FATIGUE RESISTANCE OF PARABOLIC SPRINGS

MIRCO D. CHAPETTI; SEBASTIAN JAUREGUIZAHAR; BOJAN SENCIC; NENAD GUBELJAK

Mar del Plata

Congreso; 20 Congreso Internacional de Materiales. SAM CONAMET 2022; 2022

INTEMA

Spring leaves usually fail by fatigue failure nucleated from internal inclusions. The initiationlocation depends on different parameters associated with the fatigue mechanism that gives rice to a crack, and then it is related with the weakest configuration of the material resistance. High surface compressive residual stress amplitude is usually introduced by shot-peening in order to prevent fatigue crack propagation from the surface of bending loaded spring leaf. This moves the fatigue crack nucleation process to the inner part of the leaf where the effective driving force is higher. The objective of this work is to analyze the resistance of parabolic springs produced with 51CrV4 steel. The analysis is carried out by using a fracture mechanic approach [see details of its application in [1] and [2]). Surface and internal initiation processes, that compete with each other, are compared. The mechanical and fatigue properties of the 51CrV4 steel are reported in another 1/2 work [3]. The steel parameters used here are: Kthr = 9,2 and 5,06 MPa m for R = -1 and 0,1, respectively (fatigue threshold for long cracks), VH = 1440 for R = -1 (fatigue limit of the steelmatrix), and d = 0.005 mm (grain size). Inclusions are usually MnS, Al-silicates and TiN. The sizes of inclusions vary between 0,05 to 0,5 mm. Spring leaf in its final condition was used for residual stress measurement by X-ray diffraction method. The measured compressive residual stress on the surface was -493 ± 20 MPa in longitudinal direction. The maximum compressive residual stress value was obtained at 0.25 mm in depth and was about 1100-1200 Mpa. Six track spring leaves were tested by three points bending by the producer. Applied stress range was 1200 Mpa at a stress ratio R = 0,1 (frequency of 1 Hz). Table 1 shows the number of fatigue cycles to failure, the defect size that led to the failure and the depth at which the failure nucleated (defect position).