Viscoelasticity plastics

Fiber-reinforced composite materials such as boron-epoxy and graphite-epoxy are usually treated as linear elastic materials because the essentially linear elastic fibers provide the majority of the strength and stiffness. Refinement of that approximation requires consideration of some form of plasticity, viscoelasticity, or both (viscoplasticity). Very little work has been done to implement those models or idealizations of composite material behavior in structural applications. 

Yellow, plastic, viscoelastic, amorphous mass. The strands can be stretched out to many times their Initial length, which causes partial crystallization. Repeated stretchii produces loss of elasticity. More detailed studies have been made by K. Sakurada and H. Erbring, Kolloid-Z. ]72, 129 (1935). Insoluble in water, partially soluble in CSg. 

Fig. 12. Diagram showing the time-dependent deformation behaviour of plastic> viscoelastic, and elastic substances before and after a deforming force is applied (modified according to Hessemer Dick, 1996)...

Now we turn to rubber compounds and specifically to plastic viscous and plastic viscoelastic fluids. 

For a plastic viscoelastic fluid, the dimensionless groups of Eq. (102) should be replaced by... 

Creep is related to plastics viscoelastic behavior and can be explained with the aid of a Maxwell model such as that shown in Figure 3-55 [12, 287]. When a load is applied to the system, shown diagrammatically, the spring will deform to a certain degree. The dashpot will first remain stationary under the applied load, but if the same load continues to be applied, the viscous fluid in the dashpot will slowly leak past the piston, causing the dashpot to move. Its movement corresponds to the strain or deformation of the plastic material. 

Mechanical properties of crystalline plasties are much more complex than those of amorphous plastics. Viscoelastic data, at least in theory, can be utilized to predict mechanical performance of a material under any use conditions. However it is seldom practical to carry out the necessarily large number of tests for the long time periods involved. Such limitations can be largely overcome by utilizing the principle of reduced variables embodying a time-temperature shift. Plastic usually exhibits not one but many relaxation times with each relaxation affected by the temperature. 

Plastic viscoelastic nature reacts to a constant creep load over a long period of time by an ever-increasing strain. With the stress being constant, while the strain is increasing, result is a decreasing modulus. This apparent modulus and the data for it are collected from test observations for the purpose of predicting long-term behavior of plastics subjected to a constant stress at selected temperatures. 

Suetsugu, Y. and White, J.L. (1984) A theory of thixotropic plastic viscoelastic fluids with a time-dependent yield surface and its comparison to transient and steady state experiments on small particle filled polymer melts, /. Non-Newtonian Fluid Mech., 14,121-40. 

In 1979 White [49] proposed a three-dimensional theory of a plastic-viscoelastic fluid intended to represent the behavior of small particle-filled compoimds. This was also based on the von Mises stress criterion but contained memory and predicted shear flow normal stresses. This was later generalized to include thixotropy [50, 51]. Leonov [52] developed an alternative three-dimensional tensor theory of this behavior. 

---