Stress relaxation profiles

The Alpha Technologies MV 2000" Mooney viscometer can perform a stress relaxation test after completing the viscosity test (see condition 3. above). For example, an ML 1+4 test can be done on a raw rubber sample followed by a two-minute stress relaxation test. The stress relaxation portion of the test is initiated when the Mooney rotor is suddenly-stopped. The total test time would be seven minutes (or two minutes longer). Figure 36 shows two polymers that have the same Mooney viscosity yet have different stress relaxation profiles [132],... 

Stress Relaxation Studies. Time-temperature superposition was used to generate stress relaxation profiles through the a transition for PEMA and PTFEMA. The WLF... 

FIGURE 4-37 Creep and stress relaxation profiles (three-dimensionaD... 

When a viscoelastic material is subjected to a constant strain, the stress initially induced within it decays in a time-dependent manner. This behavior is called stress relaxation. The viscoelastic stress relaxation behavior is typical of many TPs. The material specimen is a system to which a strain-versus-time profile is applied as input and from which a stress-versus-time profile is obtained as an output. Initially the material is subjected to a constant strain that is maintained for a long period of time. An immediate initial stress gradually approaches zero as time passes. The material responds with an immediate initial stress that decreases with time. When the applied strain is removed, the material responds with an immediate decrease in stress that may result in a change from tensile to compressive stress. The residual stress then gradually approaches zero. 

Figure 6. Stress relaxation comparison of two emulsion polymers having the same overall composition. The power feed example utilized a feed profile with increasing butyl acrylate (0-> 0.65), decreasing methyl methacrylate (1.0 -> 0.35), and x = 1.3. The time axis has been shifted to a reference temperature of 26°C.

Both materials were subjected to the same processing conditions. The cure profile consisted of heating from room temperature to 2 60°C at 5°C/min, holding at 260°C for 2 hours, then cooling to room temperature at 5°C/min. As can be seen in Figure 3, their stress-temperature profiles are quite different. Both films left the spin-coater with approximately zero stress. Upon heating, the polyimide film developed substantial tensile stress due to film contraction from solvent evaporation while the BCB film exhibited only mild tensile stress buildup. The stress in the BCB film relaxed at 260°C while the stress in the polyimide did not. 

Kumar B, Das A, Alagirusamy R. Prediction of internal pressure profile of compression bandages using stress relaxation parameters. Biorheology 2012 49(1) 1—13. 

The computer can also be used to provide a singular solution to the required cross section. The time-stress-strain data to generate a set of creep curves for a specific material would be provided. The computer is then programmed with the problem of the stress-time profile for the part. Using the inverse curve as the strain relaxation curve, the computer can do an iterative solution to determine the minimum cross section. That would restrict the creep to the set amount in the desired design time. The WLF transformation can be done on the basic stress-strain time data to provide the solution for different operating temperatures. 

Base rubbers may be based on silicone, fluoropolymers, or hydrocarbons. Although silicone rubbers such as silicone S and G have been applied in stacks, it has become clear that they are not sufficiently stable [83-85]. Materials like ethylene-propylene-diene-monomer (EPDM), butyl rubber (IRR), or fluororubbers (FKM such as Viton )seem better suited. Further research is carried out to optimize properties like hardness, tensile strength, and stress relaxation. Also the morphology is being considered, with apparently a preference for profiled over flat gaskets. 

Thermal relaxation occurs behind the shock front to restore thermal equilibrium. In a condensed system, this involves the reestablishment of the equilibrium distribution of both the frequencies and the velocities of the particles, i.e., their potential and kinetic energy distributions. In the simplest case without structural relaxation and the accompanying stress relaxation, we expect this process to occur at the appropriate second sound velocity. In the fully relaxed region the kinetic and potential energy distributions are in equilibrium. In the relaxing region these distributions are not steady. The shock profile as a whole is therefore unsteady. 

Figure lb. Schematics of the normal and relaxed stress profiles are shown. In the relaxed profile, the maximum in the surface compression has moved away from the surface of the sample. 

FIG. 1 -4. Geometry and time profile of a simple shear stress relaxation experiment following sudden strain. 

The narrow molecular weight distribution of metallocene homopolypropylene directly translates into narrow stress relaxation time spectra of the molten polymer. In a spinning process, where the polymer melt is stretched and subsequently crystallized, this means a more rapid decay of the stress in the fibers, and as a consequence, allows higher spinning speeds and finer fibers. A fully viscoelastic simulation of the spinning process indicates that metallocene polymers exhibit a more pronounced spinline profile, allowing the production of thinner filaments than can be achieved using conventional polypropylene (Fig. 43). 

When a cyclic temperatiore profile is applied to a sample the heat flow signal will oscillate as a result of the temperature program, and the size of the oscillation will be a function of the heat capacity of the sample. Therefore, the amplitude of the heat flow signal allows a heat capacity value to be obtained. This is similar to DMA (see Chapter 4) where the amplitude of the oscillation allows a modulus value to be obtained. Whilst other methods already exist to provide heat capacity, the value of this method is that the heat capacity measurement is separated from other potentially overlapping events, such as reactions or stress relaxations and can also be obtained with increased sensitivity compared to the slow linear scan rates of traditional DSC. 

Wave profiles in the elastic-plastic region are often idealized as two distinct shock fronts separated by a region of constant elastic strain. Such an idealized behavior is seldom, if ever, observed. Near the leading elastic wave, relaxations are typical and the profile in front of the inelastic wave typically shows significant changes in stress with time. 

Changes in polarization may be caused by either the input stress profile or a relaxation of stress in the piezoelectric material. The mechanical relaxation is obviously inelastic but the present model should serve as an approximation to the inelastic behavior. Internal conduction is not treated in the theory nevertheless, if electrical relaxations in current due to conduction are not large, an approximate solution is obtained. The analysis is particularly useful for determining the signs and magnitudes of the electric fields so that threshold conditions for conduction can be established. 

Let us stress here, that in an attempt to relax the ripple by introducing the viscosity, the distortion of the solution profile and accuracy losses occurred. 

Cheremisinoff and Davis (1979) relaxed these two assumptions by using a correlation developed by Cohen and Hanratty (1968) for the interfacial shear stress, using von Karman s and Deissler s eddy viscosity expressions for solving the liquid-phase momentum equations while still using the hydraulic diameter concept for the gas phase. They assumed, however, that the velocity profile is a function only of the radius, r, or the normal distance from the wall, y, and that the shear stress is constant, t = tw. ... 

---