9 - Mechanical properties of ceramics Why Do We Care?...

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Mechanical properties of ceramics
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hy Do We Care? eramic Armor Why Do We Care? Ceramic Armor Ceramic Armor used for over 50 years Mostly personnel and light vehicle applications 2 layered ceramic used in Afghanistan and Iraq
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eramic Armor in Practice pact Ceramic Armor in Practice • Impact • Pre-penetration • Material Failure • Penetration • Momentum Absorption
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Why Use Ceramics? Advantages • Weight efficiency Disadvantages • Low tensile • Low Density • High hardness strength • Steel armors may e better for • High compressive be better for certain pplications strength • High Elastic applications Modulus
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Mechanical Properties of Ceramics •Elastic Properties •Strength •Fracture •Toughening
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lasticity Elasticity oad on material (stress = leads to deformation Load on material (stress ) leads to deformation (strain = ) /2 Elastic deformation: = E here E = Young’s modulus where E Young s modulus hear loading = G Shear loading = = G where G = shear modulus
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lastic Modulus Elastic Modulus • Ceramics with weak ionic bonding have low E (e.g., E NaCl = 44.2 GPa ) • Ceramics with strong covalent bonds have high E (e.g., E diamond = 1035 GPa ) ond strength and E can vary in different Bond strength and E can vary in different crystallographic directions ost ceramic materials are polycrystalline so Es given Most ceramic materials are polycrystalline, so Es given are randomized and averaged
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lastic Moduli Elastic Moduli Rubber 0.0035 Concrete 13.8 aCl 4 2 NaCl 44.2 Glass 69 ZrO2 138 O2 73 UO2 173 Spinel 284 Al2O3 380 SiC 414 TiC 462 iamond 035 Diamond 1035
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Effects of Porosity on Elastic Properties Pores concentrate stress •so less stress is necessary to achieve the same strain • therefore E decreases and mechanical strength decreases; • at the same time, increased porosity decreases uctility overall and brittle fracture is more likely ductility overall and brittle fracture is more likely • also causes the mechanical strength of the material to decrease
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ore on Porosity and Elastic Modulus More on Porosity and Elastic Modulus =E - 9P+09P 2 E = E 0 (1 1.9P + 0.9P ) where E 0 = E of nonporous material P = volume fraction of pores valid for up to 50% porosity and Poisson’s ratio of 0.3
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ffect of Temperature on Elastic Modulus Effect of Temperature on Elastic Modulus • E decreases slightly as T increases – because of the increase in interatomic spacing om thermal expansion from thermal expansion (as the interatomic spacing increases, less force is needed to increase interatomic separation)
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oisson’s Ratio Poisson s Ratio - /d) / ( l) = Poisson’s Ratio = ( d/d) / ( l/l) = Poisson s Ratio d/d d/d sually 0 1- 5for l/l Usually 0.1 0.5 for ceramics ceramic is: If ceramic is: • isotropic l/l • polycrystalline =2G(1+ Tensile load E = 2G (1 + ) Theory limits E = 3G and -1< < 0.5
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Youngs Modulus 200 Metals Alloys Graphite Ceramics Semicond Polymers Composites /fibers E ceramics antalum Steel, Ni Molybdenum Si nitride Al oxide Carbon fibers only 0 600 800 1000 1200 400 Tungsten 111> Si carbide Diamond CFRE(|| fibers)* > E
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9 - Mechanical properties of ceramics Why Do We Care?...

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