Designer Materials 4
- 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:
- 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