Surface Stresses and Deformations

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. 

There are examples where control of deflection or deformation during service may be required. Such structural elements are designed for stiffiiess to control deflection but must be checked to assure that strength criteria are reached. A product can be viewed as a collection of individual elements interconnected to achieve an overall systems function. Each element may be individually modeled however, the model becomes complex when the elements are interconnected. 

To keep the body at rest there must be a system of forces acting on the cut surface to balance the external forces. These same systems of forces exist within the uncut body and are called stresses. Stresses must be described with both a magnitude and a direction. Consider an arbitrary point, P, on the cut surface in the figure where the stress, S, is as indicated. For analysis, it is more convenient to resolve the stress, S, into two stress components. One acts perpendicular to the surface and is called a normal or direct stress, o. The second stress acts parallel to the surface and is called a shear stress, t. 

Product design starts by one visualizing a certain material, makes approximate calculations to see if the contemplated idea is practical to meet requirements that includes cost, and, if the answer is favorable, proceeds to collect detailed data on a range of materials that may be considered for the new product. The application of appropriate data to product design can mean the difference between the success and failure of manufactured products made from any material. The available plastic test data requires an understanding and proper interpretation before an attempt can be made to apply them to the product design. Details on designing a product can follow a flow pattern as shown in Table 7.6. 

There are three important sources of data and information on RPs. Obtain data from products your organization previously fabricated there is the data sheet compiled by a manufacturer of the material and derived from tests conducted in accordance with standardized specifications. If suppliers data were to be applied without a complete analysis of the test data for each property, the result could prove costly and embarrassing. Final source is preparing your own test specimens and conducting your own tests. Either one or both specimen preparation and test evaluation could be conducted by an outside source. If the fiibricator prepares the specimens a duplicate of the fabricating process to be used will be produced or as close to the process as possible. The amount and degree of testing is usually related to factors such as (1) if a prototype is to be prepared and tested and (2) product requirement such as safety. 

As an example view a 3-D product that has a balanced system of forces acting on it, Fi through Fs in Fig. 3.7, such that the product is at rest. A product subjected to external forces develops internal forces to transfer and distribute the external load. Imagine that the product in 

Plastic materials subjected to a constant stress can deform continuously with time and the behavior under different conditions such as temperature. This continuous deformation with time is called creep or cold flow. In some applications the permissible creep deformations are critical, in others of no significance. But the existence of creep necessitates information on the creep deformations that may occur during the expected life of the product. Materials such as plastic, RP, zinc, and tin creep at room temperature. Aluminum and magnesium alloys start to creep at around 300°F. Steels above 650°F must be checked for creep. 

These test specimens may be loaded in tension or flexure (with some in compression) in a constant temperature environment. With the load kept constant, deflection or strain is recorded at regular intervals of hours, days, weeks, months, or years. Generally, results are obtained at different stress levels. 

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