1. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Objectives Define and use the terms stress, strain and Young modulus. Describe an experiment to determine the Young modulus of a metal wire. 1.3.8 Stress, Strain and Young’s Modulus

2. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Stretching Materials Ductility A material is said to be ductile if it can be permanently stretched. Brittleness A material is said to be brittle if it cannot be permanently stretched. It will break soon after it has passed its elastic limit (point beyond which it will not return to its original shape / length). Stress A measure of the force per unit cross sectional area applied to a material. Stress (Pa) = Force (N) / Area (m 2 )  = F / A

3. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Experiment to stretch a wire – add masses and record the length of the wire between the clamp and the marker. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13

4. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Young’s Modulus (E) E = stress / strain E = (F/A) / (  L/L) E = FL A  L The units of Young’s Modulus are……. Pa

5. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Strain is the percentage extension. Stress is the Force per unit cross sectional area. If the wire tested here was 1m long how much permanent extension was there? 8mm Permanent extension 

6. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Stress / Pa Strain (0,0) Brittle material breaks here. Ductile material stretches permanently beyond its elastic limit. Release of stress. Permanent Deformation. If a material is stiff, it produces little strain so will have a steeper gradient.

7. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Objectives Define and use the term ultimate tensile strength. Describe the shapes of stress–strain graphs for ductile, brittle and polymeric materials. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 1.3.9 Tensile Strength of materials

8. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Using pencils to stretch a copper wire © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 When you do this you can feel a change in the way the copper wire stretches. You should be able to feel what is going on in the graph on the next slide. This should also be the shape of the graph achieved when the wire was stretched in the Young’s Modulus experiment.

9. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Stress-strain graph for a ductile material © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Limit of Proportionality Elastic Limit Yeild Point Permanent Deformation

10. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Stress-strain graph for a brittle material eg cast iron or glass © Pearson Education Ltd 2008 This document may have been altered from the original Week 13

11. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Stress-strain graph for a rubber band © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Graph shows an increase in the force needed to try and stretch the band as it stretches. It “goes solid like string”. Also the band shows a very large amount of extension relative to it’s length – strain axes very long!

12. © Pearson Education Ltd 2008 This document may have been altered from the original Week 13 Stress / 10 9 Pa Strain (0,0) 6 1 Stress / 10 9 Pa Strain (0,0) 0.5 1 Rubber Polythene Hysteresis Here it matters if you are loading or unloading