446 Mechanics of Materials 2 §11.1 11.1.Fatigue Introduction Fracture of components due to fatigue is the most common cause of service failure, particularly in shafts,axles,aircraft wings,etc..where cyclic stressing is taking place.With static loading of a ductile material,plastic flow precedes final fracture,the specimen necks and the fractured surface reveals a fibrous structure,but with fatigue,the crack is initiated from points of high stress concentration on the surface of the component such as sharp changes in cross-section,slag inclusions,tool marks,etc.,and then spreads or propagates under the influence of the load cycles until it reaches a critical size when fast fracture of the remaining cross-section takes place.The surface of a typical fatigue-failed component shows three areas,the small point of initiation and then,spreading out from this point,a smaller glass-like area containing shell-like markings called "arrest lines"or "conchoidal markings" and,finally,the crystalline area of rupture. Fatigue failures can and often do occur under loading conditions where the fluctuating stress is below the tensile strength and,in some materials,even below the elastic limit. Because of its importance,the subject has been extensively researched over the last one hundred years but even today one still occasionally hears of a disaster in which fatigue is a prime contributing factor. 11.1.1.The SIN curve Fatigue tests are usually carried out under conditions of rotating-bending and with a zero mean stress as obtained by means of a Wohler machine. From Fig.11.1,it can be seen that the top surface of the specimen,held "cantilever fashion"in the machine,is in tension,whilst the bottom surface is in compression.As the specimen rotates,the top surface moves to the bottom and hence each segment of the surface moves continuously from tension to compression producing a stress-cycle curve as shown in Fig.11.2. Main beoring Ball race Motor Chuck Load Specimen Fig.11.1.Single point load arrangement in a Wohler machine for zero mean stress fatigue testing. In order to understand certain terms in common usage,let us consider a stress-cycle curve where there is a positive tensile mean stress as may be obtained using other types of fatigue machines such as a Haigh "push-pull"machine.446 Mechanics of Materials 2 511.1 11.1. Fatigue Introduction Fracture of components due to fatigue is the most common cause of service failure, particularly in shafts, axles, aircraft wings, etc., where cyclic stressing is taking place. With static loading of a ductile material, plastic flow precedes final fracture, the specimen necks and the fractured surface reveals a fibrous structure, but with fatigue, the crack is initiated from points of high stress concentration on the surface of the component such as sharp changes in cross-section, slag inclusions, tool marks, etc., and then spreads or propagates under the influence of the load cycles until it reaches a critical size when fast fracture of the remaining cross-section takes place. The surface of a typical fatigue-failed component shows three areas, the small point of initiation and then, spreading out from this point, a smaller glass-like area containing shell-like markings called “arrest lines” or “conchoidal markings” and, finally, the crystalline area of rupture. Fatigue failures can and often do occur under loading conditions where the fluctuating stress is below the tensile strength and, in some materials, even below the elastic limit. Because of its importance, the subject has been extensively researched over the last one hundred years but even today one still occasionally hears of a disaster in which fatigue is a prime contributing factor. 11 .I .I. The SIN curve Fatigue tests are usually carried out under conditions of rotating - bending and with a zero mean stress as obtained by means of a Wohler machine. From Fig. 11.1, it can be seen that the top surface of the specimen, held “cantilever fashion” in the machine, is in tension, whilst the bottom surface is in compression. As the specimen rotates, the top surface moves to the bottom and hence each segment of the surface moves continuously from tension to compression producing a stress-cycle curve as shown in Fig. 11.2. Main beoring / Boll race Chuck Saecimen Fig. I I .I. Single point load arrangement in a Wohler machine for zero mean stress fatigue testing. In order to understand certain terms in common usage, let us consider a stress-cycle curve where there is a positive tensile mean stress as may be obtained using other types of fatigue machines such as a Haigh “push-pull” machine