Stress relaxation is a manifestation of viscoelastic behavior. Although the time-temperature shift principle appears to work properly, in real applications, the material experiences temperature variation and/or thermal cycling.
As we know, Liquid Silicone Rubbers combine the generally advantageous qualities of silicone with highly effective processing of high precision articles such as seals, electrical connectors, sealing membranes, infant products, etc. Temperature is a prominent determinant in the rate of stress relaxation of polymers. High temperatures result in a faster rate of stress relaxation. The time-temperature superposition (WLF) principle is used to explain the relationship between temperature and time. The non-isothermal viscoelastic theory assumes that temperature has no effect on polymers other than changing the rate of the creep/stress relaxation process, and the actual time interval, at a particular temperature, is fully equivalent to an effective time interval at a reference temperature. In practical applications, when a seal material is subjected to temperature variation, there are temperature-dependent effects such as thermal expansion/contraction in addition to time-dependent material property changes such as the rate of stress relaxation.
To analyze this thermal mechanical behavior, Liquid Silicone Rubber is evaluated in Compression Stress Relaxation (CSR) tests. These tests consist of putting a round button of LSR inside the CSR equipment, which has three independent testing rigs. The rigs can be put in an oven and the temperature of the oven can be adjusted manually. The temperature and compressive force applied during the test can be recorded continuously.
The compressive load and the temperature profile obtained during the tests are shown below.
Load Development and Change in Temperature:
The load changed sharply when there was a temperature change, but it was fully recovered when the temperature returned to its previous value. The stiffness of the material is linearly dependent on the stress level.
At the beginning, the stress was relaxed at room temperature for about one day. Then, the temperature was increased until 120°C. With that, the load increased due to the thermal expansion of the sample. The stress relaxed quickly when the temperature was 120°C. Again, the temperature was adjusted to room temperature and the load dropped accordingly, due to a thermal contraction. Here, no significant relaxation occurred because of the temperature. A similar situation is shown when the temperature was decreased until -10°C. When the new cycle started (at 120°C), the load returned almost to the end value of the last cycle at the same temperature, indicating that the load changed due to the thermal expansion/contraction, being fully reversible. The cycle was repeated three times in 1,000 h. The stress relaxation was significant only at 120°C. At the other temperatures, the stress remained almost constant because of the very slow relaxation rate. The stiffness of the material can be calculated using the information obtained in this test and with the following equation:
From here, the material stiffness decreased with temperature and the stress level. Additionally, the values were higher than the “fresh” material stiffness. This is evident because the LSR stiffness changed during the temperature cycling; it is interesting that this demonstrates why the mechanical properties always underestimate the observed value. For that, the material stiffness is almost linearly dependent on either the level of stress or temperature. Because temperature and thermal stress are dependent on one another, more observable research is needed to investigate this relationship.
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