Safety factor: how do I calculate it? - ugly for everyone (2023)

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.

Eric Lee(Australian):

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.

Jeff Finlayson(Bing):

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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If you are a beginner in FEA simulation, please readThis article.