Technology & Engineering
Creep in Materials
Creep in materials refers to the slow, time-dependent deformation that occurs under constant load or stress at elevated temperatures. It is a phenomenon commonly observed in materials such as metals, plastics, and ceramics. Creep can lead to permanent changes in the shape and dimensions of a material, making it an important consideration in the design and performance of engineering components operating under high temperatures and loads.
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12 Key excerpts on "Creep in Materials"
eBook - PDF- Andrzej Weronski(Author)
- 1991(Publication Date)
- CRC Press(Publisher)
Creep 2 2.1 INTRODUCTION The term creep denotes slowly proceeding deformation of solid matter under a maintained load. The first systematic studies providing some quantitative informa-tion on the nature of creep were those of Andrade (1910). During World War I, research on creep became more urgent. Impetus was given by the rapid increase in steam admission temperatures in power plants (to about 670 K in the 1920s), approaching the creep range of low-alloy steels. Early researchers were concerned with finding the limiting stress below which creep would not occur, but using more accurate experimental techniques, this idea was subsequently shown to be false. The temperature at which creep becomes important for the designer is about 0.4 of the melting temperature of the material considered, but numerous exceptions bring this rule into question. For example, creep is observed in titanium at lower temperatures than in iron-based alloys, despite the higher melting point of the former. Stress levels under which creep is observable are always much lower than the strength of the material. A creep curve, which is the graphical presentation of the dependence of the strain on time under constant stress and temperature, is shown in Fig. 2.1. The strain £ 0 is developed immediately upon loading; the period of time between £ 0 and is called primary creep. Between ex and e2 the creep rate remains almost constant; this portion of the curve is termed secondary (or steady-state) creep: • * 2 S i £f = -----h ~ h The creep rate increases beyond £ 2 » an<3 this period is called tertiary creep. It is convenient to obtain experimental data under a constant tensile load. As creep proceeds, the true stress increases continually, giving rise to a pronounced change in the creep rate. The tertiary creep, where necking is appreciable, is certainly also 54 Creep 55 Time Figure 2.1 Schematic representation of a creep-rupture curve.
eBook - ePubMolecular Dynamics Simulation of Nanostructured Materials
An Understanding of Mechanical Behavior
- Snehanshu Pal, Bankim Chandra Ray(Authors)
- 2020(Publication Date)
- CRC Press(Publisher)
3Creep and Fatigue Behavior of Materials
3.1 METALLIC CREEP AND VISCOELASTICITY
Creep is defined as a time-dependent progressive deformation of metals subjected to constant stress or load [1 ]. Creep is generally considered a high-temperature phenomenon (≥0.5 Tm , where Tm is the absolute melting point of the material) in which metal deforms plastically at constant stress/load to obtain the time-dependent plasticity in terms of strain. For both metals and viscoelastic materials, the process is the same, but they differ in the way the phenomenon takes place (i.e., deformation mechanisms), which affects their response. As it is evident from the name of viscoelastic material, it exhibits inherent elasticity during the loading and unloading conditions; the metal deforms; and it returns to its original condition in slow manner. In case of polymers, crosslinks and mechanical entanglements cause the material to get back to its original form, depicting a reversible phenomenon. On the other hand, the metal acts as a viscoplastic material (displays time-dependent stress-strain behavior) during the creep through the movement of vacancies, grain boundaries, and dislocations, which doesn’t get back the metal to its original form even after the load is removed [2 ]. In the perspective of creep properties, metals exhibit an irreversible stress-strain behavior, whereas viscoelastic materials exhibit a reversible behavior.3.1.1 TENSILE CREEP CURVE: TRANSIENT AND STEADY -STATE CREEPCreep curve is obtained from the results of creep test performed on the specimen. As stated previously, creep is a time-dependent continued plastic deformation at constant load/stress. Tensile creep curve is obtained by applying constant load on tensile specimen at constant temperature, and variation of strain as a function of time is determined. In creep test, the plot obtains from strain vs. time. On the other hand, in tensile test, the plot obtains from stress vs. strain. Creep curve can be divided into three regions/stages per the slope of the curve (i.e., affected by the controlling mechanisms taking place at that particular time), which is shown in Figure 3.1
eBook - ePubDeformation and Evolution of Life in Crystalline Materials
An Integrated Creep-Fatigue Theory
- Xijia Wu(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
CHAPTER 5 Creep 5.1 OverviewCreep refers to the phenomena of time-dependent deformation under constant load or stress at elevated temperature. Following Andrade’s first observation in 1910, creep testing has been a conventional way to characterize material’s high-temperature strength. A typical uniaxial creep frame setup is shown in Figure 5.1 , where the creep coupon is loaded by a dead weight through a lever arm. Such a creep test is therefore called constant-load creep test. During the test, the coupon is enclosed in a furnace set at temperature and its elongation is measured using a linear variable differential transformer (LVDT). The elongation-time data are recorded to the point of specimen fracture, i.e., stress rupture, and hence creep life is called the rupture life.Figure 5.1. Schematic drawing of main components of creep testing machine (after Bueno, 2008).Creep deformation as high-temperature design limitations is widely recognized in industrial design codes, e.g., ASME Boiler and Pressure Vessel Code. The creep failure criteria can be given either in terms of creep strain to be reached, e.g. 1%, or hours of service, e.g. 100,000 hours, against which the allowable stress is determined. With regards to the above criteria for component design, a few questions and factors have to be completely understood.
- Rajiv Asthana, Ashok Kumar, Narendra B. Dahotre(Authors)
- 2006(Publication Date)
- Butterworth-Heinemann(Publisher)
From the material and component design perspectives, the steady-state creep rate (d ε s / dt ) is of considerable importance. The steady-state creep rate depends on the magnitude of applied stress, σ , and temperature, T, according to d ε s dt = K σ n exp − Q RT , (1-12) where Q is the activation energy for creep, K and n are empirical constants, and R is the gas constant. The exponent n depends on the dominant mechanisms of creep under a given set of experimental conditions, which include vacancy diffusion in a stress field, grain boundary migration, dislocation motion, grain boundary sliding, and others. The materials used in modern aircraft engines provide a classic example of how high-temperature creep resistance is enhanced through microstructure design in Ni-base alloys by dispersing nanometer-size oxide particles; this is discussed in Chapter 6. Time, t t r Creep strain, e Instantaneous deformation Primary Tertiary Rupture Secondary ∆t ∆e FIGURE 1-29 Schematic creep curve showing strain versus time at constant stress and temper- ature. The minimum creep rate is the slope of the linear region in the secondary creep regime. (W. D. Callister, Jr., Materials Science & Engineering: An Introduction, 5th ed., Wiley, New York, 2000, p. 226). 34 MATERIALS PROCESSING AND MANUFACTURING SCIENCE Deformation Processing A large number of manufacturing processes employ solid-state deformation of hot or cold metals and alloys to shape parts. Hot-working is the mechanical shaping operation that is per- formed at temperatures high enough to cause the processes of recovery and recrystallization to keep pace with the work hardening due to deformation. In contrast, cold-working is performed below the recrystallization temperature of the metal; however, both hot- and cold-working are done at high strain rates. Cold-working strain hardens a metal, and excessive deformation without intermediate annealing causes fracture.
- Yichun Zhou, Li Yang, Yongli Huang(Authors)
- 2013(Publication Date)
- CRC Press(Publisher)
˙ initially increases with time, but decreases when approaching the end of the first stage, and gradually approaches a constant value. This stage is called the primary stage, or the transient stage. The second stage is the steady-state stage of creep deformation. In this stage, the creep rate is constant. In the third stage, the creep rate increases rapidly with time in an unsteady manner, until the material fractures. This stage is called the creep acceleration stage.FIGURE 7.2 Schematic illustration of creep test setup. (From Dowling, N.E., Mechanical Behavior of Materials (Engineering Methods for Deformation, Fracture and Fatigue), Prentice Hall, Upper Saddle River, NJ, 1998; Taken from Figure 15.3.)FIGURE 7.3 The evolution of strain with time in creep tests under constant load. (From Dowling, N.E., Mechanical Behavior of Materials (Engineering Methods for Deformation, Fracture and Fatigue), Prentice Hall, Upper Saddle River, NJ, 1998; Taken from Figure 15.4.)The shape of the creep curve reflects the work hardening and recovery softening procedures accompanying high temperature deformation. In the initial stage of creep, the deformation rate is very fast (or the flow stress is very low), indicating that the deformation resistance of the material is low. Subsequently, due to the work hardening of deformation, the creep rate decreases gradually (or the flow stress increases gradually). With the increase of work hardening, the dynamic recovery rate gradually increases. Finally, work hardening and recovery softening reach a dynamic equilibrium and the creep rate remains constant (or the flow stress remains constant). This is the secondary stage of deformation, or steady-state creep. In the third stage, the creep rate increases (or the flow stress decreases). This can be ascribed to stress concentration caused by creep voids in the specimen, the increase of true stress due to the decrease in the specimen cross section and necking, and change in the material’s microstructure.
eBook - PDF- George Z. Kyzas, Athanasios C. Mitropoulos, George Z. Kyzas, Athanasios C. Mitropoulos(Authors)
- 2018(Publication Date)
- IntechOpen(Publisher)
Thus, even in the case of the uniaxial stress state, it is necessary to consider the four macroquantities — temperature, stress, time and strain. Creep characteristics determined in the experiments at constant temperatures can also often be used when evaluating the efficiency of structures at varying temperatures. To determine the dependences describing the creep process, it is usually necessary to use the data obtained in the standard uniaxial tension tests. Creep is mostly common with metals and alloys at absolute temperatures T higher than (0.4 – 0.5) T * ( T * is the melting point on the absolute scale, i.e., in Kelvin (K)). In the creep test, a cylindrical specimen with thermocouples attached to it is secured in the clamps of the loading machine and placed in the furnace. The temperature of the specimen is controlled using the thermocouples, and the results are sent to a tracking system. This system ensures heating of the specimen to the required level, and the temperature is then maintained constant with a specific accuracy. After complete heating of the furnace area, a tensile force is applied to the specimen. This force changes with time under a given law (in most cases, this force is constant or a fractional-constant time function). A strain measurement device is used to record the variation of the length of the specimen with time during continuous recording of the deformation diagram. The elongation of the specimen as a result of the creep of the material is accompanied by a decrease in the cross-sectional area and, consequently, the tensile stress increases continuously at a constant load. In the tests of the materials characterised by high creep strains (of 4 – 5% order or more), there are used systems where the load is self-compensated so that the stress in the specimen remains constant. When testing a number of creep-resistant alloys, it appears that the creep strains remain relatively small (approximately 1 – 2%) up to the moment of fracture.
- Koryun Karapetyan(Author)
- 2019(Publication Date)
- Butterworth-Heinemann(Publisher)
7] .Stress relaxation in a material, including concrete, is the effect of creep appearance.Experimental studies of the stress relaxation process in a concrete element are a few, and this can be explained by certain difficulties encountered in their implementation. The overwhelming majority of these studies were carried out without taking into account such important factors as the natural aging of the material, the level of the applied initial stress, etc. [4 , 8 –18 ]. At the same time, in some of the mentioned works, an attempt was made to describe the relaxation process applying creep data based on various concepts, including empirical ones [4 , 8 , 10 , 13 , 14 , 17] .Creep and associated with it stress relaxation capacity are the properties of concrete that have to be taken into account at the consideration of the load long-term effects on it. Having clear ideas about the patterns of development of the concrete element creep during the period of time under different loading conditions as well as taking into account the age of the material and the process of stress relaxation in the elements can be useful for assessment of the real mode of deformation of structures, including those operating in regions lacking humidity.
eBook - PDFThe Stress Analysis of Pressure Vessels and Pressure Vessel Components
International Series of Monographs in Mechanical Engineering
- S. S. Gill(Author)
- 2016(Publication Date)
- Pergamon(Publisher)
Although this example is a particular one the results can easily be generalized. In the second section a method of analysis for a given constitutive law is illustrated for examples of plates and shells [10.4, 10.5], Again this method could be extended to the general case. DETERMINATION OF CREEP EFFECTS IN STRUCTURES 515 10.2. Creep Laws and the Skeletal Point Concept 10.2.1. CREEP CURVES AT CONSTANT STRESS It has been found from experiment that if a metal which creeps is subjected to a constant uniaxial stress then the accumulation of creep strains with time has the form illustrated in Fig. 10.1. The curves for metals tend to have the same form composed of: (i) An initial elastic response. (ii) A region of so-called primary creep in which the creep rate de-creases with time. (iii) A region of so-called secondary or steady-state creep in which the creep rate remains constant at a minimum value. (iv) The tertiary region in which the strain rate increases very rapidly and is the prelude to rupture. FIG. 10.1. Creep curve obtained at constant temperature under constant load. The portion of interest of the creep curve depends upon the problem under consideration. For example in the traditional situation of steady loads sustained for long periods, the problem has been the determination of long-term deformations. In this situation the elastic and primary portions of the curves are usually neglected as being small with respect to the steady-state creep and the simplified creep curves and the simplified creep curves take the form shown in Fig. 10,2. It is 516 THE STRESS ANALYSIS OF PRESSURE VESSELS found that the steady-state creep rate is governed by a formula of the type: (10.1) where k and m are constants, the value of m normally being between 2 and 9 according to the metal. The structural analysis which results from the relation in equation (10.1) is known as a steady-state analysis.
eBook - ePub- A. M. Lokoshchenko(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
t ):(1.54)ε( t )=+ pσ ( t )E( t )≡ const .The relaxation process is the creep with decreasing stress which takes place so that the increase of the creep strain compensates the decrease of the elastic strain as a result of the decrease of stress. Stress relaxation is often observed in technology. A typical example is stress relaxation in any threaded joint. The bolts are tightened a preliminary force which weakens with time. The axial deformation of the bolt and the axial force in the bolt are linked by the relationship (1.54 ). The stress relaxation phenomenon should be taken into account in the operation of many structures in which the possibility of deformation of one of the elements is restricted by constraint from the side of other elements.The formulation of the relaxation experiments is far more complicated than the formulation of the creep experiments. It is necessary to ensure that the rigidity of the dynamometer and other components connected gradually with the bar is considerably greater than the rigidity of the bar; only if these conditions are satisfied, it can be assumed that the axial deformation of the bar is constant. In practice, the stress relaxation tests are carried out using the procedure in which the loading device, generating the tensile stress, is connected to the tracking system. The strain measurement device is connected with the contact of the device influencing, by means of the tracking system, the loading device in such a manner that the deformation of the bar remains constant throughout the test period. Since the entire measurement system has a certain sensitivity threshold, the stress relaxation tests is in fact a sequence of the processes with the stresses decreasing in steps.
- Karl Jakus, Shanti Nair(Authors)
- 1995(Publication Date)
- Butterworth-Heinemann(Publisher)
During the rapid application of a creep load, stress is distributed to each constituent based on their elastic properties (Young's modulus). During subsequent sustained (static) loading, the stress tends to redistribute between the constituents based on their creep properties. Following the application of a creep load, a progressive stress redistribution occurs and will continue until a steady state is reached, at which time ei = (e/)/and &i = 0 for all the constituents.~ This requires that at steady-state, the stress is distributed in such a way that the intrinsic creep rates of all constituents are equal (ei = ec, tot), and that the elastic components of strain rate equal zero (ei, et = 0). This stress redistribution process, which is a characteristic feature of the transient creep behavior of composites, may occur over a long period of time (i.e., some hundred hours), or may be only a short period (i.e., several minutes); the precise kinetics of redistribution will depend on the combination of elastic properties (Ei), creep properties (Ai, n/), and volume fractions (vi) of the constituents. The I-D model presented above can be used to study the effect of loading history and microstructural changes on composite creep behavior. For example, using the elastic strain rate term in Eqn. (5), (Ei/Ec)b'r various loading patterns can be simulated without changing the computation proce- dure. Using this model, the influence of constituent creep behavior, elastic properties and volume fractions can be studied in an attempt to optimize the microstructure of a composite for given temperature and loading histories. The number of constituents in the model is arbitrary, as is the behavior of each Creep Behavior of Continuous Fiber-Reinforced Ceramics 201 constituent. If we consider the fibers, matrix, and interracial zone as a three-component composite system, Eqn.
- Wole Soboyejo(Author)
- 2002(Publication Date)
- CRC Press(Publisher)
5000 Viscoelasticity, Creep, and Creep Crack Growth 545 4000 3000 £ 1000 0 F igure 15.28 Melting or softening temperatures for different solids. [From Ashby and Jones (1996) with permission from Butterworth-Heinemann.] strengthened m aterials. H ow ever, the precipitates or second phases m ust be stable at elevated tem perature to be effective creep strengtheners. T h e seco n d a p p ro a ch to creep stren gth en in g o f p o w er law creep in g m aterials in v o lv es the selectio n o f m aterials w ith h igh lattice resistan ce to d islo c a tio n m o tio n . S u ch m aterials are gen erally co v a le n tly b o n d ed solid s. T h ey in clu d e o xid es, carb id es, an d nitrid es. U n fo rtu n a te ly , h o w ev er, these m aterials are brittle in natu re. T h ey therefore presen t a d ifferen t set o f p rob lem s to the designer. M o st d esign s are d o n e for p o w er law creep in g solid s. H o w ev er, in m aterials w ith relatively sm all grain sizes, d iffu sio n a l creep m a y b eco m e life lim itin g. T h is is particu larly true in m aterials su b jected to lo w stresses an d elevated tem p erature. M aterial d esign a gain st creep in su ch m aterials m a y be acco m p lish ed by: ( 1 ) h ea t treatm en ts th at in crease the grain size, ( 2 ) the use o f grain b ou n d ary p recip itates to resist grain b o u n d a ry slid in g, and (3) th e ch o ice o f m aterials w ith low er d iffu sio n coefficien ts. D iffu sio n a l creep co n sid era tio n s are particu larly im p o rta n t in the d esign o f structures fab ricated from structural ceram ics in w h ich p o w er law creep is su p p ressed Ceramics Metais Polymers Composites Diamond Graprvt* SiC MgO S.,N, A lkali halide* Tjngstan Tantalum Mo*ytxJar*um N iobium Chrom ium Zirconium etatmum Titanium trorvStaai Cobalt'N
eBook - ePubPhenomenological Creep Models of Composites and Nanomaterials
Deterministic and Probabilistic Approach
- Leo Razdolsky(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
1 ] for calculating the creep of composites and nanocomposites is reduced to the following steps:- All variables and parameters (stress, strain, temperature, etc.) are presented in dimensionless form. This has a threefold effect: first, this reduces the number of parameters that should be determined based on the experimental data to a minimum; second, there is no need to include the aging function (so-called “shift factor”) in the integral creep constitutive equation (the data of material creep tests results can be used to analyze the behavior of the material in the future), and finally—the dimensionless form of the integral type constitutive creep equation can be solved in numerical (tabulated) form and be approximated by the analytical function afterwards for further use in structural engineering analysis and design.
- At any point (time and space) of the composite, the total elongation is the elongation of the material at the initial instant time, minus the elongations due to the external temperature effect, the joint action of temperature and stress, and the elongations due to heat released from the chemical reaction (for example, nanoparticles growth process in the case of nanocomposites).
- In order to simplify the formulation of creep deformation problem of a composite material, the latter will be divided conditionally into two groups: multilayered composites and dispersed composites. The modulus of elasticity of the material entering the integral type creep equation is the average weighted value of the components (the fiber and the matrix) and obeys so called “rule of mixtures”.
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