Notes

Outline

Materials and Processes As we begin the design procedure we should keep in mind manufacturing and processing concerns Will the part(s) have thin sections? Will the part(s) have thick sections? Will the geometry be symmetric or asymmetric? Will the part(s) be subject to fatigue? Will the part(s) be subject to creep? How will the final products operating environment impact our design decisions for processing and materials?

Material Properties We Need to  Understand Yield Strength, Sy Ultimate Tensile Strength, Sut Shear Strength, Ssy Ductility Brittleness Hardness Toughness Resilience Fracture Toughness

Sy, Sut, Fracture s - e Sy .002 off-set Permanent set in material after this value Sut Ultimate Tensile Strength Necking down of test part Fracture Material fails

Ductility and Brittleness Ductility is measured in % Elongation of test material %El =100* (lf – lo)/lo lf = length of test specimen at fracture lo = original length of test specimen If %El >=5%, material is ductile Brittle describes the material if  %El < 5% Brittle materials will usually exhibit different strengths in tension and compression Suc – ultimate compressive strength If Suc and Sut are not close in value the material is called Suc is typically >> Sut If Suc and Sut are close in value  the material is called even

Examples Ductile Taffy 1040 Q&T Steel Chromium and Nickel Steel Alloys Brittle Life Savers Cast Iron Cast Aluminums

What is Shear Strength? Normal Stress Member is in tension Use N = Sut/s Shear Stress Member is in shear use N = Ssy/t Ssy = .577*Sy This is for ductile materials For brittle materials:  will derive shear strength at a later date Ssu=f(Sut, Suc)

Hardness Hardness measures the capacity of  a material to resist deformation Brinell:  A hardened steel ball is pressed into the surface of a test specimen the resulting diameter of the impression determines the Brinell Hardness Number (BHN) by the way Sut ~= 500*BHN Rockwell:  The depth of the indentation is measured On both Rockwell and Brinell numbers, be sure and note testing material and testing standards, i.e., diameter of ball and material being tested

Impact Resistance A measure of how resistant a material is to suddenly applied loads A materials energy absorption capacity—its strain energy at the elastic limit

Toughness Toughness A material’s resistances to fracture Strain Energy at fracture Uf = (1/2)*(Sut + Sy)*ef As temperatures decrease, toughness decreases Charpy Impact test is used to determine toughness

Charpy Test E = mgh

Fracture Toughness Fracture Toughness is a material’s ability to resist stress at the tip of a crack. Fracture toughness is determined empirically Is brittle fracture likely? Are temperatures low? Is the ratio Sy/Sut very low? Does the material have low impact resistance?

Processing Heat treatments Increases hardenability (the higher the carbon content the better the hardenability) Quenching – medium or high carbon content heat to critical temperature, dip in oil or water atomic structure, magnetism and electrical resistance change Tempering – quench first, reheat then slowly cool Annealing – heat to critical temperature and slowly cool eliminates residual stresses created during forming the part

What properties are imparted? s-e quenched tempered annealed

Surface Hardening Carbeurizing Part takes up carbon atoms at surface Used for low carbon steels Hard shell Soft core

Cold Working Work metals at room temperature Strength increases trade off ductility Take metal into plastic region—release load Reapply load Permanent strain results—but strength increases s - e