Liquid Silicone Rubber’s Fatigue Behavior
Silicone’s fatigue behavior is sensitive to cyclic load limits and factors that influence the fatigue. Each effect has different factors which have an impact on the behavior. The load history denotes the associated mechanical severity.
The environmental factors affect the short and long term fatigue behavior. The compounding factors that affect the behavior are silicone type, filler type, and volume fraction.
In silicone materials, mechanical fatigue is defined as a progressive weakening of their physical properties as a result of a slow crack growth during the application of dynamic loads or deformations. The gradual reduction in stiffness is the most visible consequence of this. Various phenomena in the atomic and molecular levels occur during fatigue. Fatigue characteristics for silicone are empirical in some grade because of the complexity of the structure. Some tests were developed to evaluate and simulate the repeated distortions received in service by silicone products. These distortions can be in extension, compression, and bending, or in a combination of them. The most popular test is the flex fatigue test in which a static load is applied in several cycles and the peak load, elongation percentage, modulus of elongation and yield to break are calculated. This test is standardized by ASTM D623 and ASTM D430.
The resulting curve (see figure below) of the experiment relates the load with the strain, and severe cycles are presented. A significant difference between the load and unload curves is observed, which forms a hysteresis loop.
Many factors influence the mechanical fatigue life of silicone components. The three major factors are loading history, environmental effects, and silicone formulation. The understanding of these factors permits the development of durable silicone parts and the analysis and prediction of the failure.
Mechanical fatigue involves crack nucleation and growth due to fluctuating loads.
Environmental effects play a crucial role in the fatigue process, particularly at long life. The fatigue is highly dependent upon the temperature. Making the analysis using the figure below, at temperatures below 0°C, there is a difference in stiffness between -40°C and -20°C. For temperatures greater than 0°C, the magnitudes of hysteresis and stress softening decrease with increasing temperatures. Up to 60°C, the temperature does not alter the stress softening any more. The last test (150°C) is not complete because the specimen fails before the time of experiment.
Generally speaking, the strength of silicone rubber against dynamic stress is no greater than in organic rubbers. But several companies have developed new materials with a resistance that is 8 to 20 times higher than in conventional silicones.
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