عنوان مقاله [English]
In recent years, shape memory alloys, especially NiTi, have received a great deal of attention in industrial applications. Martensitic phase transformation in shape memory alloys is the most important factor in their unique behavior. In this paper, the formation of stress-induced martensite phase in the crack tip of superelastic NiTi (50.8% Ni) samples was investigated by using the digital image correlation method. In particular, single edge cracked specimens were subjected to fatigue mechanical loading, then the crack length and also displacement fields at the crack tip of specimens were measured by the digital image correlation technique. Control of the crack length was performed using a high magnification camera during the fatigue test. In the following, stress intensity factors were calculated according to ASTM standard E647-15. Obtained results from the fracture analysis show that fatigue threshold values are decreased with increasing the load ratio. In the present paper, for a load ratio of 0.05, during the crack propagation, the fatigue threshold value is 17 MPa m1/2, while stress intensity factor is estimated about 35 MPa m1/2 before the final failure. Also, as a new method in observation of the phase transformation, digital image correlation pictures indicated the formation of stress-induced martensite at the specimen crack tip.
S. Hazar, G. Anlas, Z. Moumni, Evaluation of transformation region around crack tip in shape memory alloys, International Journal of Fracture, 197(1)(2016)99-110.
G. Wang, A finite element analysis of evolution of stress–strain and martensite transformation in front of a notch in shape memory alloy NiTi, Materials Science and Engineering: A, 460(1) (2007) 383-391.
G. Wang, Effect of martensite transformation on fracture behavior of shape memory alloy NiTi in a notched specimen, International Journal of Fracture, 146(1)(2007) 93- 104.
T. Baxevanis, Y. Chemisky, D. Lagoudas, Finite element analysis of the plane strain crack-tip mechanical fields in pseudoelastic shape memory alloys, Smart Materials and Structures, 21(9) (2012) 094012.
A. McKelvey, R. Ritchie, Fatigue-crack propagation in Nitinol, a shape-memory and superelastic endovascular stent material, Journal of Biomedical Materials Research, 47(3) (1999) 301-308.
S. Robertson, Evolution of crack-tip transformation zones in superelastic Nitinol subjected to in situ fatigue: A fracture mechanics and synchrotron X-ray microdiffraction analysis, Acta Materialia, 55(18) (2007) 6198-6207.
S. Robertson, R. Ritchie, A fracture‐mechanics‐based approach to fracture control in biomedical devices manufactured from superelastic Nitinol tube, Journal of Biomedical Materials Research Part B: Applied Biomaterials, 84(1) (2008) 26-33.
S. Gollerthan, Direct physical evidence for the backtransformation of stress-induced martensite in the vicinity of cracks in pseudoelastic NiTi shape memory alloys, Acta materialia, 57(19) (2009) 5892-5897.
M. Daymond, Strain and texture evolution during mechanical loading of a crack tip in martensitic shapememory NiTi, Acta Materialia, 55(11) (2007) 3929-3942.
Y. You, Y. Zhang, Z. Moumni, G. Anlas, W. Zhang, Effect of the thermomechanical coupling on fatigue crack propagation in NiTi shape memory alloys, Materials Science and Engineering: A, 685(1) (2017) 50-56.
C. Maletta, E. Sgambitterra, F. Niccoli, Temperature dependent fracture properties of shape memory alloys: novel findings and a comprehensive model. Scientific reports, 6(1) (2016) 17.