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Mechanical expansion or compression of a material

  1. Apr 14, 2017 #1
    Would you explain what "mechanical expansion or compression of a material" is? It makes no sense to me I only know thermal expansion or compression.

    https://www.google.tl/patents/US5004946

    Thank you.
     
  2. jcsd
  3. Apr 14, 2017 #2

    berkeman

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    Staff: Mentor

    They mention piezoelectric expansion and contraction in the patent. That is not thermal -- it is electromechanical. Does that make sense?
     
  4. Apr 15, 2017 #3
    I have never heard piezoelectric expansion. I will make a search.

    Thank you.
     
  5. Apr 15, 2017 #4

    CWatters

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    Science Advisor
    Homework Helper

    Mechanical expansion or compression occurs when you apply forces to an object. Examples...

    If you walk on a grass lawn the grass is mechanically compressed.
    When you pump up a car tyre the air is compressed and the tyre expands.
    If you squeeze a sponge it is compressed.

    When forces are applied to a piezoelectric crystal it is compressed but only very slightly because they have a high modulus.
     
  6. Apr 15, 2017 #5
    I could only think this situation for elongation of springs, because in engineering we assume other elements as rigid.

    Thank you.
     
  7. Apr 15, 2017 #6

    JBA

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    Forty years of engineering and occasions of dealing with impacting components has taught me that the majority of engineering materials and structures should never be considered as being "rigid". They all have a modulus of elasticity, some very high,some very low and most somewhere between those two extremes.
     
  8. Apr 15, 2017 #7
    You are right but the frequency probably changes due to area of engineering. In mechanical engineering, I don't remember if rigidity of materials are important other than strength of materials/mechanics of materials having some other names.

    Thank you.
     
  9. Apr 15, 2017 #8

    JBA

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    The particular application does matter; but it can be a mistake not to include deflection as a part of a design analysis even when strength requirements are clearly met. In many designs deflection and/or rigidity can be controlling factors in design when strength clearly exceeds it required minimum.

    A few examples:
    In the design of gear assemblies rigidity of the supporting structure is the most important element because it is what counteracts to prevent the displacement of the tooth contact diameter of the gear teeth under load that will destroy a gear set in short order; and, as a result, the analysis of a gearbox structure will show it to be grossly over designed from a strength standpoint.
    In design of structures and buildings, excess flexibility, even when strength has been achieved can result in the transfer of loading to attachments like windows that can result in failures; or, walkway or bridge structures that bounce or oscillate under the rhythm of traffic loading.
    In pressure piping and vessel bolting, even if strength is provided the stretching of the connecting flange bolts under the tensile loading can result in reduced compressive loading of the sealing gasket and result in product leakage.
    In machinery, flexing and or deflection under loading can result in loss of accuracy and precision in the machining of components even when structural strength is sufficient.

    I only press this point to try and stress (no pun intended) the fact that material flexibility deflection should always be considered to be a possible controlling element in essentially every design analysis.
     
  10. Apr 15, 2017 #9
    Everything is deformable, but somethings are moreso than others. The assumption that particular parts are rigid is often a useful simplifying assumption, but it is never strictly true. A major part of engineering analysis is to correctly understand when such an assumption is valid and when it is not. This is where the art of engineering comes into play.
     
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