Stress relaxation 

In materials science, stress relaxation is the observed decrease in stress in response to strain generated in the structure. This is primarily due to keeping the structure in a strained condition for some finite interval of time and hence causing some amount of plastic strain. This should not be confused with creep, which is a constant state of stress with an increasing amount of strain.

Since relaxation relieves the state of stress, it has the effect of also relieving the equipment reactions. Thus, relaxation has the same effect as cold springing, except it occurs over a longer period of time. The amount of relaxation which takes place is a function of time, temperature and stress level, thus the actual effect it has on the system is not precisely known, but can be bounded.

Stress relaxation describes how polymers relieve stress under constant strain. Because they are viscoelastic, polymers behave in a nonlinear, non-Hookean fashion.This nonlinearity is described by both stress relaxation and a phenomenon known as creep, which describes how polymers strain under constant stress.

Some Non-Newtonian fluids undergo a yield behavior when deformed beyond a limit. Such fluids have an associated yield stress beyond which it undergoes a distinct change in flow behavior. Common examples of materials that possess a yield stress are foams, cross-linked gels and pastes. In this lecture we will talk about such materials and then describe some of the common applications of yielding in the transport of small particles as well as the dangers of having a dead zone in sharp corners of channels. We will then proceed to the age-old squeeze film paradox. After that we will talk about what is common between ketchup bottles and oil pipelines. Kabla & Debregeas has suggested that foams get permanently damaged upon yielding. Therefore, an understanding of the yield behavior of foams is of direct relevance to their efficacy. The figure 13 has been taken from St.Jalmes & Durian. They show how the yielding stress and strain decrease with decreasing volume fraction of gas in foam.

The rheological behavior of an unnamed cross-linked gel is shown in figure??. Both the degree of cross-linking and the concentration of the polymer molecules forming the gel will determine its yield stress. Yield stress is an important quantity when walls are being plastered. If the yield stress of the plaster is too large, it will not flow very smoothly - sticking, slipping and bringing into question the self-esteem of the painter. However, if the yield stress is too small, painting the cieling will be an exercise in making the floor dirty because all the plater paste will give in to gravity. The figure?? shows the yielding behavior of a suspension of 30% Aluminium particles in an unknown solvent. Clearly, the the yielding behavior is strongly governed by the pH of the suspension. An explanation for this is still lacking

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Nancy Ella