PSI - Issue 25

A.Yu. Smolin et al. / Procedia Structural Integrity 25 (2020) 477–485

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A.Yu. Smolin et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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c)

Fig. 5. The cracks in the model specimen shown as lines connected the centers of the pairs with broken bonds at different bending deflections Δ z / l : a) 6∙10 − 5 , b) 2.2∙10 − 4 , c) 6∙10 − 4 .

Particular time moment of the coating delamination depends on the used value of the adhesion between the silicon and the used. The results shown in Figs. 4 and 5 correspond to this value of 690 MPa, i.e. the strength of model silicon in compression. According to the simulation results obtained the general qualitative behavior of the model system does not change dramatically in case of larger values of the adhesion up to 10 GPa. Finally, the coating on the upper face of the bar also delaminates. By the beginning of the plastic deformation of the bar, all the model coating is cracked. For comparison, Fig. 6 shows the picture from scanning electron microscope of the lateral faces of a real nitinol specimen with silicon coating after a three-point bending test. One may conclude that the real silicon coating is not so brittle. However, it is clearly seen that the upper part of the coatings undergoes compression and is folded, while in the lower part of the coating the pores are elongated along the bar axis, so this region undergoes tension. Nevertheless, it is obvious that in future the at least strength parameters of the silicon coating should be corrected. More of that, because the properties of the coating may differ from the properties of bulk material it is suggested that it would be better to assume the porous silicon coating as an elastoplastic, not a brittle elastic material.

Fig. 6. SEM picture of the lateral face of the real nitinol specimen with silicon coating after three-point bending.