Introduction to Fatigue Analysis

Keywords: crack propagation; plastic strain; low cycle fatigue; elastic range; high cycle fatigue; fracture mechanics

Summary

Fatigue may be characterized as a progressive failure phenomenon that occurs by the initiation and propagation of cracks to an unstable size. Fatigue failures continue to be a major concern in engineering design. It is estimated that the annual cost of fatigue of materials to the U.S. economy in 1982 dollars was around $100 billion. These costs arise from the occurrence or prevention of fatigue failure for ground vehicles, rail vehicles, aircraft of all types, bridges, cranes, power plant equipment, offshore oil-well structures, and a wide variety of machinery and equipment.

History

In the early 1800’s there were reports of various engineers studying fatigue and failure of railroad equipment. One of the first documented fatigue investigations was reported by a German mining engineer, W. A. S. Albert, who in 1829 performed repeated loading tests on hoist chains. Later in the 1850s, August Wohler’s research on railway axle failures led to the development of strategies for averting fatigue failure caused by bending, torsion, and axial loads. Wohler also demonstrated the influence of both cyclic and accompanying steady stresses on fatigue. For a more comprehensive understanding of the historical development, read more about the History of Fatigue Analysis.

Introduction

Fatigue failure investigations over the years have led to the observation that the fatigue process embraces two domains of cyclic stressing or straining that are significantly different in character, and with each – failure is produced by different physical mechanisms. One domain of cyclic loading is when significant plastic straining occurs during each cycle, which is associated with loads exceeding the material elastic limit, resulting in low cycles to failure – commonly referred to as low-cycle fatigue.

The other domain of cyclic loading is when strain cycles are largely confined to the elastic range, and is associated with lower loads and long lives, or high numbers of cycles to failure – commonly referred to as high-cycle fatigue. Low-cycle fatigue is typically associated with cyclic lives from one up to about 10^4 or 10^5 cycles, and high-cycle fatigue for lives greater than about 10^4 or 10^5 cycles.

This curve shows the typical relationship between stress, strain and cycles to failure (Ref. 5)

In equipment design, preventing fatigue failure during the designated lifespan is imperative. Components enduring numerous cycles should be designed to maintain stress amplitudes below fatigue design curves, ensuring an infinite life. Conversely, components experiencing stresses surpassing fatigue strength are engineered for a finite life.

Presently, three major approaches exist for analyzing and designing against fatigue failures:
Stress-Based Approach: This traditional method relies on nominal (average) stresses within the component area. It determines the cyclic load’s resistible nominal stress by accounting for mean stresses and adjusting for stress-raising features like grooves, holes, and fillets.
Strain-Based Approach: This involves a more intricate analysis of localized yielding at stress-raising points during cyclic loading.
Fracture Mechanics Approach: Primarily considering material toughness, crack size, and stress levels, this approach requires an understanding of crack growth and fracture.

Fatigue Failures

Fatigue is the most common cause of fatigue failure in mechanical and thermal processing equipment, accounting for an estimated 70 percent of such failures.
The basic reason relates to the myriad of the causes of failures which include deficiencies in design, materials, processing complexities and owner/operator error. Failures have been traced to corrosion, material defects, stress risers, structural hot spots, dynamic loads, variable weather conditions and other factors not adequately covered by Design, Safety and Fabrication Codes & Standards.

References

(1) “Mechanical Behavior of Materials – Engineering Methods from Deformation, Fracture and Fatigue” Norman E. Dowling, College of Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1993, Prentice Hall, NJ 07632
(2)“Failure of Materials in Mechanical Design – Analysis, Prediction, Prevention” J.A. Collins, Ohio State University, 1981, John Wiley & Sons
(3) “Metal Failures – Mechanisms, Analysis, Prevention” Arthur J. McEvily, Professor Emeritus, Department of Metallurgy and Materials Engineering, University of Connecticut, 2002, John Wiley & Sons
(4)“Engineering Against Fatigue” J.H. Benyon, M.W. Brown, T.C. Lindley, R.A. Smith, & B. Tomkins, Eds., 1999, A.A. Balkema, Rotterdam, Netherlands
(5) “Code Design and Evaluation for Cyclic Loading – Sections III and VIII” W.J. O’Donnell, Chapter 39 Companion Guide to the Boiler & Pressure Vessel Code- Criteria and Commentary on Select Aspects of the Boiler Pressure Vessel and Piping Codes, Second Edition, Vol. 1, Ed. K.R. Rao, 2006. ASME

Fatigue Analysis Resources

– ASME Standards
– ASME Committee on Design Methods
– ASTM Fatigue Standards and Fracture Standards
– ASM Handbook, Volume 19: Fatigue and Fracture