Issue 33

A. Carpinteri et alii, Frattura ed Integrità Strutturale, 33 (2015) 376-381; DOI: 10.3221/IGF-ESIS.33.41 376 Focussed on multiaxial fatigue Time and frequency domain models for multiaxial fatigue life estimation under random loading Andrea Carpinteri, Andrea Spagnoli, Camilla Ronchei, Sabrina Vantadori Dipartimento di Ingegneria Civile, dell’Ambiente del Territorio e Architettura, Università di Parma spagnoli@unipr.it A BSTRACT . Engineering structures and components are often subjected to random fatigue loading produced, for example, by wind turbulences, marine waves and vibrations. The methods available in the literature for fatigue assessment under random loading are formulated in time domain or, alternatively, in frequency domain. The former methods require the knowledge of the loading time history, and a large number of experimental tests/numerical simulations is needed to obtain statistically reliable results. The latter methods are generally more advantageous with respect to the time domain ones, allowing a rapid fatigue damage evaluation. In the present paper, a multiaxial criterion formulated in the frequency-domain is presented to estimate the fatigue lives of smooth metallic structures subjected to combined bending and torsion random loading. A comparison in terms of fatigue life prediction by employing a time domain methods, previously proposed by the authors, is also performed. K EYWORDS . Multiaxial fatigue. Critical plane approach. Random loading. Frequency-domain. Winde-band spectrum. I NTRODUCTION he design of engineering structures subjected to cyclic loading characterized by randomly varying amplitudes is a complex and critical issue, which becomes even more complex in the case of multiaxial loading. In such a case, the procedures usually employed in the fatigue assessment of structural components are formulated in time domain or, alternatively, in frequency domain. The former procedures usually represent a generalisation of their counterparts for constant amplitude loading, by introducing a cycle counting method (e.g. rainflow method) and a damage model (e.g. the Miner rule) [1-5]. Some criteria present specific cycle counting methods to resolve multiaxial loading histories into individual cycles (e.g. see Refs [2, 6]). The knowledge of the time histories of the local stress or strain tensor components, experimentally measured on the structural component, is required, and many records are needed in order to obtain reliable statistical parameters of the loading process. Frequency-based procedures, instead, require the knowledge of cycle distribution of random loading from a statistical point of view: as a matter of fact, starting from the (PSD) power spectral density function of local stress or strain tensor components, an estimation of damage is directly obtained [7-10]. In the present paper, a stress-based critical plane criterion formulated in the frequency-domain is proposed to evaluate the fatigue lives of smooth metallic structures subjected to multiaxial random loading. It consists of the following three steps: (i) definition of the critical plane; (ii) PSD evaluation of an equivalent normal stress; (iii) determination of fatigue life. More precisely: (i) the critical plane is proposed to be dependent on the PSD matrix of the stress tensor; (ii) on such a verification plane, the PSD of an equivalent stress is defined by a linear combination of the PSD functions of both the T