FAILUREM - DESIGN FOR STRUCTURAL INTEGRITY FAILURE MODES...

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DESIGN FOR STRUCTURAL INTEGRITY: FAILURE MODES Most of what you have learned in Mechanics of Materials could best be described as analysis . You learned how to find internal stress and strain distributions and how to relate them to externally applied loads, and to the properties of the material used and the geometry of the part or structure. When you go to work as engineers, you'll probably find that most of the problems that you'll face are almost completely opposite from the textbook-style, analysis-oriented problems you've learned to solve. Real world problems often involve design , or synthesis, a process where you start with the answer and find or create a structure or part that gives that answer under the stated conditions. The answer to a typical mechanical design problem appears as a specification or set of requirements that must be met in an efficient or optimal sort of way. The design process involves choices you have to make and then evaluate. The analysis methods you've learned are the basic tools you must use to make sensible choices and to evaluate their likelihood of success in meeting the specified goals. In mechanical design, the specification relating to structural integrity usually goes something like this: "The structure must support (certain specified) loads without failing". Much of structural design is done with the express purpose of avoiding failure . To be successful in this kind of effort, the following questions must be answered: In what ways might this structure fail and what are the consequences of a failure? How can I prevent the failure of this structure? Obviously, a thorough knowledge of how things fail in general is essential. The ways in which things fail are known as failure modes . CATEGORIES OF FAILURE MODES Mechanical failures can occur because of breakage or fracture of parts, or because of excessive deformation of parts or the structure as a whole. The following is a list of some common failure modes in each of these categories, the conditions under which they occur and strategies for avoiding them. A. Failures by Fracture 1. Ductile overload fracture. The tensile stress reaches the ultimate tensile strength of the material. To avoid, design to keep stresses well below the UTS. This is the most obvious type of failure, but is actually not the most common. 2. Brittle fracture. This is a dangerous type of failure that occurs in the presence of tensile stresses. The stresses to cause brittle fracture can be much lower than the UTS or even the yield stress. Intrinsically brittle materials often fail due to the magnification of stresses by stress raisers (areas with high stress concentration factors). Cracks or other flaws can have potent weakening effects on these materials. Even ductile materials may
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fail in a brittle fashion if cracks are present (and they often are!).
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