The constant stress tensile creep behaviour of a superplastic zirconia-alumina composite

A detailed study was conducted to evaluate the constant stress tensile creep behaviour of a superplastic 3 mol% yttria-stabilized zirconia-20 wt% alumina composite. The comprehensive experimental results indicate that creep deformation may be expressed in the form exp(–585 000/8.3T), where is the steady-state creep rate, is the imposed stress, is the linear intercept grain size andT is the absolute temperature. Microstructural observations revealed that there is very little dislocation activity, or change in grain size or shape. A detailed analysis was conducted to evaluate the possible rate-controlling mechanisms in terms of the experimentally determined mechanical properties and the microstructural observations. Based on the maintenance of an equiaxed microstructure and the strong grain size and stress dependence, it is concluded that creep occurs by a grain-boundary sliding/grain rearrangement process.     

Deformation and failure of ice under constant stress or constant strain-rate

Fine-grained isotropic ice was tested in uniaxial compression at ?5°C. Tests were made under: (1) constant strain rate, and (2) constant stress, with total axial strains up to about 7%. Constant rate tests for the range 10^?7 to 10^?3 s^?1 gave stress/strain curves which exhibited two distinct yield points for rates up to about 10^?4 s^?1. The “initial yield point”, at which internal cracks begin to form at a high rate, occurred at strains in the range 0.03–0.6%, the strain for initial yielding increasing with strain rate. A secondary yield point occurred at axial strains close to 1%. However, above 10^?4 s^?1 the initial yield point became dominant and the secondary yield point disappeared. At the lowest rates (10^?7–10^?6 s^?1), the secondary yield point was distinct, but the initial yield occurred at a stress level equal to, or greater than, that for secondary yield. Constant stress tests for the range 0.8–3.8 MPa gave creep curves which had a minimum strain rate at strains close to 1%. For strains less than 0.2% the resolution and data sampling were inadequate for accurate determination of strain rate as a function of time or strain, but there were fairly clear indications of another strain rate minimum in the range of 0.01%–0.1% axial strain.Direct comparison of the results for constant stress and constant strain rate suggests that the two tests give much the same information when interpreted suitably. Detailed comparisons and interpretations of the data will be given in a subsequent paper.     

Low-stress creep behaviour of superplastic Zn-22% Al alloy

Low-stress creep behaviour of microduplex Zn-22% Al alloy was studied using spring specimen geometry. The average phase size in the specimens investigated was 0.87, 1.48 and 1.98 m. Experiments were conducted in the temperature range 393–473 K at stresses below about 1.0 MN m^–2. The present study has established that the stress exponent of the creep rate is unity and, therefore, a viscous creep process dominates the flow in Region I superplasticity. The activation energy corresponds to that for boundary diffusion. However, the phase-size exponent was found to be –2 instead of –3, as predicted by the Coble creep theory. Further, the measured creep rates are three to four orders of magnitude slower than those predicted by the Coble theory. Transmission electron microscopy revealed precipitation, along / grain interfaces, whose inhibiting action on plastic flow should at least be partly responsible for the lower values of measured creep rates. There also exist two other interfaces, namely / and /, whose comprehensive role in diffusion creep is not yet fully understood. Therefore, it seems illogical to describe the creep behaviour of Zn-22% Al by the classical Coble theory, originally developed for single-phase polycrystals.     

Thermodynamic model of creep at constant stress and constant strain rate

A thermodynamic model has been developed that describes the entire creep process, including primary, secondary, and tertiary creep, and failure for both constant stress (CS) tests ($\text{σ =}\text{const.}$) and constant strain rate (CSR) tests ($\text{? =}\text{const.}$), in the form of a unified constitutive equation and unified failure criteria. Deformation and failure are considered as a single thermoactivated process in which the dominant role belongs to the change of entropy. Failure occurs when the entropy change is zero. At that moment, the strain rates in CS tests reach the minima and stress in CSR tests reaches the maximum (peak) values. Families of creep (? vs t) and stress-strain (σ vs ?) curves, obtained from uniaxial compression CS and CSR tests of frozen soil, respectively (both presented in dimensionless coordinates), are plotted as straight lines and are superposed, confirming the unity of the deformation and failure process and the validity of the model. A method is developed for determining the parameters of the model, so that creep deformation and the stress-strain relationship of ductile materials such as soils can be predicted based upon information obtained from either type of test.     

Evaluation of the true activation enthalpy of superplastic flow including a threshold stress

A new method is suggested for the evaluation of the true activation enthalpy for alloys where the strain rate of the superplastic flow varies with a power of an effective stress _e = -_o, where and _o are the applied stress and a threshold stress, respectively. Some earlier results concerning superplastic AlMgZnCu alloys containing chromium and in which a strongly temperature-dependent threshold stress can be revealed, are reanalysed. The results are in good agreement with the previous ones. It has been shown further that for the alloys investigated the true activation energy increases with increasing chromium content.     

On the tensile creep behaviour of a directionally-solidified Ni3Al-based alloy

High temperature tensile creep behaviour of a directionally-solidified Ni₃Al-based alloy is presented. The study involved selection of nine alloy systems based on Ni₃Al. The alloys contained varying amounts of Cr and Ta, fixed amounts of 1·5 at.% Hf and 0·5 at.% Zr and doped with 0·2 at.% each of C and B. The alloys were vacuum arc-melted into buttons and homogenized at 1050°C for 68 h. The test pieces of the alloys were hot compression tested at 600, 700, 800 and 900°C. The yield strength data of some of the alloys were superior to conventionally cast Mar-M 200, a cast nickel-base superalloy widely used in gas turbine structural applications. The best alloy system was chosen based on consistent performance in the hot compression studies. The alloy so chosen was directionally solidified and vacuum-homogenization-treated for 20 h at various selected temperatures. Optimum creep properties were observed at 1120°C, 20 h treatment. The minimum creep rate data of the DS alloy showed relatively higher values even at lower temperatures and stress levels as compared to Mar-M 200. Hence, the alloy is less promising in replacing nickel-based superalloys used as structural materials in gas turbine applications.     

Equipment for creep testing under tensile stress and hydrostatic pressure

The design and operation of equipment is described that permits creep tests to be made under independently controlled tensile stresses and superimposed hydrostatic pressures at elevated temperatures. The range of usefulness and applications of the equipment are briefly indicated.     

Determination of tensile and compressive creep behaviour of ceramic materials from bend tests

We have developed equations that permit the determination of individual compressive and tensile creep rates from four-point bend test measurements made on trapezoidal bars. The equations developed are for the general stress dependence case. Finnie first proposed this type of analysis and solved the equations for the case of linear stress dependence. We present graphs that show solutions to the equations for the stress squared case. Creep measurements were performed on trapezoidal HS-130 Si₃ N ₄ bars to determine the ratio of compressive to tensile creep (). The values determined on four separate tests using two different temperatures and beam dimension ratios b ₂/b ₁ gave fairly consistent results. The s for previously unstrained material are: 0.24, 0.14, 0.17, and 0.17 with the average value of =0.18. For the case of prestrained Si₃N₄, is 0.48. To further test the usefulness of this analysis, data from a conventional (rectangular bars) four-point bend test were taken from the literature and analysed using the equations developed in this paper and the average determined value of . The individual tensile and compressive creep rates determined in this manner were found to agree very closely with other literature data measured in direct tension and compression tests.The experimental work was performed at the Metallurgy and Ceramics Laboratory, Aerospace Research Laboratories (AFSC), Wright-Patterson AFB, Ohio, USA.     

Density measurements after tensile and creep tests on pure and slightly oxidised aluminium

In order to investigate the influence of dispersed oxide particles on crack-formation in Al-Al₂O₃ alloys, series of tensile tests (20 to 550† C) and creep tests (450† C) were carried out on pure Al and on Al-0.7wt% Al₂O₃ specimens. Density measurements performed on broken samples showed no changes in pure Al but considerable decreases in the slightly oxidised Al. The observed influence of oxide is strong for the small percentage if compared with the behaviour of Al-Al₂O₃ alloys with much higher oxide contents.     

Characterization of a superplastic aluminium alloy ALNOVI-U through free inflation tests and inverse analysis

Numerical methods are widespread in forming applications since a deeper understanding and a finer calibration of the process can be reached without most of the assumptions used in analytical approaches. In this calibration procedure the characterization of the material behaviour is an important preliminary step that cannot be avoided. Experimental tests can be numerically modelled and material constants can be found by inverse methods making numerical results as close as possible to experimental ones. In this work material parameters of a superplastic aluminium alloy have been found by experimental forming tests and an inverse analysis. Constant pressure free inflation tests were firstly performed to find the optimal range for temperature and strain rate values. Material constants were then calculated, on the basis of these tests, minimizing errors between experimental and numerical data through a gradient based optimization iterative procedure. Constant strain rate experimental tests were finally used to refine material parameters and to gain a better agreement between experiments and numerical simulations.     

Room-temperature forming limit diagram and tensile behaviour up to 200° C of an AI-Ca-Zn superplastic alloy

The forming limit diagram and strain distribution under punch stretching at room temperature of an AI-Ca-Zn (superplastic) alloy have been evaluated. Tensile behaviour up to 200° C is reported. The fracture surfaces have been examined by scanning electron microscopy and the results are analysed to support the failure criterion proposed earlier by Marciniaket al.     

Predicting the tensile creep of concrete

Over a four year period, six phases of testing were performed to observe the influence of age at loading, applied stress level, mix composition and relative humidity on the tensile creep of concrete. From these investigations it was possible to develop a model which allowed the prediction of tensile creep based on a knowledge of the compressive strength of the concrete (determined at the age of loading), the applied stress level and the relative humidity. Subsequently, this model was validated using the results from three independent investigations. Compressive creep as well as tensile creep was also obtained. This allowed a comparison of compressive creep with tensile creep and illustrated that on the basis of equal stresses, tensile creep is on average between 2 and 3 times greater than compressive creep (the maximum ratio is in excess of 8). For this investigation, however, on the basis of stress/strength ratio the difference between tensile and compressive creep is less significant. Considering a simply supported flexural reinforced concrete element, the investigation suggests that it is unwise to consider actual compressive creep equal to actual tensile creep as is often the case in design practice.