Lecture25 - ME 382 Lecture 25 STRESS-CORROSION CRACKING...

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ME 382 Lecture 25 17/iii/06 1 S TRESS - CORROSION CRACKING Corrosion & oxidation are very important phenomena we don’t have time to discuss Environment + steady stress crack growth (stress-corrosion cracking) Classic examples: Oxides (glass, Al 2 O 3 , etc. ) with water (growth of cracks in a windshield) Steels with hydrogen (hydrogen embrittlement) Aluminum in sea water Consider SiO 2 Formation of crack leaves Si-O- or Si- bonds at surface SiO 2 is brittle (no crack-tip plasticity) Toughness given by surface energy of Si-O or Si bonds I.e., Γ 2 γ Si-O K IC ! 2 E " Si # O If there is water near crack tip, a chemical reaction can occur e.g. : Si-O-Si (bonds) + H 2 O 2SiOH (for silica) M-O-M (bonds) + H 2 O 2MOH (for metal oxides) Surface energy of Si-OH much lower than surface energy Si-O γ Si-OH < γ Si-O Γ (& K Ic ) in presence of H 2 O is much lower than in vacuum In presence of H 2 O, crack will propagate at much lower values of K I But , crack only propagates as fast as H 2 O can reach crack tip and react with Si-O bonds
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ME 382 Lecture 25 17/iii/06 2 Classic experiments by Wiederhorn (1967) for glass in water vapor Notes: As relative humidity decreases - behavior tends to that in a vacuum
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Lecture25 - ME 382 Lecture 25 STRESS-CORROSION CRACKING...

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