Factor of safety
Encyclopedia
Factor of safety also known as safety factor (SF), is a term describing the structural capacity of a system beyond the expected loads or actual loads. Essentially, how much stronger the system is than it usually needs to be for an intended load. Safety factors are often calculated using detailed analysis because comprehensive testing is impractical on many projects, such as bridges and buildings, but the structure's ability to carry load must be determined to a reasonable accuracy.

Many systems are purposefully built much stronger than needed for normal usage to allow for emergency situations, unexpected loads, misuse, or degradation.

Definition

There are two distinct uses of the factor of safety: One as a ratio of absolute strength (structural capacity) to actual applied load. This is a measure of the reliability
Reliability engineering
Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...

 of a particular design. The other use of FoS is a constant value imposed by law
Law
Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...

, standard, specification, contract
Contract
A contract is an agreement entered into by two parties or more with the intention of creating a legal obligation, which may have elements in writing. Contracts can be made orally. The remedy for breach of contract can be "damages" or compensation of money. In equity, the remedy can be specific...

 or custom
Convention (norm)
A convention is a set of agreed, stipulated or generally accepted standards, norms, social norms or criteria, often taking the form of a custom....

 to which a structure must conform or exceed.

The first sense (a calculated value) is generally referred to as a factor of safety or, to be explicit, a realized factor of safety, and the second sense (a required value) as a design factor, design factor of safety or required factor of safety, but usage is inconsistent and confusing.
The cause of much confusion is that reference books and standards agencies use the term factor of safety differently. Design codes and structural
Structural engineering
Structural engineering is a field of engineering dealing with the analysis and design of structures that support or resist loads. Structural engineering is usually considered a specialty within civil engineering, but it can also be studied in its own right....

 and mechanical engineering
Mechanical engineering
Mechanical engineering is a discipline of engineering that applies the principles of physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the...

 textbooks often use the term to mean the fraction of total structural capability over that needed (first sense). Many undergraduate Strength of Materials
Strength of materials
In materials science, the strength of a material is its ability to withstand an applied stress without failure. The applied stress may be tensile, compressive, or shear. Strength of materials is a subject which deals with loads, deformations and the forces acting on a material. A load applied to a...

 books use "Factor of Safety" as a constant value intended to be a minimum target for design (second sense).

factor of safety= ultimate stress/working stress

Calculation

There are several ways to compare the factor of safety for structures. 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). The difference between the methods is the way in which the values are calculated and compared. Safety factor values can be thought of as a standardized way for comparing strength and reliability between systems.

Design factor and safety factor

The difference between the safety factor and design factor (design safety factor) is as follows: The safety factor is how much the designed part actually will be able to withstand. The design factor is what the item is required to be able to withstand. The design factor is defined for an application (generally provided in advance and often set by regulatory code 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.

This may sound similar, but consider this: Say a beam in a structure is required to have a design factor of 3. The engineer chose a beam that will be able to withstand 10 times the load. The design factor is still 3, because it is the requirement that must be met, the beam just happens to exceed the requirement and its safety factor is 10. The safety factor should always meet or exceed the required design factor or the design is not adequate. Meeting the required design factor exactly implies that the design meets the minimum allowable strength. A high safety factor well over the required design factor sometimes implies "overengineering
Overengineering
Overengineering is when a product is more robust or complicated than necessary for its application, either to ensure sufficient factor of safety, sufficient functionality, or due to design errors...

" which can result in excessive weight and/or cost. In colloquial use the term, "required safety factor" is functionally equivalent to the design factor.

For ductile materials (e.g. most metals), it is often required that the factor of safety be checked against both yield and ultimate strengths. The yield calculation will determine the safety factor until the part starts to plastically deform. The ultimate calculation will determine the safety factor until failure. On brittle materials these values are often so close as to be indistinguishable, so is it usually acceptable to only calculate the ultimate safety factor.

The use of a factor of safety does not imply that an item, structure, or design is "safe". Many quality assurance
Quality Assurance
Quality assurance, or QA for short, is the systematic monitoring and evaluation of the various aspects of a project, service or facility to maximize the probability that minimum standards of quality are being attained by the production process...

, engineering design, manufacturing
Manufacturing
Manufacturing is the use of machines, tools and labor to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale...

, installation, and end-use factors may influence whether or not something is safe in any particular situation.



  • Design load being the maximum load the part should ever see in service.


Margin of safety

Many government agencies and industries (such as aerospace) require the use of a margin of safety (MoS or M.S.) to describe the ratio of the strength of the structure to the requirements. There are two separate definitions for the margin of safety so care is needed to determine which is being used for a given application. One usage of M.S. is as a measure of capacity like FoS. The other usage of M.S. is as a measure of satisfying design requirements (requirement verification). Margin of safety can be conceptualized (along with the reserve factor explained below) to represent how much of the structure's total capacity is held "in reserve" during loading.

M.S. as a measure of structural capacity: This definition of margin of safety commonly seen in textbooks basically says that if the part is loaded to the maximum load it should ever see in service, how many more loads of the same force can it withstand before failing. In effect, this is a measure of excess capacity. If the margin is 0, the part will not take any additional load before it fails, if it is negative the part will fail before reaching its design load in service. If the margin is 1, it can withstand one additional load of equal force to the maximum load it was designed to support (i.e. twice the design load).






M.S. as a measure of requirement verification: Many agencies such as NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 and AIAA define the margin of safety including the design factor, in other words, the margin of safety is calculated after applying the design factor. In the case of a margin of 0, the part is at exactly the required strength (the safety factor would equal the design factor). If there is a part with a required design factor of 3 and a margin of 1, the part would have a safety factor of 6 (capable of supporting two loads equal to its design factor of 3, supporting six times the design load before failure). A margin of 0 would mean the part would pass with a safety factor of 3. If the margin is less than 0 in this definition, although the part will not necessarily fail, the design requirement has not been met. A convenience of this usage is that for all applications, a margin of 0 or higher is passing, one does not need to know application details or compare against requirements, just glancing at the margin calculation tells whether the design passes or not.




Design Safety Factor = [Provided as requirement]







For a successful design, the realized Safety Factor must always equal or exceed the required Safety Factor (Design Factor) so the Margin of Safety is greater than or equal to zero. The Margin of Safety is sometimes, but infrequently, used as a percentage, i.e., a 0.50 M.S is equivalent to a 50% M.S. When a design satisfies this test it is said to have a "positive margin," and, conversely, a “negative margin” when it does not.

Reserve factor

A measure of strength frequently used in Europe is the Reserve Factor (RF). With the strength and applied loads expressed in the same units, the Reserve Factor is defined as:


RF = Proof Strength / Proof Load


RF = Ultimate Strength / Ultimate Load


The applied loads have any factors, including factors of safety applied.

Choosing design factors

Appropriate design factors are based on several considerations, such as the accuracy of predictions on the imposed loads
Structural load
Structural loads or actions are forces, deformations or accelerations applied to a structure or its components.Loads cause stresses, deformations and displacements in structures. Assessment of their effects is carried out by the methods of structural analysis...

, strength, wear
Wear
In materials science, wear is erosion or sideways displacement of material from its "derivative" and original position on a solid surface performed by the action of another surface....

 estimates, and the environmental
Environment (systems)
In science and engineering, a system is the part of the universe that is being studied, while the environment is the remainder of the universe that lies outside the boundaries of the system. It is also known as the surroundings, and in thermodynamics, as the reservoir...

 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. For example, components whose failure
Failure
Failure refers to the state or condition of not meeting a desirable or intended objective, and may be viewed as the opposite of success. Product failure ranges from failure to sell the product to fracture of the product, in the worst cases leading to personal injury, the province of forensic...

 could result in substantial financial loss, serious injury, or death may use a safety factor of four or higher (often ten). Non-critical components generally might have a design factor of two. Risk analysis
Risk analysis (engineering)
Risk analysis is the science of risks and their probability and evaluation.Probabilistic risk assessment is one analysis strategy usually employed in science and engineering.-Risk analysis and the risk workshop:...

, failure mode and effects analysis
Failure mode and effects analysis
A failure modes and effects analysis is a procedure in product development and operations management for analysis of potential failure modes within a system for classification by the severity and likelihood of the failures...

, and other tools are commonly used. Design factors for specific applications are often mandated by law, policy, or industry standards.

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
Redundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....

. Pressure vessel
Pressure vessel
A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...

s use 3.5 to 4.0, automobiles use 3.0, and aircraft and spacecraft use 1.2 to 3.0 depending on the application and materials. Ductile
Ductility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...

, metallic materials tend to use the lower value while brittle
Brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...

 materials use the higher values. The field of aerospace engineering
Aerospace engineering
Aerospace engineering is the primary branch of engineering concerned with the design, construction and science of aircraft and spacecraft. It is divided into two major and overlapping branches: aeronautical engineering and astronautical engineering...

 uses generally lower design factors because the costs associated with structural weight are high (e.g. an aircraft with an overall safety factor of 5 would probably be too heavy to get off the ground). This low design factor is why aerospace parts and materials are subject to very stringent quality control
Quality control
Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production. This approach places an emphasis on three aspects:...

 and strict preventative maintenance schedules to help ensure reliability. 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.

In aerospace, there are additional requirements. Before and up to Limit Load, the structure may not have failed nor have permanent deformation. In excess of this, plastic deformation is allowed, but failure is not. Reaching the Ultimate Load (usually the Limit Load multiplied by the Safety Factor), the structure is allowed to fail. Civilian aircraft structures are required to meet both Limit Load and Ultimate Load criteria.

For loading that is cyclical, repetitive, or fluctuating, it is important to consider the possibility of metal fatigue
Metal Fatigue
Metal Fatigue , is a futuristic science fiction, real-time strategy computer game developed by Zono Incorporated and published by Psygnosis and TalonSoft .-Plot:...

 when choosing factor of safety. A cyclic load well below a material's yield strength can cause failure if it is repeated enough.

See also

  • Limit state design
    Limit state design
    Limit state design refers to a design method used in structural engineering. A limit state is a condition of a structure beyond which it no longer fulfills the relevant design criteria. The condition may refer to a degree of loading or other actions on the structure, while the criteria refers to...

  • Redundancy (total quality management)
    Redundancy (total quality management)
    In total quality management, TQM, redundancy in quality or redundant quality means quality which exceeds the required quality level. Tolerances may be too accurate, for example, creating unnecessarily high costs of production....

  • Probabilistic design
    Probabilistic design
    Probabilistic design is a discipline within engineering design. It deals primarily with the consideration of the effects of random variability upon the performance of an engineering system during the design phase. Typically, these effects are related to quality and reliability...

  • Sacrificial part
    Sacrificial part
    A sacrificial part is a part of a machine or product that is intentionally engineered to fail under excess mechanical stress, electrical stress, or other unexpected and dangerous situations...

  • Statistical interference
    Statistical interference
    When two probability distributions overlap, statistical interference exists. Knowledge of the distributions can be used to determine the likelihood that one parameter exceeds another, and by how much....

  • Verification and validation
    Verification and Validation
    In software project management, software testing, and software engineering, verification and validation is the process of checking that a software system meets specifications and that it fulfills its intended purpose...


Further reading

  • Lalanne, C., Specification Development - 2nd Ed., ISTE-Wiley, 2009
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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