Download
1 / 18
180 likes | 298 Views
This outline covers mechanical testing methods such as yield and tensile strength tests, bend and hardness tests for brittle materials, impact test for toughness, and fatigue life testing. It also includes information on hardness measurement techniques using Rockwell and Brinell scales. Examples and calculations for hardness values, Toughness, and Fatigue Life are discussed to demonstrate the practical application of material properties in engineering. Material property variability, factor of safety calculations, and a design problem are also covered to provide a comprehensive understanding of mechanical testing.
E N D
Engineering 45 Mechanical Properties Testing Bruce Mayer, PE Licensed Electrical & Mechanical EngineerBMayer@ChabotCollege.edu
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
Tensile Test • A Pull Test Leading to an Engineering Stress-Strain Plot • See Last Lecture
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
Many Types Hardness Test • RockwellIndenters Brale Ball • Brinell Indenter • Brinell Eqn for HB • Most Common • Rockwell • Brinell
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.
Rockwell Scales • As With Most Hardness Measurements Rockwell has NO units
As Noted Previously Hardness and Strength Are Closely Related For STEELS with HB = 200-350 Steel Hardness & Strength
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
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
Impact Tests are Used to Measure Toughness; the ability to Absorb Energy Toughness V-Notch
IZOD Charpy Toughness • DUCTILE & STRONG Materials are Toughest Lo-C Embrittles
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
Fatigue Behavior 33 ksi load →2 Mcycle Fatigue-Life • Endurance Limit Stress Which allows Unlimited Cycling
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)
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
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
