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SUMMARY:Crack Propagation in Brittle Crystals under Combined Tensile and S
 hear Stresses - Dr Dov Sherman: Technion\, Israel
DTSTART:20091030T143000Z
DTEND:20091030T153000Z
UID:TALK19803@talks.cam.ac.uk
CONTACT:Anita Clayson
DESCRIPTION:The way cracks in brittle crystals initiate and propagate is a
  fundamental problem in fracture\, not yet elucidated. The main complicati
 on is the fact that two opposing mechanisms dictate crack path\; the tende
 ncy of the crack to propagate on the preferred low energy cleavage plane a
 nd the requirement to reduce the shear stresses following propagation. Ano
 ther complication is attributed to the anisotropy of the elastic propertie
 s. Among the materials properties\, the cleavage energies of the preferred
  planes play a major role in the fracture processes.\nWe study the way cra
 cks initiate and propagate in brittle crystals using a special designed ex
 perimental methods. Specimens are fractured under three point bending and 
 under combined tensile and shear modes in controllable manner. \nWe fractu
 re silicon as the most studied\, nearly ideal brittle crystal. As the two 
 low energy cleavage planes of silicon\, {111} and {110}\, have comparable 
 cleavage energies\, complex occurrences during fracture are assured.  The 
 phenomenon of crack deflection from {110} plane of silicon to {111} plane 
 from energy consideration was identified. It was found that the deflection
  is velocity and crystallographic orientation dependent. The generation of
  new type of surface instabilities at low crack velocity on the {111}<112>
  crack system of silicon were revealed recently under bending. Same new su
 rface instabilities were revealed under tension. This is in contrast to th
 e assumption that slow cracks are stable. We showed that contrary\, fast c
 racks in crystals are stable. \nThe path and surface instability occurring
  in {111}<112> and {110}<110> cleavage systems of silicon under tensile an
 d tensile and shear modes were studied. We show that cracks propagating at
  low speed maintain propagating on the cleavage system even when the shear
  to tensile modes ratio\, GII/GI\, is relatively high. Faster cracks do no
 t propagate on the cleavage plane. Instead\, they deflect to the path that
  maximizes GI. This maximum is crack velocity dependent. This is attribute
 d to the atomistic arrangement and atomistic vibrations at the crack tip. 
 \n
LOCATION:Department of Engineering - LR6
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