33MechanicalBehaviorofMetalMaterials.pptx

第第8次课次课Mechanical Behavior of Metal MaterialsCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA3.4 Mechanical Behavior-MetalswFor most metallic materials,elastic deformation persists only to strains of about 0.005.As the material is deformed beyond this point,the stress is no longer proportional to strain,and permanent,nonrecoverable,or plastic deformation occurs.Plastic deformation:When the stress is removed,the material does not return to its previous dimension but there is a permanent,irreversible deformation(不可回复的变形不可回复的变形).College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA FigureFigure:typical stress-strain behavior for a metal showing elastic and plastic deformations,the proportional limit P,and the yield strength y,as determined using the 0.002 strain offset残余，偏移method.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAUnderstanding plastic deformation in an atomic perspective Plastic deformation corresponds to the breaking of bonds with original atom neighbors and then reforming bonds with new neighbors as large numbers of atoms or molecules move relative to one another;upon removal of the stress they do not return to their original positions.wThis permanent deformation for metals is accomplished by means of a process called slip(滑移滑移)College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA(1)Yielding and yield strength(2)Tensile strength(拉伸强度）拉伸强度）(3)Ductility(延展性延展性)(4)Resilience(回弹性回弹性)(5)Toughness(韧性韧性)Tensile PropertiesCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAwThree conceptsThree concepts:1.YieldingYielding(屈服):the phenomenon that plastic deformation begins.2.Proportional limitProportional limit(比例极限):the point of the initial departure from linearity of the stress-strain curve.3.Yield strengthYield strength:the stress corresponding to the intersection of the line,which parallels to the elastic portion of the stress-strain curve at some specified strain offset,and the stress-strain curve as it bends over in the plastic region.Yielding and yield strength College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAFor nonlinear elastic behavior materials:wUse of the strain offset method is not possible;wThe usual practice is to define the yield strength as the stress required to produce some amount of strain(=0.005)College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAwThe elastic-plastic transition is very well defined and occurs abruptly in what is termed a yield point phenomenon.wAt the upper yield point,plastic deformation is initiated with an actual decrease in stress.wContinued deformation fluctuates slightly about some constant stress value,termed the lower yield point;stress subsequently rises with increasing strain.wFor metals,the yield strength is taken as the average stress that is associated with the lower yield point.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINATensile strengthConcept:Concept:wthe stress at the maximum on the engineering stressstrain curve.wcorresponds to the maximum stress that can be sustained by a structure in tension;if this stress is applied and maintained,fracture will result.wAll deformation up to this point is uniform throughout the narrow region of the tensile specimen.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAFor applications,the yield strength is usually a more important property than the tensile strength,since once the it is passed,the structure has deformed beyond acceptable limits.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAExample problem 3College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAProblem:From the tensile stress-strain behavior for the brassspecimen shown in above fig.,determined the following:College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINADuctilitywIt is a measure of the degree of plastic deformation that has been sustained at fracture.wA material that experiences very little or no plastic deformation upon fracture is termed brittle.Figure:Schematic representations of tensile stress-strain behavior for brittle and ductile materials loaded to fracture.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA Ductility may be expressed quantitatively as either percent elongation(延伸率延伸率)or percent reduction(收缩率收缩率)in area.wThe percent elongation%EL is the percentage of plastic strain at fracture,orWhere lf is the fracture length and l0 is the original gauge length as above.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAwPercent reduction in area%RA is defined asWhere A0 is the original cross-sectional area and Af is the cross-sectional area at the point of fracture.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAImportance of the ductility of materials1.it indicates to a designer the degree to which a structure will deform plastically before fracture.2.it specifies the degree of allowable deformation during fabrication operations.Brittle materials are approximately considered to be those having a fracture strain of less than about 5%.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAResilience（回弹性）wTwo concept:1.Resilience is the capacity of a material to absorb energy when it is deformed elastically and then,upon unloading,to have this energy recovered.2.Modulus of resilience,Ur,the strain energy per unit volume required to stress a material from an unloaded state up to the point of yielding.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAwAssuming a linear elastic region,in which y is the strain at yielding.Incorporation of above Equations yieldsCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAToughness（韧性）Toughness is a measure of the ability of a material to absorb energy up to fracture.For the static situation,toughness may be ascertained from the results of a tensile stress-strain test.It is the area under the strain-stress curve up to the point of fracture.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINATrue Stress and StrainThe decline in the stress necessary to continue deformation past the maximum,point M,is the metal becoming is the metal becoming weaker?weaker?College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAConceptswTrue stress T the load F divided by the instantaneous cross-sectional area Ai over which deformation is occurring(i.e.,the neck,past the tensile point),orwTrue strainTCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINATrue vs.EngineeringwIf no volume change occurs during deformation,that is,iftrue and engineering stress and strain are related according toCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA For some metals and alloys the region of the true stress-strain curve from the onset of plastic deformation to the point at which necking begins may be approximated byK and n are constant,which will vary from alloy to alloy,and will also depend on the condition of the material.College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAExample problem 4College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINASolutionCollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINACollege of Materials Science&EngineeringNORTH UNIVERSITY OF CHINAExample problem 5College of Materials Science&EngineeringNORTH UNIVERSITY OF CHINA1(1)Historically,the development and advancement of societies have been intimately tied to the members ability to produce and manipulate materials to fill their needs.(2)automobiles would not have been possible without the availability of inexpensive steel or some other comparable substitute