Issue 23

G. De Pasquale et alii, Frattura ed Integrità Strutturale, 23 (2013) 114-126; DOI: 10.3221/IGF-ESIS.23.12 117 ( a) ( b) Figure 1 : Geometrical shape of the testing device for shear and flexural fatigue loading with enlarged gap thickness ( a) and SEM image of the actual device ( b) . At the specimen connections with the rest of the device the sharp corners were rounded to avoid the notch-effect in correspondence with specimen-support and specimen-plate conjunctions. The plate is clamped on the side opposite the specimen. A stress distribution combining shear and flexural components affects the material of the film during its bending; this stress distribution is variable across the beam thickness. The device is supplied by an alternate voltage of actuation that causes the gold beam deflection; the amplitude of the alternate bending force is variable and is proportional to the amplitude of the input voltage. The number of loading cycles can easily be determined by the knowledge of the input voltage frequency and the loading time. Nominal dimension [μm] Measured dimension [μm] Specimen length 50.0 46.5 Specimen width 10.0 12.4 Specimen thickness 1.800 1.945 Plate length 420.0 417.2 Plate width 180.0 183.1 Plate thickness 4.800 4.695 Holes side 20.0 18.4 Holes interspace 20.0 21.3 Number of holes 10 x4 10 x4 Lower electrode length 420.0 420.8 Lower electrode width 190.0 192.3 Lower electrode thickness 2.300 2.360 Gap thickness 3.000 3.009 Table 1 : Nominal dimensions and dimensions measured by optical profilometer on actual samples for shear and flexural fatigue loading. Design 2: tensile fatigue loading The schematic of testing device is represented in Fig. 2a and an optical microscope image of the actual test structure is shown by Fig. 2b. The specimen is the small, suspended double-clamped beam located in the center of the device; the specimen has a rectangular cross section and is connected to the structure with rounded corners to avoid local stress concentration effects. The specimen is connected to two movable plates acting as electrostatic actuators, which are constrained at the outer edges by means of trapezoidal beam elements. The trapezoidal shape is pretty original and was