Question

Linear-elastic fracture mechanics method for fatigue life estimation is more appropriate for which stage(s) of fatigue?

Answer 1

Usually fatigue is divided into three stages initiation, propagation, and final rupture.

To make life estimations for fatigue crack growth and damage tolerant design, the following information are often needed:

– The stress intensity factor, K.

– The fracture toughness, Kc.

– The applicable fatigue crack growth rate expression.

– The initial crack size, ai (ao).

– The final or critical crack size, af (ac).

Linear-elastic fracture mechanics method for fatigue life estimation is more appropriate for propagation stage of fatigue. And also fatigue crack propagation is divided in 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 at 45 degrees. This is known as stage I or the short crack growth propagation stage. The crack propagates until it is caused to decelerate by a microstructural barrier such as a grain boundary, inclusions, 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 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. Whilst stage I is orientated 45 degrees in relation to the applied load, propagation in stage II is perpendicular to the load direction.

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; however, they are clearly seen in pure metals and many ductile alloys such as aluminum. In steels, they are frequently observed in cold-worked alloys. The most commonly 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 tip.

• Stage III: Finally, stage III is related to unstable crack growth as Kmaxapproaches KIC. At this stage, crack growth is controlled by static modes of failure and is very sensitive to the microstructure, load ratio, and stress state (plane stress or plane strain loading). Macroscopically, the fatigue fracture surface can be divided into two distinct regions.