Factor of safety (FoS) can mean either the fraction of structural capability over that required, or a multiplier applied to the maximum expected load (force, torque, bending moment or a combination) to which a component or assembly will be subjected. The two senses of the term are completely different in that the first is a measure of the reliability of a particular design, while the second is a requirement imposed by law, standard, specification, contract or custom. Careful engineers refer to the first sense as a factor of safety, or, to be explicit, a realized factor of safety, and the second sense as a design factor, but usage is inconsistent and confusing, so engineers need to be aware of both. The Factor of Safety is given to the engineer as a requirement. The Design Factor is calculated by the engineer.
Appropriate factors of safety are based on several considerations. Prime considerations are the accuracy of load, strength, and wear estimates, the consequences of engineering failure, and the cost of overengineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury or death usually can use a safety factor of four or higher (often ten). Non-critical components generally might have a design factor of two. Risk analysis, failure mode and effects analysis, and other tools are commonly used.
Buildings commonly use a factor of safety of 2.0 for each structural member. The value for buildings is relatively low because the loads are well understood and most structures are redundant. Pressure vessels use 3.5 to 4.0, automobiles use 3.0, and aircraft and spacecraft use 1.4 to 3.0 depending on the materials. Ductile, metallic materials use the lower value while brittle materials use the higher values. The field of aerospace engineering uses generally lower design factors because the costs associated with structural weight are high. This low design factor is why aerospace parts and materials are subject to more stringent quality control. The usually applied Safety Factor is 1.5, but for pressurized fuselage it is 2.0 and for main landing gear structures it is often 1.25.
A design factor of 1.0 implies that the design meets but does not exceed the minimum requirements, with no room for variation nor error. A high design factor sometimes implies "overengineering" which results in excessive weight and/or cost. In aerospace there is another criterium. At Limit Load the structure may not fail neither have permanent (structural) deformation of the structure. At Ultimate Load (usually the Limit Load multiplied with the Safety Factor) the aircraft structure is allowed to fail. Before Ultimate Load no failure is allowed but permanent deformation is allowed. An (civil) aircraft structure has to meet both Limit Load and Ultimate Load criteria.
Faraday's law of induction
Faraday's law of induction describes a basic law of electromagnetism, which is involved in the working of transformers, inductors, and many forms of electrical generators. The law states:
The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow — the natural science of fluids (liquids and gases) in motion. It has several subdisciplines itself, including aerodynamics (the study of gases in motion) and hydrodynamics (the study of liquids in motion). Fluid dynamics has a wide range of applications, including calculating forces and moments on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns, understanding nebulae in interstellar space and reportedly modeling fission weapon detonation. Some of its principles are even used in traffic engineering, where traffic is treated as a continuous fluid.
Fluid dynamics offers a systematic structure that underlies these practical disciplines, that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to a fluid dynamics problem typically involves calculating various properties of the fluid, such as velocity, pressure, density, and temperature, as functions of space and time.
Historically, hydrodynamics meant something different than it does today. Before the twentieth century, hydrodynamics was synonymous with fluid dynamics. This is still reflected in names of some fluid dynamics topics, like magnetohydrodynamics and hydrodynamic stability — both also applicable in, as well as being applied to, gases
Faraday's law of induction
For the relationship between a time-varying magnetic field and an induced electric field, see Maxwell's equations.
Faraday's law of induction describes a basic law of electromagnetism, which is involved in the working of transformers, inductors, and many forms of electrical generators. The law states:
The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency.
Fourier transform
In mathematics, the Fourier transform (often abbreviated FT) is an operation that transforms one complex-valued function of a real variable into another. In such applications as signal processing, the domain of the original function is typically time and is accordingly called the time domain. That of the new function is frequency, and so the Fourier transform is often called the frequency domain representation of the original function. It describes which frequencies are present in the original function. This is in a similar spirit to the way that a chord of music can be described by notes that are being played. In effect, the Fourier transform decomposes a function into oscillatory functions. The term Fourier transform refers both to the frequency domain representation of a function and to the process or formula that "transforms" one function into the other.
The Fourier transform and its generalizations are the subject of Fourier analysis. In this specific case, both the time and frequency domains are unbounded linear continua. It is possible to define the Fourier transform of a function of several variables, which is important for instance in the physical study of wave motion and optics. It is also possible to generalize the Fourier transform on discrete structures such as finite groups, efficient computation of which through a fast Fourier transform is essential for high-speed computing.
No comments:
Post a Comment