1. Tutorial 2 Materials

2. Question 1 In your own words, explain the difference between hardness and toughness. When a steel knife and a diamond is made to collide at high speed, which of the two is more likely to break? Why (explain in terms of the molecular structure)?

3. Q1. Toughness Definition: the amount of energy that a material can absorb before rupturing (the resistance to fracture of a material when stressed). It can be found by finding the area (i.e. by taking the integral) underneath the stress-strain curve

4. Q1. Hardness Definition: the resistance of a material to permanent deformation (plastic deformation). Hardness can be measured in various scales, depending on needs. For example, in mineralogy, the hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material (Mohs scale). Diamond is the hardest material in the world that can sustain high pressure without getting deformed. However, steel knife is tougher than diamond, in the sense that it can sustain high impact. So when the two collide at high speed, diamond will likely to shatter first.

5. Question 2 Compare and contrast the structural makeup of a samurai sword and a human tooth.

6. Hard on the outside, soft on the inside =) Q2. Human Tooth VS Katana Sword The Japanese sword blade is formed from a combination of two different steels, a harder outer jacket steel wrapped around a relatively softer, inner core of steel - http://en.wikipedia.org/wiki/Katana http://en.wikipedia.org/wiki/Katana Dentin is softer than enamel - http://en.wikipedia.org/wiki/Teeth http://en.wikipedia.org/wiki/Teeth

7. Question 3 If you strike a lock at room temperature with a hammer, you may make small dents, but will not break the lock. If you pour liquid Nitrogen (-196 o C) onto the locks and then hammer it, you may shatter the lock. Why does the characteristic change so dramatically?

8. Question 3 The atoms at high temperature have higher mobility than those at low temperature. As a result, at low temperature, the atoms in the material cannot move so much as to accommodate the stress applied on it. Hence, if the stress applied is too great, the material will shatter. On the other hand, at high temperature, the atoms can move so as to accommodate the stress before it breaks.

9. Q3 Regular VS Reinforced Concrete Cement: hard, yet brittle Steel: tough, high tensile-strength, but easy to deformed

10. Question 4 What is tempered glass? Why can’t glass be cut to size after being tempered? Give examples of where tempered glass is normally used, and other examples of where it is NOT used.

11. Q4. Tempered Glass Tempered glass is made by heating a normal/annealed glass above its melting point, then rapidly cool it down (quenching), causing the surface to harden and contract immediately while core is still free to flow for some time. Later when the core cools down, it will further contract and compress the surface. The greater contraction of the inner layer induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. It is this compressive stress that gives the toughened glass an increased strength. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin

12. Annealed Glass VS Tempered Glass The crack spreads along the defect lines in annealed glass (left pictures). But in tempered glass, it spreads equally well in all direction, causing it to collapse altogether (right picture)

13. Question 5 Why does an ice-cube usually crack when you take it directly from the freezer and drop it into a glass of water?

14. Q5. Crystal Grains Crystal grows in grain structure. The boundaries between the grains are actually the micro-defects. And when the material expand, each grain will expand independently, causing the material to crack along the grain boundaries (see next slide).

15. Q5. Micro-defect in Ice Crystal

16. Question 6 A modern alternative to wooden or steel cooking utensils is rubber (polymer). My experience with a frying spatula made of rubber reveals that it will scorch if used on a VERY hot pan, but that it will not melt. Why not?

17. Q6. Thermoplastics VS Thermosets Thermoplastics are composed of entangled polymer-chains. When it’s heated up, the polymers will automatically disentangle themselves through a back-and-forth sliding motion along its length, a process called reptation. Hence they will melt. Thermosets, however, will not melt when heated up, because the cross-links between their chains prevent them from reptating. Hence, they will only get burnt and charred.

18. Question 7 The semi-transparent plastic cup covers provided by fast food restaurants often have buttons you can push to indicate the cup’s contents. When you push the buttons, you bend the plastic and it turns white. Why?

19. Q7. The plastic crazes when it’s bent, forming gaps that scatter light (hence it looks white to you) http://science.howstuffworks.com/invisibility-cloak2.htm Invisibility cloak and metamaterial http://en.wikipedia.org/wiki/Metamaterial

20. Question 8 While a thin Kevlar fiber has an enormous tensile strength, a large block of solid Kevlar exhibits a similar tensile strength only along one direction. Why is that so?

21. Q8. Kevlar Kevlar is strong along the direction of their aligned chains. When you pull on the material, you stress the chain themselves If you pull perpendicular to the chains, they separate relatively easily