Dear Researchers;
I have reviewed the mechanisms which cause fatigue failure, including the Wood and the Newman mechanisms, but they are both based on the movement of dislocations and slip-planes. BUT, as I know, the fatigue failure occurs below the yield stress, which means no dislocation movement occurs. Am I wrong? if yes, what is the main reason for fatigue failure?
Imagine there is an ideal surface of a component, without any surface microcracks or so. will there be any fatigue failure in it? if so, how does it occur?
Dear Ali:
Dr. Mandal is correct - this was the dilemma that faced the early engineers studying the first recognized fatigue failures in the mid 1800s - how could a component fail at stresses below the yield strength? The answer of course is local stress concentrations particularly at the surface, i.e., from microstructural defects, inclusions, pre-existing microcracks, notches - and often tensile residual stresses, all of which generate locally high stresses (exceeding the yield strength) to promote dislocation activity in the form of cyclic plasticity to initiate and grow fatigue cracks. Since stress is the most important factor promoting fatigue failures - the fatigue lifetime is invariably a function of the stress raised to a high power - lowering the far-field (applied) stresses has to be combined with considerations of design, e.g., by minimizing the presence of notches (the first commercial jet airlines, the De Havilland Comets in the 1950s, failed by fatigue cracks in the fuselage which initiated at the corners of the plane's square windows), reducing the presence of inclusions and other microstructural defects, smoothing the surface of the component to try and remove nicks, scratches, cracks, and ideally putting in beneficial compressive residual stresses - by such processes as shot-peening, laser-shocking, burnishing, cold rolling, etc. - and/or hardened surface layers - by the same types of procedures.
The notion of an ideal surface without imperfections is naturally purely hypothetical. However, if you can polish a surface together with imparting beneficial surface residual stresses, it is possible to suppress the initiation of fatigue cracks at the surface, which transitions the initiation process to occur from sub-surface cracks. As internal cracks have to be essentially twice as large to surface cracks to have the same effect (all other things being equal) and that the initial growth of such internal cracks will not be subjected to external environmental effects, i.e., they will initially grow in vacuo (these sub-surface initiated fatigue cracks are often referred to as "fisheyes"), the fatigue life of the component can be dramatically extended.
Just a final note, whereas metal fatigue is dominated by the role of dislocation plasticity, ceramics can also fatigue by an entirely different process which has nothing to do with dislocations. The effect is much smaller - more subtle may be a better description - but can occur in toughened ceramics and their composites by the cyclic loading induced degradation of their toughening mechanisms, specifically from crack-tip shielding by such mechanisms as crack bridging. If this is of interest, the attached article can provide further details.
ROR
Hi Ali Mohammadnejad,
It is true that fatigue failure takes place at a stress well-below yield stress but that doesn't mean it does not involve dislocations. Yield stress is reached when 102-103 dislocations/m2 have already been moved. Fatigue failure occurs in three stages, namely crack initiation, crack propagation, and final failure. The initial development of a fatigue crack is attributed to the generation and movement of discontinuities (dislocations), which actually strengthens the metal through plastic deformation i.e. work hardening. As the metal becomes work hardened, it loses its ability to plastically deform in the localized area where cyclic stresses are evident. Once the metal reaches its plastic deformation limit, the discontinuity becomes a small crack. The initial crack formation takes place entirely within the elastic deformation regime.
Dear Ali:
Dr. Mandal is correct - this was the dilemma that faced the early engineers studying the first recognized fatigue failures in the mid 1800s - how could a component fail at stresses below the yield strength? The answer of course is local stress concentrations particularly at the surface, i.e., from microstructural defects, inclusions, pre-existing microcracks, notches - and often tensile residual stresses, all of which generate locally high stresses (exceeding the yield strength) to promote dislocation activity in the form of cyclic plasticity to initiate and grow fatigue cracks. Since stress is the most important factor promoting fatigue failures - the fatigue lifetime is invariably a function of the stress raised to a high power - lowering the far-field (applied) stresses has to be combined with considerations of design, e.g., by minimizing the presence of notches (the first commercial jet airlines, the De Havilland Comets in the 1950s, failed by fatigue cracks in the fuselage which initiated at the corners of the plane's square windows), reducing the presence of inclusions and other microstructural defects, smoothing the surface of the component to try and remove nicks, scratches, cracks, and ideally putting in beneficial compressive residual stresses - by such processes as shot-peening, laser-shocking, burnishing, cold rolling, etc. - and/or hardened surface layers - by the same types of procedures.
The notion of an ideal surface without imperfections is naturally purely hypothetical. However, if you can polish a surface together with imparting beneficial surface residual stresses, it is possible to suppress the initiation of fatigue cracks at the surface, which transitions the initiation process to occur from sub-surface cracks. As internal cracks have to be essentially twice as large to surface cracks to have the same effect (all other things being equal) and that the initial growth of such internal cracks will not be subjected to external environmental effects, i.e., they will initially grow in vacuo (these sub-surface initiated fatigue cracks are often referred to as "fisheyes"), the fatigue life of the component can be dramatically extended.
Just a final note, whereas metal fatigue is dominated by the role of dislocation plasticity, ceramics can also fatigue by an entirely different process which has nothing to do with dislocations. The effect is much smaller - more subtle may be a better description - but can occur in toughened ceramics and their composites by the cyclic loading induced degradation of their toughening mechanisms, specifically from crack-tip shielding by such mechanisms as crack bridging. If this is of interest, the attached article can provide further details.
ROR
Agreed with above explanations. Crack initiation is from dislocations below yielding.
In addition to comments by Arka Manda and Robert O Ritchie which are correct, I wish to add that:
1. Defects exist even in the nanoscale range. Broken surface bonds are also material defects. No matter how well a material is polished, defects cannot be said to be totally removed. For example, for steel, a well polished surface will still contain broken chemical bonds, phases having different mechanical and chemical properties – different from the average value commonly quoted for the whole steel material.
2. These defects will amplify externally imposed cyclic stress (fatigue). If the amplification is high enough to cause local yielding (dislocation motion) on a particular phase, a microplastic zone is said to be formed. Dislocations are emitted from these zones which cause the defect to grow and the material to fail at externally imposed cyclic stress far less than the yield strength of the material. Note that apart from micro defects mentioned by ROR, broken surface bonds and vacancies in a steel, for example, are pre-existing thermodynamically initiated defects.
The growth of these defects can be stopped or delayed by imposing surface residual stress or the external cyclic stress should not be high enough to concentrate stress at the defects tips that could cause yielding or dislocation motion.
Dear Ali Mohammadnejad, just to reach out with another angle here: when thinking of fatigue, a key feature to bear in mind is the statistically large number of loading/unloading which take place: Those very small energies we put into the sample, although might not be enough to make a deformation, they can be large enough to help very small defects (as big as atomic steps on the surface or point defects like vacancies or solute atoms in the crystal lattice) to rearrange, relax and/or cluster. So that the next loading/unloading step then may localize a bit further. When added in large numbers, this self-amplifying procedure eventually results in an opening as big as microcracks which then enable plastic activities and develop toward failure.
A mechanism of fatigue failure in this article
Article A mechanism of fatigue failure
Fatigue Failure: Mechanism and Theories | Metallurgy
https://www.engineeringenotes.com/metallurgy/fatigue-failure/fatigue-failure-mechanism-and-theories-metallurgy/41610
Fatigue Failure Mechanisms and Fatigue Testing in the attached file
A review of the fatigue failure mechanism of metallic materials under a corroded environment in this link
Article A Review of the Fatigue Failure Mechanism of Metallic Materi...
Fatigue Failure is presented here]
https://www.slideshare.net/kevalpatil/failure-mechanics-fatigue-failure
Recently, An analysis of fatigue failure mechanisms was presented in June 2020
Article An analysis of fatigue failure mechanisms in an additively m...
This chapter presents a thoroughly description of the fatigue phenomena in engineering materials, starting with an historical synopsis of fatigue failures, the Wöler’s concept of fatigue life and a description of the S-N curves.
More details here
Chapter Fatigue
Hi Ali,
Although I totally agree the professor's comments below, I would like to add a simple comment, which may be a reasonable answer to your question.
Everyone knows a fatigue crack generally initiates at a surface (except for VHCF and other special cases). This is mainly because the strength at the surface is much lower than the strength of bulk (i.e., the ordinary yield strength) due to the free-surface effect. Near the surface, dislocations can move without the effect of the grain boundary strengthening. That is, the strength at the surface can be regarded as that equivalent with the friction strength to move dislocations of the matrix.
The friction strength to move dislocation of the matrix can be approximately estimated based on Hall-Petch law. For instance, the tensile friction strength to move dislocations of a mild steel (averaged grain size: 24.5 μm, bulk yield strength: 260 MPa) was only 111 MPa. It can simply explain why a crack initiate under the loading condition lower than the yield strength.
Based on the above consideration, we have developed a physics-based multiscale model for predicting fatigue life and limit of steels. Although the model does not need any adjustable material constants, the predicted results showed sufficiently good accuracy in strict validation.
Article Multiscale model prediction of ferritic steel fatigue streng...
Extrusion and extrusion are the main causes of fatigue crack initiation. I agree with Kazuki Shibanuma that the reason of the slip below the yield point.
Dear colleage,
I have a book entitled "Elastoplastic behavior of highly ductile materials" which was published by Springer in 2019, both failures of ductile fracture and fatigue are concerned in some sense in this book.
Please read it and give your kind comments.
Yours
M. Zheng
Agreed with Arka Mandal and Kazuki Shibanuma .
Good explanation.
Failure of material under fluctuating stresses at a stress magnitude which is lower than the ultimate tensile strength of the material called fatigue failure. A fatigue failure always starts as a small crack and grows in size under repeated loading of the stress. It is generally occurs in turbines, aircraft etc.
https://www.sciencedirect.com/topics/engineering/fatigue-failure
Please refer to the following paper. Reliable fracture mechanics, IJRRS 3(1): 1-15. You may find new idea regarding fracture mechanics. Fracture is a property of the system not the cause! So the mechanism of the fracture is the same for all leadings, including the fatigue!
Dear Dr Ali:Fatigue is a failure mechanism that involves the cracking of materials and structural components due to cyclic (or fluctuating) stress. While applied stresses may be tensile, compressive or torsional, crack initiation and propagation are due to the tensile component.
For more details please see:
https://inspectioneering.com/tag/fatigue#:~:text=Fatigue%20is%20a%20failure%20mechanism,due%20to%20the%20tensile%20component.
Dear Ali
Fatgue phenomenon of solid objects does occur because the movement of dislocation from the bulk of solids to the free surface. This movement of dislocation is known to take place way under the yield strength of the material of solids. The phenomenon of fatigue does occur in solids desbite of no present of surface microcracks, as long as the movement of dislocations to the free surface is present leading to extrusion/intrusion of the surface, i.e., initiation of fatigue-
microcacks ,then subsequently to fatigue crack probagation and eventually to fatigue failure.
Dr Ali ---please have a look at the following useful links:
https://www.researchgate.net/post/fatigue_failure_S-N_curve
https://www.umflint.edu/sites/default/files/groups/Research_and_Sponsored_Programs/MOM/d.hendrickson.pdf
and most importantly:
https://www.academia.edu/35866739/Solution_Manual_Defo_and_Fracture_Mech_of_Eng_Mat_5th_ed
Regards--
In a stainless steel, a deformation is composed of the elastic deformation of crystal lattices and plastic deformation of moving dislocations which can be expressed by the Ramberg–Osgood model. Cyclic loading causes small local plastic strain and elastic modulus degradation even below the proof stress. The alternative moving dislocations form a fatigue crack. Micro and macro defects, local stress concentration also induce a crack. A reference:
Article Mean and residual stress effects on fatigue behavior in a pr...
Dr Ali----kindly have a look at the following useful link:
https://www.sciencedirect.com/topics/engineering/stress-concentration-feature
Regards-
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