Issue 30

V. Crupi et alii, Frattura ed Integrità Strutturale, 30 (2014) 569-577; DOI: 10.3221/IGF-ESIS.30.68 569 Focussed on: Fracture and Structural Integrity related Issues Investigation of very high cycle fatigue by thermographyc method V. Crupi, G. Epasto, E. Guglielmino, G. Risitano University of Messina, Department of Electronic Engineering, Chemistry and Industrial Engineering (DIECII) crupi.vincenzo@unime.it ; gabriella.epasto@unime.it ; eguglie@unime.it ; grisitano@unime.it A BSTRACT . Nowadays, many components and structures are subjected to fatigue loading with a number of cycles higher than 10 7 . In this scientific work, the behaviour of two kinds of tool steel was investigated in very high cycle fatigue regime. The fatigue tests were carried out at the frequency of 20 kHz and in fully reversed tension- compression mode ( R = -1) by means of an ultrasonic fatigue testing equipment. The radiometric surface temperature was detected during all the test by means of an IR camera in order to extend the Thermographic Method and the Energetic Approach in very high cycle fatigue regime. The failure mechanism of the investigated steels was evaluated by means of several experimental techniques (scanning electron microscopy, Energy Dispersive X-ray spectroscopy and Optical Microscopy). K EYWORDS . Very High Cycle Fatigue; Thermographic method; Energetic approach; Failure analysis; Microscopy. I NTRODUCTION ith the increasing progress of the technological development in structural applications of tool steels, the required fatigue life has increased, so it is very important to determine a safe fatigue strength for 10 9 cycles. Nowadays, the very high cycle fatigue (VHCF) constitutes one of the main fatigue design criteria. Many researches published important scientific works based on the results of ultrasonic fatigue testing. In 1999 Bathias [1] experimentally proved that there is no infinite life in metallic materials and in 2007 Sonsino [2] showed that a continuous decrease of fatigue strength occurs in the range of large cycle numbers. The fatigue crack initiation in the gigacycle regime seems to occur essentially inside the specimen [1] and not at the surface as it generally occurs in the low cycle fatigue (LCF) and high cycle fatigue (HCF) regime. This means that the effect of environment is quite small in the gigacycle regime as the initiation of short cracks is inside the specimen and the surface plays a minor role especially if it is smooth. The effect of internal hydrogen trapped by non-metallic inclusions on high cycle fatigue was first indicated by Murakami et al. [3]. They observed that around the internal inclusion, from which the failure starts, an optical dark area (ODA) can be seen by an optical microscope. The rule of a crack initiation located inside the specimen in VHCF regime is not rigid, but has several exceptions. According to Bayraktar et al. [4], the crack initiation site in some automotive metallic alloys is not always in the interior of the specimen. Moreover, the cause of the initiation site can be not only inclusions, but also the pores. Sohar et al. [5] carried out ultrasonic fatigue testing on AISI D2 type wrought cold work tool steel, finding that crack growth behaviour can follow two mechanisms: the so-called fisheye (internal crack initiation) or half-of fish-eye (near-surface crack initiation), depending to the presence of primary carbides (clusters). Moreover the temperature evolution during the fatigue tests in HCF [6 - 10] and LCF [11] regimes has been investigated by several researchers, but there are only few studies in VHCF regime. Xue et al. [12] investigated fatigue damage W