Factor of Safety
Factor of Safety is a measure used in engineering to ensure the safety and reliability of a structure or component. It is calculated by dividing the maximum load a material can handle by the actual load applied. A higher factor of safety indicates a lower risk of failure, providing a margin of safety against unexpected variations in load or material properties.
4 Key excerpts on "Factor of Safety"
eBook - ePub- Alan Hendrickson(Author)
- 2012(Publication Date)
- Routledge(Publisher)
...If the cylinders are appropriately sized and the pressure is set properly (see Chapter 14) the failure of one cylinder should prevent the remaining one from lifting the load, and the counterbalance valves will be sufficient to keep the load from falling. In this case, the failure of the lift to operate will alert the operator to the existence of a problem without a catastrophic failure. Similarly, in the example of a sheave with keeper bolts, the operator should (hopefully) feel the increased drag or hear the sheave dragging on the keepers and stop to investigate the problem, or torque limits on the motor drive should prevent the effect from running due to the excessive friction. While machine control is not the subject here, it is worth mention that it is quickly becoming common that safety critical parts of a control system involve single failure proof design. For instance, the signals from two encoders, one perhaps on the motor shaft and one on the drum, are monitored to ensure that they both track together. If they ever do not, something is wrong, and the machine in emergency stopped. Safety relays and programmable logic controllers, PLCs, are constructed so that they monitor themselves for their own failures as well as failure in the components connected to them. The use of this equipment will only grow. Design Factor and Factor of Safety The terms design factor and Factor of Safety are often used interchangeably (if not specifically correctly) to refer to the ratio of the breaking strength of a material or component to the expected load. Both are used to compensate for inconsistencies in manufacturing, unanticipated loads, and other unknown factors which affect equipment after it has been put in service...
eBook - ePub- J. Michael Duncan, Stephen G. Wright, Thomas L. Brandon(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
...Although and are equally logical measures of stability, there is less experience with their use, and therefore less guidance regarding acceptable values. Another consideration regarding use of reliability and probability of failure is that it is sometimes easier to explain the concepts of reliability or probability of failure to people who do not have technical backgrounds and experience. However, some find it disturbing that a slope has a probability of failure that is not zero, and may not be comfortable hearing that there is some chance that a slope might fail. Factors of safety and reliability complement each other, and each has its own advantages and disadvantages. Knowing the values of both Factor of Safety and probability of failure is more useful than knowing either one alone. 13.1 Definitions of Factor of Safety The most widely used and most generally useful definition of Factor of Safety for slope stability is 13.2 Uncertainty about shear strength is often the largest uncertainty involved in slope stability analyses, and for this reason it is logical that the Factor of Safety—called by George Sowers the factor of ignorance —should be related directly to shear strength. One way of judging whether a value of provides a sufficient margin of safety is by considering the question: What is the lowest conceivable value of shear strength? A value of for a slope indicates that the slope should be stable even if the shear strength was 33 percent lower than anticipated (if all the other factors were the same as anticipated). When shear strength is represented in terms of and, or and, the same value of is applied to both of these components of shear strength. It can be said that this definition of Factor of Safety computed using limit equilibrium procedures is based on the assumption that is the same for every point along the slip surface...
eBook - ePub- Kailash C. Kapur, Michael Pecht(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
...Formulaically, we can say that The reliability of the component is the probability that the strength of the component will be greater than the stress to which it will be subjected. The Factor of Safety, represented by number n, is the ratio of strength (Y) and the stress (X). Since both Y and X are random variable, one definition of the Factor of Safety is (11.1) There are four basic ways in which the designer can increase reliability: Increase Mean Strength. This is achieved by increasing size or weight of materials, using stronger materials, and so on. Decrease Average Stress. This can be done by controlling loads or using higher dimensions. Decrease Stress Variations. This variation is harder to control, but can be effectively truncated by putting limitations on use conditions. Decrease Strength Variation. The inherent part-to-part variation can be reduced by improving the basic process, controlling the process, and utilizing tests to eliminate less desirable parts. 11.3 Reliability Models for Probabilistic Design For a certain mode of failure, let f (x) and g (y) be the probability density functions for the stress random variable X and the strength random variable Y, respectively. Also, let F (x) and G (y) be the cumulative distribution functions for the random variables X and Y,. respectively. Then the reliability, R, of the product for a failure mode under consideration, with the assumption that the stress and the strength are independent random variables, is given by (11.2) Consider a product where the stress and strength are normally distributed. Specifically, stress random variable X is normally distributed, with mean μ X and with the standard deviation as σ X. Similarly, the strength random variable Y is normally distributed, with mean μ Y and standard deviation σ Y...
- Ali Jamnia, Khaled Atua(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
...It suggests that so long as the applied loads remain below a value, as calculated and shown earlier, the member will function as expected. In contrast, the second case study focuses on the applied stresses and indicates that so long as these stresses remain below a certain threshold, the snap-fit arm functions as projected. If we were to generalize these case studies, we would say that any product has an inherent strength that is represented by a probability distribution function. Similarly, the same product when used or in operation undergoes a degree of stress represented by another probability distribution function. Should these two distributions overlap, the product will fail. The probability of failure is determined by the area under the overlap (Sadlon 1993; Dudley 1999). This concept is shown graphically in Figure 7.4. FIGURE 7.4 The relationship between stress and strength and their potential overlap. Safety Factor or Design Margin Often, we, as engineers, either design for a Factor of Safety (also known as design margin) or calculate it for a design that has been developed to ensure that there are no shortcomings. We define this factor as the ratio of the nominal value of strength and the nominal value of stress: Factor of Safety or Design Margin = Nominal Value of Strength Nominal Value of Stress It should be clear that unless the distribution of both the stresses and strengths of a design is known, simply providing or calculating a design margin purely on the basis of a nominal value may not provide much of a protection against unexpected failures. Another often forgotten factor is that the strength of a design may be reduced by a number of different physical and/or environmental influences. Fatigue, creep, and even corrosion impact strength reduction...
