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How to tell if a spring is fatigue, creep or degrade?

  1. Nov 1, 2016 #1
    I'm doing some spring endurance test.

    If the spring is exceeding its cycles limit and causes fatigue failure, how normally the spring constant changes? For example, after the expected limit cycles (lets say 2million load-unload cycles), will the spring constant gradually decreases? or fluctuated as the cycles continue (after 2million)?

    In simple words, how the spring constant changes (with cycles continue) if a spring is fatigue, creep or degraded? How to determine if a spring steel is degrading?

    (Please assume everything is in ideal condition, as there are many factors might affect the result)
     
  2. jcsd
  3. Nov 1, 2016 #2

    Nidum

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    What research have you done on this subject ?

    There is a large amount of easily found information on the internet relating to spring technology .
     
  4. Nov 1, 2016 #3

    Ranger Mike

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    simple
    take a new unused spring and measure the height
    next find out how much weight it takes to compress it one inch
    find out how much weight to compress two inch
    now you have the bench marks of new spring
    do same tests on cycled spring
     
  5. Nov 1, 2016 #4

    JBA

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    If you referring to wire coil compression springs, these will principally only lose their initial set (free length and spring rate) if they are exposed to extreme temperatures that can anneal the material or to an environment that is corrosive to their material.
    The principal failure of this type of spring due to cycling fatigue is fracturing of the wire in one of the coils not loss of spring rate or free length; but extremely rapid cycling over an extended period can result in internal heating of the spring wire that can result in a loss of spring rate and free length.

    For critical applications, presetting (compressing the spring through its full travel for a few cycles) is a standard step during the spring manufacturing process.
     
  6. Nov 1, 2016 #5
    Thanks and appreciates!

    It is a spring steel (compression), in high and rapid load-unload cycles (100ms per cycle and 2million cycles) and spring constant (k) is to be tested for each 100k cycles. (assume environment is ideal)

    Since its load-unload, fatigue failure which causes fracture in the material itself might occurred. So, does it means spring constant should remained even fatigue failure occurred or ...? If internal heating exists, does the spring constant would have observable decreases?

    Will spring constant fluctuates through cycles..... or perhaps it is just the tolerance of data recording?
     

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  7. Nov 1, 2016 #6

    JBA

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    First, if I understand what you have asked about a chance in spring constant after fracture that would not make any since the spring is now in two broken parts and spring constant is dependent upon the spring's total number of coils.

    With sufficient internal haeting some portion of the spring could approach the annealing temperature of the spring alloy and this would result in a softening and loss of temper in the spring wire so there would be a corresponding loss of spring constant and could result in permanent deformation of coil in that region.

    Quality manufactured compression springs are very consistent and repeatable in their performance, so I would expect any variances during testing to be related to the testing fixture and recording not to a variance in the spring itself.

    While having said all of the above, there is no such thing as a perfectly manufactured spring so there can be some variance in the finished free length, spring constant and fatigue life between test samples of springs manufactured to the same design and specifications.
     
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