Failure of Beam Specimens Examples

1
Failure of Beam Specimens (Examples)
Notched Beams

• 60-mm multi-axial fatigue device, the frequency
  was set to 23.885 kHz so that the amplitude at
  the end of the beam specimen was 1-2 µm.
• With the notch, a maximum von Mises stress of
  about 1.5 GPa was achieved at the notch.
• Failure occurred after approximately 8.3 x 108
  cycles.

Unnotched Beams

• Two beam, 40 mm x 5 mm multi-axial fatigue
  device,
• The amplitude at the end of the beam specimen was
  2 µm, yielding an equivalent stress of 2.80 GPa.
• Failed after 2.14 x 105 cycles at 23.885 kHz
• Crack occurs where predicted by FEM analysis
• Smooth crack initiation site, brittle fracture

2
Stress Lifetime Curve

• Stress-life results for polysilicon in
  vacuum conditions reveal a possible endurance
  limit for this material close to 1.5 GPa, in
  agreement with published data.