![]() ![]() For example, components whose failure could result in substantial financial loss, serious injury, or death may use a safety factor of four or higher (often ten). On brittle materials these values are often so close as to be indistinguishable, so it is usually acceptable to only calculate the ultimate safety factor.Īppropriate design factors are based on several considerations, such as the accuracy of predictions on the imposed loads, strength, wear estimates, and the environmental effects to which the product will be exposed in service the consequences of engineering failure and the cost of over-engineering the component to achieve that factor of safety. The ultimate calculation will determine the safety factor until failure. The yield calculation will determine the safety factor until the part starts to deform plastically. most metals), it is often required that the factor of safety be checked against both yield and ultimate strengths. The applied loads have many factors, including factors of safety applied.įor ductile materials (e.g. The design factor is defined for an application (generally provided in advance and often set by regulatory building codes or policy) and is not an actual calculation, the safety factor is a ratio of maximum strength to intended load for the actual item that was designed.įactor of safety = yield stress working stress The design factor, or working stress, is what the item is required to be able to withstand (second "use"). The difference between the safety factor and design factor (design safety factor) is as follows: The safety factor, or yield stress, is how much the designed part actually will be able to withstand (first "use" from above). Many quality assurance, engineering design, manufacturing, installation, and end-use factors may influence whether or not something is safe in any particular situation. The use of a factor of safety does not imply that an item, structure, or design is "safe". Safety factor values can be thought of as a standardized way for comparing strength and reliability between systems. The difference between the methods is the way in which the values are calculated and compared. All the different calculations fundamentally measure the same thing: how much extra load beyond what is intended a structure will actually take (or be required to withstand). There are several ways to compare the factor of safety for structures. Many undergraduate strength of materials books use "Factor of Safety" as a constant value intended as a minimum target for design (second use). Those are realized factors of safety (first use). Building codes, structural and mechanical engineering textbooks often refer to the "factor of safety" as the fraction of total structural capability over what is needed. The cause of much confusion is that various reference books and standards agencies use the factor of safety definitions and terms differently. ![]() However, between various industries and engineering groups usage is inconsistent and confusing there are several definitions used. The realized factor of safety must be greater than the required design factor of safety. This can be referred to as a design factor, design factor of safety or required factor of safety.
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