Bruce Mayer, PE Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege

1. Engineering 45 Mechanical Properties Testing Bruce Mayer, PE Licensed Electrical & Mechanical EngineerBMayer@ChabotCollege.edu

2. Mechanical Testing OutLine • Yield & Tensile Strength → Tensile Test • Flexural Strength Bend → Strength Test for BRITTLE Materials • Hardness → Hardness Test • Toughness → Impact Test • Fatigue Life → Rotating Beam Test

3. Tensile Test • A Pull Test Leading to an Engineering Stress-Strain Plot • See Last Lecture

4. Brittle Materials are Difficult to Grasp without creating Crack-Initiation Sites → Hard to Tensile Test Use a flex test Flexural Test – Brittle Materials • F  Load at Fracture

5. Many Types Hardness Test • RockwellIndenters Brale Ball • Brinell Indenter • Brinell Eqn for HB • Most Common • Rockwell • Brinell

6. Major Hardness Tests

7. Hardness Test Indenter is pre-loaded (minor Load) into the component to be measured. A major load is then applied, and the indenter causes localized plastic deformation. The size/depth of the indentation (usually 1-2 mm) is measured and is inversely related to the Hardness.

8. Rockwell Scales • As With Most Hardness Measurements Rockwell has NO units

9. As Noted Previously Hardness and Strength Are Closely Related For STEELS with HB = 200-350 Steel Hardness & Strength

10. Example  Estimate Rc & HB • Given Naval-Brass σ-εCharacteristic • Estimate Rockwell and Brinell Hardness Values for this Material • Plan • Determine σu for the Material • 450 Mpa Per σ-ε Behavior • Consult σu vs Hardness Correlations

11. 82 450 130 Example  Est. Rc & HB cont. • Consult Fig 6.19 for σu vs Hardness • ID 450 MPa on Brass Curve • Cast Downward for HB • HB  130 • Cast Upward for HRB • HRB  82

12. Impact Tests are Used to Measure Toughness; the ability to Absorb Energy Toughness  V-Notch

13. IZOD Charpy Toughness • DUCTILE & STRONG Materials are Toughest Lo-C Embrittles

14. Fatigue Strength  Ability of a Material to Resist REPEATED Stress CYCLING Destructive Load Cycling is Almost always Tensile Fatigue Testing • Rotation of slightly bent Specimen Results in Load Reversals

15. Fatigue Behavior 33 ksi load →2 Mcycle Fatigue-Life • Endurance Limit  Stress Which allows Unlimited Cycling

16. Material Properties are Statistical Averages Actual Values Can Vary 1-2 Standard Deviations Use Factor of Safety, N, Based on Working Stress, σw and YIELD Strength, σy Variability of Matl Properties f Std Dev, s y (MPa)

17. d 1045 plain carbon steel: L o s = 310 MPa y 5 TS = 565 MPa F = 220,000N Factor of Safety Example • Factor of safety, N Often N is between 1.2 and 4 • Example: Calculate a diameter, d, to ensure that yield does NOT occur in the 1045 carbon steel rod below. Use a factor of safety of 5. d = 0.067 m = 6.7 cm

18. WhiteBoard Work • Problem 6.22 • Design Criteria • Round Bar, Ø 15mm x 120mm Long • Tensile Load = 35 kN • NO Plastic Deformation • Øo-Øf  12 µm • Available Matls