Chapter 6 part b notes Mechanical properties of ceramics o Strong in compession and weak in tension o Stress strain graphs on straight not lines not curves Crack in Ceramics applied tensile stress o Om max stress at the tip of crack o Oapp o C crack length o P radius of the tip smaller the tip the greater stress exerted at the crack tip Bend test is usually used for brittle materials o Graph of stress vs strain Hardness o A measure of a materials resistance to localized plastic deformation dent scratch o Depth and size of the indentation is measured and it corresponds to a hardness level o Softer the material larger and deeper the indentation lower hardness level o In applications we want a high hardness value Resistance to permanently indenting the surface Large hardness means resistance to plastic deformation or cracking in compression better wear properties Both hardness and tensile strength are indicators of a metal s resistance to plastic deformation Stress and strain are size independent measure of load and displacement Elastic behavior reversible behavior often shows a linear relation between stress and strain Plastic behavior the permeant deformation occurs when the tensile stress reaches the yield point Toughness the energy needed to break a unit volume of material Ductility the plastic strain at failure