Chapter 7:Slide 2Questions to Think AboutStress-Strain TestTensile TestImportant Mechanical Properties from a Tensile TestSlide 7Elastic DeformationPlastic Deformation (Metals)c07f10abPlastic Deformation (permanent)c07f25Slide 13Permanent DeformationYield Strength, sySlide 16Slide 17Slide 18Slide 20Slide 21c07tf02Yield Strength: ComparisonTensile Strength, TSSlide 25Tensile Strength: ComparisonEngineering StressVMSESlide 29Slide 30Ductility, %ELc07f13Toughnessc07f05c07probc07f09Slide 37Stress-Strain Results for Steel SampleSlide 39Example 2: Young’s Modulus - Aluminum Alloy - continuedYoung’s Moduli: ComparisonSlide 42c07f17Strain Hardening (n, K or C values)c07f12c07f33Mechanical Behavior - CeramicsSlide 48Slide 49Slide 50c07f18MEASURING ELASTIC MODULUSMEASURING STRENGTHStress-Strain Behavior: ElastomersSlide 55HardnessSlide 57Hardness Testersc07tf05c07f30c07f31Slide 62Summary1Chapter 7:MECHANICAL PROPERTIESChapter Outline Terminology for Mechanical PropertiesThe Tensile Test: Stress-Strain DiagramProperties Obtained from a Tensile TestTrue Stress and True StrainThe Bend Test for Brittle MaterialsHardness of Materials3Questions to Think About• Stress and strain: What are they and why are they used instead of load and deformation? •Elastic behavior: When loads are small, how much deformation occurs? What materials deform least?•Plastic behavior: At what point do dislocations cause permanent deformation? What materials are most resistant to permanent deformation?•Toughness and ductility: What are they and how do we measure them? •Ceramic Materials: What special provisions/tests are made for ceramic materials?4Stress-Strain Testspecimenmachine5Tensile Test6Important Mechanical Properties from a Tensile Test •Young's Modulus: This is the slope of the linear portion of the stress-strain curve, it is usually specific to each material; a constant, known value. •Yield Strength: This is the value of stress at the yield point, calculated by plotting young's modulus at a specified percent of offset (usually offset = 0.2%). •Ultimate Tensile Strength: This is the highest value of stress on the stress-strain curve. •Percent Elongation: This is the change in gauge length divided by the original gauge length.TerminologyLoad - The force applied to a material during testing.Strain gage or Extensometer - A device used for measuring change in length (strain).Engineering stress - The applied load, or force, divided by the original cross-sectional area of the material.Engineering strain - The amount that a material deforms per unit length in a tensile test.8Fbonds stretchreturn to initial1. Initial 2. Small load 3. UnloadElastic means reversible.FLinear- elasticNon-Linear-elasticElastic Deformation91. Initial 2. Small load 3. UnloadPlastic means permanent.Flinear elasticlinear elasticplasticplanes still shearedFelastic + plasticbonds stretch & planes shearplasticPlastic Deformation (Metals)10Typical stress-strain behavior for a metal showing elastic and plastic deformations, the proportional limit P and the yield strength σy, as determined using the 0.002 strain offset method (where there is noticeable plastic deformation). P is the gradual elastic to plastic transition.11Plastic Deformation (permanent)•From an atomic perspective, plastic deformation corresponds to the breaking of bonds with original atom neighbors and then reforming bonds with new neighbors. •After removal of the stress, the large number of atoms that have relocated, do not return to original position.•Yield strength is a measure of resistance to plastic deformation.12(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.• Localized deformation of a ductile material during a tensile test produces a necked region. • The image shows necked region in a fractured sample14Permanent Deformation•Permanent deformation for metals is accomplished by means of a process called slip, which involves the motion of dislocations.•Most structures are designed to ensure that only elastic deformation results when stress is applied. •A structure that has plastically deformed, or experienced a permanent change in shape, may not be capable of functioning as intended.15tensile stress, engineering strain, yp = 0.002Yield Strength, ytensile stress, engineering strain, Elastic initiallyElastic+Plastic at larger stresspermanent (plastic) after load is removedpplastic strainStress-Strain Diagram Strain ( ) (L/Lo)41235Stress (F/A)Elastic RegionPlasticRegionStrainHardeningFractureultimatetensile strengthSlope=EElastic region slope =Young’s (elastic) modulus yield strengthPlastic region ultimate tensile strength strain hardening fractureneckingyieldstrengthUTSyεEσ εσE 12yε εσEStress-Strain Diagram (cont) • Elastic Region (Point 1 –2) - The material will return to its original shape after the material is unloaded( like a rubber band). - The stress is linearly proportional to the strain in this region.εEσ  : Stress(psi)E : Elastic modulus (Young’s Modulus) (psi) : Strain (in/in)σε- Point 2 : Yield Strength : a point where permanent deformation occurs. ( If it is passed, the material will no longer return to its original length.) εσE or• Strain Hardening - If the material is loaded again from Point 4, the curve will follow back to Point 3 with the same Elastic Modulus (slope). - The material now has a higher yield strength of Point 4. - Raising the yield strength by permanently straining the material is called Strain Hardening.Stress-Strain Diagram (cont)• Tensile Strength (Point 3) - The largest value of stress on the diagram is called Tensile Strength(TS) or Ultimate Tensile Strength (UTS) - It is the maximum stress which the material can support without breaking.• Fracture (Point 5) - If the material is stretched beyond Point 3, the stress decreases as necking and non-uniform deformation occur. - Fracture will finally occur at Point 5.Stress-Strain Diagram (cont)(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.The stress-strain curve for an aluminum alloy.21•Stress-strain behavior found for some steels with yield point phenomenon.22TENSILEPROPERTIES23Graphite/