Conservatism in Stress Engineering
A majority, if not all, aerospace stress engineering activities are dominated by classical hand calculations of margins of safety. They also include a decent dose of analysis conservatism.
But what exactly is ‘conservatism’ in stress engineering? In stress analysis, it is embedded in various forms and methods. In this post, I would like to get a little deeper into some of those methods. What are the reasons, the benefits and downsides to conservatism in stress engineering? Without further delay, let us get into it.
We touched on this concept in some other blog posts, such as those listed below:
- http://www.stressebook.com/stress-engineering-interview-questions-part-3/
- https://www.stressebook.com/14-cfr-subpart-c-section-25-365/
- https://www.stressebook.com/14-cfr-subpart-c-section-25-303/
- https://www.stressebook.com/14-cfr-subpart-c-section-25-301/
Click here to access pdf versions of the latest blog posts…
Definition:
If you asked me what conservatism in aerospace stress engineering is, then I’d define it this way:
Conservatism in aerospace stress analysis generally involves slightly simpler analysis methods at the cost of more penalizing component margin of safety (M.S).
Why do we use it?
There are many reasons to use it in aerospace stress engineering. Listed below are some common reasons in my experience.
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Ease of analysis
Typically, it helps simplify the sizing of a component using simpler or less detailed analysis methods or techniques.
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Uncertainty
Assume that for a certain component we do not have a measured or FEM based or tested flight load available. In such cases, design loads are used. These prototype design loads could be much higher than what the component will ever experience in its lifetime.
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Easier approvals from approving authorities
It usually helps appease the concerns of approving authorities and helps with easier approvals for such analyses. Of course, the assumption is that your conservatism is sound, based on accepted industry methods and techniques, but also not overly conservative.
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Opposite direction
This may sound a bit obtuse but let us take the example of a critical lug fitting in a joint. The penalty on weight is minimal compared to the safety of the joint and how critical that particular component is to that joint. So in such cases, it is better to have high margins of +0.50 or greater. This type of conservatism is opposite to the typical definition I stated above but worth it.
What are the sources of conservatism?
Before you get into the material below, I suggest reading through this article. Now that we have discussed why we employ it, let us look at some of the sources.
I like to use the saying “my stress analysis is only as good as the loads I am given”. There are entire loads groups in many large aerospace companies that work on nothing but loads. Loads may come from finite element analysis loads models, or classical hand calculations. In both cases, the way things are modeled or calculated may include conservatism.
I have an entire blog post dedicated to this, but an additional fitting factor in stress analysis is mandated by the FAA (and the military as well) to account for uncertainty in loads at a joint.
Again, I have various blog posts on this topic so I won’t go too deep in this blogpost on it. Click the link above to learn more on this safety factor.
In general, for all component section checks (M.S sections) and critical joint checks, A-Basis material properties are recommended to be conservative. There are exceptions to this, the DER may allow the use of B-Basis allowable values in cases with redundant load paths. Click the link above to learn more about A-Basis and B-Basis properties.
When considering the section properties of a component, it is best practice to use either the maximum or minimum tolerance values, whichever produces a lower M.S. The same applies to stack up tolerances. In other words, the tolerance values of an assembly of components producing a longer moment arm dimension for example. In other cases, it may be the smallest hole tolerance dimension for a bearing check, or the smallest tolerance diameter for a bolt check etc., I think you get the point.
In certain analysis cases, for example a bolted joint, one of the bolts that is expected to see the largest load is assumed to fail. This is sometimes referred to as a fail-safe load case. This is one way to conservatively analyze the remaining bolts in the joint for the ‘fail-safe’ loading condition. In other words, the reaming bolts need to safely take the redundant load after failure of one of the critical bolts. Another example may be to ignore certain fastener along a riveted joint to simplify the analysis.
It is not uncommon to have complicated geometry for certain closed or open sections at a critical cross section of a part. If classical hand calculations are employed to check such a section, then to make life easier certain portions of the section may be truncated to simply the geometry. Care needs to be taken though not to ignore extremities of the section where the highest bending stresses may occur.
Disadvantages:
The benefits obviously include those aspects already covered in the ‘why do we use it section’ above. But what may be some disadvantages? Some of them are listed below.
- M.S too low for downstream customers such as MRB or repairs
- Sizing ends up too heavy or inefficient
- Over simplification may ignore important aspects of analysis methods
- Inaccurate analysis method is also a possibility
- Sometimes what you think is conservative may not be the case for another engineer or a customer
- Lack of detail or precision in the analysis methodology may turn some people off
Conclusions:
In conclusion, I think you need to use it as a balancing act to be as efficient as possible with your analysis. Your method needs to be convincing, not just conservative. If you take all of the above aspects into consideration, then you may end up being just fine.
I hope you enjoyed this post. Please feel free to comment below.
Cheers.