3.11 MECHANICS OF MATERIALS F03Lecture #33Professor Christine OrtizOFFICE : 13-4022 PHONE : 452-3084WWW : http://web.mit.edu/cortiz/wwwIMPACT TESTING OF MATERIALSSUMMARY : LAST LECTUREMultiaxial Yield Criterion123:(,,) YYpppYYielding takes place in 1D when : whereyield stress in uniaxial tensionDefine "equivalent stress" : fYielding takes place in 3D when :1. Tresca or Maxmimum Shear Stress Cσσσσσσσσσ≥==≥( ) ( ) ( )max13222132331221122212(2) TRESCAppYVMppppppYVMppppYriterion :2. Von Mises Criterion:Dστσσσσσσσσσσσσσσσσσ==−≥=−+−+−≥=++≥IIIIIIIVVVIUNYIELDEDYIELDEDTRESCA polygonVM ellipseYield LocusSome More on Metal Plasticityhttp://www.msm.cam.ac.uk/doitpoms/tlplib/BD3/results1.phpuniaxial tensionSome More on Metal Plasticityuniaxial tensionSome More on Metal Plasticityuniaxial tensionSome More on Metal Plasticity? Work hardening: The process of plastically deforming a sample by rolling or drawing the material at low temperatures (less than half the melting temperature, Tm), which increases the number of dislocations and the amount that they are entangled, resulting in reduced ductility and increased hardness and strength. ? Annealing is a heat treatment process that brings about a softer or more relaxed state in worked materials with an increased grain size.IMPACT RESISTANCE AND TESTINGBASIC DEFINITIONSSPEEDSMiles/HourLight 670.57 millionBullet 818-2727Concorde 1336 Speed of Sound in Air 758 Boeing 747 566World Fastest Helicopter 250 High Speed “Bullet” Train 170Fastest Bird: peregrine 100-200falconCar 100Cheetah 70 Fastest fish (sailfish) 68Fastest insect (dragonfly) 35Snail 0.03Pendulum Impact TestsCharpy Pendulum Impact TestsCharpy Pendulum Impact TestsThe energy absorbed is equal to the loss in potential energy of the system, according to :Potential energy lost = MgH ( sin (θ1- 90) + cos(θ2) )Charpy Pendulum Impact TestsDrop Weight Impact TestsDrop Weight PhysicsInstrumented Impact
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