This article covers the concept of safety factor and how it is calculated. It is a basic idea that every mechanical engineer must understand well.
How do I know if my part design is "safe" enough?
That's a tough question that many designers ask themselves.
One of the possible answers concerns the degree ofsafety factor.
For some people this term is still not well understood, so I decided to write about it!
Important NOTE: This article is mainly about "stress-based" safety factors, but you should be aware that there are different definitions of "safety factor" margins that are not necessarily related to the state of stress. Furthermore, the definitions of stress can vary… there is von Mises stress, but also Tresca, etc…. (Thanks toBoyd Mckayfor helping to improve this article by mentioning it)
Get safe designs
First off, I think it's worth remembering that one of the purposes of simulation is to achieve safe designs...
Of course, if a part fails, it is aDanger to human life, but also amajor financial damage to the companywho created this part (think exploding Samsung batteries and you will understand what I am talking about)
FEA simulation helps to understandWhy does a design fail?,where it failedjhow to improve it.
This is why FEA is so important for companies designing products.
To assess a design's security, designers need a simple factor to help them understand if a design is secure enough.
This factor is called the safety factor.
How is the safety factor calculated?
The definition of the safety factor is simple. It is defined as the relationship between the resistance of the material and the maximum stress in the part.
If the stress at a given location becomes greater than the resistance of the material, the safety factor becomes less than 1 if there is a hazard.
What it is basically telling us is that in a certain area of the model, the stress is greater than the resistance the material can withstand.
If the stress in the model is still much less than the strength of the material, the safety factor remains greater than 1 and the model is "safe".
Note that if the safety factor is much greater than 1 anywhere in your model, this also indicates that your part may be reworked. In this case, it is also undesirable, since you will only waste material resources and increase costs.
Now let's talk about the 2 important values you need to calculate this safety factor: stress and strength
What is stress?
If you still have doubts about it, don't be embarrassed, it's not an easy concept for beginners, but it's important.
In short, stress is a value that measures the internal pressure inside a solid caused by an external load. If the stress within a part is too high, the part can fail.
The term stress is not that different from what we experience every day at work... When we receive a workload, we become stressed. When we are too stressed we can experience nervous breakdown and many health problems.
If you want to understand more about stress and how stress is actually calculated, I wrote a whole article about it a few months ago.
read the article:what is stress
What is the resistance of a material?
Stress and violence are different and this is where a lot of people don't get it.
Stress in a body is always a function of the applied load and cross-section, while strength is an inherent property of the body's material/manufacturing process.
Similar to other material properties, strength is determined, for example, by a standard tensile test in which a test bar is subjected to a uniaxial load. We can then plot the stress-strain curve for the material by extracting the strain data and plotting it against the stress data.
Note that if you need high precision, the test must be performed under conditions similar to the operating conditions of the part or system (temperature, strain rate, material grain, flow direction,...).
There are a few key points to understand about this curve:
- Point P is the proportional limit, it delimits the part of the curve governed by Hooke's law
- Point E is the elastic limit. The material continues to behave elastically up to point E, but stress and strain are no longer proportional.
- Point Y is the elastic limit, which corresponds to the elastic limit of the material.
- The point U indicates the maximum stress that the material can reach. Corresponds to its breaking or tensile strength.
- Point F is the breaking point.
Note that for some types of materials, such as e.g. B. ferrous materials, may collapse.
The elastic limit is not necessarily very clear and is usually obtained by a compensation method:
Y is taken as the intersection of a shifted line, parallel to the linear portion of the stress-strain curve, typically at an axial strain of 0.002, and the plastic portion of the curve.
As you read, there are several values for material strength: yield strength, ultimate strength, and ultimate strength.
The safety factor is calculated using the elastic limit, so this is the parameter you should know first.
Is this relationship a perfect indicator of a model's safety?
I would like to say that nothing is really perfect... As engineers we have to learn to live with mistakes ;-)
Bugs are everywhere:
- In the testing process, it provides you with the material's stress-strain curve and yield strength, which are used to calculate the safety factor
- In the FE model you create, it is likely that the boundary conditions and/or meshing will introduce some level of error.
- The error is contained (and hopefully controlled) in the FEA software itself and the algorithms it uses.
Therefore, it is always better to consider a safety factor that is not exactly 1, but maybe slightly higher (2-3) depending on the hypothesis.
Additional note:The safety factor only describes the failure of the material. It is sufficient in some layouts, but if you are designing a thin element, some form of stability error (eg.kinks) can occur. The safety factor quoted does not take this into account, as buckling can occur if the stress is much less than the material's ultimate stress.
Notable comments from other FEA specialists
David Hinterhaus(Backhouse Technical Service LTD):
There is no established "factor of safety" as such. This is too simplistic a concept since there are so many failure modes. For example, there are cases where stresses above the yield point are acceptable. You really need to look at the appropriate design standard for the structure, its use, and the stress rating.
Karl vanAswegen(FZ LLE liquid codes):
Safety factors are not necessarily maximum voltage/power. Often the industry you work in will dictate how you calculate safety factors for the design. Most of the time, your biggest problem is fatigue, not performance.
In my opinion, safety factors are only important in certain cases. In the industries I've worked in (shipbuilding/offshore), safety factors are not the criteria we work with. We have our allowable stresses, but safety factors are never definitive as there are always stress concentrations that can be omitted due to geometry, mesh/aspect ratio, loading conditions, etc.
The only real safety factors where the design is totally safe are in lifting applications where an SF of 3 to 5 is required. That said it's always good to check, especially when doing rough hand calculations.
It is best to understand what a factor of safety really is and what the requirements actually state.
In general, the ultimate safety factor is the maximum load/load applied, and for elongation SF it is yield load/load applied. Stresses may not be linearly related to load due to local plasticity effects. Static biases may receive little or no safety depending on the uncertainty requirements.
Vlad Kertschmann(Independent consultant):
When evaluating the possible lastloads/effortsPeople often look for quasi-static or steady-state conditions with extreme overload or preload. In operation/real life, it usually happens when there is a drastic change in load: inLet's say dynamicVehicle collides with an obstacle on the road orbulk goods,or seismic loading of a structure, explosion, etc. This is where FEA can really help to replace difficult and expensive tests.
because neil(Allen Vanguard):
I think the safety factor should be expressed in terms of something. For single use, it can be in terms of maximum durability. Similarly, it can be expressed in terms of fatigue strength for many cycles.
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How do you calculate the safety factor? ›
The Factor of Safety of the structure is defined as F = C/D and failure is assumed to occur when F is less than unity.What does a safety factor of 1.5 mean? ›
1.3 – 1.5. For use with reliable materials where loading and environmental conditions are not severe. 1.5 – 2. For use with ordinary materials where loading and environmental conditions are not severe. 2 – 2.5.How do you calculate safety factor for lifting? ›
The safety factor for lifting is the ratio between force applied to a component in a system, and the minimum breaking strength of the component. The safety factor is calculated by dividing the minimum breaking strength of the gear by the maximum force that can be supported by the lifting gear.What is a 10% safety factor? ›
As an example, if we have a system that requires 80 psi and the available city pressure of 88 psi, a 10% safety factor on the required pressure would be 80 psi x 10% = 8 psi safety required.Why do we calculate factor of safety? ›
A factor of safety increases the safety of people and reduces the risk of failure of a product. When it comes to safety equipment and fall protection, the factor of safety is extremely important. If a structure fails there is a risk of injury and death as well as a company's financial loss.What does a safety factor of 0.5 mean? ›
You can express the safety factor as a number (e.g., "10") or a ratio (e.g., "10:1"). If the safety factor is less than 1 it means that the force exceeds the strength and the component will fail. For example, if you hang 60 kN a 30 kN rope, the safety factor will be 0.5.What does a safety factor of 2.0 mean? ›
Structures or components with FoS < 1 are not viable; basically, 1 is the minimum. With the equation above, an FoS of 2 means that a component will fail at twice the design load, and so on.What is normal safety factor? ›
A 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 some cases it is impractical or impossible for a part to meet the "standard" design factor.What is the safety factor of 5? ›
Safety Factor Meaning
It is commonly stated as a ratio, such as 5:1. This means that the wire rope can hold five times their Safe Work Load (SWL) before it will break. So, if a 5:1 wire rope's SWL is 10,000 lbs., the safety factor is 50,000 lbs. However, you would never want to place a load near 50,000 lbs.
Multiple lift rigging procedure.
This capacity, certified by the manufacturer or a qualified rigger, shall be based on the manufacturer's specifications with a 5 to 1 safety factor for all components.
What is safety factor for strength? ›
The factor of safety is defined as the ratio of ultimate stress of the component material to the working stress. It denotes the additional strength of the component than the required strength to carry that load. It tells us how much stronger a system is or needs to be for an intended load.Is a safety factor of 1 good? ›
A factor of safety of 1 represents that the stress is at the allowable limit. A factor of safety of less than 1 represents likely failure. A factor of safety of greater than 1 represents how much the stress is within the allowable limit.What does safety factor 2.5 mean? ›
2.5 - 3. For less tried materials or for brittle materials under average conditions of environment, load and stress. 3 - 4. For untried materials used under average conditions of environment, load and stress.What is a safety factor of 3? ›
A safety factor 3.0 in bearing capacity is about the same that a factor of safety 1.5 in retaining walls or slopes. This is so because the bearing capacity equation is higly nonlinear (Exp(pi*tan(fi)...). You reduce your friction angle by 30%, your bearing capacity falls by 60%. Hope it helps.What does safety factor depend on? ›
The safety factor depends on the materials and use of an item. Different industries have varying ideas on what FoS should be required. Although there is some ambiguity regarding safety factors, there are some general guidelines across multiple verticals.What is the maximum safety factor? ›
Factor of Safety Equation
For a structure to be considered safe, its factor of safety must be greater than 1. A factor of safety that is equal to 1 means that the structure's maximum strength or capacity is equal to its determined design load. This means that the structure would fail if any additional load was applied.
The 100-fold factor is considered to represent the product of a 10-fold factor to allow for species differences between the test animal and humans and a 10-fold factor to allow for inter-individual differences.Is factor of safety always less than 1? ›
Factor of safety=Ultimate Load (Strength)/Allowable Load (Stress) As understood from the above equation the allowable stress is always less than the ultimate failure stress. Hence, the factor of safety is always greater than 1.What is a 4 to 1 safety factor? ›
In the section covering leaf chain, the Machinery Directive states that the minimum safety factor when lifting a weight should be 4:1. In other words, the leaf chain should be able to lift four times the maximum weight it will be lifting in its working life.How much can one person lift OSHA? ›
The lifting equation establishes a maximum load of 51 pounds, which is then adjusted to account for how often you are lifting, twisting of your back during lifting, the vertical distance the load is lifted, the distance of the load from your body, the distance you move while lifting the load, and how easy it is to hold ...
How much can an employee lift OSHA? ›
Based off the NIOSH Lifting Equation, the Occupational Safety and Health Administration (OSHA) recommends the weight limit for individual lifting be 50 pounds. When lifting more than 50 pounds, it is recommended to use a lifting device or two or more people. Don't hold your breath while lifting.What is the maximum safe load to lift? ›
The guidelines suggest that the maximum weight men should lift at work is 25kg. This relates to loads held close to the body at around waist height. The recommended maximum weight is reduced to 5kg for loads being held at arms length or above shoulder height. Maximum weight guidelines recommend lower weights for women.What is allowable stress and safety factor? ›
The allowable stress is defined as the material failure stress (a property of the material) divided by a factor of safety greater than one.What happens if factor of safety is below 1? ›
A factor of safety of less than 1 represents likely failure. A factor of safety of greater than 1 represents how much the stress is within the allowable limit.What does a factor of 1.2 mean? ›
Factor Pairs of Decimal Number
A decimal number is a number whose whole number part and the fractional part is separated by a decimal point. For example: 1.2, 3.456, 5.28 etc. For example: For the decimal 1.2. Factors of 1.2 are 1, 2, 3, 4, 6, 12, 0.1, 0.2, 0.3, 0.4, 0.6, 1.2.
These components should have a minimum FOS equal to or more than 2. In other words, the value of the safety factor is always greater than or equal to the design factor.What is a safety factor 4 to 1? ›
The factor of safety is represented as a ratio. For example, if your equipment needs to lift 2,000 kg, and its safety factor is 4:1, the equipment will be designed up to 8,000 kg. The factor of safety depends on the materials and the use of equipment.What is the factor of safety for mild steel? ›
Partial safety factor for concrete and steel are 1.5 and 1.15 respectively, because. No worries!What is 1 factor of 100? ›
Solution: Factors of 100 = 1, 2, 4, 5, 10, 20, 25, 50, and 100.What does a factor of 3 mean? ›
The factors of 3 are the numbers that divide 3 evenly without leaving any remainder. As we know that the number 3 is the first odd prime number. The factors and the pair factors of 3 can be expressed in positive and negative forms. For example, the pair factor of 3 is represented by (1, 3) or (-1, -3).
Is 1 the factor of 25 true or false? ›
The factors of 25 are 1, 5 and 25. The factors of 24 are 1, 2, 3, 4, 6, 8, 12, and 24.Can factor of safety be 15? ›
By default, the maximum value is selected which is automatically set to 15.Can factor of safety be 1000? ›
Higher safety factors such as 1,000, 2,000, and even 5,000 can be used in the regulation of substances believed to induce severe toxic effects in humans. The effect of a safety factor on the actual risk depends on the dose–response relationship.