Hysteresis heating failure

The hysteresis heating failure occurs more commonly in TP type members subject to dynamic loads. An example is a plastic gear. With the gear teeth under load once per revolution, it is subjected to a bending load that transmits the power from one gear to another. Another example is a link that is used to move a paper sheet in a copier or in an accounting machine from one operation to the next. The load may be simple tensile or compressive stresses, but more commonly it is a bending load. 

An example will be given to show how dynamic loading can lead to product failure by hysteresis heating. When this condition exists the failure will be catastrophic rather than... 

Be alert to possible external disturbances—such as high humidity, mechanical vibrations, stray electric fields, voltage fluctuations, unusual local heating—and internal misbehaviors—such as vacuum leaks, nonhnear meter readings, hysteresis. Some of these may cause equipment failure or result in noisy, erratic readings. Some can introduce subtler troubles in the form of systematic errors, where the data look fine but are not. See Chapter II for a detailed discussion of errors. 

The change in properties extends beyond the static properties of the material. Under dynamic loading, the material behaves much more elastically and the tendency to destruction by hysteresis induced heating effects is reduced. On the other hand, because the polymer chains are stretched severely in the stress transfer process, there are much higher levels of bond breakage and recrystallization effects with typical fatigue failure performance. In the balance, the net result is a substantial improvement in the performance of the material in dynamic loading. 

Increasing the load level is another way of accelerating the test. Here, again, it is important to do this cautiously and base it on the performance curves for the materials. The creep of the plastics is sensitive to the stress level and the range under consideration may be such that a relatively small increase in stress level will result in a large increase in creep level. In addition, since the part is also dynamically loaded, the increased stress level may lead to severe heat build up due to increased size of the hysteresis loop at increased stress levels and this could lead to catastrophic failure of the part. In any event, the data for the material should be carefully examined to see that the acceleration method used does not lead to erroneous results because any critical level factor is exceeded. 

The rotating beam fatigue test is generally not used to test plastics materials due to hysteresis phenomena. When used, the test is conducted at a very low testing speed to avoid premature failure due to heat buildup. The test is conducted by mounting both ends of the dumbbell specimen in the testing machine. The specimen is rotated between two spindles, and stress in the form of tension and compression is applied. The specimen is subjected to the number of cycles of stress specified or until fracture occurs. 

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