16.26 Problem Set #6

4/7/97

 

Due 4/16/97


  1. Consider the material model below:

     
    1.  Sketch the response of this material to
      1. a constant deflection (plot F vs. t, given x = xo for t > 0)
      2. a constant load (plot x vs. t, given F = Fo for t > 0).

      Comment on what happens at t = 0, small t, and t -> in each case.

    2. If, in the above cases, the load or deflection is later removed, does the material eventually return to its original shape?

    3. If h = ho/T, (where T is the absolute temperature) can we simply scale the time to predict the temperature dependence of stress relaxation?

  2. Consider the T300/934 [+/- 45]s laminate, restrained while cooling from a stress-free temperature of 350°F, discussed in class. Find the stresses as functions of time if we cool the laminate at constant rates of 1°F/min, 3°F/min, or 10°F/min. In all cases, cool until the laminate reaches 70°F, then hold at 70°F forever.

  3. A jet engine component made of Inconel 718 must support a constant tensile stress of 80ksi while the engine is operating. The component normally operates for 4000 hrs between inspections. In this time it should suffer no more than 0.5% creep strain. What is the allowable operating temperature?

    How would the above answer be affected if the component design was altered to account for the following: The component may be subjected to incidents of abnormal operation at higher temperature. These incidents could occur at any time. One 10 hr. incident of abnormal operation at 100°F above the operating temperature should be survived without requiring any additional inspections. The component should also be able to survive 10 hrs at 200°F above the operating temperature without rupture, although in this case the 0.5% creep rule may be violated. Make sure you clearly expain your approach to this combined-creep case.