Creep Dynamic properties

The data included provides examples of what are available. As an example static properties (tensile, flexural, etc.) and dynamic properties (creep, fatigue, impact, etc.) can range from near zero too extremely high values. They can be applied in different environments from below the surface of the earth, to over the earth, and into space. 

While creep experiments of the type described above are often ployed in the analysis of mechanical properties, they are not the only (mes used for that purpose. Two other important techniques are the stress-relaxation method and the measurement of dynamic properties. 

Special requirements, with examples being stress relaxation, creep, wear, compression stress-strain behavior, dynamic properties, staining, and metal corrosion. 

Non-reinforcing fillers, for a given increase in vulcanizate stiffness, generally give better permanent set, creep and dynamic properties than reinforcing types. The basic properties of natural rubber also show up to advantage in the fabrication of components. 

Dynamic mechanical analysis provides a measurement of the mechanical response of a material as it is deformed under periodic stress. Material properties of primary interest include modulus ( ). loss modulus ( "), tan (E"IE ), compliance, viscosity, stress relaxation, and creep. These properties characterize the viscoelastic performance of a material. 

For undiluted polymers, unlike dilute solutions, the viscoelastic behavior in stress relaxation and creep is of considerable interest as well as dynamic properties. The functions G t) and J t) as predicted by the Rouse theory have been numerically evaluated by Tschoegl, and are tabulated in reduced dimensionless form in Appendix E. 

In other studies of creep of solutions of polystyrene in tricresyl phosphate and creep and dynamic properties of polystyrene in benzyl -butyl phthalate, a slightly higher concentration dependence has been observed. An example is shown in Fig. 17-15, where points are plotted from various integration procedures like equations 3 to 5 of Chapter 13. The slope of the logarithmic plot is 2.2 to 2.3. In these studies, data at different temperatures must be reduced to a reference temperature, and it is uncertain whether G% should be directly proportional to cT (cf. Fig. 13-10 and associated discussion) if it is, an exponent near 2 is obtained. In recent measurements on solutions of polybutadiene, which do not require temperature reduction for comparison, an exponent between 2.2 and 2.3 has also been obtained. ... 

Mechanical testing procedures in common use involve other patterns of stress history than the simple creep and relaxation experiments on which the definitions of the transient viscoelastic functions are based, and the sinusoidally varying stress which is inherent in the definitions of the so-called dynamic properties. Certain relations between the behavior under coniplicated conditions and the basic viscoelastic functions are presented here together with some related problems. They are limited to linear viscoelastic systems and hence small strains, but in some cases could be extended to describe larger deformations, especially for simple extension. 

In the discussion on dynamic properties (Section 2.4), it was pointed out that there is an energy loss per cycle of stress and strain proportional to the loss creep compliance J". This is represented by a hysteresis loop on the stress-strain diagram. Figure 4.7. 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

Depending on construction and orientation of stress relative to reinforcement, it may not be necessary to provide extensive data on time-dependent stiffness properties since their effects may be small and can frequently be considered by rule of thumb using established practical design approaches. When time dependent strength properties are required, creep and other data are used most effectively. There are many RP products that have had super life spans of many decades. Included are products that have been subjected to different dynamic loads in many different environments from very low temperatures to very high corrosive conditions, etc. An example is aircraft primary structures (10,14,62). 

Distributions of relaxation or retardation times are useful and important both theoretically and practicably, because // can be calculated from /.. (and vice versa) and because from such distributions other types of viscoelastic properties can be calculated. For example, dynamic modulus data can be calculated from experimentally measured stress relaxation data via the resulting // spectrum, or H can be inverted to L, from which creep can be calculated. Alternatively, rather than going from one measured property function to the spectrum to a desired property function [e.g., Eft) — // In Schwarzl has presented a series of easy-to-use approximate equations, including estimated error limits, for converting from one property function to another (11). 

An instrument for measuring the mechanical properties of rubbers in relation to their use as materials for the absorption and isolation of vibration. These properties are resilience, modulus (static and dynamic), kinetic energy, creep and set. The introduction of an improved version has recently been announced. 

---