PSI - Issue 25

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

482

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

6

2,b) downwards in the vertical direction (negative along the Z -axis) with a constant velocity of 1 m/s. The automaton size is 2 µm, the total number of particles in the model is 1 363 523. Adhesion between the silicon coating and the nitinol substrate is characterized by the critical value of the tensile strength in the pairs of silicon and nitinol automata and was varied in this study from 16 MPa up to 10 GPa.

4. Simulation Results

Fig. 3 shows the fields of the equivalent (shear) stresses and mean (hydrostatic) stresses in the specimen just before appearing the first damages. It can be seen that the shear stresses are distributed symmetrically relative to the mid-plane of the specimen. The field of mean stresses (hydrostatic pressure with the opposite sign) is also symmetrical, but the upper part of the specimen undergoes compression while the lower part tension as it should be under flexural loading. Thus, the results depicted in Fig. 3 confirm that the stress fields in the model bar are correct and correspond to the assumed properties of the materials.

a)

b)

Fig. 3. The fields of (a) equivalent and (b) mean stresses (Pa) in the model specimen just before the first cracks initiation.

Fig. 4 shows the loading curve, which is plotted as bending stress σ b =3 Fl /(2 bh 2 ) versus normalized bending deflections Δ z / l , where F is the resistance force to the loading, b = h = 10 5 µm is the width and height of the bar (with the coating), l = 0.5 mm is the distance between fixing lines, Δ z is the displacement of the loading plane. The appearance of the first cracks in the coating corresponds to a jump in the plot at approximately 16.6 MPa; further cracking and delamination of the coating leads to a “waviness” of the loading curve. After 500 MPa the bar undergoes plastic deformation and the curve deflects according to the assumed work hardening of the material.