Download
1 / 100
1.04k likes | 1.11k Vues
AE 1354 - HIGH TEMPERATURE MATERIALS. UNIT 1. UNIT 1. CREEP. CREEP. Definition.
E N D
UNIT 1 CREEP
CREEP • Definition Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses. It occurs as a result of long term exposure to levels of stress that are below the yield strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Creep always increases with temperature
Creep The rate of this deformation is a function of the material properties, exposure time, exposure temperature and the applied structural load. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function — for example creep of a turbine blade will cause the blade to contact the casing, resulting in the failure of the blade.
Creep Stress Strain Time
Mechanisms of Creep • High rates of diffusion permit reshaping of crystals to relieve stress • Diffusion significant at both grain boundaries and in the bulk • High energy and weak bonds allow dislocations to “climb” around structures that pin them at lower temperature
Mechanisms of Creep in metals • Mechanisms of Creep in metals: There are three basic mechanisms that can contribute to creep in metals, namely: • (i) Dislocation slip and climb. • (ii) Grain boundary sliding. • (iii) Diffusional flow.
Grain Boundary sliding: Grain Boundary sliding: The onset of tertiary creep is a sign that structural damage has occurred in an alloy. Rounded and wedge shaped voids are seen mainly at the grain boundaries and when these coalesce creep rupture occurs. The mechanism of void formation involves grain boundary sliding which occurs under the action of shear stresses acting on the boundaries.
Creep deformation is important not only in systems where high temperatures are endured such as nuclear power plants, jet engines and heat exchangers, but also in the design of many everyday objects
Generally, the minimum temperature required for creep deformation to occur is 30-40% of the melting point for metals and 40-50% of melting point for ceramics • creep can be seen at relatively low temperatures for some materials. Plastics and low-melting-temperature metals, including many solders, creep at room temperature as can be seen marked in lead and zinc
Practical example • An example of an application involving creep deformation is the design of tungsten lightbulb filaments. Sagging of the filament coil between its supports increases with time due to creep deformation caused by the weight of the filament itself. If too much deformation occurs, the adjacent turns of the coil touch one another, causing an electrical short and local overheating, which quickly leads to failure of the filament
Other some examples • Other examples • Though mostly due to the reduced yield stress at higher temperatures, the Collapse of the World Trade Center was due in part to creep from increased temperature operation. • The creep rate of hot pressure-loaded components in a nuclear reactor at power can be a significant design-constraint, since the creep rate is enhanced by the flux of energetic particles. • Creep was blamed for the Big Dig tunnel ceiling collapse in Boston, Massachusetts that occurred in July 2006
In steam turbine power plants, steam pipes carry superheated vapour under high temperature (1050°F/565.5°C) and high pressure often at 3500 psi (24.131 MPa) or greater. In a jet engine temperatures may reach to 1000°C, which may initiate creep deformation in a weak zone. For these reasons, it is crucial for public and operational safety to understand creep deformation behavior of engineering materials.
Stages of creep • In the initial stage, known as primary creep, the strain rate is relatively high, but slows with increasing strain. • The strain rate eventually reaches a minimum and becomes near-constant. This is known as secondary or steady-state creep. This stage is the most understood. The characterized "creep strain rate", typically refers to the rate in this secondary stage. The stress dependence of this rate depends on the creep mechanism.
Mechanisms of creep The mechanism of creep depends on temperature and stress. The various methods are: • Thermally activated glide - e.g., via cross-slip • Climb assisted glide - here the climb is an enabling mechanism, allowing dislocations to get around obstacles • Climb - here the strain is actually accomplished by climb • Grain boundary diffusion
General creep equation where ε is the creep strain, C is a constant dependent on the material and the particular creep mechanism, m and b are exponents dependent on the creep mechanism, Q is the activation energy of the creep mechanism, σ is the applied stress, d is the grain size of the material, k is Boltzmann's constant, and T is the absolute temperature
Dislocation creep At high stresses (relative to the shear modulus), creep is controlled by the movement of dislocations. When a stress is applied to a material, plastic deformation occurs due to the movement of dislocations in the slip plane. Materials contain a variety of defects, for example solute atoms, that act as obstacles to dislocation motion.
Creep arises from this because of the phenomenon of dislocation climb. At high temperatures vacancies in the crystal can diffuse to the location of a dislocation and cause the dislocation to move to an adjacent slip plane. By climbing to adjacent slip planes dislocations can get around obstacles to their motion, allowing further deformation to occur. Because it takes time for vacancies to diffuse to the location of a dislocation this results in time dependent strain, or creep.
Effect of stress on creep curves at constant temperature
Unit III Fracture
