fatigue

Topics: Elasticity, Fatigue, Fracture mechanics Pages: 33 (5588 words) Published: January 9, 2014
Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

FATIGUE CONSIDERATION IN DESIGN
OBJECTIVES AND SCOPE
In this module we will be discussing on design aspects related to fatigue failure, an important mode of failure in engineering components. Fatigue failure results mainly due to variable loading or more precisely due to cyclic variations in the applied loading or induced stresses So starting from the basic concepts of variable (non-static) loading, we will be discussing in detail how it leads to fatigue failure in components, what factors influence them, how to account them and finally how to design parts or components to resist failure by fatigue

WHAT IS FATIGUE?
Fatigue is a phenomenon associated with variable loading or more precisely to cyclic stressing or straining of a material. Just as we human beings get fatigue when a specific task is repeatedly performed, in a similar manner metallic components subjected to variable loading get fatigue, which leads to their premature failure under specific conditions.

WHAT IS FATIGUE LOADING?
Fatigue loading is primarily the type of loading which causes cyclic variations in the applied

stress or strain on a component. Thus any variable loading is

basically a fatigue loading.

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

Variable Loading
Variable loading results when the applied load or the induced stress on a component is not constant but changes with time i.e load or stress varies with time in some pattern. Most mechanical systems and devices consists moving or rotating components. When they are subjected to external loadings, the induced stresses are not constant even if the magnitude of the applied load remains invariant.

In reality most mechanical components experience variable loading due to -Change in the magnitude of applied load Example: punching or shearing operations-Change in direction of load application Example: a connecting rod -Change in point of load application Example: a rotating shaft There are different types of fatigue/variable loading. The worst case of fatigue loading is the case known as fully-reversible load. One cycle of this type of loading occurs when a tensile stress of some value is applied to an unloaded part and then released, then a compressive stress of the same value is applied and released.

Indian Institute of Technology Madras

Machine Design II

Prof. K.Gopinath & Prof. M.M.Mayuram

A rotating shaft with a bending load applied to it is a good example of fully reversible load. In order to visualize the fully-reversing nature of the load, picture the shaft in a fixed position (not rotating) but subjected to an applied bending load (as shown here). The outermost fibers on the shaft surface lying on the convex side of the deflection (upper surface in the picture) will be loaded in tension (upper green arrows), and the fibers on the opposite side will be loaded in compression (lower green arrows). Now, rotate the shaft 180° in its bearings, with the loads remaining the same. The shaft stress level is the same, but now the fibers which were loaded in compression before you rotated it are now loaded in tension, and vice-versa. Thus if the shaft is rotated let us say at 900 revolutions per minute then the shaft is cyclically stressed 900 times a minute. To illustrate how damaging such type load is, take a paper clip, bend it out straight, then pick a spot in the middle, and bend the clip 90° back and forth at that spot (from straight to "L" shaped and back). When you bend it the other way, you reverse the stresses (fully reversing fatigue). You can notice that the clip will break in a few to about a maximum of 10 cycles.

When you are bending it you are plastically-deforming the metal, you are, by definition, exceeding its yield stress. When you bend it in one direction, you are applying a high tensile stress to the fibers on one side of the OD, and a...
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