Issue 21

C. Maletta et alii, Frattura ed Integrità Strutturale, 21 (2012) 5-12; DOI: 10.3221/IGF-ESIS.21.01 9 Fig. 3.a represent an average estimate of the different regions near the indenter, i.e. they are obtained from the macroscopic stress field and do not take into account the real microstructure of the alloy. In fact, local stress-induced transformation mechanisms, due to different orientations of the martensite variants, and dislocation movement, could occur around the contact region as a consequence of the stress field. Fig. 3.b presents a comparison of the indented profile of a SMA and an equivalent elastic-plastic material having the same Young’s modulus as the austenite phase and a yield stress equal to the start transformation stress Y s AM    . In particular, the profiles at the maximum load of 300 mN and upon unloading, normalized with respect to the maximum depth , are compared. The figure clearly illustrates a smaller residual depth in the SMA after unloading, i.e. it exhibits higher recovery deformation as a consequence of the reversible stress-induced martensitic transformation in the indentation region. In fact, the recovery mechanisms in SMAs can be attributed to both elastic and pseudoelastic properties. a) b) Figure 3 : Preliminary FE results: a) stress-induced transformation contours near the contact region; b) comparison of the indentation profile between an elastic-plastic material and a SMA. Indentation tests Fig. 4 shows the load-displacement (P-  ) curves obtained from indentation tests carried out at increasing values of maximum load: 50 mN (a), 150 mN (b), 300 mN (c) and 450 mN (d). It is worth noting that good repeatability of the P-δ curves was observed, especially at higher values of indentation load; this results from an optimal choice of the test parameters. In fact, the load-displacement curves become smoother and differences between repeat tests decrease with increasing indentation load, due to both a reduction in experimental errors and the greater amount of material undergoing phase transformation. The reversible stress-induced phase transition mechanisms are also demonstrated by the pop-out events [18] which occur in the unloading stage. In addition, as observed from the preliminary FE simulations, the residual depth upon unloading is a useful measure of the functional behavior of the SMA in terms of its pseudoelastic recovery capability. Fig. 5.a shows the values of the maximum depth ( h max ), residual depth ( h r ) and residual depth ratio ( h r / h max ) as a function of the indentation load. The figure illustrates that both the residual depth and the residual depth ratio increase with increasing indentation load, which indicate an overall reduction of the pseudoelastic response of the SMA due to an increased volume fraction of dislocations and stabilized martensite (region D in Fig. 3.a) immediately beneath the indented surface. Similar considerations can be made from an energetic point of view, as illustrated in Fig. 5.b. Specifically, this figure presents the recovery energy ( E e ), i.e. the energy associated with the unloading path, the dissipated energy ( E d ), i.e. the area between loading and unloading curve, the total energy ( E t = E e +E d ), the recovery energy ratio ( E e / E t ) and the dissipated energy ratio ( E d / E t ) as a function of the indentation load. The figure clearly shows an increase of both dissipated and recovery energy with increasing indentation load, which indicate an overall increase of both permanent and recovery deformation, as is also illustrated in Fig. 5.a. Fig. 5.b shows that the difference between the recovery energy and dissipated energy increases gradually with indentation load above 150 mN. This indicates that, although the residual depth increases with increasing load (Fig. 5.a) the recovery energy also increases counteracting the energy dissipated in the formation and movement of dislocations. Fig. 5.b also demonstrates almost constant values of the Dissipated energy ratio and the A : Untransformed austenite B : Transformation region C : Transformed martensite D : Stabilized martensite f AM    s f AM AM      s AM   