Transient Creep Strain Research Articles

Transient creep in iron at low stresses and homologous temperatures 0.40–0.54

Transient creep of high purity iron was investigated at low stresses and homologous temperatures 0.40–0.54 using the technique of helicoid specimens. At the intercept grain sizes less than ~ 123 μm, Coble creep operates and the transient creep strain ϵ i increases linearly with applied stress and systematically with temperature. The duration of the transient creep period t i is independent of applied stress and increases exponentially with decreasing temperature. The transient creep strain is caused primarily by bowing out of links of the three-dimensional dislocation network accompanied by releasing and moving of some of these links. The relative importance of the latter of these processes increases with temperature. The climb of the links is most probably controlled by dislocation core diffusion. At the intercept grain sizes greater than ~123 μm, Harper-Dorn creep operates. In this region, the transient creep strain ϵ i increases linearly with applied stress. The influence of temperature on t i and also the transition strain ϵ i , is eventually dominated by the effects of mechanical and thermal treatment before creep exposure. Identical processes are suggested to be responsible for transient creep in Regions 1 and 2.

Primary and recoverable creep in [formula omitted] stainless steel

2025 Niobium stabilised stainless steel las crept in tension at temperatures and stresses in the range 600–800°C and 7–62 MPa respectively. The tests lere confined to small strains of the order of several elastic deflections. The initial transient creep strain rates on applying a stress lere found to be identical in form to the reverse strain rates obtained on removing the stress. The kinetics of the anelastic strain and of the ‘steady’ creep rates under stress can be described by a dislocation link model. Here the transient strain rates result from the boling by climb of dislocations. The ‘steady’ creep results from the climb of dislocations escaping from pinning points.

Transient stage in low-stress viscous creep of titanium and cobalt

Transient creep data recorded during low-stress viscous creep of α-Ti and β-Co have been presented in terms of four important variables, namely (i) prior specimen treatment, (ii) specimen grain size, (iii) test temperature, and (iv) temperature cycling during test. An analysis of the observations provides further experimental evidence for two of the existing notions, namely that dislocation activity and thermal anisotropy are contributory causes of transient creep strains.

Transient creep and semibrittle behavior of crystalline rocks

We review transient creep and semibrittle behavior of crystalline solids. The results are expected to be pertinent to crystalline rocks undergoing deformation in the depth range 5 to 20 km, corresponding to depths of focus of many major earthquakes. Transient creep data for crystalline rocks at elevated temperatures are analyzed but are poorly understood because of lack of information on the deformation processes which, at low to moderate pressure, are likely to be semibrittle in nature. Activation energies for transient creep at high effective confining pressure are much higher than those found for atmospheric pressure tests in which thermally-activated microfracturing probably dominates the creep rate. Empirical transient creep equations are extrapolated at 200° to 600°C, stresses from 0.1 to 1.0 kbar, to times ranging from 3.17 × 102 to 3.17 × 108 years. At the higher temperatures, appreciable transient creep strains may take place but the physical significance of the results is in question because the flow mechanisms have not been determined. The purpose of this paper is to stimulate careful research on this important topic.

Effects of Temperature and Stress on Transient Creep Behavior of Alpha Iron

The transient creep characteristics of polycrystalline alpha iron were investigated mainly at temperatures ranging from 500 to 650°C using the constant stress creep equipment with a servo divider and the rapid heating method developed newly for determining the activation energy for transient creep. The results obtained are summarized as follows: (1) The transient creep strain exhibits a normal time dependence and inverted transient creep has not been observed. The stress dependence of the initial creep rate, Vi, which is the rate just after applying a load is larger than that of the steady-state creep rate, Vs, and the value of Vi⁄Vs, increases as the stress increases within the range up to about twice the 0.01% offset stress. (2) Of some proposed equations for creep time relation, an equation based on the assumption given by Akulov satisfies the experimental results fairly well. The calculated values of the creep rate approximate the experimental ones at the creep strain more than 1%. (3) The activation energy for transient creep depends on the creep strain. The dependence can be divided into three regions at 700°C as reported previously. However, the first region is not observed at 800°C, and the values of the activation energy obtained at the creep strain of nearly zero are smaller than that of self-diffusion on the contrary to the results at 700°C.

The effect of crystal orientation on the creep behaviour of magnesium oxide

Abstract The activation energy for creep and the strain/time behaviour following small stress changes during creep have been found to be similar for MgO single crystals with ⟨100⟩ and ⟨111⟩ stress axes, tested in compression at 1596 K. For both orientations, it is proposed that the rate-determining process is the diffusion-controlled growth of the three-dimensional dislocation network to generate links which are sufficiently long to act as dislocation sources, allowing slip to occur. The instantaneous strain on loading, the transient creep strain and the steady-state creep rate are markedly lower for the ⟨111⟩ crystals than for the ⟨100⟩ crystals under comparable conditions. These observations have been interpreted in terms of the stress required to initiate slip and the coefficient of strain hardening being significantly higher for the ⟨111⟩ crystals, which deform plastically on {001}⟨110⟩ slip systems, than for the ⟨100⟩ crystals, for which the {110} ⟨110⟩ slip systems operate.

Transient and steady state creep behaviour of polycrystalline MgO

Stress change experiments during compressive creep tests at high stresses on polycrystalline MgO at 1596 K have shown that the creep rate at any instant during transient and steady state creep is predicted by the ratio,r/h, wherer is the rate of recovery (=−∂σ/t6t) andh is the coefficient of strain hardening (=∂σ/∂e). Over most of transient and steady state creep, whenh is constant and the decrease in creep rate,\(\dot \in\), is a direct result of a decrease inr, the variation of the creep strain,e, with time,t, is accurately described as $$ \in = \in _0 + \in _T (1 - e^{ - mt} ) + \dot \in _s t$$ wheree0 is the instantaneous strain on loading,eT the transient creep strain,m relates to the rate of exhaustion of transient creep and\(\dot \in _s\) is the steady creep rate. Deviations from this equation occur during the initial 10 to 15% of the transient creep life, whenh increases rapidly.

Materials Creep Behavior and Elevated Temperature Design

New elevated temperature design criteria are based on time-dependent material deformation behavior. These criteria establish time-dependent allowable stresses which are evaluated from a consideration of three quantities: 1. the stress to produce rupture2. the stress to produce the onset of tertiary creep3. the stress to produce 1% total strain.These quantities must be determined over a time range from 10 to 200 000 h for temperatures from 800 to 1300°F. Allowable stress values for Types 304 and 316 stainless steel are derived from an analysis of experimental creep data. Stress quantities associated with rupture and tertiary creep are largely determined from existing data and analysis methods, while new data and analysis procedures are employed to evaluate the stress to produce 1% strain. An analytical formulation describing the transient and steady-state creep strain response observed in constant load, constant temperature tests is developed for use in structural analysis. This formulation is used with an equation-of-state approach based on strain hardening for application to variable load or temperature conditions.

Inelastic deformation of an aluminum alloy under combined stress at elevated temperature

Biaxial stress tests were performed on thin-wall tubes of polycrystalline 2024-T81 aluminum at temperatures of 150°C and 250°C. The nominal metallurgical stabilization temperature for this alloy is 190°C. Transient and steady-state creep strain rates exhibited a considerable dependence on load path history. For a prescribed history it is possible to determine unique surfaces of constant creep strain rate. For the zero history, involving a single loading from the origin to a prescribed point in stress space, surfaces of constant steady-state strain rate, at elevated temperature, have the same shape as room temperature yield surfaces of moderate offset. In the temperature and small strain regions considered here, room-temperature yield surfaces were found to be unaffected by elevated temperature deformation. The changes in shape of room-temperature yield surfaces, due to room-temperature plastic deformation caused corresponding changes in the elevated temperature surfaces of constant steady-state creep rate. At a given stress point, an outward local motion of the yield surface resulted in a corresponding outward local motion of the steady-state creep rate surfaces. The experimental determination of surfaces of constant flow potential was also attempted.

Relaxation of Compression Springs at High Temperatures

An analysis is developed to determine the relaxation of cylindrical compression springs at temperatures where creep is predominantly steady state and the effect of transient creep and anelastic strain is small. Springs which operate under these conditions must be designed for limited life. Equations are derived which predict the relaxation of springs directly from tensile creep data for various materials. Using creep data for 18-8 stainless steel and Inconel-X, families of design curves are presented which give the time-temperature initial-stress relationships for various stress-relaxation ratios. These curves are useful in selecting an initial design stress for a specific operating temperature.

Creep behavior of molybdenum single crystals

The creep behavior of molybdenum single crystals oriented with a [011] direction parallel to the tensile axis was investigated over the temperature range 1200° to 1917°C (0.5 to 0.76 T m). The activation energy for creep at applied stresses of 900 psi and 2050 psi was 108 and 105 kcal/mol, respectively. No steady state creep was observed, although the creep rate was approaching a constant value at 0.45 creep strain. The transient creep strain was fitted to a relation ϵ = ϵ 0 + bt m , yielding b ∝ σ 3.8 at 1410°C, and 0.37 < m < 0.71, with m increasing with increasing temperature. The analysis of slip bands and Laue back reflection patterns indicated that {112}〈111〉 was the primary slip mode for [011] crystals crept at these temperatures. The slip distribution was very non-uniform, being concentrated into regularly spaced, narrow bands. The slip band spacing decreased with increasing stress.

Dynamic Strain Amplitude During Fatigue Process of Mild Steel due to Rotary Bending

Annealed mild steel specimens have been subjected to rotation under the bending stress of 21∼31kg/mm2 with the speed of stress repetition at 3∼3000rpm at room temperature to examine its dynamic strain amplitudes and creep strains. A wire strain gage and an ocillogram have been employed by means of a slip ring to record the process.(1) Fatigue process, which is expressed by the dynamic strain amplitude, can be divided into three stages: first stage of softening, second stage of hardening, and third stage of apparent softening.(2) When the speed of stress repetition is slow and the stress amplitude is high, the amplitude of dynamic strain increases to a larger value, and the effect of stress amplitude on the specimens is larger than that of the speed of stress repetition.From the experimental results, it has been made clear that fatigue life is in inverse proportion to the peak of dynamic strain amplitude, i. e. the maximum value of softening process.(3) With the exception of the third stage, the dynamic strain amplitude on fatigue process is proportional to the transient creep strain during fatigue process.

The mechanisms of irradiation creep in graphite

Abstract The motion of edge and screw dislocations in a graphite polycrystal subjected to external forces and fast neutron irradiation is considered. A transient creep strain, proportional to the external stress and proportional to the ratio of the dimensions of the component crystallites in the a and c directions, is deduced. The calculated and experimentally observed magnitudes agree. From the time constant of this transient a value for the number of defect pairs created by a neutron during its moderation is calculated. There is fair agreement with accepted estimates. Graphite crystals show irradiation growth and a polycrystal must therefore either disintegrate or show yielding creep. The equations of yielding creep predict a creep rate greater than that observed, but they do not take into account that graphite is not a continuous solid. The discrepancy may thus be resolved. The possibility of an increase in the number of modes of deformation during neutron irradiation is discussed.

温度変動下のクリープおよびクリープ破断

Structure members which are subjected to load at high temperature are, in general, not in a steady condition of applied stress and temperature. Therefore, the studies on creep of structures under cyclic temperatures or cyclic stressing are important problem. In this paper, the influence of temperature history on creep is discussed. Experimental and analytical relationships of the creep and the rupture life under temperature cycling or under combined variation of stress and temperature to the static creep and the rupture life are presented.(1) The effect of temperature history on creep is composed of transitional change in internal yield stress, that is, recovery or aging, where the internal yield stress is defined as the local stress at which deformation begins without the aid of thermal fluctuation. The influence of the temperature history results in the existence of “induction period” or transitional increase in creep rate immediately after the change of temperature. However, these effects are negligible small in most metallic materials and the concept of “mechanical equation of state in solid”or the Robinson's hypothesis is applicable for preduction of creep or rupture life under cyclic temperatures.(2) By adopting the assumption the creep strain in the stage of transient or steady state creep under temperature cycling can be predicted analytically from the data of static creep test in transient stage, by introducing the equivalent steady temperature. The equivalent steady temperature means the temperature at which the transient creep strain under cyclic temperatures is equivalent to the strain in the static creep test. In like manner, creep rupture life under temperature cycling can also be estimated analytically from the data of static creep rupture test by introducing the equivalent steady temperature for creep rupture life. These analysises are usefull if severe metallurgical change such as microstructural phase change or corrosion does not occur. The assumption is also applicable for prediction of creep or the rupture life under combined variation of stress and temperature. However, under the condition that the stress amplitude which is synchronized with temperature cycle is large, the influence of recovery must be considered.

An Analytical Theory of the Creep Deformation of Materials

Abstract This paper reports on the formulation of an analytical theory of creep. This theory is proposed for an idealized material and may be applied to those materials whose behavior conforms to that of this ideal material. The theory takes into account the initial elastic strain, the transient creep strain, and the minimum rate creep strain. Unlike previous theories, this theory is capable of representing the simultaneous action of creep and creep recovery and may be used for conditions of varying as well as constant stresses. In this respect the theory is more general than those presented in the past. The new theory is of particular importance in the design of many new military and domestic applications where high temperatures over short periods of time make the initial short-time creep strains of importance.