Recovery from deformation

The following problem provides a simple illustration of the approach. 

The appropriate formula from classical elastic analysis is 

Creep strain when load is removed Recovery time 

---

Resilient Diners. Resilient liners reduce the impact of the hard denture bases on soft oral tissues. They are designed to absorb some of the energy produced by masticatory forces that would otherwise be transmitted through the denture to the soft basal tissue. The liners should adhere to but not impair the denture base. Other critical properties include total recovery from deformation, retention of mechanical properties, good wettability, minimal absorption of... 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

Recovery from deformation and general high-elasticity properties. 

The ratio of the energy given up on recovery from deformation to the energy required to produce the deformation, expressed as a percentage. See Heat Build-Up, Hysteresis and Rebound Resilience. 

Polyurethane multiblock copolymers of the type described by Eqs. 2-197 and 2-198 constitute an important segment of the commercial polyurethane market. The annual global production is about 250 million pounds. These polyurethanes are referred to as thermoplastic polyurethanes (TPUs) (trade names Estane, Texin). They are among a broader group of elastomeric block copolymers referred to as thermoplastic elastomers (TPEs). Crosslinking is a requirement to obtain the resilience associated with a rubber. The presence of a crosslinked network prevents polymer chains from irreversibly slipping past one another on deformation and allows for rapid and complete recovery from deformation. 

Elasticity. Their recovery from deformation is superior to almost all conventional materials. 

Elastomeric finishes are also referred to as stretch or elastic finishes and are particularly important for knitwear. These finishes are currently achieved only with silicone-based products. The main effect is durable elasticity, because not only must extensibility be enhanced, but recovery from deformation is of crucial importance. After all stresses and disturbing forces have been released, the fabric should return to its original shape. 

The individual fibres of an elastomeric fabric must be completely covered with a thin film of an elastic material without any fibre-to-fibre bonding. The film causes a high degree of recovery from deformations owing to its particular structure of widely spaced crosslinks. 

The effect on resilience (approximate rate of recovery from deformation) of reducing is more complex. At relatively low degrees of cross linking, the system exhibits rubbery elasticity. As decreases due to further cross linking, T increases and as it approaches the test temperature, a point of maximum damping is achieved. Here the resilience is at a minimum. Further decrease in Me increases resiliency until the sample become an elastic solid. 

Thus, for an ideal solid, the maximum strain is observed at the same instant as the maximum applied stress (Figure 10.6). The applied energy has therefore been employed to ensure instantaneous recovery from deformation. 

Urethane rubbers exhibit excellent recovery from deformation. Parts made of urethane rubbers may build up heat when subjected to high-frequency deformation, but their high strength and load-bearing capacity may permit the use of sections thin enough to dissipate the heat as fast as it is generated. 

These compounds possess the rapid recovery from deformation, or resilience, of a true elastomer and exhibit mechanical properties of the same order of magnitude as those of conventional S5mthetic rubbers. 

Williamson, W.O., 1955, Oriented aggregation, differential drying-shrinkage and recovery from deformation of a Kaolinite-Ilhte clay, Transactions of the British Ceramic Society, 54(7), 413-442. 

---