Creep in Metals
Creep in metals refers to the gradual deformation of a material under constant stress at elevated temperatures. It is a time-dependent process that occurs over an extended period, leading to the material's elongation or distortion. Creep is a critical consideration in engineering applications, particularly in high-temperature environments, as it can affect the structural integrity and performance of metal components.
7 Key excerpts on "Creep in Metals"
eBook - ePubPractical Guide to the Packaging of Electronics
Thermal and Mechanical Design and Analysis, Third Edition
- Ali Jamnia(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
...Appendix F: Creep When a material is subjected to external loads, the internal structure (i.e., grain lattice, crystals, or molecules, in the case of many plastics) has to move in order to be aligned in a formation that would withstand the external loads. This new alignment may be called a state of stress, considering that there would be a tendency for the material to revert back to its natural state once the loads are removed. If enough energy (generally in the form of heat) is provided, this internal structure tends to accept the new formation as a natural state, and thus the state of stress is either lessened or relieved altogether. This phenomenon is called creep and is defined by Lau (1993) as a mathematical model to represent the behavior of rate-sensitive elastoplastic materials at elevated temperatures. Clearly, this behavior depends on the material’s properties and temperature levels, along with residence time at those levels and applied loads. In and of itself, creep may not be considered a failure mechanism; however, it may lead to catastrophic failures. If creep is allowed to continue, deformations are enlarged and, eventually, strains become so large that the structure loses its load-carrying capacity and fails either in a buckling mode or a stress rupture. Nearly all materials exhibit creep near their melting points. Although this proximity to the melting point varies from material to material, in general, once temperature value surpasses 50% of the melting point, creep and strain rates associated with creep become important factors that may not be ignored. In electronics packaging, solders and plastics are particularly at risk because of their relatively low melting temperatures and operating temperatures that may exceed the 50% threshold (Pecht 1991; Lau 1993; Lau et al. 1998). Stages of Creep The process of creep development is not uniform or constant. Figure F.1 depicts a typical creep curve with the three stages of creep identified...
eBook - ePubAdditive and Traditionally Manufactured Components
A Comparative Analysis of Mechanical Properties
- Joshua Pelleg(Author)
- 2020(Publication Date)
- Elsevier(Publisher)
...The rate of deformation is a function of exposure time, temperature, and applied load and depends on material properties. Creep is the tendency of a solid material to move slowly under the influence of mechanical stress but still below the yield strength and deform permanently. Creep is more severe in materials that are subjected to heat for long periods and generally increase as their temperature increase until a critical failure (dimensional and shape changes, which makes a component useless) is reached near the melting point. Therefore, creep is of concern to engineers when designing components to operate under high stress or high temperatures. However, creep may occur at relatively moderate temperatures in many cases. Even some ceramics with low-temperature ductility may creep ~ 0.5 T m. An important consideration is the choice of the type of material, namely, single or polycrystalline material. In order to eliminate grain boundary sliding in the material while exposed to some elevated temperature single crystal is the obvious choice. However, for many reasons such as other mechanical properties or cost factor consideration, polycrystalline materials are used despite their limitations for high-temperature applications. Grain size is a factor. A compromise must be made between small grain size, which enhances most of the mechanical properties, and large grained material, which is preferred for long-time exposure (creep) at high temperature. Ti6Al4V alloy also known as Ti64 is the most commonly used titanium alloy. It exhibits good mechanical and physical properties and due to its lightweight it is used in aerospace applications. These unique properties are strongly affected by chemical composition, microstructure, deformation, and heat-treatment history. The most commonly used titanium alloy is the two-phase (α + β) alloy. Significant research was devoted to improving the microstructure and attaining appropriate mechanical properties...
eBook - ePub- Michael E. Kassner(Author)
- 2015(Publication Date)
- Butterworth-Heinemann(Publisher)
...Introduction 1.1. Description of Creep Creep of materials is classically associated with time-dependent plasticity under a fixed stress at an elevated temperature, often greater than roughly 0.5 T m, where T m is the absolute melting temperature. The plasticity under these conditions is described in Figure 1 for constant stress (a) and constant strain rate (b) conditions. Several aspects of the curve in Figure 1 require explanation. First, three regions are delineated: Stage I, or primary creep, which denotes that portion where (in (a)) the creep rate (plastic strain rate), ε ˙ = ⅆ ε / ⅆ t is changing with increasing plastic strain or time. In Figure 1(a), the primary creep rate decreases with increasing strain, but with some types of creep, such as solute drag with “3-power creep,” an “inverted” primary occurs where the strain rate increases with strain. Analogously, in (b), under constant strain rate conditions, the metal hardens, resulting in increasing flow stresses. Often, in pure metals, the strain rate decreases or the stress increases to a value that is constant over a range of strain. The phenomenon is termed Stage II, secondary, or steady-state (SS) creep. Eventually, cavitation and/or cracking increases the apparent strain rate or decreases the flow stress. This regime is termed Stage III, or tertiary, creep and leads to fracture. Sometimes, Stage I leads directly to Stage III and an “inflection” is observed. Thus, care must sometimes be exercised in concluding a mechanical SS. The term “creep” as applied to plasticity of materials likely arose from the observation that at modest and constant stress, at or even below the macroscopic yield stress of the metal (at a “conventional” strain rate), plastic deformation occurs over time as described in Figure 1(a)...
eBook - ePub- Fujio Abe, Torsten-Ulf Kern, R Viswanathan(Authors)
- 2008(Publication Date)
- Woodhead Publishing(Publisher)
...Because of these reasons, time-dependent plastic deformation, namely creep, occurs below the athermal yield stress as depicted in Fig. 8.5(b). The strain–time curve is called the creep curve. 8.5 (a) Athermal stress–strain curve and (b) creep curves at stresses σ 1 (below athermal yield stress σ a) and σ 2 (above σ a). Slope of the creep curve (dε/d t) is called creep rate. On the creep curve, creep rate first decreases owing to strain hardening (primary creep) and reaches a stationary value (secondary creep). Degradation of microstructures and accumulation of creep damage, such as cavities and micro cracks, proceed during creep, resulting in creep acceleration to rupture. This stage is called tertiary creep. Since the stationary creep rate in the secondary creep stage is minimum during the whole creep process, the creep rate is often called the minimum creep rate and used as a measure of creep deformation resistance. The minimum (or steady state) creep rate ε ˙ m is expressed as: ε ˙ m = ε ˙ 0 σ / E n exp − Q c / R T [8.6] where ε ˙ 0 is a material constant, σ is the creep stress and Q c is the activation energy for creep. The values of n and Q c are closely related to creep mechanism as will be explained in Section 8.7. Practical heat-resistant steels are strengthened by various obstacles, such as dislocation substructures, precipitates and so on. Degradation of these obstacles starts at a very early stage of creep. In such materials the minimum creep rate is attained by dynamic balance between strain hardening and microstructural degradation. 2, 3 At applied stress σ 2 higher than σ a, the specimen deforms elastically to point B and then plastically to point C within a short period of time. The specimen is strain hardened during the plastic deformation by introducing dislocations. The athermal plastic deformation stops at this point and no further deformation continues at room temperature...
- William N. Findley, Francis A. Davis(Authors)
- 2013(Publication Date)
- Dover Publications(Publisher)
...CHAPTER 2 HISTORICAL SURVEY OF CREEP 2.1 Creep of Metals The experimental and metallurgical aspects of creep of metals have been treated in a number of books beginning with Norton [2.1] and Tapsell [2.2] and followed by Hoff [2.3], Finnie-Heller [2.4], Kachanov [2.5], Lubahn-Felgar [2.6], Dorn [2.7], Odqvist-Hult [2.8], Kennedy [2.9] and more recently Odqvist [2.10], Hult [2.11], Rabotnov [2.12], and Smith-Nicholson [2.13]. In recent publications on creep of metals the concept of dislocations has been employed to explain the physical phenomena of creep in terms of the structure of metals (see, for example [2.9]). These books have generally concentrated their attention on the primary and secondary stages of creep. 2.2 Creep under Uniaxial Stress A systematic observation of the creep phenomenon was first reported in 1834 by Vicat [2.14]. Thurston [2.15] seems to be the first to propose three stages of creep as shown in Fig. 1.2 and to study stress relaxation. Andrade [2.16] made a systematic investigation of creep of lead wires under constant load and in 1910 proposed the first creep law as follows: (2.1) where l 0 and l are initial and current length of the specimen, respectively, t is the time under load, β and k are material constants which depend on stress. Since then various empirical equations have been proposed for the stress dependence of creep of metals based on experimental observations. The steady state of creep (secondary creep) particularly has been studied in great detail. Bailey [2.17] and Norton [2.1] suggested the empirical equation that yielded good agreement with experimental data for steady creep under low stresses, (2.2) where is the steady creep rate, σ is the applied stress, k and p are material constants. Values of p have been found to lie between 3 and 7. Equation (2.2) is called the power law or Norton’s creep law...
eBook - ePubPhenomenological Creep Models of Composites and Nanomaterials
Deterministic and Probabilistic Approach
- Leo Razdolsky(Author)
- 2019(Publication Date)
- CRC Press(Publisher)
...In order to analyze the engineering creep deformations of composite materials, one should classify them from the macro-structural point of view, living aside the manufacturing process and dealing with the final composite material product. It has to be mentioned again that the final product of any engineering creep analyses is limited in most cases to obtaining the stress-strain relationship as function of temperature (time) and its application to structural analysis and design. The matrix material can enhance the stiffness and therefore creep performance of the structure and it can be regulated by the mathematical expression of modulus of elasticity Ε(θ) in Equation (2.1) [ 4 ]. It is assumed here that the memory function K (t, t’) in this case is affected by high temperature and it might be (or might be not) affected by the rheological composition of the material, for instance, nonNewtonian nonlinear viscosity. A strong dependence on temperature (time) of the properties of soft composites (PMS) exists compared with those of other materials such as metals and ceramics (MMC and CMC). This strong dependence is due to the viscoelastic nature of PMC. Generally, PMC behave in a more elastic fashion in response to a rapidly applied temperature rise (force) and in a more viscous fashion in response to a slowly applied temperature increase. Viscoelasticity means behavior similar to both purely elastic solids in which the deformation is proportional to the applied force and to viscous liquids in which the rate of deformation is proportional to the applied force. The behavior of composites is very complex due to this dual nature...
eBook - ePub- W. Bolton, R.A. Higgins(Authors)
- 2014(Publication Date)
- Routledge(Publisher)
...Plastics materials, however, are much more prone to this type of deformation since they undergo the viscoelastic type mentioned earlier (Section 20.3.1). Creep rates, like other mechanical properties for polymers, are very temperature dependent. Even for a plastics raincoat, creep must be considered since it must be capable of supporting its own weight so that it can hang in a wardrobe possibly for long periods without slowly flowing towards the floor during a period of warm weather. The creep curves shown in Figure 20.8 are typical of those for most thermoplastics materials. These show that the initial rate of creep is rapid but then the rate decreases to a lower constant rate, which for small stresses might mean no further change in creep. Naturally, the greater the applied force the greater the creep rate and elongation and quite small rises of temperature will produce much greater rates of creep. Figure 20.8 Creep curves for cellulose acetate at 25°C. At a stress of 18 MPa, rapid creep soon leads to catastrophic failure, whilst at a stress of 7 MPa a small initial amount of creep ceases after about 1000 hours and no further measurable creep occurs. 20.3.5 Other mechanical tests Other mechanical tests are used to measure compressive properties, tear strength, shear strength and deflection in bend under an applied load. All of these tests, together with those described briefly here, are dealt with in BS 2782: Part 3. 20.4 Additives Various materials can be introduced into a polymer at the moulding stage by mixing the additive thoroughly with the moulding powder. 20.4.1 Fillers Fillers are used principally in the interests of economy when up to 80% of the moulding powder might be filler. Generally, fillers are finely powdered, naturally occurring, non-polymeric substances which are readily available at low cost...
