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The public today is clamoring for faster, cleaner, safer, and quieter machines. And design engineers, in their efforts to satisfy these demances with products at a lower cost, are being sued in the courts throughout the land for designing things that are unsafe, that are dangerous to human life. One of the real problems we shall face in this book is that of relating the strength of an element to the external loads which are imposed upon it. These external loads cause internal stresses in the element. To avoid product liability the design engineer must have positive assurance that these stresses will never exceed the strength. In this chapter we shall concern ourselves with the determination of these stresses. then, in other chapters to follow, we shall introduce the concept of strength and begin the process of relating strenfth and stress to achieve safety.
It is worth observing that the stresses which are calculated are only as reliable as are the external loads and geometries which are used in the determination. Once a product has been manufactured and placed in the hands of the consumer or user, the manufacturer has relinquished control over that product, and to some extent over the external loads to which it may be subjected. It is for this reason, among others, that testing and experimental proving are so important in the design and development of a new machine or device.