Some Aspects of the Mechanical Response of BMI 5250-4 Neat Resin at 191 Degrees C: Experiment and Modeling
Master's thesis Sep 2004-Mar 2006
AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH DEPT OF AERONAUTICS AND ASTRONAUTICS
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The mechanical response of BMI 5250-4 neat resin at 191 degrees C was studied using both creep and recovery tests where several variables were allowed to change. In these tests, the effect of stress rate, prior history, and panel variability were all taken into account. During the creep test, the material showed both primary and secondary creep over 20 h. The recovery tests showed full recovery after it was subjected to 80 UTS. The higher stress rate caused a much greater response in both creep and recovery tests. The prior history was studied by allowing the specimens to go through a stepwise creep test. They behaved similar to the single step creep test when preceded by a loading segment. During creep tests preceded by unloading, the material showed a decrease in creep strain. This decrease grew as the creep stress approached zero. The only difference that could be seen with panel variability was that the UTS dropped dramatically between the panels. A nonlinear viscoelastic model was created that was based on the work by Schapery. This model included four constants that were material specific and stress dependant. These constants were obtained by viewing the response during a two- step program including creep and recovery. The model was verified by comparing the predictions to previous tests including the creep and stepwise creep tests. The model predicted the creep strain preceded by load up well. It was capable of showing the response of the negative creep but the error grew as more steps were introduced. The model could not take into account stress rate. Therefore it could only predict the results at the higher stress rate.
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