How does creep differ from fatigue

Coffin, S. Manson, Fatigue at elevated temperatures. STP , — Google Scholar. Larson, J. Miller, Trans. Ostergren, A damage function and associated failure equations for predicting hold time and frequency effects in elevated temperature low cycle fatigue. ASTM J. Test Eval. Sabour, Ph. Mohammad Hossein Sabour 1 Email author 1. The traditional view of creep-fatigue damage development and interaction involves the development of transgranular fatigue cracking from the surface and intergranular creep damage from sub-surface to the point where the intensity of grain boundary damage is sufficient to deflect crack propagation onto the grain boundaries e.

It is now appreciated that this type of creep-fatigue damage interaction does not always occur for creep ductile alloys e. Multiple microstructurally short fatigue crack development occurs at the surface, while creep damage evolves from the center of the specimen. When the dominant fatigue crack meets the creep damage zone advancing from the axis, interaction occurs in the way represented in Figure 1 d.

It is important not to underestimate the influence of oxidation on creep-fatigue crack development. While oxidation can enhance the rate of cracking, it can also consume any microcrack development to the point where thermo-mechanical crack propagation is entirely creep dominated e. In such cases, strain-enhanced oxidation can consume all evidence of microcrack development and become detached by spalling due to large cyclic thermal transients.

Creep dominated crack development in a 1CrMoV rotor steel the upper and lower profiles are from two specimens following thermo-mechanical fatigue testing under identical conditions: the upper one breaking open when cold due to the high density of internal creep cavities ; and the lower one remaining unbroken. It has already been acknowledged that microstructural evidence of a strong creep-fatigue interaction is not always only apparent in the form of physical damage i. Familiarity with the metallurgical characteristics of the material of the failed component is therefore invaluable for effective and efficient failure diagnosis.

Failure diagnosis invariably involves consideration of both the associated material condition and the results of a mechanical analysis of prior operating history. This review has focused on these aspects with particular reference to creep-fatigue failure diagnosis. The effectiveness and efficiency of the diagnosis of failures due to creep-fatigue loading are enhanced by a familiarity with the characteristics of the material of the failed component which can come from the routine post-test examination of laboratory specimens.

The concept of routine laboratory specimen post-test examination to quantitatively characterise the detail of deformation and damage accumulation under known and well-controlled loading conditions is strongly advocated, and illustrated with examples.

National Center for Biotechnology Information , U. Journal List Materials Basel v. Materials Basel. Published online Nov Stuart Holdsworth. Robert Lancaster, Academic Editor.

Author information Article notes Copyright and License information Disclaimer. Received Oct 22; Accepted Nov 6. Abstract Failure diagnosis invariably involves consideration of both associated material condition and the results of a mechanical analysis of prior operating history.

Keywords: failure diagnosis, creep-fatigue, material condition, mechanical analysis. Introduction The diagnosis of failures invariably involves consideration of both the associated material condition and the results of a mechanical analysis of prior operating history. Material Condition 2. Mechanism of Creep-Fatigue Cracking The development of creep-fatigue damage in most power plant steels depends on temperature, strain range, strain rate, hold time, and the creep strength and ductility of the material [ 1 , 2 , 3 , 4 ].

Open in a separate window. Figure 1. Figure 2. Creep Ductility Creep ductility is influential in determining the extent of creep-fatigue interaction Figure 3. Figure 3. Figure 4. Figure 5. Influence of creep-ductility on creep-fatigue cracking mechanisms.

Figure 6. Mechanical Analysis of Creep-Fatigue Cracking 3. Crack Initiation While there are a number of published and in-house procedures available to assess the risk of creep-fatigue crack initiation in high-temperature components e. Crack Development Crack development due to creep-fatigue loading may occur i within the confines of a cyclic plastic zone when the crack is physically small, i.

Figure 7. Figure 8. Figure 9. Post-Test Examination An effective quantitative interpretation of the evidence associated with a component failure requires familiarity with the microstructural and damage conditions of the constituent material s under known and well-controlled loading conditions.

Oxide Dating Measurements from the post-test examination of specimens tested under known conditions provide invaluable evidence for subsequent failure diagnosis. Figure Creep Damage Assessment Creep damage condition can be represented in terms of feature type, development state, or size.

Development Creep damage is most commonly characterized in terms of a quantity representing its extent or degree of development, e. Size The characterisation of damage size may involve sizing of i the damage feature e. Fatigue Damage Assessment Microstructurally short Stage I fatigue cracks typically extend along persistent slip bands [ 29 ] to a depth of 1—2 grain diameters before becoming Stage II cracks propagating in a transgranular manner normal to the maximum principal stress Figure 11 [ 30 , 31 ].

Creep-Fatigue Damage Assessment The assessment of creep-fatigue damage has tended to focus on materials for which creep damage is intergranular, and for circumstances represented by Figure 1 b—d e. Concluding Remarks Failure diagnosis invariably involves consideration of both the associated material condition and the results of a mechanical analysis of prior operating history.

Conflicts of Interest The author declares no conflict of interest. References 1. Thomas G. Holdsworth S. Miller D. A review of material response and life prediction techniques. High Temp. Bicego V. Low Cycle Fatigue. Low cycle fatigue characterisation of a HP-IP steam turbine rotor; pp. Fournier B. Microstructure evolutions. Yan W. Component assessment data requirements from creep-fatigue tests; pp. Skelton R. Creep-fatigue damage accumulation and interaction diagram based on metallographic interpretation of mechanisms.

Behaviour of Defects at High Temperatures. Fatigue and creep 1. It is the failure of a material by fracture when subjected to a cyclic stress. Fatigue is distinguished by three main features. All rotating machine parts are subjected to alternating stresses. Example: aircraft wings are subjected to repeated loads, oil and gas pipes are often subjected to static loads but the dynamic effect of temperature variation will cause fatigue. There are many other situations where fatigue failure will be very harmful.

Because of the difficulty of recognizing fatigue conditions, fatigue failure comprises a large percentage of the failures occurring in engineering. To avoid stress concentrations, rough surfaces and tensile residual stresses, fatigue specimens must be carefully prepared. UNIT V Lecturer4 5 Fatigue The point at which the curve flatters out is termed as fatigue limit and is well below the normal yield stress. The significance of the fatigue limit is that if the material is loaded below this stress, then it will not fail, regardless of the number of times it is loaded.

Materials such as aluminium, copper and magnesium do not show a fatigue limit; therefore they will fail at any stress and number of cycles. Other important terms are fatigue strength and fatigue life. The fatigue strength can be defined as the stress that produces failure in a given number of cycles usually The fatigue life can be defined as the number of cycles required for a material to fail at a certain stress. UNIT V Lecturer4 6 Factors affecting fatigue properties Surface finish: Scratches dents identification marks can act as stress raisers and so reduce the fatigue properties.

Electro-plating produces tensile residual stresses and have a deterimental effect on the fatigue properties. Temperature: As a consequence of oxidation or corrosion of the metal surface increasing, increase in temperature can lead to a reduction in fatigue properties. Residual stresses on the surface of the material will improve the fatigue properties. Heat treatment: Hardening and heat treatments reduce the surface compressive stresses; as a result the fatigue properties of the materials are getting affected.

Stress concentrations: These are caused by sudden changes in cross section holes or sharp corners can more easily lead to fatigue failure. The simplest type of load is the alternating stress where the stress amplitude is equal to the maximum stress and the mean or average stress is zero.

The bending stress in a shaft varies in this way. UNIT V Lecturer4 9 Fatigue Failure Fatigue fracture results from the presence of fatigue cracks, usually initiated by cyclic stresses, at surface imperfections such as machine marking and slip steps.

The initial stress concentration associated with these cracks are too low to cause brittle fracture they may be sufficient to cause slow growth of the cracks into the interior. Eventually the cracks may become sufficiently deep so that the stress concentration exceeds the fracture strength and sudden failure occurs. The extent of the crack propagation process depends upon the brittleness of the material under test.

In brittle materials the crack grows to a critical size from which it propagates right through the structures in a fast manner, whereas with ductile materials the crack keeps growing until the remaining area cannot support the load and an almost ductile fracture suddenly occurs.

For a typical fracture ,Two distinct zones can be distinguished — a smooth zone near the fatigue crack itself which, has been smoothened by the continual rubbing together of the cracked surfaces, and a rough crystalline-looking zone which is the final fracture.

Occasionally fatigue cracks show rough concentric rings which correspond to successive positions of the crack. Iron, nickel, copper and their alloys exhibited this property at elevated temperature. The primary stage is of great interest to the designer since it forms an early part of the total extension reached in a given time and may affect clearness provided between components of a machine.

The work hardening and recovery processes are exactly balanced. It is the important property of the curve which is used to estimate the service life of the alloy.

Tertiary creep can occur due to necking of the specimen and other processes that ultimately result in failure. The calculation of creep limit includes the temperature, the deformation and the time in which this deformation appears. At high temperature, the influence of work hardening is weakened and there is a possibility of mechanical recovery.

As a result, the creep rate does not decrease and the recovery creep curve is obtained.