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FATIGUE MECHANICAL LIFE

DESIGN-A REVIEW CLASS NOTES

 Abstract

Fatigue is due to cyclic loading and

unloading of one kind or the other.

It is due to the presence of discontinuities

in the material. Mostly fatigue failure is

progressive and plastic in nature. It is due

to the nucleation, growth and propagation

of a micro crack at the point of a

discontinuity. Plain low carbon steels have

unlimited fatigue life. Nonferrous &

ferrous materials have limited fatigue life.

Fatigue is mostly due to tensile stresses

and is random as well as sudden without

any warning. 90 % of the service failures

are due to fatigue. There is lot of

information on fatigue failures. It is being

continued because of complex nature of

fatigue failures. This results in loss of life

and property. Avoid fatigue failures by a

proper selection of material & surface

finish.  Stress raisers, residual stresses,

reliability, surrounding  environment and

temperature affects fatigue. Fatigue is due

to cyclic loading and unloading. However,

the fatigue reduces by proper selection of

fatigue resistant material like composites.

Further, fatigue also decreases by drilling a hole at

the point of a probable crack. Use of laser

peeing and high frequency mechanical

impact (HFMI) treatment of welds reduce

fatigue. Use stress strain fatigue life

approaches for plastic and elastic

deformations respectively. This short

review paper cannot treat the vast subject

thoroughly.  The reader may consult

consult more references for additional

knowledge.

Introduction

Under cyclic loading and unloading, failure is due to fatigue. Fatigue/endurance limit (σ_e) represents a stress level.  Below which the material does not fail even after infinite number of cycles. Fatigue is reduction in strength due to a progressive and localized structural damage. Fatigue takes place in a moving component only. For example, in automobiles, ships , aircraft wings and nuclear reactors, jet engines, and turbines.

First time, fatigue  became known as early 1800 in Europe. It was observed that bridge and railroad components were cracking subjected to repeated loading[1-10].

Three basic factors to cause fatigue are

(i) a sufficiently high tensile stress

(ii) a large variation in the applied stress

(iii) a sufficiently large number of repetitions in loading and unloading.

The nominal maximum stress which causes fatigue is much less than the ultimate tensile strength of a brittle material. It is less than the yield stress of a ductile material. If the stress present is above a certain threshold value, microscopic cracks will start at the points of stress concentrations. For example, like a scratch, key way, square holes or sharp corners. The crack then travels along weaker points.  Ultimately it results in a fracture. Fatigue is thus a progressive plastic failure. This phenomenon occurs in three phases namely

(i) crack initiation

(ii) crack propagation

(iii) catastrophic overload failure

There are two types of materials experiencing fatigue. One type which has a fixed endurance limit as plain low carbon steels. These steels do not undergo fatigue even for infinite life. It happens when  the actual stress present in the component is slightly less than the fatigue limit. There are also brittle or ductile materials which do not have a fixed fatigue limit. For example, Cast iron, Copper, Aluminum and their alloys. The design for such materials for a fixed number of cycles 5 x 10⁸(500 million cycles). If the component has 750 RPM with one reversal per cycle, it will have a life of about four years. If the RPM increases, life will reduce [1-16].

Thus importance of fatigue is that it directly governs the useful life of a component under cyclic loading. Continuous research is there on fatigue. It is because number of well-known catastrophic fatigue failures which took place all over the world. Avoid fatigue failures by a proper selection of material. Surface finish, stress raisers, residual stresses, reliability, surrounding  environment and temperature help in the selection of material.

Salient features of fatigue include randomness and sudden failure without any warning. It is mostly due to tensile stress and the presence of a stress raiser. There is a strong affect of the surrounding environment & temperature. Surface finish and residual stresses also affect it. Fatigue reduces by proper selection of fatigue resistant material like

(i) composites

(ii) drilling a hole at the point of a probable crack

(iii) use of laser peeing and use of high frequency mechanical impact (HFMI) treatment of welds.

Out of different fatigue design approaches, stress life and strain life has been used for plastic and elastic deformations respectively [17-21].

PRACTICAL FATIGUE FAILURES

1.  Shafts, buckets, disks and blades

           of jet engines

1.  Crank shafts of ground vehicles

2.  Gears used in ground vehicles

3. Gears used in mining equipment

          and marine equipment

4.  Compression springs in ground

automobiles

5. Anything or everything in motion

under cyclic loading of one kind or

the other.

6. Low amplitude and high cycle

loading is the common cause for fatigue.

7. It exists in

(i) jet engines Vanes

(ii) Spacers

(iii) Disks

(iv) Blades

(v) Sheet metal work

(vi) Compressors

(vii) pumps

(viii) turbines

(ix) bridges

STEPS TO REDUCE FATIGUE

1. Drill a hole at the point of a probable crack.

2.  Use a fatigue resistant material like composites.

3. Utilize laser penning

4. Employ high frequency mechanical impact (HFMI) treatment of welds

PRINCIPAL CONSIDERATIONS IN DESIGN AGAINST FATIGUE

Fatique requires a durable and dependable design. It requires thorough deep knowledge and practical experience. Thus, while designing for fatigue, it is important to know which loads are more  frequent. Which loads are occasional and exceptional?  Past experience is very helpful in this determination. Fatigue exists in every sphere of life. There are a few important principal considerations in fatigue design.

1.  Keep design stress below threshold of endurance limit.

2.  Select materials free from discontinuities.

3.  Shape selected should be free of stress raisers.

4. Assume limited safe life say for 5/10 years.

5. Predict the fatigue life based on fatigue crack growth rates for a crack of a certain size.

6. Check fatigue design on the basis of

(i) Strength

(ii)Stiffness

(iii) Stability

(iv)Wear and

(v)Various theories of elastic failures

CONCLUSIONS

1. Fatigue behavior is based on many factors which are random in nature.

2. Design related to fatigue is closely related to the geometrical shape and dimensions. It depends on quality of the fabrication, type and size of acceptable defects.

3.  Do the fatigue load analysis in detail. It is to done to know the stress strain behavior in actual use.

4. Designer be knowledgeable and experienced. He should  be able to interpret main factors affecting fatigue resistance. He selects

5.  Material of construction after considering all possible considerations affecting fatigue.

6.   Proper fatigue strength curve as per details of use of the component.

7. Select life with utmost care.

8. Empirical design should be based analytical research. Experimental findings and experience help in design.

9. Before designing, consult the following codes and standards.

(i) AASHTO for steel bridge

(ii)  ASTM fatigue and fracture standards

(iii) Consult FEM analysis of welded joints.

1. Do the design on the basis of

(i) Strength

(ii) Stiffness

(iii) Stability

(iv) Wear

(v) Various theories of elastic failures as per selected material of construction.

References

1. C. Juvinall, “Engineering Considerations of Stress, Strain, and Strength”, 1967

2. A. Graham, “Fatigue Design Handbook”, SAE, 1968

3. F. Madayag, “Metal Fatigue: Theory and Design” 1969

4. Little, R.E. &Jebe, E. H., “Statistical design of fatigue experiments”,1975

5. Kim, W.H.; Laird, C. Crack, “Nucleation and State I Propagation in High Strain Fatigue- II Mechanism”. pp. 789–799, 1978

6. O Fuchs and R. I. Stephens, “Metal Fatigue in Engineering”, 1980

7. C. Osgood, “Fatigue Design”, 2nd Ed. 1982

8. A. Ballantine, J.J. Conner, and J.L. Handrock,”Fundamentals of Metal Fatigue Analysis”, 1990

9. Bäumel, Jr and T. Seeger,“Materials data for cyclic loading, supplement 1. Elsevier”(1990).

10. E. Dowling, “Mechanical Behavior of Materials”, 1993

11. Schutz, W. “A history of fatigue”.Engineering Fracture Mechanics, 54: 263–300.1996

12. Subra Suresh,“Fatigue of Materials”, Second Edition, Cambridge University Press, 1998

13. Stephens, Ralph I.; Fuchs, Henry O.,“Metal Fatigue in Engineering (Second Ed.)”. John Wiley & Sons, Inc. 69.2001

14. Mott, “Machine Elements in Mechanical Design”, 2003

15. Ali Fatemi – University of Toledo, “Fatigue Design Methods”, Chapter 2,2004

16. Pugno et al. / J. Mech. Phys. “Solids”, 54, 1333–1349, 2000.

17. Tapany Udomphol. “Fatigue of Metals”, p. 54. sut.ac.th, 2007.

18. Pook, Les.“Metal Fatigue, What it is, why it matters”, Springer.

19. Draper, John,“Modern Metal Fatigue Analysis”, EMAS, 2008

20. Schijve, J.,“Fatigue of Structures and Materials”, 2nd Edition with Cd-Rom. Springer, 2009

21. Lalanne, C.,“Fatigue Damage”, ISTE – Wiley, 2009