Improved Fatigue Strength through POLIGRAT Processes
Fatigue fractures mostly start in a small way at specific crack risers. Then the fracture usually continues to progress steadily but unrecognized, until it finally precipitates a sudden violent break in the weakened remaining section.
The fracture strength, in other words the strength to resist fatigue fractures in dynamically stressed components, is determined to a considerable extent by the form and structure of the material layers near the surface.
Over 80% of fatigue fractures start at crack risers on or near the surface. In many cases the targeted use of POLIGRAT processes can considerably reduce the danger of fatigue fractures in metal components and increase their useful life many times over.
POLIGRAT processes remove or reduce most of the external crack risers, which can be divided into two groups:
- Crack risers as a result of the finishing. These are essentially burrs and tears along edges, sharp-edged structures of the surface profile or deepened grain boundaries. Notch effects at the base of these structures lead to a local concentration of tensile strain with the danger that a tear with a subsequent fatigue fracture may begin there.
- Crack risers of several μm depth within damaged material layers near the surface as a result of the mechanical, thermic and chemical influences during processing. These material layers, also known as Beilby layers, differ considerably with regard to composition, structure and internal stress compared with the undamaged material. They often involve local tensile strain, embedded foreign matter and oxides, coarse-grain formation and local emissions at grain boundaries which can precipitate fatigue fractures.
POLIGRAT processes remove damaged material layers together with all the foreign matter, tensile and pressure strain and other defects completely and reliably, without stress and in a controlled manner. They bring to bear the undisturbed basic structure with its best qualities on the surface. At the same time, the edges and surfaces are deburred across the entire area. The surface profile is smoothed and rounded on a micro-level. Cracks and sharp-edged structures are either entirely removed or, if they are larger, are opened and rounded in such a way that they will no longer cause a notch effect. At the same time the opening of cracks permits a reliable quality control.
POLIGRAT processes can largely be applied independently of the form and size of the components and the hardness of the material. Tried and tested processes are available for most metals and alloys in industrial use.
Extensive expeirence proves that in components that are subject to major dynamic stress, processing with POLIGRAT processes will result in an increase in the fatigue strength by a factor of 4 to 10, in extreme cases by a factor of 30. Typical applications from a wide range of examples include mechanical springs, hydraulic components under pressure threshold loading, components for turbine and engine construction and electrical switching contacts.
POLIGRAT offers comprehensive advice for all applications and proof of achievable results through the processing of samples of original parts in our own technical centre.
|Fig. 1: Springs||Fig. 2: Springs – detail view|
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