Reinforced-plastic material

The yield point is the first point on the stress-strain curve at which an increase in strain occurs without an increase in stress. The stress at which a material exhibits a special limiting deviation from the proportionality of stress-to-strain is the yield strength. A material whose stress-strain curve exhibits points of zero slope may be considered to have a yield point such as described in Fig. 2-11. The data sheets usually omit the yield strength when there is a zero slope point on the stress-strain curve in the yield region. In reinforced plastic materials, the values of the yield strength and the tensile strength are very close to each other. 

It is also found that in short-fiber-reinforced plastics the impact strength is maximum when the fiber length has the critical value. The requirements for maximum impact strength (i.e., short fiber and relatively weak interfacial bond) are thus seen to be contrary to those for maximum tensile strength (long fibers and strong bond). The structure of a reinforced plastic material should therefore he tailored in accordance with the service conditions to be encountered by the material. 

L. W. Toth, T. J. Boiler, I. R. Butcher, A. H. Kariotis, and F. D. Yoder, Program for the Evaluation of Structural Reinforced Plastic Materials at Cryogenic Temperatures, NASA CR-80061 (N67-12051), Goodyear Aerospace Corp., Akron, Ohio (1966). 

Only a few reinforced plastics materials are sufficiently translucent for visual inspection. Glass fibre reinforced resins can in some instances be... 

Figure 2.5 Laser-based technique to check ply orientation in reinforced plastics laminates (By permission S Hill, Rapid nondestructive testing of carbon fibre reinforced plastics. Materials World, August 1996, Institute of Materials, London)...

Thick laminates are obviously much less affected than thin ones in a given period and this explains the durability of many early reinforced plastics structures. It has been calculated that an epoxy-based reinforced plastics material with a typical diffusivity towards moisture of lO m s would require 13 months to reach saturation if left in a tropical climate at 35°C and 95% relative humidity (RH) if the thickness was 2 mm, but a 90 mm thick section would need 1342 years. During the approach to saturation, there is a through-the-thickness variation in moisture content and therefore in properties. 

A reaction between the reinforced plastics material and the surrounding medium is defined here as an event which breaks and makes new covalent chemical bonds in the participating substances. One illustration is the hydrolysis reaction which breaks ester and amide groups in polyesters, polyamides and certain polyurethanes, when they are exposed to dilute acids or alkalis. Another example is the oxidation reaction that attacks carbon fibres whenever they are in contact with dilute nitric acid, acid/ dichromate, hypochlorite solutions, and so on. Such reactions can easily be foreseen and avoided by the use of more appropriate resins, fibres, and so on. The rate of chemical reactions involving resins is much more difficult to predict and they can sometimes be so slow that the effects are not necessarily unacceptable at ambient temperatures. One obvious illustration is the... 

Fatigue strength of reinforced plastic materials is generally explored with laboratory tests of specimens in cycling machines. Test specimens are specially prepared or cut from the full structure. Such tests, however, cannot provide accurate information about the durability of a complete structure under actual operating conditions. Thus composite structures are sometimes built and subjected to full scale testing. The subject of fatigue is discussed fully in Chapter 5. 

Glass reinforced plastics materials have been used on this site since the late 1970s, and a considerable amount of experience in their use has been... 

Chang C-S (2006) Turning of glass-fiber reinforced plastics materials with chamfered main cutting edge carbide tools. J Mater Process Technol 180(1-3) 117-129... 

PROGRAM FOR THE EVALUATION OF STRUCTURAL REINFORCED PLASTIC MATERIALS AT CRYOGENIC TEMPERATURES. FINAL REPT.JUL 1,1963 TO JUL. 1,1966. 

Short-beam shear strength n. Shear strength of reinforced-plastics materials as measured in three-point bending of a specimen whose length between end supports is about five times the specimen depth. ASTM D 2344 describes one such test for flat specimens and short arcs cut from relatively large rings, while ASTM D 4475 describes a similar test for pultruded round rod. 

Tridirectional laminate n. A reinforced-plastics material having reinforcing fibers running in three principal directions. 

Examples are polystyrene and polyisoprene, used in reinforced plastic materials. 

Table C.6 Typical properties of copper cold laminate. (Ref Sanyson, R.N., Laminate and Reinforced Plastic Materials and Processes, Handbook of Plastics, Elastomers, and Composites, 3rd ed., C.A. Harper (Ed.), McGraw-Hill, New York, 1996)... 

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