TABLE OF CONTENTS When the concept of mechanical causes of materials degradation is invoked, mechanical forces are necessarily involved. With the exception of a stationary object in a weightless environment, there are few examples of a mechanical structure, which does not sustain any force. Most of these forces, e.g. the weight force are not destructive but some forces and their attendant stresses can cause rapid material degradation and failure. Principal examples of these destructive forces and stresses are oscillating forces and stresses associated with fatigue, extremely localized areas of very high stress at contact areas between solid bodies that cause wear, internal stresses that accumulate within a material to initiate delamination and peeling. Degradation mechanisms that are dependent on mechanical forces, i.e. wear, fatigue and fracture are discussed below in this chapter.
Direct experience of wear and the importance of controlling it are probably as old as the invention of the wheel and axle. Ancient civilizations were familiar with failure of axles by wear and devised rudimentary lubricants to suppress the wear. Prehistoric cavemen in northern countries probably resented the fact that their primitive foot coverings of un-tanned animal skin were rapidly destroyed by wear. An adequate description of what wear is, i.e. its fundamental causes and mechanisms are in contrast only a recent discovery. A primary reason for this historical delay in understanding, is that the collection of experimental evidence of wear mechanisms relies on sophisticated microscopes and surface profilometry, which were only developed not so long ago.
