Showing papers on "Creep published in 1981"

Thermal Stress and Low Cycle Fatigue

Abstract: Text on thermal stress and low-cycle fatigue covering elastic stresses, plastic flow and creep, stress cycling, etc

A Continuum Theory of Creep and Creep Damage

Abstract: It is shown that the material damage in creep can be represented by a second rank symmetric tensor. The constitutive equations of creep and creep damage are formulated by employing the damage tensor as an internal state-variable. The difference between the effects of the material damage on creep and damage growth is incorporated. Anisotropic damage law, Prager-Drucker type flow law and strain-hardening hypothesis are assumed to specify the resulting constitutive equations. The numerical results under constant and non-proportional loadings are compared with those of the corresponding experiments on copper at 250°C.

Failure of materials in mechanical design : analysis, prediction, prevention

Abstract: The Role of Failure Prevention Analysis in Mechanical Design. Modes of Mechanical Failure. Strength and Deformation of Engineering Metals. State of Stress. Relationships Between Stress and Strain. Combined Stress Theories of Failure and Their Use in Design. High-Cycle Fatigue. Concepts of Cumulative Damage, Life Prediction, and Fracture Control. Use of Statistics in Fatigue Analysis. Fatigue Testing Procedures and Statistical Interpretations of Data. Low-Cycle Fatigue. Stress Concentration. Creep, Stress Rupture, and Fatigue. Fretting, Fretting Fatigue, and Fretting Wear. Shock and Impact. Buckling and Instability. Wear, Corrosion, and Other Important Failure Modes. Index.

The asymptotic stress and strain field near the tip of a growing crack under creep conditions

Abstract: The asymptotic stress and strain fields near the tip of a slowly growing crack are derived for elastic-nonlinear viscous materials, which deform in tension according to the law % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGafuyTduMbai% aacqGH9aqpcuqHdpWCgaGaaiaac+cacaWGfbGaey4kaSIaamOqaiab% fo8aZnaaCaaaleqabaGaamOBaaaaaaa!428D!\[\dot \varepsilon = \dot \sigma /E + B\sigma ^n \]. The nonlinear viscous term describes power law creep. Based on (small strain) continuum mechanics, a stress analysis is carried out for anti-plane shear (Mode III), plane stress and plane strain (Mode I).

A model of dislocation-controlled rheology for the mantle

Abstract: The dislocation microstructure of mantle materials can account simultaneously for long-term steady-state creep, and for stress wave attenuation at seismic frequencies. The hypothesis that a single microstructural model explains the rheology for characteristic times ranging from 1 to 10^(10) seconds can be used to restrict the class of permissible rheological models for the mantle. We review steady-state dislocation damping models in order of increasing complexity, and reject those which do not satisfy laboratory data or geophysical constraints. This elimination procedure leads us to consider an organized microstructure, in which most dislocations are found inside subgrain walls. The cells contain relatively few dislocation links. These are free to bow under small, i.e. seismic, stresses. The time constant of this mechanism is controlled either by the diffusion of kinks or of point defects bound to the dislocation line. The glide of intragrain dislocations explains the magnitude and frequency range of seismic attenuation. Steady-state creep is governed by recovery through climb and annihilation in cell walls. Under conditions of jog undersaturation, climb is controlled by jog formation in addition to self-diffusion, and the model requires a higher creep activation energy than for self-diffusion, in agreement with observations on olivine. Quantitative agreement with laboratory data is achieved if the density of cell-wall dislocations is one to two orders of magnitude higher than the density of intracell dislocations. Self-diffusion is probably controlled by silicon diffusion at low pressure and by oxygen diffusion at high pressure. The long-term tectonic stress is the dominant factor determining scale lengths; as a result, the total strength of the relaxation associated with bowing of intracell dislocation links is fixed by the geometry and is of the order of 10%. This limits the width of the seismic absorption band to 2-3 decades in frequency for each mantle mineral. The actual position of the seismic absorption band is determined primarily as a result of a trade-off between temperature, pressure and tectonic stress. This model provides a physical framework within which the quality factor Q and viscosity are related via the dislocation microstructure.

The glass transition in basalt.

Abstract: The glass transition has been experimentally detected in basalt as (1) an increase in the aggregate linear thermal expansion coefficient αL, (2) an abrupt change in the temperature dependence of Young's modulus dE/dT, and (3) a change in stress relaxation behavior that effectively separates the T> TG and T TG. Collectively, the mechanical results suggest that for Hawaiian olivine tholeiite at 1-atm pressure, the principal material responses are (1) elastic (T ≤ 600°C), (2) reduced creep (600 980°C). Fracture surface morphologies developed during solidification suggest that the presence of the supercooled melt grain boundary phase may participate in the regulation of the thermal cracking process. Well-preserved fracture surfaces formed by incremental crack growth are found to be covered with striations that correspond to the inferred sequential stopping positions of the advancing fracture front. These striae may be rationalized in terms of experimental analogues that have been produced in viscoelastic polymers by cyclic tension-tension loading. The fracture surface morphology, mechanical data, and the controlled crack growth analogues suggest that thermal fracture in solidifying basalt is an incremental and cyclic process, involving three steps: (1) the accumulation of elastic strain energy in cooling rock at temperatures below that required for stress relaxation due to viscous flow in the intercrystalline liquid phase, (2) fracture at a ΔT determined primarily by the aggregate thermal expansion coefficient αυ and Young's modulus E, (3) the penetration by the advancing crack tip, of the thermal horizon capable of relaxing stress due to the creep of intercrystalline supercooled melt, producing the rough surface texture associated with the termination of a striation. Further crack growth must now await the migration of the solidus. The cycle then repeats. Striations measured in deep Hawaiian lava lakes have been compared with the crack advance increments expected in the vicinity of the glass transition, based on two tests: (1) thermal gradients measured in Kilauea Iki, combined with the mechanical properties of olivine tholeiite evaluated near TG, and (2) the crack advance required to match the recorded seismic stopping phases for prexisting cracks of the dimensions expected for Kilauea Iki. The observed versus predicted comparisons are (1) 31 versus 36 cm; and (2) 31 versus 30 cm. We envision this incremental crack growth process as contributing to the control on the downward movement of the thermal cracking front—and its associated hydrothermal circulation zone—in the upper portions of solidifying subaerial and submarine ponded basalt.

Influence of strain rate on dilatancy and strength of Oshima granite under uniaxial compression

Abstract: Uniaxial compression tests have been conducted on Oshima granite under various constant axial strain rates ranging from 10−8 to 10−4. The results showed that the strength and the acoustic emission rate increased exponentially with increasing strain rate. The inelastic volumetric strain rate defined by the differentiation with respect to the stress increased with decreasing strain rate. The redistribution of microcracks due to subcritical crack growth was considered theoretically, and the equations derived from the theory were compared with the experimental results. The agreement between the theoretical and experimental results shows that stress corrosion plays not only a major role in the brittle creep under constant load but also dominates the strain rate effects on strength and dilatancy observed in the constant strain rate loadings.

Creep deformation at crack tips in elastic-viscoplastic solids

Abstract: The evaluation of crack growth tests under creep conditions must be based on the stress analysis of a cracked body taking into account elastic, plastic and creep deformation. In addition to the well-known analysis of a cracked body creeping in secondary (steady-state) creep, the stress field at the tip of a stationary crack is calculated for primary (strain-hardening) or tertiary (strain-softening) creep of the whole specimen. For the special hardening creep-law considered, a path-independent integral C ∗ h , can be defined which correlates the near-tip field to the applied load. It is also shown how, after sudden load application, creep strains develop in the initially elastic or, for a higher load level, plastic body. Characteristic times are derived to distinguish between short times when the creep-zones, in which creep strains are concentrated, are still small, and long times when the whole specimen creeps extensively in primary and finally in secondary and tertiary creep. Comparing the creep-zone sizes with the specimen dimensions or comparing the characteristic times with the test duration, one can decide which deformation mechanism prevails in the bulk of the specimen and which load parameter enters into the near-tip stress field and determines crack growth behavior. The governing load parameter is the stress intensity factor K 1 if the bulk of the specimen is predominantly elastic and it is the J -integral in a fully-plastic situation when large creep strains are still confined to a small zone. The C ∗ h -integral applies if the bulk of the specimen deforms in primary or tertiary creep, and C ∗ is the relevant load parameter for predominantly secondary creep of the whole specimen.

Creep and fabrics of polycrystalline ice under shear and compression

Abstract: Creep tests were performed in torsion and torsion–compression on polycrystalline ice at temperatures near the melting point Syntectonic recrystallization occurs at strains of the order of 2–3%, leading to a rapid increase in strain-rate It is shown that the increase of creep-rate during tertiary creep arises from the development of fabrics favouring the glide on basal planes but also from the softening processes associated with recrystallization The c-axis fabric of recrystallized ice developed in simple shear consists of two-maxima, one at the pole of the permanent shear plane and the other between the normal of the second plane of maximum shearing stress and the principal direction of compression In torsion–compression, a three- or four-maximum fabric is formed according to the intensity of different components of the stress tensor The maxima are clustered around the principal direction of compression Processes of fabric formation are discussed The experimentally developed fabrics are probably produced by the strain-induced recrystallization, for which the driving force is provided by differences in stored plastic strain energy However the degree of preferred orientation of ice c-axes must be a function of the total strain when syntectonic recrystallization becomes less important In this case, fabrics are principally formed by plastic flow and a steady state is obtained for very high strains

High‐temperature creep of forsterite single crystals

Abstract: Creep of forsterite single crystals has been studied with respect to the orientation of the differential stress. Three orientations have been investigated: [110]c, [101]c, and [011]c. Specimens were deformed at high temperature (T ≥ 1400°C) and moderate stresses (15 < σ < 110 MPa) in a dead load creep apparatus at room pressure and under controlled atmosphere. Assuming that the creep law has the general form = Aσn exp −(Q/RT), two different tests were performed. Stress step and temperature step experiments gave the stress exponent n and the activation energy Q, respectively. A thorough analysis of the dislocation microstructures was carried out in the potential glide planes using the optical microscope (decorated thin section) and transmission electron microscope. Comparison of both mechanical and microstructural data allowed the determination of the different flow laws for the three orientations studied. Forsterite and olivine data are consistent; both flow laws and dislocation microstructures are similar. In light of these results, extrapolation of flow laws to the stress conditions of the mantle is not straightforward. Very high activation energies at low stresses (σ < 15 MPa) may suggest a change of flow mechanisms. Accordingly, further investigations in the low-stress domain are required.

Creep mechanism in Mg2GeO4: Effects of a phase transition

Abstract: We deformed polycrystalline Mg2GeO4 olivine and spinel in a Griggs-type solid medium apparatus. Flow of Mg2GeO4 olivine can be represented by = 6.5×l07 σ3.5 exp(−105(kcal/mol)/ RT) where σ (kbar) is the differential stress and (s−1) is the natural strain rate. For Mg2GeO4 spinel the flow law is = 2.6×104 σ2exp(−73(kcal/ mol)/RT). The low stress exponent and activation enthalpy coupled with fine grain size (3 μm) suggest that Mg2GeO4 spinel deformed by a superplastic mechanism. Flow parameters for the olivine phase suggest a dislocation creep mechanism. Comparison of theoretical superplastic flow laws for Mg2GeO4 olivine with the spinel phase data suggests that strain rates in Mg2GeO4 spinel are only about a factor of 3 lower than for Mg2GeO4 olivine of the same grain size. A similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion. Transformation from Mg2GeO4 olivine to spinel reduced grain size to approximately 3 μm. Thus we might expect a similar reduction in grain size in the earth's transition zone which could result in superplastic deformation of the transformed phase and cause a weak ‘decoupling’ zone at the transition boundary in the mantle. Superplasticity brought about by transformation-induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle.

Salt-constitutive modeling using mechanism maps

Abstract: Numerous constitutive models have been proposed for the low temperature creep of salt This work was the first to develop such a model within the framework of a deformation-mechanism map Use of this framework permits unfolding of the rather complicated low temperature steady-state creep behavior into three simple responses involving separate regimes with individual controlling mechanisms The constitutive model developed in this report incorporates primary (transient) creep as a two parameter modification to the steady-state creep equations The constitutive model presented can be applied to deep mines and, more importantly, to design of the petroleum storage and radioactive waste isolation facilities