PECM Issue 36 2019 | Page 124

PROCESS TECHNOLOGIES FATIGUE FAILURE REDUCTION POLIGRAT HIGHER FATIGUE RESISTANCE DUE TO POLIGRAT-PROCESSES Fatigue failures, also called fatigue fractures, mostly start small crack initiating structures and defects. Cracks grow mostly undetected until they finally cause a sudden forced rupture of the reduced remaining cross section. The fatigue resistance i.e. the resistance to fatigue cracking of dynamically loaded components is largely determined by shape and structure of near surface layers of material. More than 80% of fatigue cracks start at local surface related structures. In many cases the application of POLIGRAT-Processes significantly can reduce the risk of fatigue cracking and correspondingly increase lifetime by several times. POLIGRAT Processes eliminate or reduce most of the near surface crack initiating structures, which can be divided into two groups: • Cracks initiated by micro notches. Essentially these are cracks at burrs and edges and sharp structures of the profile or crevices at grain boundaries. Stress concentration at the base of notches cause local concentration of tensile stresses with the risk, that there will start a fatigue crack with the consequence of later rupture. • Crack initiating structures within a surface related material layer with a thickness of several micro meters, which are caused by mechanical, thermal or chemical influence from fabrication. These layers, also called Beilby-layers, significantly differ from the original base material with respect to composition, structure and stresses. They often contain local tensile stresses, embedded foreign matter, oxides coarse grains and precipitations at grain boundaries, which are capable to initiate cracks. POLIGRAT-Processes controlled and stress free remove all damaged material layers from the surface with all the inherent stresses, foreign matter and other defects. They bring the undisturbed base metal with its best properties to the surface. At the same time edges and surfaces are deburred and the micro structures smoothened and rounded. Existing micro cracks and sharp structures are removed or if they are larger in size opened and rounded to a degree, that they are no more notch factors. Revealing of cracks also provides a reliable quality control. POLIGRAT-Processes are applicable widely independent of shape and size of parts and hardness of material. For most of the commonly used metals and alloys industrially proven processes are available. 124 PECM Issue 36 Long time experience show, that on dynamically heavily loaded components an increase of fatigue resistance by the factor 4 to 10 or more will result from the application of POLIGRAT-Processes. Typical applications are springs, turbine blades, hydraulic parts and nozzles. POLIGRAT provides comprehensive advice for all applications and verification of achievable results by processing samples of original parts in their own laboratory. Valentin-Linhof-Straße 19 D-81829 München Tel. +49 (0)89 42778-0 / 01252 336337 Fax +49 (0)89 42778-309 email: [email protected] or [email protected] www.poligratuk.co.uk