Elastic-plastic behavior, resin

Elastic Behavior The assumption that displacement strains will produce proportional stress over a sufficiently wide range to justify an elastic-stress analysis often is not valid for nonmetals. In brittle nonmetallic piping, strains initially will produce relatively large elastic stresses. The total displacement strain must be kept small, however, since overstrain results in failure rather than plastic deformation. In plastic and resin nonmetallic piping strains generally will produce stresses of the overstrained (plasfic) type even at relatively low values of total displacement strain. 

Figure 4. Model of elastic-plastic, stress-strain behavior of the epoxy resin.

The pitch fiber and rough laminar pyrocarbon exhibited plastic behavior that can be attributed to the low shear resistance due to weak bonding between the well-organized graphene sheets. On the other hand, the PAN fiber, charred resin and isotropic pyrocarbon all exhibited almost full elasticity within the applied range of indentation depths. 

The stiffness response of RPs can be identified as viscoelasticity. RPs are nearly elastic in behavior and tend to reduce the importance of the time-dependent component of viscoelastic behavior. Also, the stiffness of fiber reinforcements and the usual TS resin matrices are less sensitive to temperature change than most unreinforced plastics. The stiffness of both the fibers and the matrices are frequently more stable on exposure to solvents, oils, and greases than TPs although for certain composites water, acids, bases, and some strong solvents still may alter stiffness properties significantly. 

In order to obtain samples to be observed by SEM, the mixture PVC-plasticizer is heated until the desired temperature is reaehed and then it is rapidly cooled in hquid nitrogen. The specimen is fractured at a low temperature to obtain smfaces ready to be observed. The behavior of a PVC plastisol of a cotmnercial resin and diisodecyl phtha-late, DIDP, is given in Figure 9.3 which shows elastic and viscous moduli cirrves. The same plastisol was analyzed by SEM. The results are given Figure 9.4. 

Historically, most discussions on polymer flow focus only on viscosity. However, with many of the newer resins that are highly impact modified or have high elasticity, viscosity tells only part of the story for flow behavior. Two plastics can have identical viscosity characteristics and yet mold differently. Elastic response and the polymer s ability to transmit pressure become increasingly important as part designs increase in complexity. Both viscous and elastic response are required to properly characterize flow through complex cavities (22). 

When an engineering plastic resin is used with the structural foam process, the material produced exhibits behavior that is predictable over a large range of temperatures. Its stress-strain curve shows a significantly linearly elastic region like other Hookean materials, up to its proportional limit. 

The second major assumption is that the material is elastic, meaning that the strains are directly proportional to the stresses applied and when the load is removed the deformation will disappear. In engineering terms the material is assumed to obey Hooke s Law (see Chapter 2). This assumption is probably a close approximation of the material s actual behavior in direct stress below its proportional limit, particularly in tension, if the fibers are stiff and elastic in the Hookean sense and carry essentially all the stress. This assumption is probably less valid in shear, where the resin carries a substantial portion of the stress. The resin may then undergo plastic flow, leading to creep or relaxation of the stresses, especially when the stresses are high. 

Both Young s modulus and the modulus of rigidity show a marked increase with decreasing temperature for the expanded epoxy resin and for the expanded polystyrene in each density. This behavior is typical of solid plastic materials and it is not too surprising to find it duplicated in the foams when we consider that the plastic cell walls of the expanded materials govern their elastic properties. 

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