Reciprocal deformation of rubber
For an ideally elastic material, such as a steel spring, when it is stretched, the function performed by the external force is stored as potential energy. When the force is removed, the potential energy becomes kinetic energy and the spring returns to its original dimensions. Viscous liquids are another extreme example, where the energy used to deform it cannot be stored and is dissipated in the form of heat. Rubber is somewhere in between. If a rubber specimen is stretched (without breaking it) and then retracted at the same rate, the stress-strain curves for stretching and retraction do not coincide, as shown in Figure 5-39. In the figure, OA is the elongation curve, stretch to point A and then let it retract, AC is the retraction curve. The elongation curve OA and the area OAB surrounded by the horizontal coordinate is to make the rubber elongation, the outside world on its mechanical work done. The retraction curve is carried out according to AC, and the area ABC surrounded by AC and the transverse axis is the mechanical work done to the outside world when the rubber is retracted. And OAB area and ABC area of the difference between OAC for the expansion and contraction of a week of mechanical work lost. The lost mechanical work is converted into heat, when the rubber itself rises in temperature. The lost mechanical work is called hysteresis loss.
If it is an ideal rubber, there is no viscous resistance between molecules, the elongation and retraction curves should all be carried out according to the OCA line (i.e., the equilibrium line), such as Fig. 5-40. However, the actual rubber is viscous resistance, and the deformation falls behind the acting force. If the force per unit area of the specimen is P, the equilibrium deformation hard is 1-0, the deformation of the specimen can only reach 1-1; and retraction, the stress is reduced to P, the specimen can not be restored to 1-0, can only be restored to 1-2. in an extension and retraction of a cycle, the relationship between the stress - strain constitutes a loss of energy circle, known as the hysteresis circle.
Hysteresis loss rings are the result of the viscous resistance nature of rubber as it deforms. In the force machine, the speed of expansion and contraction for a certain, while the rubber has a series of relaxation mechanism. In this series of relaxation mechanism, the relaxation time is shorter than the strong machine round-trip cycle, in the rubber deformation process has its own relaxation. This part of the relaxation mechanism to keep up with the strong machine round trip, does not produce hysteresis loss. There are some relaxation time than the strong machine round trip time is much longer than the relaxation mechanism, in the expansion and contraction process is too late to move, there is no hysteresis loss. Only those mechanisms that have a cycle comparable to the round-trip operation of the force machine play a major role in sticking and blocking.
Thus, the hysteresis loss of vulcanized rubber is related to the type of rubber, the degree of vulcanization, the cooperating agent, etc., as well as to the speed and temperature of deformation. If the rubber expansion is carried out very quickly, than the relaxation time t of all molecular chains are small, the molecular chain is too late to open, the rubber presents a glassy state, there is no hysteresis loss. If the amount of deformation is very small, the movement of the deformation time is longer than the time of all molecular chains, all the relaxation mechanism are in the cycle of the expansion and contraction of the movement of the relaxation is complete, close to the equilibrium condition, the hysteresis loss is also very small. The effect of temperature is still as mentioned above, at high and very low temperatures, the hysteresis loss is very small.
If the specimen is deformed several times on the force machine, the hysteresis circle will be gradually reduced and the amount of capacity loss will be fixed. In the discussion of hysteresis loss should be based on a fixed circle, after a few reciprocating motion, the molecules participating in the deformation are moved to a fixed position, and then in this equilibrium position on the reciprocating motion.
Rubber is often subjected to cyclic forces in actual use, and the experimental results of the general static force often do not correspond to the actual use of the product. Rubber presents the glass state temperature and the frequency of cyclic deformation, natural rubber vulcanized rubber glass transition temperature and frequency of the relationship is as follows:
If the car driving speed of 60 kilometers per hour, the general frequency of deformation of the outer tire is about 100 weeks / second, and the general strength of the machine is equivalent to the time in 5 minutes a week to carry out, the test piece of the tensile speed and the actual use of the situation is nearly 30,000 times the difference between the two cases of the cold temperature difference of nearly 30 degrees Celsius. Therefore, the performance of rubber products should be used in close proximity to the actual use of the situation, or according to the temperature, time interchangeable principle for the necessary conversion.