in Vol. 5 - January Issue - Year 2004
Fatigue Failures in Machine Parts
Author: Michele Bandini, Director of Peen Service, Norblast Group
Fig. 2: Surfaces of hypoidal bevel gear fracture shows the beach marks that the crack causes as it develops
Fig. 3: Hypoidal bevel gear; pulsed fatigue shows an incredible propagation of fatigue cracks
Fig. 4: Fatigue test part; monoaxial fatigue
Norblast has been manufacturing shot blasting systems since 1975. In 1985 the new branch Norblast „Service“ was completely dedicated to studying and applying the shot peening process. In 2000 the branch Norblast „Service“ acquired its own identity and became Peen Service, a research centre that cooperates with the utmost important aeronautic industries, formula 1 teams and many others to develop and apply shot peening.
The knowledge of fatigue failures in machine parts is relatively recent and certainly does not cover the whole panorama of tribological features. Fatigue means varying levels of operating loads over time. Fatigue is therefore a single term that comprises phenomena such as mechanical fatigue, heat induced fatigue, pitting, fretting, scoring, etc. To talk about fatigue without specifying the type of fatigue is therefore rather vague.
Fatigue Failures and Enucleation of the Defect
All these tribological phenomena have in common the same behaviour of the material. As a matter of fact fatigue failure occurs under modest loads, generally after the component has been in use for a large number of cycles.
As we know the fatigue failure of a machine part is caused by defects in the bulk material. These defects may have occurred during manufacturing. For example, there may be microgrooves in the welding beads. Or the defects may be caused by intrinsic causes in the material itself such as vacancies, dislocations, etc.
Metallic materials are crystalline. This means that they consist of atoms arranged in an ordered pattern. Most metallic materials are polycrystalline. In other words, they are made up of a large number of crystals or grains, each one arranged in its own individual pattern. Each grain has its own features such as molecular orientation, mechanical properties, etc. Defects in the material alter the original properties of the crystal. The main effect of these defects is to greatly increase the possibility of reciprocal slipping of the atomic planes. According to authoritative sources such as „Fuchs“, the first slips occur in those grains with the reticulated planes orientated towards the direction of the maximum applied shear stress. These orientations are the most favourable ones for the slips. After that the slips will occur on the other planes with different orientation.
Both slips that occur under static and cyclical loads progressively alter the surface and create intrusion and extrusion bands, as illustrated in Fig.1.
These bands form significant stress concentrations. A microcrack is thus formed, that is at first subject to shearing strain. At a few grains depth, the crack deviates and extends as a zigzag in a direction perpendicular to the maximum normal stress.
The microcrack proceeds under the cyclical load, at first slowly and then faster and faster. If the area is still under tensile stress, beach marks will appear. These are lines left by the crack as it advances. The crack proceeds until the useable cross sectional area has been excessively reduced in relation to the external load; at this point, fragile impact failure occurs. This phenomenon is extremely complex and no single fatigue failure theory totally explains the mechanics of fatigue failure.
It goes without saying that any treatment which may reduce this effect is highly appreciated.
Shot peening seems to be the most effective method available today. The photographs (Fig.2 - Fig.4) on the left side show some typical fracture surfaces.