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B.B. Guzina, D.H. Timm, V.R. Voller

PREDICTION OF THERMAL CRACK SPACING. B.B. Guzina, D.H. Timm, V.R. Voller. Civil Engineering University of Minnesota. PROBLEM: Thin Film placed on a Substrate When the film cools and shrinks OR the substrate is strained a uni-axial stress will develop in the film. At some point this

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B.B. Guzina, D.H. Timm, V.R. Voller

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  1. PREDICTION OF THERMAL CRACK SPACING B.B. Guzina, D.H. Timm, V.R. Voller Civil Engineering University of Minnesota

  2. PROBLEM: Thin Film placed on a Substrate When the film cools and shrinks OR the substrate is strained a uni-axial stress will develop in the film. At some point this stress will exceed the film “strength” S, and transverse cracks will form. What controls EVOLUTION and SPACING ?

  3. The Phenomenon Occurs at all Scales Asphalt Road Cooled by DT = -15 oC Ceramic Film 2.56% substrate strain Chen et al 7.5m L=150mm L=150m

  4. Elastic Rod with an Elastoplastic Restraint imposed at the film/substrate interface Interface shear stress at failure Spring coefficient (Winkler Foundation) Half crack space

  5. (limit case as ) Rigid Substrate: The Shear lag Model region of linear stress increase When intact segment length bounded Minimum length for stress to reach value of S

  6. Elastoplastic Restraint elastic plastic x =l x =l/2 xt substrate stiffness

  7. Substrate Stiffness, k Match overall stiffness of model system with benchmark Benchmark: Elastic half-plane with shear over length lref Valid if substrate is relatively thick Intact length before new crack forms

  8. Bounds on Crack Spacing Cracks will occur if lo General Crack Spacing Length Scale In limit of large cooling or stiff substrate Average spacing is A given crack spacing is bound by

  9. Ceramic Film on Strained Steel Substrate applied strain Chen et al set Crack Densification Through to Saturation sres residual stress in film

  10. Current Extensions Prediction of Spacing in Jointed Rock Cracks in Bench Scale Systems ~10mm Bai, Pollard &Gao, Nature, 403, 753-756 Cracks in wax strip (w = 20mm, h = 5mm) On wood plank cooled to -18 oC Supported by NSF National Center for Earth-surface Dynamics

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