Crack initiation life in notched Ti-6Al-4V titanium bars under uniaxial and multiaxial fatigue: synthesis based on the averaged strain energy density approach

Abstract

ABSTRACT. The fatigue behaviour of circumferentially notched specimens made of titanium alloy, Ti-6Al-4V, has been analysed. To investigate the notch effect on the fatigue strength, pure bending, pure torsion and multiaxial bending-torsion fatigue tests have been carried out on specimens characterized by two different root radii, namely 0.1 and 4 mm. Crack nucleation and subsequent propagation have been accurately monitored by using the direct current potential drop (DCPD) technique. Based on the results obtained from the potential drop technique, the crack initiation life has been defined in correspondence of a relative potential drop increase V/V0 equal to 1%, and it has been used as failure criterion. Doing so, the effect of extrinsic mechanisms operating during crack propagation phase, such as sliding contact, friction and meshing between fracture surfaces, is expected to be reduced. The experimental fatigue test results have been re-analysed by using the local strain energy density (SED) averaged over a structural volume having radius R0 and surrounding the notch tip. Finally, the use of the local strain energy density parameter allowed us to properly correlate the crack initiation life of Ti-6Al-4V notched specimens, despite the different notch geometries and loading conditions involved in the tests.

Recent advances on notch effects in metal fatigue: A review

“…Since the first work performed by Beltrami, the concept of strain energy density has been widely utilized in both static and fatigue behaviour assessment of smooth and notched structures . Considering the methods developed by averaging or weighting the stress/strain‐based control parameters, one may not quantify well the real fatigue damage and link the corresponding failure mechanism.…”

“…Since the first work performed by Beltrami, the concept of strain energy density has been widely utilized in both static and fatigue behaviour assessment of smooth and notched structures. [131][132][133][134] Considering the methods developed by averaging or weighting the stress/strain-based control parameters, one may not quantify well the real fatigue damage and link the corresponding failure mechanism. Various researchers intended to analyse the notch effect from the perspective of energy dissipation, which shows superiority in correlating microscopic and macroscopic experimental evidences.…”

Fracture mechanics approach to fatigue crack initiation at uniaxially loaded notches

“…Researchers developed empirical formulas for fatigue strength reduction factors to illustrate the effect of the notch size on fatigue strength, for example, Härkegård and Halleraker 3 and Luo et al, 4 and to quickly assess its fatigue strength. Further, methods for notch fatigue analysis exist in the relevant literature, for example, the nominal stress approaches (e.g., Luo et al 4 and Yao 5 ), which assume that two parts made of the same material have identical fatigue life if they possess the same K T and nominal stress; local stress–strain approaches (e.g., Visvanatha et al 6 and Saskatchewan 7 ), which assume that the number of loading cycles required for crack initiation of notched components and smooth specimens made of the same material are equal if the local stress–strain response at the notch tip and of smooth specimens are the same; critical distance theory and its variants (e.g., Taylor 8 and Wang et al 9 ), which assume an effective stress for fatigue evaluation equal to (a) the stress at a certain distance from the notch root (the point method) 10 or (b) the average stress over a deterministic (i) distance ahead of the notch root (the line method), 10 (ii) the area in the vicinity of the notch within a deterministic radius (the area method), 11 or (iii) the hemispherical volume within a deterministic radius (the volume method) 12 ; and weighting control parameter‐relevant approaches (e.g., Liao and Zhu 13 ), which evaluates the fatigue damage by using either averaged or weighted control parameters defined by (a) stress (the stress field approach developed by Yao 14 ), (b) strain (the local stress–strain field intensity approach developed by Shang et al 15 ), or both, like strain energy density 16–19 A successful approach to notch fatigue should be linked to real fatigue failure mechanism.…”

Influence of Notch Sensitivity and Crack Initiation Site on Low Cycle Fatigue Life of Notched Components under Multiaxial Non-proportional Loading

“…Several methodologies for the evaluation of the local values of strain and stress for non-proportional loading at the notch tip are reported in previous researches [25][26][27][28]. Some works demonstrated also that energetic approaches such as the evaluation of the average strain energy over a control volume around the notch [29][30][31][32] can be applied both for static and multiaxial fatigue field [33][34][35][36][37]. Recent and past suggested that the key for a correct evaluation of fatigue life are the strain and stress gradients in the proximity of the notch [38][39].…”