Literature on Fatigue Failure
Fatigue Crack Propagation
Basically, fatigue crack propagation can be divided into three stages: stage I
(short cracks), stage II (long cracks) and stage III (final fracture). • Stage I: Once initiated, a fatigue crack propagates along high shear stress planes (45 degrees), as schematically represented in Fig. 1. This is known as stage I or the short crack growth propagation stage. The crack propagates until it is decelerated by a microstructural barrier such as a grain boundary, inclusions, or pearlitic zones, which cannot accommodate the initial crack growth direction. Therefore, grain refinement is capable of increasing fatigue strength of the material by the insertion of a large quantity of microstructural barriers, i.e. grain boundaries, which have to be overcome in the stage I of propagation. Surface mechanical treatments such as shot peening and surface rolling, contribute to the increase in the number of microstructural barriers per unit of length due to the flattening of the grains. • Stage II: When the stress intensity factor K increases as a consequence of crack growth or higher applied loads, slips start to develop in different planes close to the crack tip, initiating stage II. While stage I is orientated 45 degrees in relation to the applied load, propagation in stage II is perpendicular to the load direction, as depicted in Fig. 1. An important characteristic of stage II is the presence of surface ripples known as “striations,” which are visible with the aid of a scanning electron microscope. Not all engineering materials exhibit striations. They are clearly seen in pure metals and many ductile alloys such as aluminum. In steels, they are frequently observed in cold-worked alloys. Figure 2 shows examples of fatigue striations in an interstitial-free steel and in aluminum alloys. The most accepted mechanism for the formation of striations on the fatigue fracture surface of ductile metals, is the successive blunting and re-sharpening of the crack
Basically, fatigue crack propagation can be divided into three stages: stage I
(short cracks), stage II (long cracks) and stage III (final fracture). • Stage I: Once initiated, a fatigue crack propagates along high shear stress planes (45 degrees), as schematically represented in Fig. 1. This is known as stage I or the short crack growth propagation stage. The crack propagates until it is decelerated by a microstructural barrier such as a grain boundary, inclusions, or pearlitic zones, which cannot accommodate the initial crack growth direction. Therefore, grain refinement is capable of increasing fatigue strength of the material by the insertion of a large quantity of microstructural barriers, i.e. grain boundaries, which have to be overcome in the stage I of propagation. Surface mechanical treatments such as shot peening and surface rolling, contribute to the increase in the number of microstructural barriers per unit of length due to the flattening of the grains. • Stage II: When the stress intensity factor K increases as a consequence of crack growth or higher applied loads, slips start to develop in different planes close to the crack tip, initiating stage II. While stage I is orientated 45 degrees in relation to the applied load, propagation in stage II is perpendicular to the load direction, as depicted in Fig. 1. An important characteristic of stage II is the presence of surface ripples known as “striations,” which are visible with the aid of a scanning electron microscope. Not all engineering materials exhibit striations. They are clearly seen in pure metals and many ductile alloys such as aluminum. In steels, they are frequently observed in cold-worked alloys. Figure 2 shows examples of fatigue striations in an interstitial-free steel and in aluminum alloys. The most accepted mechanism for the formation of striations on the fatigue fracture surface of ductile metals, is the successive blunting and re-sharpening of the crack