Creep-ratcheting behavior of PTFE gaskets under various temperatures

Compressive creep-fatigue experiments on PTFE gaskets with small stress amplitude were tested. The effects of peak holding times and temperatures on creep-ratcheting behaviors were further discussed. Results showed that at room temperature, the accumulated creep-ratcheting strain corresponding to 400 cycles for peak holding 1min is nearly 1.06 times that for peak holding time 0.5min and 1.3 times that without peak holding. While, at 100 °C, 150 °C and 200 °C, they are almost 1.1 times and 1.6 times, 1.1 times and 1.7 times, 1.05 times and 1.9 times, respectively. This means although the accumulated creep-ratcheting strains for peak holding 0.5min and 1min are very close to each other, but short peak holding time still has obvious effect on the accumulated deformation comparing that without peak holding. Moreover, the creep-ratcheting deformation with short peak holding time and small stress amplitude for PTFE gaskets can be simply estimated by the corresponding compressive static creep strain at the same peak stress in practical engineering with relatively good accuracy.     

Compressive ratcheting effect of expanded PTFE considering multiple load paths

Uniaxial stress-controlled ratcheting behaviors of expanded PTFE (ePTFE) under cyclic compressive loads were tested. The effects of temperature, stress rate and mean stress on the ratcheting behaviors of ePTFE considering multiple load paths were discussed in detail. Results present that the steady ratcheting strain is rate-independent when the stress rate is less than about 0.1 MPa/s, while it approximately linearly decreases with increasing the stress rate for greater stress rate. Additionally, the steady ratcheting is temperature-independent when the temperature is greater than about 150 °C, but it nearly linearly increases with enhancing the temperature for lower temperature. Especially, the stress rate almost has little effect on the ratcheting strain of ePTFE at 200 °C. Moreover, the accumulated ratcheting strain enhances rapidly in about the first 80 cycles, and subsequently tends to shakedown in the subsequent cycles for each load path. Furthermore, if a higher stress is used in the prior cycling, the greater ratcheting strain may be produced, and a negative ratcheting strain rate can be obtained in the subsequent cycling with lower mean stress due to the greater strain hardening and deformation resistance produced by the previous higher stress.