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  • Manufacturing ProcessesMechanical Properties

  • Mechanical Properties in Design and ManufacturingMechanical properties determine a materials behavior when subjected to mechanical stresses Properties include elastic modulus, ductility, hardness, and various measures of strength

    Dilemma: mechanical properties desirable to the designer, such as high strength, usually make manufacturing more difficult The manufacturing engineer should appreciate the design viewpoint and the designer should be aware of the manufacturing viewpoint

  • ImportanceInfluence function and performance

    Reflects the capacity to resist deformation

  • StressTensileStretch the materialCompressiveSqueeze the materialShearSlide of adjacent portions of the material

  • Conceptual Model30 kNFBCFBCFABFAB30 kN5 m==4 m3 mFAB= 40 kNFBC= 50 kN

  • Conceptual Model (cont.)BCFBCFBCFBCFBCDDStress- internal force per unit area

  • Strain(e)BCFBCFBCloLStrain elongation per unit of length

    e=Llo-lo

  • Measurement UnitsInternational System (SI)Axial force (F) in Newtons (N)Area (A) in squared meters (m2)Stress () in N/m2 or Pascals (Pa)1 N/m2 = 1 Pa US Customary System (USCS)Axial force (F) in pounds-force (lbf)Area (A) in squared inches (in.2)Stress () in lbf/in.2 or psi

    internal force per unit area

  • Tensile TestMost common test for studying stressstrain relationship, especially metals In the test, a force pulls the material, elongating it and reducing its diameter

    Figure 3.1 Tensile test: (a) tensile force applied in (1) and (2) resulting elongation of material

  • ASTM (American Society for Testing and Materials) specifies preparation of test specimen Figure 3.1 Tensile test: (b) typical test specimen

  • Figure 3.1 Tensile test: (c) setup of the tensile test

  • Figure 3.2 Typical progress of a tensile test: (1) beginning of test, no load; (2) uniform elongation and reduction of crosssectional area; (3) continued elongation, maximum load reached; (4) necking begins, load begins to decrease; and (5) fracture. If pieces are put back together as in (6), final length can be measured

  • Stress-Strain RelationshipStressstrain curve - basic relationship that describes mechanical properties for all three types.

  • Figure 3.3 Typical engineering stressstrain plot in a tensile test of a metalprior to yielding of the materialafter yielding of the material

  • Elastic Region in StressStrain CurveRelationship between stress and strain is linearMaterial returns to its original length when stress is removed

    Hooke's Law: = E e where E = modulus of elasticity; slope of the curve E is a measure of the inherent stiffness of a materialIts value differs for different materials

  • Yield Point in StressStrain CurveAs stress increases, a point in the linear relationship is finally reached when the material begins to yieldYield point Y can be identified by the change in slope at the upper end of the linear region Y = a strength propertyOther names for yield point = yield strength, yield stress, and elastic limit

  • Figure 3.3 Typical engineering stressstrain plot in a tensile test of a metalprior to yielding of the materialafter yielding of the material

  • Plastic Region in StressStrain CurveYield point marks the beginning of plastic deformationThe stress-strain relationship is no longer guided by Hooke's Law As load is increased beyond Y, elongation proceeds at a much faster rate than before, causing the slope of the curve to change dramatically

  • StressEngineering stress

  • Tensile Strength in StressStrain CurveElongation is accompanied by a uniform reduction in crosssectional area, consistent with maintaining constant volume Finally, the applied load F reaches a maximum value, and engineering stress at this point is called the tensile strength TS or ultimate tensile strength

    TS =

  • Figure 3.3 Typical engineering stressstrain plot in a tensile test of a metalprior to yielding of the materialafter yielding of the materialUTSYS

  • Ductility in Tensile TestAbility of a material to plastically strain without fracturewhere EL = elongation; Lf = specimen length at fracture; and Lo = original specimen lengthLf is measured as the distance between gage marks after two pieces of specimen are put back together

  • Figure 3.3 Typical engineering stressstrain plot in a tensile test of a metalprior to yielding of the materialafter yielding of the materialUTSYSDuctility

  • True StressStress value obtained by dividing the instantaneous area into applied loadwhere = true stress; F = force; and A = actual (instantaneous) area resisting the load

  • True StrainProvides a more realistic assessment of "instantaneous" elongation per unit length

  • If previous engineering stressstrain curve were plotted using true stress and strain valuesFigure 3.4 True stressstrain curve for the previous engineering stressstrain plot in Figure 3.3

  • Strain Hardening in Stress-Strain CurveNote that true stress increases continuously in the plastic region until neckingIn the engineering stressstrain curve, the significance of this was lost because stress was based on an incorrect area valueWhat it means is that the metal is becoming stronger as strain increases This is the property called strain hardening

  • When the plastic region of the true stressstrain curve is plotted on a loglog scale, it becomes linearFigure 3.5 True stressstrain curve plotted on loglog scale

  • Flow Curve Because it is a straight line in a log-log plot, the relationship between true stress and true strain in the plastic region is

    where K = strength coefficient; and n = strain hardening exponent

  • Types of stress-strainPerfectly elasticbehavior follows Hookes law; fractures rather than yielding to plastic flowElastic and perfectly elasticBehave as indicated by E; once yield is reached deforms plastically at same stress level.Elastic and strain hardeningObeys Hookes Law in the elastic region; begin to flow at yield strength (Y); continued deformation requires ever-increasing stress

  • Behavior is defined completely by modulus of elasticity EIt fractures rather than yielding to plastic flow Brittle materials: ceramics, many cast irons, and thermosetting polymers

    Figure 3.6 Three categories of stressstrain relationship: (a) perfectly elasticPerfectly Elastic

  • Stiffness defined by E Once Y reached, deforms plastically at same stress level Flow curve: K = Y, n = 0Metals behave like this when heated to sufficiently high temperatures (above recrystallization)

    Figure 3.6 Three categories of stressstrain relationship: (b) elastic and perfectly plasticElastic and Perfectly Plastic

  • Hooke's Law in elastic region, yields at YFlow curve: K > Y, n > 0Most ductile metals behave this way when cold worked

    Figure 3.6 Three categories of stressstrain relationship: (c) elastic and strain hardeningElastic and Strain Hardening

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