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Designer Materials 4

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Designer Materials 4 Presentation Transcript

  • 1. Testing Materials Stress and Strain Graphs May 29, 2009
  • 2. To Review Homework...
    • Calculations on Stress, Strain and Y.M.
    • 1) Strain = Extension / Original Length
      • = 25mm / 75mm = 0.33
    • 2) mm / mm : Cancel Out
    • 3) Stress = Force / Area
      • Therefore Force = Stress x Area
      • =10 9 Nm -2 x 10 -8 m 2 = 10N
  • 3. To Review Homework...
    • Calculations on Stress, Strain and Y.M.
    • 4) Stress = Force / Area
      • so Area = Force / Stress = 15kN / 75MNm -2
      • = 2 x 10 -4 m 2
      • Area =  d 2 / 4
      • therefore d =  4 x 2 x 10 -4 m 2 / 3.14
      • d = 1.6 x 10 -2 m or 1.6 cm
  • 4. To Review Homework...
    • 5) Stress = Force / Area
      • Area =  d 2 / 4
      • 3.14 x (50 x 10 -3 m) 2 / 4
      • = 1.96 x 10 -3 m 2
      • Therefore Stress = 4kN / 1.96 x 10 -3 m 2
      • = 2.0 x 10 6 Nm -2 or 2.0 MPa
  • 5. To Review Homework...
    • 6) Stress = 2MPa
    • 7) Stress is same for nylon and cable
      • The Y.M for nylon is about 2 - 4GPa and 200 GPa for steel
      • Therefore Strain would be greater for nylon - it would extend more..!
    • 8) Y.M = Stress / Strain =
    • (Force / Area) / (Extension / Orig. Length)
      • Therefore Extension = (F x OL)/(Area x Y.M.)
      • =2.5 x 10 -3 m
  • 6. To Review Homework...
    • Exam Question 10a
      • Brittle - Shatters on impact / cracks propagate easily
      • Hard - Difficult to scratch or dent
      • Strong - Large breaking stress
      • Tough - Difficult to crack / large energy to break
  • 7. To Review Homework...
    • Exam Question 10
    • bi) Steel is 4 times tougher
    • bii) Toughness is (energy) per new surface (area)
    • ci) B is best fit
        • Tougher than A, less tougher than C
  • 8. To Review Homework...
    • Exam Question 1
    • Car bodies e.g. steel - B - Strong and Tough
    • Weak material e.g. biscuit - C Weak and Brittle
    • Brittle Metal e.g. cast iron under tension - A - Strong but brittle
  • 9. To Review Homework...
    • Exam Question 7
    • a) Elastic and Tough
    • b) Area = Force / Stress = 550 / 2.0 x 10 9 = 2.8 x 10 -7 m 2
    • ci) Elastic limit is past the elastic behaviour of material. Behaviour is plastic beyond this
    • cii) Y.M. = Stress / Strain 1.6 x 10 9 Nm -2 / 0.35m = 4.6 x 10 9 Pa
  • 10. Graphs of Young’s Modulus
    • In pairs, summarise the key characteristics of the graphs you obtained from your practical results:
  • 11.  
  • 12. Graphs for Different Characteristics
    • Sketch graphs for the following different materials:
    • An elastic rubber band
    • A piece of steel under tension
    • A piece of cast iron under tension (more brittle than steel)
    • A glass fibre (brittle and stiff)
    • A plastic ruler
  • 13. Strain Stress
  • 14. Key Terms
    • Elastic Limit - point at which elastic behavior is replaced by plastic deformation
    • Yield Point - Stress at which elastic limit is reached
    • Breaking Stress - the stress at which a material breaks
    • Plastic Region - Deformation of material is permanent (non-elastic). It can result in:
      • Uniform extension along length of material
      • Necking at a point
  • 15. Questions
    • Answer questions 1 to 6 on page 91
    • Show working out