A comparison between time- and frequency-domain Carpinteri criteria for fatigue life estimation under multiaxial random loading

Structures and mechanical components are frequently subjected to time-varying loading characterised by a random nature. Their fatigue design is complex, especially in the case of a multiaxial local stress state. A time-domain criterion is often applied to evaluate the fatigue life. Time-domain criteria are based on cycle counting methods. Under both uniaxial loading and multiaxial proportional loading, the Rain-Flow Counting (RFC) procedure is considered as the most accurate method by the scientific community. However, when loading cannot easily be described by using time signals, a frequency-domain criterion has to be employed. Frequency-domain criteria are based on the statistical properties of a suitable counting variable. In the present paper, two critical plane-based criteria proposed by Carpinteri et al. are discussed. Such criteria are able both to perform the fatigue assessment and to estimate the nucleation crack paths. More precisely, one is formulated in time-domain whereas the other one in frequency domain. Such criteria are here modified in order to improve their accuracy in terms of fatigue life estimation, and then applied to experimental fatigue data related to 18G2A structural steel tests available in the literature, performed on smooth specimens under random non-proportional bending and torsion loading.