P

sf (effective) = x test strength composite A,

Fiber efficiency can now be defined as the rado of the developed fiber stress to the base strength of the fibers. Thus,

RP efficiency is based on the total glass content plus the total resin matrix content, and fiber efficiency is based on the glass area oriented in the load direction. The average tensile strength of glass fibers in their several common forms are about 500 000 psi for a virgin single filament, 40 000 psi for single glass roving, and 250 000 psi for glass strands (as woven into cloth). As a basis for comparing fiber geometries, the base glass strength will be assumed as 400 000 psi for glass roving. Single-glass filaments are not practical to handle, and glass strands that are used in cloth have undergone the first phase of fabrication.

Stiffness-Viscoelasticity

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.

Stiffness properties of RPs are used (as with other materials) for the usual purposes of estimating stresses and strains in a structural design, and to predict buckling capacity under compressive loads. Also, stiffness properties of individual plies of a layered "flat plate approach" may be used for the calculation of overall stiffness and strength properties. The relationship between stress and strain of unreinforced or reinforced plastics varies from viscous to elastic. Most RPs, particularly RTSs are intermediate between viscous and elastic. The type of plastic, stress, strain, time, temperature, and environment all influence the degree of their viscoelasticity.

Creep and Stress Relaxation

Properties of unreinforced plastics are strongly dependent on temperature and time. This is also true, to a lesser degree, for RPs, particularly RTSs, fiber efficiency =

developed fiber stress basic fiber strength x 100

compared with other materials, such as steel. This strong dependence of properdes on temperature and how fast the material is deformed, based on a time scale, is a result of the viscoelastic nature of plastics. Consequendy, it is important in practice to know how the product is likely to be loaded with respect to time.

In structural design, it is important to distinguish between various modes in the product. The behavior of any material in tension, for example, is different from its behavior in shear, as with plastics, metals, concrete, etc. For viscoelastic materials such as plastics, the history of deformation also has an effect on the response of the material, since viscoelastic materials have time- and temperature-dependent material properties.

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