| 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 |
| 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 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 | ||
| Ductile | ||
| Taffy | ||
| 1040 Q&T Steel | ||
| Chromium and Nickel Steel Alloys | ||
| Brittle | ||
| Life Savers | ||
| Cast Iron | ||
| Cast Aluminums | ||
| 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 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 | ||||
| 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 | |||
| 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 | |||
| 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? | ||
| 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 | |||||
| s-e | |
| quenched | |
| tempered | |
| annealed |
| Carbeurizing | ||
| Part takes up carbon atoms at surface | ||
| Used for low carbon steels | ||
| Hard shell | ||
| Soft core | ||
| 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 | ||