Influence of loading pattern in fatigue life for notched round bars subjected to bending-torsion

¹ Department of Mechanical Engineering, ISEC, Polytechnic Institute of Coimbra, Quinta da Nora, 3030-199 Coimbra, Portugal
² CEMUC, Department of Mechanical Engineering, University of Coimbra, Rua Luís Reis Santos, Pinhal de Marrocos, 3030-788 Coimbra, Portugal

^a Corresponding author: rbranco@isec.pt

Abstract

Fatigue behaviour of DIN 34CrNiMo6 lateral notched round bars subjected to in-phase bending-torsion loading was investigated. Experimental tests were conducted under constant amplitude loading with stress ratios close to zero. Crack initiation and crack growth were monitored in-situ using a high-resolution digital system. Fracture surfaces were examined by scanning electron microscope (SEM). A multi-crack initiation phenomenon from material defects at the notch surface was found. The SEM analyses revealed the presence of relatively high inclusions with sizes ranging from 2 to 30 μm. Finally, fatigue life predictions were carried out using both the Coffin-Manson (CM) and the Smith-Watson-Topper (SWT) models. The notch effect was simulated with the Theory of Critical Distances (TCD) and the Equivalent Strain Energy Density (ESED) concept. Regardless of the model, very good correlations between experimental and predicted fatigue lives were observed, particularly for lives greater than 10⁴ cycles.