# Study of Cyclic Plasticity and Creep Ratchet Behavior of PTFE
## 3 Results and Discussion
### The Cyclic Plasticity Behavior at Different Temperatures  
The cyclic plasticity test was performed on PTFE specimens at different temperatures with different average stresses and amplitude stresses. The cyclic plasticity hysteresis curves are only given for the 23 \({}^{\circ}\)C condition as examples, which shown in Figure 4. Ratchet strains apparently appeared in each loading condition, and the area of the hysteresis loop gradually decreased with the increase in the number of cycles. This indicates that the material shows a more pronounced hardening when plastic deformation occurs. Figure 5 illustrates the variation in ratchet strain and strain rate for each loading condition. It reveals that the ratchet strain increases gradually with the number of cycles, but the ratchet strain rate decreases rapidly. Among the cycles, it accumulates rapidly in the first 20 cycles and gradually increases at a relatively stable rate after 20 cycles. Under different average stresses, the material initial single tensile rise section curve change trend is basically the same, indicating that PTFE in uniaxial ratchet test conditions under the stability of the stress-strain response and the rapid accumulation of ratcheting strain still occurs in the first 20 cycles. After 50 cycles, the evolution of the ratchet strain rate during loading is more or less the same, and the ratchet strain and ratchet strain rate are very sensitive to the stress response. At the same average stress, the ratchet strain and strain rate increased gradually with the increase in amplitude stress. However, at different average stresses, with the increase in average stress at the same amplitude stress, the increment of ratchet strain after 200 cycles changed significantly, a larger level of increase was observed relative to the different amplitude stresses, and the change in the ratchet strain rate was more prominent. Notably, the ratchet strain and the strain rate at 23 \({}^{\circ}\)C are greater than those at 50 \({}^{\circ}\)C. This is because PTFE exhibits three different crystalline phases at atmospheric pressure. The three polymorphs are usually denoted as form II, stable at temperatures below 19 \({}^{\circ}\)C, form IV, stable at temperatures between 19 and 30 \({}^{\circ}\)C, and form I, stable at temperatures higher than 30 \({}^{\circ}\)C up to the melting point of 330 \({}^{\circ}\)C [32]. Furthermore, by looking at the electron diffraction patterns of form IV and form I, it was found that the structures of the two forms are indeed different [33]. The present experimental data illustrate that the ratchet strain and ratchet strain rate of crystalline type IV at 23 \({}^{\circ}\)C are larger than those of crystalline type I at 50 \({}^{\circ}\)C.  
Figure 3: The uniaxial tensile curves of PTFE at different temperatures. Figure 4: The cyclic hysteresis curves under different stress amplitudes at 23 \({}^{\circ}\)C: (**a**) LC1; (**b**) LC2; (**c**) LC3; (**d**) LC4; (**e**) LC5. Figure 5: Ratchet strain and strain rate under different loading conditions Ratchet strain: (**a**) 23 \({}^{\circ}\)C; (**c**) 50 \({}^{\circ}\)C; (**e**) 100 \({}^{\circ}\)C; (**g**) 150 \({}^{\circ}\)C; (**i**) 200 \({}^{\circ}\)C. Ratchet strain rate: (**b**) 23 \({}^{\circ}\)C; (**d**) 50 \({}^{\circ}\)C; (**f**) 100 \({}^{\circ}\)C; (**h**) 150 \({}^{\circ}\)C; (**j**) 200 \({}^{\circ}\)C.  
Moreover, throughout the temperature data, for both the ratchet strain and strain rate, LC3 is in the middle, with LC2 below and LC4 above, which indicates that for the same level of average stress, both the ratchet strain and ratchet strain rate increase with the increase in amplitude stress level. Similarly, LC3 is in the middle, with LC1 below and LC5 above, which indicates that for a given level of amplitude stress, the ratchet strain and ratchet strain rate increase as the average stress level rises.