Design load-bearing products

Kohlrausch-Williams-Watts Equation n This empirical equation aids designers of load-bearing products made of plastics and reinforced plastics. 

One of the earliest steps in product design is to establish the configuration of the product that will form the basis on which a suitable material is selected to meet performance requirements. During this phase certain design features have to be kept in mind to avoid problems such as reduction of properties. Such features are called detractors or constraints. Most of them are responsible for the unwanted internal stresses that can reduce the available stress level for load bearing purposes. Other features may be classified as precautionary measures that may influence the favorable performance of a product if they are properly incorporated. 

For those not familiar with this type information recognize that the viscoelastic behavior of plastics shows that their deformations are dependent on such factors as the time under load and temperature conditions. Therefore, when structural (load bearing) plastic products are to be designed, it must be remembered that the standard equations that have been historically available for designing steel springs, beams, plates, cylinders, etc. have all been derived under the assumptions that (1) the strains are small, (2) the modulus is constant, (3) the strains are independent of the loading rate or history and are immediately reversible, (4) the material is isotropic, and (5) the material behaves in the same way in tension and compression. 

It should be evident that the full spectrum of the possible materials and applications in load-bearing situations involves many factors that may have to be taken into account. Fortunately, most products involve only a few factors, and others will not be significant or relevant. Regardless, the methods of design analysis must be made available to handle any possible combinations of such factors as the materials characteristics, the product s shape, the loading mode, the loading type, and other service factors and design criteria. 

The product designer should caution the tool designer to keep the gate area away from load-bearing surfaces and to make the gate size such that it will improve the quality of the product. It so happens that the product wall in the gate area develops the minimum tolerance due to the high melt pressure in that area. 

As can be seen from the above, the design of RP products, while essentially similar to conventional design, does differ in that the materials are combined when the product is manufactured. The RP designer must consider how the load-bearing fibers are placed and ensure that they stay in the proper position during fabrication. 

It can be said that the design of a product involves analytical, empirical, and/or experimental techniques to predict and thus control mechanical stresses. Strength is the ability of a material to bear both static (sustained) and dynamic (time-varying) loads without significant permanent deformation. Many non-ferrous materials suffer permanent deformation under sustained loads (creep). Ductile materials withstand dynamic loads better than brittle materials that may fracture under sudden load application. As reviewed, materials such as RPs can exhibit changes in material properties over a certain temperature range encountered by a product. 

The HDT test results are a useful measure of relative service temperature for a polymer when used in load-bearing parts. However, the deflection temperature test is a short-term test and should not be used alone for product design. Other factors such as the time of exposure to elevated temperature, the rate of temperature increase, and the part geometry all affect the performance. 

Consideration of creep properties is very important to predict the long-term load-bearing capacity, and to design polymer-based products for such applications. When a polymer material is subjected to a constant load, it deforms quickly to a strain depending on its physicochemical properties and then continues to deform slowly until failure. This property is called creep . A plastic thread, very tightly tied... 

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