Fatigue Monitoring of Austenitic Steels with Electromagnetic Acoustic Transducers (EMATs)

Abstract

Early detection of fatigue processes in the cyclically loaded metastable austenitic steel AISI 347 was performed by electromagnetic acoustic transducer measurements in total strain-controlled low-cycle fatigue tests at ambient and elevated temperature. The changes in the physical data were interpreted via microstructural changes observed by scanning- and transmission-electron microscopy, as well as x-ray investigations. The application of physically based measurement data, e.g., time-of-flight from electromagnetically activated ultrasonic signals in austenitic fatigue specimens and total strain, enables measurements of a new hysteresis relationship. In analogy to the common stress–strain hysteresis, this hysteresis gives information about the actual state of fatigue and shows significant changes in shape before specimen failure.

Fatigue Behavior of Metastable Austenitic Stainless Steels in LCF, HCF and VHCF Regimes at Ambient and Elevated Temperatures

“…Figure 7 presents the development of the stress amplitude σa versus the number of cycles N. At 300 °C the cyclic deformation behavior of the investigated steel was characterized by initial cyclic hardening, followed by slight cyclic softening before the final stress amplitude drop associated with macro crack propagation. The microstructural changes due to cyclic plastic deformation are described elsewhere [46,63]. At these load parameters, no αd-martensite formation occurred and, compared to ambient temperature, the stress amplitude was significantly lower (compared Figure 6a with Figure 7).…”

“…M d30 is the temperature at which 50 vol% of α´-martensite is developed by 30% of true plastic deformation [14] and was introduced for the comparison of the metastability of austenitic stainless steels. However, the deformation-induced phase transformation from γ-austenite in to α´-martensite can also take place at higher temperatures than M d30 [46,58,59,62,63]. The amount of deformation induced α´-martensite depends: (i) on the initial conditions, given by production process, such as chemical composition and initial microstructure e.g.…”

“…However, δ-ferrite, in the initial state of metastable austenite, has to be clearly separated from deformation-induced α´-martensite because both phases are ferromagnetic [7][8][9][10]. Deformation-induced phase transformations in metastable austenite affect significantly the fatigue process and are influenced by chemical composition [12][13][14][15][16], temperature [14,[27][28][29][30] (encompassed by stacking fault energy (SFE) [31][32][33][34][35][36][37][38][39]), as well as grain size [32,[40][41][42][43] strain amplitude [29,[44][45][46][47] strain rate [48][49][50] and strain/stress state [51]. Additionally, SFE in fcc materials influences the slip character of dislocation, which has been summarized in the cyclic deformation map as a function of SFE and plastic strain amplitude given in [52][53][54].…”

Fatigue monitoring of metals based on mechanical hysteresis, electromagnetic ultrasonic, electrical resistance and temperature measurements

“…Due to the increase of the dislocation density, the formation of deformation-induced α´-martensite, the development of intrusions and extrusions at the specimen surface and finally the formation of micro and macro cracks a continuous increase of the mean value of time of flight ∆tof mean from the first cycle until specimen failure was observed at ambient temperature. Additionally to the described microstructural changes, residual stresses can influence the ultrasonic velocity and attenuation (Sorich et al, 2015). Figure 7 shows the development of the stress amplitude σ a ( Fig.…”

“…The electrical resistance depends on the resistivity, which is strongly influenced by the defect structure and defect density of each individual material (Kramer et al, 2014;. At temperatures around ambient temperature (AT), plastic deformation of metastable austenitic steels often results in a phase transformation from paramagnetic austenite into ferromagnetic α´-martensite Eifl er, Smaga and Klein, Mechanical Engineering Journal, Vol.3, No.6 (2016) [DOI: 10.1299/mej.16-00303] (Altpeter et al, 2012;Bayerlein et al, 1989;Lo et al, 2009;Mughrabi et al, 1997;Sorich et al, 2015). The cyclic deformation behavior of metastable austenitic steels is predominantly influenced by changes in dislocation structure and phase transformations.…”