characteristics of dislocation structure. Subgrain structure. Subgrain structure and long-range internal stress. Behaviour of sub-boundaries. Interaction of dislocations with sub-boundaries. Generation of dislocations. Structural steady state. Concept of hard and soft regions and measured internal stress. 9. Dislocation Creep in Pure Metals. Creep controlled by recovery. Creep controlled by dislocation glide. Models based on thermally activated glide and diffusion controlled recovery. Relation between constants A and n in the dorn creep equation and the natural third power law. Harper-Dorn creep. 10. Creep in Solid Solution Alloys. Introduction. Mechanisms of creep strengthening in solid solutions. Creep controlled by viscous dislocation glide. 11. Creep in Precipitation and Dispersion Strengthened Alloys. Models of Ansell and Weertman. Back stress concept. 12. Diffusional Creep. Nabarro-Herring and Coble creep. Subgrain boundaries as sources and sinks for vacancies. Diffusional creep and grain boundary sliding. Reactions on grain boundaries. Diffusional caritational creep. 13. Deformation Mechanism Maps. Equations used for construction of deformation mechanism maps. Examples of deformation mechanism maps. ``Generalized'' deformation mechanism map for pure metals. 14. Grain Boundary Sliding.

Mathematical modeling of creep and shrinkage of concrete

Abstract: Part 1 Memorials: Robert L'Hermite and his legacy, M.Fickelson the impact of Robert L'Hermite on the evolution of creep and shrinkage theory, Z.P.Bazant a tribute to Douglas McHenry, B.Bresler Hubert Rusch and his legacy, H.Hilsdorf. Part 2 State-of-the-art in mathematical modelling of creep and shrinkage in concrete: physical mechanisms and their mathematical description, J.F.Young et al material models for structural creep analysis, Z.P.Bazant et al creep and shrinkage analysis of structures, O.Buyukozturk et al finite element analysis of creep and shrinkage, C.A.Anderson et al probabilistic models, T.Tsubaki et al conclusions for structural analysis and for formulation of standard design recommendations. Part 3 Summaries of discussions from the symposium: physical mechanisms and their mathematical description, U.Schneider material models, P.E.Roelfstra structural analysis, V.Kristek finite element analysis, L.Cedolin uncertainty of creep and shrinkage predictions, S.G.Reid current research in material modelling, J.W.Dougill current research in structural analysis, B.Espion. Appendices: list of lectures and papers presented at the symposium list of participants.

Creep of ceramics

Abstract: This review analyses a wide range of experimental data on the creep of ceramic materials and reveals many similarities with the creep of metals. It is demonstrated that there are two important differences in the creep behaviour of ceramics: (1) there is an enhanced role of diffusion creep, and (2) in the power-law regime, ceramics divide into two categories with stress exponents of ∼ 5 and ∼ 3, respectively. It is concluded that the behaviour with an exponent of ∼ 5 represents fully ductile behaviour as in f cc metals, whereas the behaviour with an exponent of ∼ 3 is due to dislocation climb from Bardeen-Herring sources under conditions where there is either a lack of five independent slip systems or, if five independent slip systems are available, a lack of interpenetration of these systems.

Deformation and fracture processes and the physical metallurgy of WC–Co hardmetals

Abstract: Deformation and fracture processes have been examined in WC–Co tool materials, still the most commonly used hardmetal. The relevant microstructural parameters are described in detail and an assessment is made of their effects on strength and fracture tests at ambient temperature, for the constituent phases separately and in combination. The results of properties more relevant to service behaviour, such as fatigue and creep, are summarised. Finally, recommendations are made with regard to the need for further fundamental research.

A Simple Way to Reduce Hysteresis and Creep When Using Piezoelectric Actuators

Abstract: We present a simple way to compensate for hysteresis and creep in piezoelectric actuators. By inserting a capacitor in series with the piezoelectric actuator, we find a reduction in the size of the hysteresis loop. For a suitable small capacity in series, creep is eliminated.

Damage‐Enhanced Creep in a Siliconized Silicon Carbide: Phenomenology

Abstract: The creep behavior of a commercial grade of reaction-bonded silicon carbide was characterized at a temperature of 1300°C. Creep occurred more easily in tension than in compression. At a given applied stress, the steady-state creep rate in tension was found to be at least 20 times that obtained in compression. In both tension and compression, the stress exponent for steadystate creep was found to increase with increasing applied stresses. At low applied stresses, the stress exponent was ∼4, suggesting some kind of dislocation mechanism operating in the two-phase composite. At high stresses, the stress exponent was ∼11 in tension. The increase in the stress exponent was attributed to damage accumulation in the form of cavities. An effective threshold stress for cavitation of less than 100 MPa was suggested. In compression, the cause of the increase of stress exponent with stress cannot be attributed to cavitation.

Mechanical Properties of Discontinuous SiC Reinforced Aluminum Composites at Elevated Temperatures

Abstract: High temperature mechanical properties of discontinuous, whisker and particulate, SiC reinforced aluminum composites, including 2124 and 6061 alloy matrices, are reviewed. It is shown that the behavior of these composites is similar to conventional oxide dispersion strengthened alloys. Namely, they exhibit a low strain rate senstivity and a high apparent activation energy for creep deformation. Despite the fact that the addition of SiC significantly improves the mechanical properties of aluminum at room temperature, the mechanical strength of the composite at elevated temperatures is dominated by the strength of the aluminum matrix This is because the SiC dispersoids are, in general, too coarse and they are not effective barriers for dislocation motion. It is also demonstrated that SiC particulate composites are less creep resistant than SiC whisker composites.

Identification of optimum binder phase compositions for improved WC hard metals

Abstract: The possibility has been investigated of using measurements of the mechanical properties of alloys with binder phase compositions to identify binder phases that will provide high hardness and toughness in WC hard metals based on cobalt, nickel, CoNi, NiAl, NiCrMo and NiCrMoAl. The results showed that, in WCCo and WC(CoNi) hard metals with constant grain size and binder phase content, an optimum grain size and binder phase content, an optimum combination of fracture toughness and compressive strain could be obtained at particular tungsten and carbon contents which could be determined from measurements of magnetic saturation and lattice parameters. As predicted from tests on NiWC alloys, it was found that WCNi hard metals had lower toughness and strength than WCCo hard metals do. Additions of aluminium to nickel binder phases to form γ′ precipitates raised the strength and conferred creep resistance but decreased the fracture toughness. Solid solution strengthening by chromium and molybdenum raised the hardness without reducing toughness as measured by Palmqvist tests and gave properties which matched those of WCCo hard metals. By combining γ′ hardening and solid solution hardening by chromium and molybdenum in nickel-based binder phases, better values of hardness and toughness were obtained than those of WCCo hard metals. The new compositions also offered the possibilities of enhanced resistance to creep and corrosion, and properties that could be varied by heat treatments to meet specific requirements.

Dynamics of fault creep

Abstract: Observations along the surface traces of active faults in California and elsewhere indicate that tectonic displacement can occur as seismic slip or as aseismic fault creep. Fault creep occurs as secular displacement and as displacement episodes, called “creep events,” which last a few hours to days and include from a millimeter or less to a few tens of millimeters of displacement. Instrumental measurements of displacement versus time during creep events show that many events have a characteristic, simple form, including a steep beginning, followed by a gradual decay in the rate of displacement. Some creep events display small amounts of precursory slip, and many seem to be composed of several discrete “simple” events. Propagation of creep events along the San Andreas, Calaveras, and Imperial faults in California has been inferred from the nearly simultaneous observation of creep events at nearby creepmeters and from kinematic modeling of continuous measurements of strain during creep events. Fault creep can be viewed as arising from the interplay of three factors: stress applied to the fault from external sources; stress caused by the geometry and distribution of displacement on the fault, arising from the elastic response of the surrounding medium to the displacements within the anelastic fault zone itself; and the constitutive relations characterizing the resistance to slip on the fault. Analytic solutions are presented for a simple, rectangular dislocation, quasi-static model of nonpropagating fault creep assuming viscous and quasi-plastic (power law creep) fault zone rheology and for a multi-element (composed of several rectangular or striplike dislocation surfaces), quasi-static model assuming a viscous rheology. Viscous rheology implies solutions composed of exponentials that decay with time. The simple, one-element, quasi-plastic model is in agreement with the form of creep events observed at Melendy Ranch, California. A matrix formulation is presented to calculate dislocation stresses on a fault zone in a half-space or plate. The formulation involves the division of the dislocation surface into strips or rectangles and the use of published solutions to calculate the stress resulting from displacement on each individual element. Results from the matrix method agree with analytic results for equilibrium displacements on “stress-free” cracks. The matrix formulation is then used as the basis for a numerical method to simulate one- and two-dimensional propagating creep assuming a quasi-plastic rheology on the fault zone. Results from this method agree reasonably well with observations of afterslip from the 1966 Parkfield, 1975 Oroville, and 1979 Imperial Valley, California, earthquakes and of a propagating creep event observed along the southern Calaveras fault, central California, during July 1977. This method of analysis, and the assumption that creep at each point on the fault zone follows a power law rheology after the applied stress reaches a spatially varying yield stress, can explain the secular and episodic nature of fault creep; afterslip; the simple shape of some individual events and the multiple, composite shape of others; and the propagation of creep events. On the southern Calaveras fault, observations of propagating creep indicate a yield stress near zero at the surface and throughout much of the fault zone. However, they also indicate the presence of a zone at a depth of about 0.5 km, and about 5 km long, with a yield stress of about 15 bars (1.5 MPa). The methods of solution presented here should be applicable to many problems in the dynamics of faulting.

A creep-rupture model for two-phase eutectic solders

Abstract: An analytical framework to predict failure of solders under creep conditions is proposed. A creep-rupture model for two-phase eutectic solders, based on both micromechanics and fracture mechanics, has been developed. The general agreement between the model predictions and reported creep-rupture data in the literature for lead/tin eutectic solder indicates that the mode and mechanisms proposed in the model may control the solder creep-rupture process. >

Kinetics and Mechanisms of High‐Temperature Creep in Silicon Carbide: III, Sintered α‐Silicon Carbide

Abstract: The operative and controlling mechanisms of steady-state creep in sintered α-SiC have been determined both from kinetic data within the ranges of temperature and constant compressive stress of 1670 to 2073 K and 138 to 414 MPa, respectively, and from the results of extensive TEM and other analytical analyses. Dislocations in glide bands, B4C precipitates, and the interaction of these two entities were the dominant microstructural features of the crept material. The stress exponent increased from 1.44 to 1.71 with temperature; it was not a function of stress at a given temperature. The curves of In ɛ vs 1/T showed a change in slope at 1920 ± 20 K. The respective activation energies below and above this temperature interval were 338 to 434 and 802 to 914 kJ/mol. A synthesis of all this information leads to the conclusion that the controlling creep mechanism at low temperatures is grain-boundary sliding accommodated by grain-boundary self-diffusion; at high temperatures, the controlling mechanism becomes grain-boundary sliding accommodated by lattice diffusion. The parallel mechanism of dislocation glide contributes increasingly to the total strain as the number/volume of precipitates declines as a result of progressive coalescence with increasing temperature.

Influence of time and temperature dependent processes on strain controlled low cycle fatigue behavior of alloy 617

Abstract: Strain controlled low cycle fatigue tests have been conducted in air to ascertain the influence of strain rate(e = 4 × 10-6'to 4 × 10-3 s-1) and temperature(T = 750/850/950 °C) on LCF behavior of Alloy 617. A strain range of 0.6 pct and a symmetrical triangular wave form were employed for all the tests. Crack initiation and propagation modes were studied. Microstructural changes that occurred during fatigue deformation were evaluated and compared with the results obtained on isothermal aging. Deformation and damage mechanisms which influence the endurance have been identified. A reduction in fatigue life was observed with decreasing e at 850 °C and with increasing temperature at e = 4 × 10-5 s-1. Cyclic stress response varied as a complex function of temperature and strain rate. Fatigue deformation was found to induce cellular precipitation of carbides at 750 and 850 ‡. Dynamic strain aging characterized by serrated flow was observed at 750 °C (e = 4 × 10-5 s-1) and in the tests at higher e at 850 °C. Strengthening of the matrix due to dynamic strain aging of matrix dislocations by precipitation of M23C6 carbides led to fracture of grain boundary carbide films formed at 750 °C, producing brittle intergranular crack propagation. At 850 °C transgranular crack propagation was observed at the higher strain rates e≥4× 10-4 s-1. At 850 and 950 °C even at strain rates of 4 × 10-5 s-1 or lower, life was not governed by intergranular creep rupture damage mechanisms under the symmetrical, continuous cycling conditions employed. Reduction of endurance at lower strain rates is caused by increased inelastic strain and intergranular crack initiation due to oxidation of surface connected grain boundaries.

Fatigue crack growth resistant nickel-base article and alloy and method for making

Abstract: An article having improved fatigue crack growth resistance is provided through an improved nickel-base superalloy and an improved method which controls grain size and a strain rate found to be critical in processing. The alloy is selected to have a gamma prime content in the range of about 30-46 volume percent and a resistance to cracking upon rapid quenching from a selected supersolvus solutioning temperature to a selected quenching temperature. The article produced has an improved balance and combination of fatigue crack growth resistance and tensile, creep, and stress rupture properties.

Fatigue of solder joints in surface mount devices

Abstract: Lifetime studies of a 16 I/O surface-mounted solder joint array undergoing isothermal cyclic fatigue in torsion shear under fixed plastic strain range show a strong correlation with creep fatigue and a creep-cracking mechanism. Experimental lifetime data follow an inverse dependence on matrix creep. Experimental measurement of the steady-state shear creep rate versus shear stress defines the creep characteristic that is sensitive to changes in metallurgical structure. The amounts of grain boundary and matrix creep taking place during a fatigue cycle are derived from experimental creep data combined with stress-strain hysteresis data obtained in steady-state cycling. Initially, thicker solder joints have a larger grain size than thinner solder joints, giving more matrix creep during fatigue and a faster failure rate. Fatigue increases the mean grain size of the solder joint as determined by the creep-rate-versus-stress characteristic and microstructure. Effects of grain size and joint thickness on lifetime are discussed. A maximum in the creep fatigue rate occurs at 333 K (60°C).

Creep-fatigue interaction of inconel 617 at 950°C in simulated nuclear reactor helium

Abstract: Strain-controlled fatigue tests have been conducted in impure helium, simulating the primary-circuit coolant of a high temperature gas-cooled reactor to ascertain the influence of strain rate ( e = 4 × 10 −3 to 2 × 10 −5 s −1 ), hold condition at peak strains (tension-only, compression-only and tension-plus-compression holds) and hold time (up to 120 min) on the low cycle fatigue behaviour of Inconel 617. A strain range of 0.6% and a temperature of 950°C were employed for all the tests. Microstructural changes which occurred during fatigue deformation were evaluated and damage mechanisms which influence fatigue life identified. A small reduction in fatigue life was found with decreasing e. Irrespective of the position of hold at peak strain in a cycle, the hold time always reduced the fatigue life in comparison with continuously cycled tests at lower strain rates but of equal cycle duration. Tensile holds were found to be most damaging, followed by compression holds. Symmetrical tension-plus-compression holds led to fatigue lives which were very close to those of the continuously cycled tests. In the continuously cycled tests with strain rates down to e = 6.7 × 10 −5 s −1 , failure was always transgranular with no indication of creep damage. However, for tests with tensile holds, creep damage was evidenced by grain boundary cavitation and oxidation, which were responsible for a reduction in the fatigue life. The formation of thick oxide scales at the surface observed at longer hold times led to chromium-depleted surface zones in which the carbide precipitates were dissolved. The loss of carbides increased grain boundary sliding which enhanced the formation of grain boundary cracks. These cracks shortened the critical length of surface fatigue cracks which, during fracture of the specimen, linked with the intergranular creep cracks in tensile hold tests. The damaging effect of compression hold was attributed to increased inelastic strain and deformation ratcheting found in the cycle; failure occurred by local accumulation of tensile plastic strain which finally caused tensile necking.

Crack-enhanced creep in polycrystalline material: strain-rate sensitive strength and deformation of ice

Abstract: A non-linear viscoelastic creep equation for polycrystalline material is presented. It incorporates the effect of cracking and is capable of describing primary, secondary and tertiary behaviour. The model predicts the formation of microcracks and thus the damage state due to the high-temperature grain-boundary embrittlement process. This paper describes its application in formulating crack-enhanced creep and material response under constant strain-rate loading conditions (theoretically the simplest case but actually the most difficult to maintain). The formulation makes it possible to define the rate effect on stress-strain response and the rate sensitivity of strength, failure time, failure strain, damage and damage rate, strain recovery, etc. Numerical correspondence between theory and experiment was observed when predictions were compared with available closed-loop, controlled, constant strain-rate strength and deformation data on pure ice. Calculations made use of material constants determined from independent constant-load creep tests.