Forces, Distribution, and Materials

Forces, Distribution, and Materials

MECHANICS AND STRUCTURES

 The magnitude of a force acting on a body is obviously important. As a rule, large forces are more likely to cause failure or damage than small ones. How a force acts on a body is also important. The direction of a force, its location or point of application, and the area over which it acts are also important in safety. A 50-lb force applied to the edge of a sheet of glass and parallel to it may not break it. If a hammer strikes the center of the sheet with the same force, the glass will probably break. A wood panel of the same size undergoing the same force will not break. When evaluating the strength of a material, it is essential to evaluate the distribution or concentration of forces as they act on bodies. Figure 10-1 gives some examples of distributed and concentrated loads.

Experience tells us that different materials have different strength properties. Striking a glass panel will cause it to shatter, whereas striking a wood panel will cause a dent. The effect of a force is related to the strength of a material and its ability to deform. Important properties of materials include strength, brittleness, ductility (ability to bend or deform), thermal expansion and contraction, shape, age, exposure to environmental conditions, and exposures to chemicals. Even strength can vary, depending on whether forces are pulling, crushing, twisting, or cutting.

 A key relationship between a force F and a body on which it acts is

 F = sA,

where s = force per unit area or stress (such as pounds per square inch) and A = area (such as square inches) over which a force acts. The stress that a material can withstand is a function of the material’s strength properties and the type of loading. If the material and the area over which the load acts are given, the designer must determine what forces the object can withstand safely. In other cases, one estimates the expected force first and then selects the material and designs for the load area.

 Examples of distributed and concentrated forces. In (a) tire flexion distributes the load over a larger road area than does the steel wheel in (b). The hole in the plate in (d) concentrates the load over a smaller internal area compared with the plate without a hole in (c).

A designer must envision the use environment. For example, building designers must determine the weight of building components and potential loads from building contents, wind, snow, rain, ice, and earthquakes. The designer of a wrench must consider how hard a user can pull on it. The designer of a toy must estimate how hard a child (young or old) can push or pull on it and how the toy’s surfaces interface with human tissue. The toy designer should even consider the impact forces of someone falling on the toy. The forces that an object can encounter are often different from the forces that an object should be able to withstand. For example, designers of breakaway sign posts and light poles along highways want the structure to fail at loads much lower than they could possibly encounter. The designer of a toy may want the toy to fail and fail safely rather than damaging body tissue when a child falls on it. In other cases, the designer may want a structure to withstand the greatest possible load.

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