PSI - Issue 16

Volodymyr Iasnii et al. / Procedia Structural Integrity 16 (2019) 67–72 Volodymyr Iasnii, Petro Yasniy / Structural Integrity Procedia 00 (2019) 000 – 000

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Nomenclature A f

austenitic finish temperature

N R

number of cycles

stress ratio

stress range at half-cycles to failure stress range in the first cycle



 1  min  max  UTS

minimum values of stress maximum values of stress  ultimate tensile strength

yield strength

 y

Pseudoelastic SMA, due to high ability to dissipate the energy, are increasingly used in parts of machines, implants (Auricchio et al. (2015)), damping devices (Yasniy et al. (2017), Torra et al. (2012), Ozbulut et al.(2011)) and other structural elements (Menna et al. (2015), Mohd Jani et al. (2014)). SMA that exhibits pseudoelastic behaviour is subjected to cyclic loading, accumulation of permanent strain ensues on the critical stresses of forward and reverse transformation decrease (Kang et al. (2012), Iasnii and Yasniy (2019)). This behaviour is called functional fatigue. Functional fatigue has been attributed to either the accumulation of dislocations (Miyazaki et al. (1986)) or the stabilization of martensite variants (Brinson et al.(2004), Moumni et al. (2005)) or a combination of these (Wagner et al. (2008)). Since SMA are subjected to intense cyclic loadings during operation, it is important to ensure their reliability and durability under low-cycle fatigue with respect to stress ratios. Literature analysis shows influence of mean stress, stress ratio and strain ratio on structural fatigue, for instance in the following works: Matsui et al. (2006); Robertson et al. (2012), Moumni et al. (2009), Predki et al. (2006). However, there is a gap in the study of the influence of stress or strain ratio on functional fatigue (Kang et al. (2012), Mahtabi and Shamsaei (2016), Predki et al. (2006)). In particular, to investigate the cyclic behaviour of pseudoelastic NiTi shape memory alloys subjected to partly and fully reversed torsional loading hollow and solid shaft cylindrical specimens are used (Predki et al. (2006)). The ratio between the dissipated energy and the elastic strain energy with the increasing of cycle number for torsional loading/unloading tests is similar to fully reversed torsional loading experiments. Thus, it is important to study the influence of stress ratio on functional properties of pseudoelastic NiTi alloy. The influence of stress ratio on functional properties was studied on pseudoelastic Ni 55.8 Ti 44.2 alloy. Particularly, the changes of residual strain, strain ratio and dissipated energy on the stress ratio of shape memory alloy are studied. Material has the following mechanical properties at 0°С ( A f = – 38. 7°С): yield strength,  0.2 = 447 MPa, ultimate tensile strength,  UTS = 869 MPa (Iasnii and Junga (2018), Iasnii et al. (2018)). Chemical composition of material is given in the paper of Iasnii and Junga (2018). Cylindrical specimens with the diameter of 4 mm and gauge length of 12.5 mm were tested under uniaxial cyclic loading at temperature 0°С and stress ratio R =  min /  max = 0 and R = 0.5 (here  min and  max are the minimum and maximum stresses). Tests were carried out under displacement – controlled mode at stress ratio R = 0. In this case, the maximum stress, except for the first twenty loading cycles, remains constant (Iasnii et al. (2018)). The dependences of the stress range on the number of loading cycles at different initial values of stress range in first cycle  1 are shown in Fig. 1. During the first cycles under displacement – controlled mode the stress range increase, i.e., the material is strengthening. Also, one can note weakening and stabilization regions followed by the stress range decrease. An exception is only for specimen (12) with the initial stress range Δσ 1 = 748 MPa, where the value decreases continuously during testing. Fatigue tests were carried out at stress ratio R = 0.5 under stress-controlled mode. Thus, it can be assumed that during testing the maximum and minimum stresses were constant. 2. Material and methodology