Analytical prediction of the phase transformation onset zone at a crack tip of a shape memory alloy exhibiting asymmetry between tension and compression

Abstract

Shape Memory Alloys (SMAs) such as NiTi exhibit stress induced martensitic phase transformation. The purpose of this paper is to provide a better understanding of SMA (such as NiTi) fracture behavior, by considering the vicinity of the crack tip where the transformation occurs. This analysis integrates the asymmetry between tension and compression in an analytical prediction of the surface of phase transformation around the crack tip for loading modes 1, 2, 3 and mixed 1+2. The influence of the asymmetry between tension-compression is more important in plane stress conditions than in plane strain conditions, particularly for mode 1 loading. In order to validate this model, we are currently setting up an experimental investigation to observe strain localization during crack propagation (transformation and martensitic saturation regions) on NiTi thin sheets.

Crack tip stress distribution and stress intensity factor in shape memory alloys

“…As expected, the shape of the transformation region is similar to the plastic zone in common metals, as a direct consequence of the application of plasticity concept and common yield criteria to define the phase transformation conditions in SMAs. However, it is worth noting that the adopted constitutive model does not take into account the asymmetries between tension and compression in SMAs and slightly different contours could be observed, depending on the degree of asymmetry, triaxiality and grain texture as demonstrated in previous works . Figure illustrates the results of isothermal mechanical tests of two SEC specimens in terms of applied load, P , and applied mode I SIF, K I *, as a function of the crosshead displacement, d , of the testing machine.…”

“…However, it is worth noting that the adopted constitutive model does not take into account the asymmetries between tension and compression in SMAs and slightly different contours could be observed, depending on the degree of asymmetry, triaxiality and grain texture as demonstrated in previous works. 7,27,28 Figure 8 illustrates the results of isothermal mechanical tests of two SEC specimens in terms of applied load, P, and applied mode I SIF, K I *, as a function of the crosshead displacement, d, of the testing machine. In particular, Fig.…”

“…As a consequence, a direct transferability of the results to the engineering community is not possible, and proper design criteria that are consistent with the unique properties of NiTi SMAs should be developed. To this aim, some analytical models have been proposed recently, [23][24][25][26][27][28][29][30][31][32][33][34] which are mainly based on modified LEFM concepts.…”

Phase Transformation Surfaces Analysis for SMA Around a Crack Tip with Curvature

“…Analytical prediction of the phase transformation onset zones was performed on a crack tip without curvature for the three modes (I, II and III) and mixed mode I?II [5]. The two cases of plane stress (CP) and plane strain (DP) were investigated.…”

Fracture mechanics of pseudoelastic NiTi alloys: review of the research activities carried out at University of Calabria

“…Furthermore, several numerical studies have been carried out, by using standard finite element codes and plasticity concepts [13][14][15] as well as by special constitutive models for SMAs [16][17]. Finally, some analytical models have been proposed recently [18][19][20][21][22][23][24][25][26][27], which are mainly based on modified linear elastic fracture mechanics concepts. In the present paper a review of the research activities on fracture mechanics of NiTi alloys, carried out at the Department of Mechanical Engineering at University of Calabria (Italy), is illustrated.…”