7.1 Failure and strength of materials

Alloying

An alloy is a mixture of a metal with other substances. In other words, alloying involves adding impurities to a metal sample. The other substances can be metal or non-metal. An alloy often has properties that are very different to the properties of the substances from which it is made. The added substances strengthen the metal by preventing dislocations from spreading. Ordinary steel is an alloy of iron and carbon. The carbon impurities trap dislocations. There are many types of steel that also include other metals with iron and carbon. Brass is an alloy of copper and Zinc. Bronze is an alloy of copper and tin. Gold and silver that is used in coins or jewellery are also alloyed.

Tempering

Tempering is a process in which a metal is melted then quickly cooled. The rapid cooling is called quenching. Usually tempering is done a number of times before a metal has the correct properties that are needed for a particular application.

Sintering

Sintering is used for making ceramic objects among other things. In this process the substance is heated so that its particles stick together. It is used with substances that have a very high melting point. The resulting product is often very pure and it is formed in the process into the shape that is wanted. Unfortunately, sintered products are brittle.

Steps of roman swordsmithing

• Purifying the iron ore.
• Heating the iron blocks in a furnace with charcoal.
• Hammering and getting into the needed shape. The smith used a hammer to pound the metal into blade shape. He usually used tongs to hold the iron block in place.
• Reheating. When the blade cooled, the smith reheated it to keep it workable. While reheated and hammered repeatedly.
• Quenching which involved the process of white heating and cooling in water. Quenching made the blade harder and stronger. At the same time it made the blade quite brittle, which was a considerable problem for the sword smiths.
• Tempering was then done to avoid brittleness the blade was tempered. In another words it was reheated a final time to a very specific temperature. How the Romans do balanced the temperature? The smith was guided only by the blade's color and his own experience.

Failure and strength of materials

1. List the similarities and differences between the brittle and ductile modes of failure.
2. What is meant by the following terms:
   1. vacancies
   2. dislocations
   3. impurities
   4. grain boundaries
3. What four terms can be used to describe a material's mechanical properties?
4. What is meant by the following:
   1. cold working
   2. annealing
   3. tempering
   4. introduction of impurities
   5. alloying
   6. sintering

Summary

1. Hooke's Law gives the relationship between the extension of a spring and the force applied to it. The law says they are proportional.
2. Materials can be classified as plastic or elastic depending on how they respond to an applied force.
3. Materials can fracture or undergo creep or fatigue when forces are applied to them.
4. Materials have the following mechanical properties to a greater or lesser degree: strength, hardness, ductility, malleability, brittleness, stiffness.
5. Materials can be weakened by have the following problems in their crystal lattice: vacancies, dislocations, impurities, difference in grain size.
6. Materials can have their mechanical properties improved by one or more of the following processes: cold working, annealing, adding impurities, tempering, sintering.

End of chapter exercise

1. State Hooke's Law in words.
2. What do we mean by the following terms with respect to Hooke's Law?
   1. elastic limit
   2. limit of proportionality
3. A spring is extended by 18 cm by a force of 90 N. Calculate the spring constant for this spring.
4. A spring of length 8 cm stretches to 14 cm when a load of 0,8 N is applied to it.
   1. Calculate the spring constant for the spring.
   2. Determine the extension of the spring if a load of 0,7 N is applied to it.
5. A spring has a spring constant of $-150$  N.m ${}^{-1}$ . By how much will it stretch if a load of 80 N is applied to it?
6. What do we mean by the following terms when speaking about properties of materials?
   1. hardness
   2. toughness
   3. ductility
   4. malleability
   5. stiffness
   6. strength
7. What is Young's modulus?
8. In what different ways can we improve the material properties of substances?
9. What is a metal alloy?
10. What do we call an alloy of:
    1. iron and carbon
    2. copper and zinc
    3. copper and tin
11. Do some research on what added substances can do to the properties of steel. Present your findings in a suitable table.