Introduction to Fatigue

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Overview of Fatigue. Many different mechanical failure modes exist in all fields of engineering. These failures can occur in simple, complex, inexpensive, or expensive components or structures. Failure due to fatigue, i.e., repeated loading, is multidisciplinary and is the most common cause of

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Introduction to Fatigue

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1. Introduction to Fatigue

2. Overview of Fatigue

3. The principles of fatigue behaviour and fatigue design have been developed, used, and tested by engineers and scientists in all disciplines and in many countries. The current capability of computers and simulated testing has a pronounced influence on the efficiency and quality of today's fatigue design procedures. However, in proper fatigue design, both computer synthesis and analysis must be integrated with proper simulated and field testing, along with continued evaluation of product usage and maintenance, including non-destructive inspection.

4. Tips in Design for Fatigue 1. Do recognize that fatigue failures are the most common cause of mechanical failure in components, vehicles, and structures and that these failures occur in all fields of engineering. 2. Do recognize that proper fatigue design methods exist and must he incorporated into the overall design process when cyclic loadings are involved. 3. Do not rely on safety factors in attempting to overcome poor design proce≠dures. 4. Do consider that good fatigue design, with or without computer-aided design, incorporates synthesis, analysis, and testing. 5. Do consider that fatigue durability testing should be used as a design verification tool rather than as a design development tool. 6. Do not overlook the additive or synergistic effects of load, environment, geometry, residual stress, time, and material microstructure

5. STRATEGIES IN FATIGUE DESIGN Fatigue design methods have many similarities but also differences. The differences exist because a component, structure, or vehicle may be safety critical or non-safety critical, simple or complex, expensive or inexpensive, and failures may be a nuisance or catastrophic. The product may be a modification of a current model or a new product. Significant computer-aided engineering (CAE) and computer-aided manufacturing, CAM) capabilities may or may not be available to the design engineer.


7. Choosing the fatigue life model

8. Purposes of Design 1. Designing a device, perhaps a special bending tool or a test rig, to be used in the plant where it was designed. It is called by an "in-house tool." 2. Changing an existing product by making it larger or smaller than previously, using a different material or different shapes, perhaps a linkage and coil spring in place of a leaf spring. It is called by a "new model." 3. Setting up a major project that is quite different from past practice. A spacecraft or an ocean drilling rig or a new type of tree harvester is example. It is called by a "new product." 4. Designing a highway bridge or a steam boiler. The expected loads, acceptable methods of analysis, and permissible stresses are specified by the customer or by a code authority. It is called by "design to code."

9. Tips in Design Related to Crack Initiation 1. Do recognize that fatigue is a localized, progressive, and permanent behaviour involving the nucleation and growth of cracks to final, usually sudden fracture. 2. Do recognize that fatigue cracks nucleate primarily on planes of maximum shear and usually grow on the plane of maximum tensile stress. 3. Do examine fracture surfaces as part of a post-failure analysis, since substantial information concerning the cause of the fracture can be gained. The examination can involve a small magnifying glass or greater magnification up to that of the electron microscope. Do not put fracture surfaces back together again to see if they fit or allow corrosive environments (including rain and moisture from fingers) to reach the fracture surface.

10. Tips in Design Related to Crack Initiation (cont.) 5. Do consider that stress-strain behaviour at notches or cracks under repeated loading may not be the same as that observed under monotonic tensile or compressive loading. 6. Do take into consideration that your product will very likely contain cracks during its design lifetime. 7. Do recognize that most fatigue cracks nucleate at the surface, and therefore that surface and manufacturing effects are extremely important. 8. Do not assume that a metal that has good resistance to crack nucleation also has good resistance to crack growth and vice versa.

11. Fatigue Loading

12. Tips in Design for Fatigue Test and the Stress-Life (S-N) Approach 1. Do consider the wide range of test systems and specimens available for fatigue testing. Tests can range from those performed on small, highly polished specimens for material characterization to full-scale durability tests of large structures. 2. Do not neglect to refer to ASTM, ISO, or similar standards on fatigue testing and data reduction techniques. 3. Do consider that the fully reversed fatigue strength, Sp at 106 to 108 cycles for components can vary from about 1 to 70 percent of the ultimate tensile strength and that the engineer can substantially influence this value by proper design and manufacturing decisions. 4. Do note that cleaner metals, and generally smaller grain size for ambient temperature, have better fatigue resistance.

13. Tips in Design for Fatigue Test and the Stress-Life (S-N) Approach (cont.) 5. Do recognize that frequency effects are generally small only when corrosion, temperature, or other aggressive environmental effects are absent. 6. Do consider that surface finish can have a substantial influence on fatigue resistance, particularly at longer lives. 7. Do not neglect the advantages of compressive mean or compressive residual stresses in improving fatigue life and the detrimental effect of tensile mean or tensile residual stresses in decreasing fatigue life, and that models are available to account for these effects. 8. Do attempt to use actual fatigue data in design; however, if this is not possible or reasonable, approximate estimates of median fatigue behaviour can be made.

14. Tips in Design for the Strain-Life (?-N) Approach 1. Do consider that inelastic stress-strain behaviour under repeated loading is not the same as that determined under monotonic tensile or compressive loading. Under repeated loading the difference between materials is less than that under monotonic loading. 2. Do not ignore the role of material hardening or softening in cyclic loading applications. Using a monotonic stress-strain curve of a cyclic softening material in a cyclic loading application can significantly underestimate the extent of plastic deformation present. 3. Do consider the importance of material ductility on low-cycle or plastic strain dominated fatigue resistance and the importance of material strength on the high-cycle or elastic strain dominated fatigue resistance. 4. Do recognize that strain-life fatigue data of smooth uni-axial specimens are based on cycles to failure, where failure represents the formation of cracks on the order of 1 mm in depth, which may or may not have caused fracture. 5. Do recognize that mean strains generally affect fatigue resistance only if they produce a non-relaxing mean stress. The greatest effect of mean stress is in the high-cycle fatigue regime.

15. End

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