Material characteristics creep deformation

In computing ordinary short-term characteristics of plastics, the standard stress analysis formulas may be used. For predicting creep and stress-rupture behavior, the method will vary according to circumstances. In viscoelastic materials, relaxation data can be used in Eqs. 2-16 to 2-20 to predict creep deformations. In other cases the rate theory may be used. 

Viscoelasticity is a phenomenon observed in most of the polymers since they possess elastic and viscous characteristics when deformed. The properties such as creep, stress relaxation, mechanical damping, vibration absorption and hysteresis are included in viscoelasticity. If a material shows linear variation of strain upon the application of stress on it, its behavior is said to be linear viscoelastic. Elastomers and soft biological tissues undergo large deformations and exhibit time dependent stress strain behavior and are nonlinear viscoelastic materials. The non-linear viscoelastic properties of solid polymers are often based on creep and stress-... 

In Figure 13.30, we compare the creep and stress relaxation responses of such a material with the responses of an ideal elastic solid and a viscous Newtonian fluid. It is easily realized from the creep behavior that a small value ofand large value of G (and thus a small value of k) correspond to the viscoelastic response approximating to the viscous response. Further, the creep response of the viscoelastic material at short times (for example, instantaneously) closely approximates to the elastic behavior, because viscous flow occurs only over time. At long times, the accumulated viscous response represents the overall deformation. Similarly, the stress relaxation behavior of the viscoelastic material suggests that 2 is a measure of time taken for stress to relax to 1/e of the original stress, and hence this is called the relaxation time of the material. The effect of material characteristics k and process time (t) are thus easily put together in the dimensionless parameter called the Deborah number De, as in Eq. (37). 

Creep. The creep characteristic of plastic foams must be considered when they are used in stmctural appHcations. Creep is the change in dimensions of a material when it is maintained under a constant stress. Data on the deformation of polystyrene foam under various static loads have been compiled (158). There are two types of creep in this material short-term and long-term. Short-term creep exists in foams at all stress levels however, a threshold stress level exists below which there is no detectable long-term creep. The minimum load required to cause long-term creep in molded polystyrene foam varies with density ranging from 50 kPa (7.3 psi) for foam density 16 kg/m (1 lb /ft ) to 455 kPa (66 psi) at foam density 160 kg/m (10... 

Mechanical properties per se concerns with the qualities which determine the behaviour of a material towards applied forces. The ability to support weight without rupture or permanent deformation, to withstand impact without breaking, to be mechanically formed into different shapes - all these depend upon a combination of mechanical properties characteristic of metals. Four types of behaviour of a material under stress are very important linear or elastic behaviour, plastic behaviour, creep behaviour and fatigue behaviour. 

Chapters 5 and 6 discuss how the mechanical characteristics of a material (solid, liquid, or viscoelastic) can be defined by comparing the mean relaxation time and the time scale of both creep and relaxation experiments, in which the transient creep compliance function and the transient relaxation modulus for viscoelastic materials can be determined. These chapters explain how the Boltzmann superposition principle can be applied to predict the evolution of either the deformation or the stress for continuous and discontinuous mechanical histories in linear viscoelasticity. Mathematical relationships between transient compliance functions and transient relaxation moduli are obtained, and interrelations between viscoelastic functions in the time and frequency domains are given. 

The analysis of structural diagrams shows that at creep in aggressive environments, the extent of a viscous phase in the composite structure is increased due to penetration of the environment into the material pores and its influence on the structure and rheological properties of RubCon. The increase of a viscous phase results in a decrease in strength and deformation characteristics of RubCon. 

Viscoelastic material such as polymers combine the characteristics of both elastic and viscous materials. They often exhibit elements of both Hookean elastic solid and pure viscous flow depending on the experimental time scale. Application of stresses of relatively long duration may cause some flow and irrecoverable (permanent) deformation, while a rapid shearing will induce elastic response in some polymeric fluids. Other examples of viscoelastic response include creep and stress relaxation, as described previously. 

The most characteristics features of viscoelastic materials are that they exhibit time-dependent deformation or strain when subjected to a constant stress (creep) and a time-dependent stress when... 

Mechanisms of creep in FRP materials are related to the progressive changes in the internal balance of forces within the materials resulting from the behaviour of the fibre, adhesion and load transfer at the resin—fibre interface, and from the deformation characteristics of the matrix. Thus any factors which either directly or indirectly cause changes to any of these key areas will affect the creep process. 

Non-linear, time-dependent characteristics of viscoelastic materials such as polyethylene have been mathematically modelled and the model compared with experimental results. Mechanical properties such as creep and stress relaxation are non-linear because they include time-dependent and irreversible components. The time-dependent component is non-linear when relaxation time is longer than the timeframe of the experiment. This becomes increasingly so at high stress. Low stress will act on faster responding deformation modes and as stress increases slower modes will respond. The slower modes will be non-linear relative to the timescale of the experiment. Some slower modes such as relative translation of molecules are irreversible. Stress relaxation is complementary to creep in that strain is applied creating a stress that may relax according to the relative times of the experiment and molecular processes. 

The characteristic of most plastics, and especially unfilled thermoplastics, is that under load they exhibit creep. When a component is subjected to a load stresses are created in it and it will deform or deflect, i.e. a strain will result. In traditional materials like metals, stone, concrete, etc., these quantities are easily manipulated because the... 

---