Effect of Temperature on Materials

3 Key excerpts on "Effect of Temperature on Materials"

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  eBook - ePub

  Applied Strength of Materials

  • Robert L. Mott, Joseph A. Untener(Authors)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  14 Thermal Effects and Elements of More than One Material

  The Big Picture

  14–1 Objectives of This Chapter

  14–2 Deformation due to Temperature Changes

  14–3 Thermal Stress

  14–4 Members Made of More than One Material

  The Big Picture Discussion Map

  • Engineering drawings typically represent parts in one given condition, but when the temperature changes, the material responds accordingly. In many applications, these changes must be considered for proper performance and safety.
  • Thermal changes when parts are free to grow or shrink without restriction will result in a change in part dimensions. If thermal changes occur while a part is restricted from growth or shrinkage, internal stresses will be induced.
  • Some elements that carry loads are made of more than one material. Those materials share the load, but not necessarily evenly. Analysis of each material is required to ensure proper performance and safety.

  To this point, elements considered in this text have been assumed to remain at the same temperature. Automotive engine components like those shown in Figure 14-1 , though, can go through extreme temperature conditions. When sitting in the morning in a northern climate, the entire engine might be well below freezing. Within minutes of starting the engine, however, an internal component near the combustion of fuel will reach extremely high temperatures. As a designer, you must anticipate the full range of conditions and ensure that your designs will perform as specified.

  FIGURE 14–1

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  eBook - ePub

  Introduction to the Electronic Properties of Materials

  • David C. Jiles(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  In the Hall effect, the application of a magnetic field normal to the passage of an electric current leads not only to a transverse voltage but also to a transverse temperature gradient. The appearance of this temperature gradient is known as the Ettingshausen effect. This arises because charge carriers with different energies (velocities) are deflected differently by the magnetic field. This is a small effect which adds to the Hall voltage.

  8.5 Thermal properties of materials

  Which factors determine the thermal properties of materials?

  The thermal properties of materials can be determined principally by the electrons, as in the case of thermal conductivity of metals, or principally by the lattice, as in the case of thermal conductivity of insulators or of specific heat capacity [5 ].

  8.5.1 Thermal conductivity

  How does thermal conduction take place in materials?

  Thermal conductivity of materials varies from 6 × 103 W.m−1 K−1 in silver and copper to 5 × 10−2 Wm−1 K−1 in sulphur [6 ]. In the case of metals, the thermal conduction mechanism is similar to the electrical conduction mechanism and proceeds via the free electrons which migrate throughout the material. In semiconductors, conduction can take place by the electrons which are thermally stimulated into the conduction band.

  In insulators, another mechanism must be involved and in this case the thermal conduction is due to phonons which are created at the hot part of a solid and destroyed at the cold part. These phonons provide the mechanism by which energy is transfered though the material. In metals the phonon contribution to thermal conductivity is also present, but the electronic contribution is so much greater that in these cases the phonon contribution is neglected.

  We have already defined the thermal conductivity Κ in Section 1.5.1 . It is the ratio of the heat flux J Q( = Q/A ) to the thermal gradient dT/ dx

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  eBook - ePub

  Thermoplastic Material Selection

  A Practical Guide

  • Eric R. Larson(Author)
  • 2015(Publication Date)
  • William Andrew

    (Publisher)

  [3] .

  5.4. Environmental Effects

  In any material selection process, one must consider how the environment that a product is used in will affect its performance. One aspect of this is in the measurement of specific performance criteria, such as the road handling of a car using a specific tire on a wet road. On a material selection level, we are not only concerned with how the specific tire performs, but also with how the environment affects the material used in the tire. There are a number of environmental phenomena that can affect materials. These effects can be loosely grouped into two main categories: those that are reversible and those that are not.

  Reversible changes in materials happen all the time, in all types of materials. As an example, almost all materials expand with the application of heat and contract when they are exposed to cold. Also, most materials become more flexible at high temperature and get stiffer when they are cold. Some materials may soften when they get wet, but will return to their original hardness when they dry out. These types of changes are common, and under most situations they are fully reversible as long as the material has not gone through a permanent phase change (such as cement turning into concrete). While these kinds of reversible changes need to be accounted for in the selection of the material (and in the design), they are normally not a big deal.

  What is a big deal is when exposure to the environment causes irreversible changes in the material itself. These changes include chemical reactions, structural changes in the polymer matrix, degradation of the polymer, and sometimes even a complete depolymerization of the polymer molecules (a breakdown of the polymer chain into its base monomers).

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