The Wonderful World of TECHNOLOGY MATERIALS External forces produce various effects inside a material. EXAMPLE: *Consider what would happen if an elephant walked in the room and stepped on your pencil. What would the effect be? EXTERNAL FORCE = Elephant weight EFFECT = pencil being crushed Different materials react differently to different external forces. MECHANICAL CONSTRAINTS A mechanical constraint describes the stress produced within a material when it is subjected to external forces. 5 types of constraints: Compression, Tension, Torsion, Bending, Shearing COMPRESSION: Force that crushes materials TENSION: Force that stretches materials TORSION: Force that twists materials BENDING: Force that bends materials SHEARING: Force that cuts or tears materials EXAMPLES: SHOES: CLICKER: WATER BOTTLE: What happens when there is TOO MUCH STRESS (constraint) on material? Elastic deformation Temporary deformation of material. Resumes shape. Plastic deformation Remains deformed. Fracture Material ruptures. Your Pencils Have you ever asked yourself why pencils are never made out of rubber? Or out of glass? MECHANICAL PROPERTIES Determines how a material will react when it is subjected to one (or more) mechanical constraints. Examples of properties: Hardness, elasticity, ductility, malleability, tensile strength Resists penetration Reacts to pressure force by elastic deformation Regains initial shape Reacts to force by elastic deformation Resists physical impacts Reacts to temporary force by elastic deformation. =j87UWgvetRg Building Design Pulls Structure Upright after Earthquake Stretches without breaking Reacts to tension force by plastic deformation. Flattens or bends without breaking Reacts to bending or compression force by plastic deformation. Resists tension without becoming permanently deformed Reacts to tension force by elastic deformation HARDNESSResists penetration ELASTICITYRegains initial shape RESILIENCEResists physical impacts DUCTILITYStretches w/o breaking MALLEABILITYFlattens/bends without breaking TENSILE STRENGTH Resists tension without permanently deforming WOOD: p.374 METALS: p.379 ALLOYS: p.380 PLASTICS: p.382