Answer EKG Question 35
Q35. A. With reference to the fatigue of engineering components;
A. Explain the influence of stress level at a cyclical frequency on expected operating life.;
B. Explain the influence of material defects on the safe operating life of engineering components;
A. Explain the influence of stress level at a cyclical frequency on expected operating life.;
B. Explain the influence of material defects on the safe operating life of engineering components;
C.
State the factors which influence the possibility of fatigue cracking
of an auxiliary boiler feed water pump shaft and explain how the risk of
such cracking can be minimized.
Answer: Fatigue may be defined as the failure of a material due to repeatedly applied stress. The stress required to bring about such a failure may be much less than that required to break the material in a tensile test.
Materials have varying fatigue limits. The limit can be increased by suitable treatment, use of alloy steels, etc. It can be reduced due to 'stress raisers'; change of section, oil holes, fillets, etc. Environments alter the limit, if it is corrosive the limit could be reduced by about a third.
Load on many structural components varies repeatedly. This can lead to fracture, even though the maximum load (stress) is very much lower than the material's ultimate tensile strength, even below its nominal yield stress. This type of failure is known as fatigue failure. It occurs in all classes of materials, except glass and is one of the most common causes of failure of engineering components, in service.
In the simplest type of laboratory fatigue test, a test piece is rotated continuously, whilst Supporting deadweight loads, and the specimen is subjected to alternating bending moments. Various loading arrangements can be used.
The maximum stress at any section occurs at the surface and fluctuates harmonically about zero, i.e. between equal maximum tensile and compressive stresses. The specimen is slightly ‘waisted’, to prevent the fracture from developing at the loading or Support points — in other words, the region of failure is ‘selected’ by inducing a change in section.
Very gentle changes of the section must be used to avoid stress concentrations, which would seriously lower the observed fatigue strength. A series of identical specimens are tested to fracture, starting at a high-stress level and progressively reducing stress on successive specimens. The stress amplitudes (S) are plotted against the number of cycles to fracture (N), using semi-log or log-log scales. These are called S-N Diagrams. Typical S-N diagrams for non-ferrous and ferrous metals are shown.
.
.
The S-N diagram indicates that the fatigue strength, or endurance strength, decreases with an increasing number of cycles. The S-N curve is sometimes divided into two regions.
Below N = 104 cycles, the effect is known as high stress, or low cycle, fatigue.
Above N = 104 cycles, it is known as low stress, or high cycle, fatigue.
For ferrous metals and very few others, the S-N curve approaches a finite stress aptitude, called the fatigue limit. Below this, a fracture will not develop, however great the number of cycles.
The appearance of a fatigue fracture has several characteristic features(at least in ductile materials such as Mild steel). In materials with less ductility, such as aluminum alloys, recognition of a fatigue fracture is not so easy. Unlike the tensile fracture, there is no apparent plastic deformation, adjacent to the fracture.
The decrease in usable strength under cyclic loading is directly attributed to the fact that the material is not an ideal homogenous solid. In each half cycle, minuscule strains that are not completely recoverable are produced. Due to this crack is to concentrate stresses, until failure occurs.
The minute strains tend to be at grain boundaries and around surface irregularities. The surface finish has a tremendous effect on fatigue strength:-
..
Materials have varying fatigue limits. The limit can be increased by suitable treatment, use of alloy steels, etc. It can be reduced due to 'stress raisers'; change of section, oil holes, fillets, etc. Environments alter the limit, if it is corrosive the limit could be reduced by about a third.
Load on many structural components varies repeatedly. This can lead to fracture, even though the maximum load (stress) is very much lower than the material's ultimate tensile strength, even below its nominal yield stress. This type of failure is known as fatigue failure. It occurs in all classes of materials, except glass and is one of the most common causes of failure of engineering components, in service.
In the simplest type of laboratory fatigue test, a test piece is rotated continuously, whilst Supporting deadweight loads, and the specimen is subjected to alternating bending moments. Various loading arrangements can be used.
The maximum stress at any section occurs at the surface and fluctuates harmonically about zero, i.e. between equal maximum tensile and compressive stresses. The specimen is slightly ‘waisted’, to prevent the fracture from developing at the loading or Support points — in other words, the region of failure is ‘selected’ by inducing a change in section.
Very gentle changes of the section must be used to avoid stress concentrations, which would seriously lower the observed fatigue strength. A series of identical specimens are tested to fracture, starting at a high-stress level and progressively reducing stress on successive specimens. The stress amplitudes (S) are plotted against the number of cycles to fracture (N), using semi-log or log-log scales. These are called S-N Diagrams. Typical S-N diagrams for non-ferrous and ferrous metals are shown.
.
.
The S-N diagram indicates that the fatigue strength, or endurance strength, decreases with an increasing number of cycles. The S-N curve is sometimes divided into two regions.
Below N = 104 cycles, the effect is known as high stress, or low cycle, fatigue.
Above N = 104 cycles, it is known as low stress, or high cycle, fatigue.
For ferrous metals and very few others, the S-N curve approaches a finite stress aptitude, called the fatigue limit. Below this, a fracture will not develop, however great the number of cycles.
The appearance of a fatigue fracture has several characteristic features(at least in ductile materials such as Mild steel). In materials with less ductility, such as aluminum alloys, recognition of a fatigue fracture is not so easy. Unlike the tensile fracture, there is no apparent plastic deformation, adjacent to the fracture.
The decrease in usable strength under cyclic loading is directly attributed to the fact that the material is not an ideal homogenous solid. In each half cycle, minuscule strains that are not completely recoverable are produced. Due to this crack is to concentrate stresses, until failure occurs.
The minute strains tend to be at grain boundaries and around surface irregularities. The surface finish has a tremendous effect on fatigue strength:-
..
|
Type of finish surface roughness (micro inches) |
Endurance (pound per sq inch) |
|
Ground (16-25) |
91.000 |
|
Lapped (12-20) |
100.000 |
|
Super finish (3-6) |
116.000 |
..
Designing against failure by fatigue is very much more complex and difficult than designing for static strength. There are a number of reasons for this which include the high sensitivity to local stress Concentrations and the large dependence on the Corrosive environment. Small specimens of representative material give different results in laboratory fatigue tests than larger specimens or actual structures. The larger the specimen, the lower the fatigue strength, particularly where there are stress concentrators. This explains the minute attention that is required to be given to the cross-section, coarseness of the surface, scratches, too-small fillet radii, and poor distribution of load between bolts and weld flashes. These are only a few of the frequent causes of fatigue failure.
The influence of chemical action is complex and difficult to reduce to quantitative terms. Fatigue cracks usually start from some point of stress concentration, such as a key-way, sharp fillet, micro-structural defect, or even a bad tool mark. Any "locked-in" stress from bad welding cool out or thermal stress can make a significant contribution. Fatigue cracks are not necessarily the result of faulty material. sometimes, bad design will limit the working cross-section of components subjected to alternating stress. Knowledge of the behavior of the material in fatigue will allow an assessment of the useful life of the component to be made so that it can be replaced, after an appropriate working period, such as a bottom-end bolt of a four-stroke engine. For plain steel, the fatigue strength in air is much higher than in water, even in freshwater. The corrosion resistance of a material is more important than its static tensile strength, in determining the corrosion fatigue strength. For example, plain carbon steels show a marked reduction of fatigue strength in freshwater; while chromium steels are only slightly affected by water and the corrosion fatigue strength is unaffected.
A similar phenomenon is fretting corrosion when two components are pressed against each other. Thus slight but repeated relative motion occurs, for example, in holding-down bolts, when the fretting corrosion destroys the joint faces when running. The corrosion products formed, like reddish brown dust of Ferric oxide, in the case of steel, can help to detect this condition.
Designing against failure by fatigue is very much more complex and difficult than designing for static strength. There are a number of reasons for this which include the high sensitivity to local stress Concentrations and the large dependence on the Corrosive environment. Small specimens of representative material give different results in laboratory fatigue tests than larger specimens or actual structures. The larger the specimen, the lower the fatigue strength, particularly where there are stress concentrators. This explains the minute attention that is required to be given to the cross-section, coarseness of the surface, scratches, too-small fillet radii, and poor distribution of load between bolts and weld flashes. These are only a few of the frequent causes of fatigue failure.
The influence of chemical action is complex and difficult to reduce to quantitative terms. Fatigue cracks usually start from some point of stress concentration, such as a key-way, sharp fillet, micro-structural defect, or even a bad tool mark. Any "locked-in" stress from bad welding cool out or thermal stress can make a significant contribution. Fatigue cracks are not necessarily the result of faulty material. sometimes, bad design will limit the working cross-section of components subjected to alternating stress. Knowledge of the behavior of the material in fatigue will allow an assessment of the useful life of the component to be made so that it can be replaced, after an appropriate working period, such as a bottom-end bolt of a four-stroke engine. For plain steel, the fatigue strength in air is much higher than in water, even in freshwater. The corrosion resistance of a material is more important than its static tensile strength, in determining the corrosion fatigue strength. For example, plain carbon steels show a marked reduction of fatigue strength in freshwater; while chromium steels are only slightly affected by water and the corrosion fatigue strength is unaffected.
A similar phenomenon is fretting corrosion when two components are pressed against each other. Thus slight but repeated relative motion occurs, for example, in holding-down bolts, when the fretting corrosion destroys the joint faces when running. The corrosion products formed, like reddish brown dust of Ferric oxide, in the case of steel, can help to detect this condition.
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