Simple Creep Tests Research Articles

Transient creep behavior and dislocation cell structure development during creep-fatigue deformation of fully annealed Cu–Cr–Zr alloy

Creep-fatigue tests of Cu–0.7Cr–0.09Zr (mass%) alloy, which include stress-holding-type creep and strain-controlled fatigue, were conducted at elevated temperatures. A simple creep test was also carried out in order to compare the results with the results of the creep-fatigue test. The creep strains accumulated during creep deformation in creep-fatigue and simple creep tests were 310% and 12%, respectively. Such a large difference was due to the stacking of normal/inverse-transient creep in the creep-fatigue test. A dislocation cell structure smoothly developed in the simple creep test, whereas the cell intermittently developed in the creep-fatigue test because compressive plastic deformation immediately after creep partly broke apart the cell walls and prevented the cell structure from smoothly developing. Compressive stresses necessary for −1.5% strain were changed as cycling progressed in the creep-fatigue test. A much higher compressive stress would introduce a much higher dislocation density immediately before creep deformation. At the start of creep, these dislocations recovered and their number gradually decreased, which resulted in the appearance of inverse-transient creep.

Damage propagation mechanism in low-cycle creep fatigue of Cu–Cr–Zr alloy

This paper describes a characteristic damage propagation mechanism in low-cycle creep–fatigue of Cu–0.7Cr–0.09Zr (mass%), as investigated by creep–fatigue tests including strain controlled fatigue and stress-holding type creep, and following microstructural observations by scanning electron microscopy (SEM). The total stress-holding time until rupture in the creep–fatigue test was shorter than one-tenth of the rupture life in the simple creep test, and the rupture life of the specimen in the creep–fatigue test was shorter than half of that in the simple fatigue test. The SEM images suggest that the connection between fatigue crack propagating along grain boundaries and intergranular creep voids rapidly accelerates crack propagation.

Petrophysical and Capillary Properties of Compacted Salt

This paper reports experimental results that demonstrate petrophysical and capillary characteristics of compacted salt. The measured data include porosity, gas permeability, pore size distribution, specific surface area, and gas-brine breakthrough and capillary pressure. Salt samples employed in the experiments were prepared by compacting sodium chloride granulates at high stresses for several hours. They represent an intermediate consolidation stage of crushed salt under in-situ conditions. The porosity and permeability of compacted salt showed similar trends to those expected in backfilled regions of waste repositories excavated in salt rock. The correlation between the measured porosity and permeability seems to be independent of the compaction parameters for the range examined in this study. The correlation also shows a different behaviour from that of rock salt. The data of all petrophysical properties show that the pore structure of compacted salt can be better characterized by fracture permeability models rather than capillary bundle ones. Simple creep tests, conducted on the fully-brine-saturated compacted salt samples, yielded similar strain rates to those obtained by a steady-state mechanical model developed from the tests on fully brine-saturated granular salt. A modified procedure is proposed for the evaluation of restored-state capillary pressure data influenced by the material creep. The characteristic parameters for the capillary behaviour of compacted salt are determined by matching the Brooks-Corey and van Genuchten models with the measured data. The Leverett functions determined with different methods agree well.

A study of nonlinear viscoelasticity of an unsaturated polyester resin. Part 1. Uniaxial model

A nonlinear viscoelastic theory is proposed to model the creep behaviour of an unsaturated polyester resin. The theory is based on Schapery's single-integral model, modified by an exponential kernel function. Further Struik's theory of physical ageing has been included to describe long-term behaviour. A series of tests has been carried out in order to characterize the model parameters and verify the established theory. The model which contains only two nonlinearizing parameters accurately describes the creep-recovery behaviour of the resin over a tensile stress range up to 40 MPa. The model has been verified for various cyclic loading conditions. Under these loading conditions, the strains predicted with the model correspond within an error band of 1% with the measured strains. Although the model parameters are obtained by simple creep tests, the successful prediction of the cyclic loading cases and long term creep deformation demonstrates the wider applicability of the model.

A simple endochronic transient creep model of metals with applications to variable temperature creep

Similar to the theory of endochronic plasticity, modified by Valanis in 1980, a simple endochronic transient creep model of metals is proposed by using a definition of intrinsic time ζ, measured within the creep strain tensor space, whose metric tensor is treated as a simple power form of creep strain-rate sensitive material function. The resulting constitutive equation of creep (Endocreep) contains only three material constants whose values can be determined completely by a simple creep test. An incremental form involving isothermally constant creep stress, with or without jump, and constant stress with temperature jump, are then formulated. In the applications of Endocreep on 304SS under variable temperature creep, data of simple creep tests, provided by Ohashi et al. at 650°C, Ohno et al. at 600°C, Findley and Cho at 593°C–649°C, are employed to determine material constants. The computational results in the simulation of creep tests under step-up and step-down temperature with constant axial stress are found in very good agreement with data provided by Findley and Cho. However, the results reveal that the model is too simple to deal with the recovery response of unloading. Beside this deficiency the model and its computational method proposed have a potential in the future FEM creep analysis of general thermomechanical loading history.

High temperature creep behaviour of 12Cr-20Mn-W austenitic stainless steel

Abstract Results of a simple creep test of three FE-12Cr-20Mn-1W alloys are presented, performed at temperatures from 873 to 973 K and at applied stresses corresponding to dislocation creep conditions. The influence of the carbon content on creep behaviour is established. The creep behavior of the steels is also comparedwith that of some standdard Fe-Cr-ni and Fe-Cr-Mn-Ni austenitic stainless steels.

Asphalt Concrete Creep as Related to Rutting

Rutting is one of the major modes of distress in flexible pavements. It is caused by plastic deformation that can occur in any of pavement layers. Historically, rutting of asphalt concrete has been considered a mix design and/or construction problem. Furthermore, current methods of mix design are independent from pavement thickness design procedures. The focus of this paper is on permanent deformation of asphalt concrete layer. The primary objective is to develop a methodology for characterization of rutting potential of asphalt mixtures that can be easily implementable at state highway agencies. Permanent deformation characterization procedures are to be based upon parameters that can be integrated into mechanistic pavement design procedures. A mechanistic methodology is developed for permanent deformation characterization of asphalt paving materials. Rheological properties of hot mix asphalt (HMA) are thoroughly studied by means of a simple creep test that is originally developed by Shell researchers. Procedures are developed in order to account for nonlinear, viscoelastic, and viscoplastic deformation components of HMA. Results indicate that structural arrangement of pavement layers has a significant influence on rutting performance of the HMA layer. Findings of this study can be easily implemented in an integrated mix-thickness design procedure.

Constitutive relationships for anisotropic high-temperature alloys

A constitutive theory is presented for representing the anisotropic viscoplastic behavior of high-temperature alloys that possess directional properties resulting from controlled grain growth or solidification. The theory is an extension of a viscoplastic model that has been applied in structural analyses involving isotropic metals. Anisotropy is introduced through the definition of a vector field that identifies a preferential (solidification) direction at each material point. Following the development of a full multiaxial theory, application is made to homogeneously stressed elements in pure shear and to a uniaxially stressed rectangular block in plane stress with the stress direction oriented at an arbitrary angle with the material direction. It is shown that an additional material parameter introduced to characterize the degree of anisotropy can be determined on the basis of simple creep tests.

Creep and relaxation resistance of electrical composition board in oil at 80�C

1. Full information regarding the creep of electrical composition board may be derived from experiments carried out at only two stress levels, using an empirical equation. 2. For certain values of time and stress σ depends linearly on σ0. Hence in order to describe stress relaxation in electrical composition board it is sufficient to carry out experiments at only one initial stress σ0. 3. The use of yield theory enables us to obtain reasonably accurate stress-relaxation curves on the basis of simple creep tests.

Tensile Strength of Plastics above the Glass Temperature

A theory is presented for the tensile strength of rubbery plastics under dynamic test conditions. It is found that a nearly universal curve may be plotted relating the tensile strength of rubbers to the time of application of the load in a simple creep test or to the shear rate in a conventional type tensile test. The dependence on chain length, degree of cross linking, and temperature is also predicted. Experimental results for the tensile strength of poly butyl methacrylate in the range 30–95°C are presented. These are shown to agree with the theory within the experimental error. The experimental testing scale extends over a range of eight decades when use is made of a temperature—time superposition principle predicted by the theory. By comparing the tensile creep and tensile strength curves for poly butyl methacrylate, it is possible to find the size of a segment as defined in the theory of viscoelasticity. One segment is found to contain about six monomer units.