Issue 41

P. Lorenzino et alii, Frattura ed Integrità Strutturale, 41 (2017) 191-196; DOI: 10.3221/IGF-ESIS.41.26 191 Focused on Crack Tip Fields Influence of forging conditions on the fatigue mechanisms of low alloy steels: a 3D study Pablo Lorenzino, Jean-Yves Buffiere, Catherine Verdu INSA Lyon MATEIS Bat Saint Exupery 25 Av. Jean Capelle F-69621 Villeurbanne Cedex France jean-yves.buffiere@insa-lyon.fr A BSTRACT . The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 μm wide x 50 μm deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface. K EYWORDS . Short cracks; 3D propagation; Crack closure; Synchrotron X-ray tomography; Forging. Citation: Lorenzino, P., Buffiere, J.-Y., Verdu, C. , Influence of forging conditions on the fatigue mechanisms of low alloy steels: a 3D study, Frattura ed Integrità Strutturale, 41 (2017) 191-196. Received: 28.02.2017 Accepted: 15.04.2017 Published: 01.07.2017 Copyright: © 2017 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION ith the enforcement, in the very near future, of new European regulations in terms of fuel consumption, car manufacturers have put a large effort in trying to reduce the weight of vehicles in the last ten years. This has led to a global increase of the load level experienced by many components in the body, the panels, the wheels or the engine of cars. Being able to accurately predict the fatigue life of those components has now become a key issue for keeping the same trend and reducing the vehicle weight further. A good example, in the field of metallic components, is given by the connecting rods or crankshafts which are submitted to higher service load for a very large number of cycles. Although it is well accepted that the methods used to produce such components (mainly hot and cold forging) have a strong effect on the material microstructure and, therefore, on their mechanical properties, the thermo-mechanical history experienced by the metals during processing is not accurately taken into account at the design stage leading to over conservative safety coefficients. W