Section 1d - Macro-Micro Aspects

Section 1d - Macro-Micro Aspects - Chapter 3 Macro/Micro...

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Chapter 3 – Macro/Micro Aspects of atigue of Metals Fatigue of Metals 1
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Outline Review key parameters involved in the fatigue process. Fatigue (crack) surfaces and macroscopic features. Fatigue mechanisms and microscopic features. 2
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Definition of Fatigue The definition of fatigue as currently stated by ASTM: The process of progressive localized permanent damage in a material subjected to fluctuating (cyclic) stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of cycles. 3
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Definition of Fatigue (2) “Progressive” implies the fatigue process occurs over a period of time or usage. A fatigue failure is often sudden with no external warning; however, the mechanisms involved may have been operating since the beginning of the component or tructural usage structural usage. “Localized” implies that the fatigue process operates at local areas rather than throughout the entire component or gp structure. These local areas can have high stresses and strains due to: xternal load transfer - external load transfer - abrupt changes in configuration - temperature differentials, residual stresses, and 4 - material imperfections.
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Definition of Fatigue (3) “Permanent” implies that once there is a structural change or material damage due to fatigue, the process is irreversible. “Fluctuating” implies that the process of fatigue involves stresses and strains that are cyclic in nature and requires more than just a sustained load. Ultimate cause of all fatigue failures is that a “crack” has rown to a point at which the remaining material can no longer grown to a point at which the remaining material can no longer tolerate the stresses or strains, and sudden fracture occurs. “Fracture” plies the last stage of the fatigue process is Fracture implies the last stage of the fatigue process is separation of a component or structure into two or more parts. 5
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Fatigue (Crack) Surfaces and Macroscopic Features Typical fatigue failures exhibit the following common aspects. istinct crack nucleation site or sites - Distinct crack nucleation site or sites. - Beach marks indicative of crack growth. - Distinct final fracture region. Review some representative macroscopic fatigue (crack) surface features. 6
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Typical Fatigue (Crack) Surface on Mountain yp ca at gue (C ac ) Su ace o ou ta Bike Seat Post Robert “Bob” Stephen’s Bike 7
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Fatigue Failure of End Bearing due to Slay Inclusions 8
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atigue Failure of a Torsional Shaft Fatigue Failure of a Torsional Shaft T T 9
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Fatigue Failure of Low-Carbon Steel Keyhole ompact Specimens Compact Specimens 10
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Flat-to-Slant Crack-Growth Characteristics Single shear mode Double shear mode Tensile mode Crack front Tensile mode rack front Fatigue surface Transition region Transition gion Crack front w B c w c region Fatigue surface L B Crack-closure damage parameter: ( K eff ) T = 0.5 o B (Newman, 1992) L
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Fatigue Crack Growth Patterns 12
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Summary of Fatigue (Crack) Surfaces
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Section 1d - Macro-Micro Aspects - Chapter 3 Macro/Micro...

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