Source
r/fea•
ProjectWheee

Fixed constraints more conservative?

I'm wondering why my lead keeps insisting that fixed is ALWAYS more conservative?

Any time I suggest giving the constraints degrees of freedom to move around, he gets very dismissive and patronizing, but won't explain the reasoning. It seems to me that, sometimes, by fixing constraints, you are actually giving the structure more support and not allowing the same load paths as in real life.

Does anyone have any good sources for info on this? If my lead can't explain it, is like to become more familiar myself.

Thank you

Posted Mar 19, 2021, 4:28 AM UTC

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biscuiteer307•

If you’re fixing every mounting point, then you’re assuming a pretty even load distribution... which could technically mean less conservative when you perform individual bolt analysis

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bvs_304•

I agree with your point of even load distribution

2
Qiadalga•

In general, I would say that any deformation that is constrained results in stresses which can make the fixed constraint more conservative.

It always depends on what your objective it, though. For determining whether the clamping force of a bolt connection is enough to prevent sliding of the parts, you need to allow these parts to move. For this, we need frictional, nonlinear contacts instead of fixed bonded ones. Here, the nonlinear approach would be the more conservative one in terms of displacement.

So I would make another general statement and say that if you look at deformation/displacement, fixed supports are not necessarily conservative, rather the opposite.

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bvs_304•

It depends on the type of structure you are simulating and actual boundary conditions.unless untill specifically looking at the structure or component or its way of working I can't say anything about your leads opinion , however

Let's say your component or structure is attached to next level assembly by means of bolts or pins partially restricted to rotate with respect to axial direction ...it's always better to fix completely ..as facilitating any rotational or sliding in bolted joint or pin location makes structure a little flexible and there by reducing stresses and deformation....so you fix all dof ..get high stresses which may not be the field experience , thus by more conservative and over design..........

Another thing is if you fix your bolted joint or partially restricted pin locations , mostly people are fine with linear analysis , which results in no need for nonlinear analysis, detailed modeling of bolts , contacts and interactions etc etc...there by reducing lot of efforts cost time and circumventing the convergence issues.........

After all it's just a business for cost reduction 😉

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MrJatt•

I think your are referring to stress analysis. There's loads and stress analysis. If you fix everything as I use to, I learned this leads to the system being overconstrained. So loads analysis with this approach gives bad results.

For stress analysis the same logic applies. You're initially solving for deformations and if you constrain things that shouldn't be, well your deformation won't represent the expected deformation in reality and you get highly stressed areas. This gives us the illusion of conservatism but really it's an erroneous way to model. Also this drives up weight and won't fly in aerospace.

There is value in fixing everything as a starting point, then remove constraints to make it consistent with the thing you're modeling in iterations. This is done to model complex systems. But I certainly wouldn't fix everything, all the time.

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billsil•

well your deformation won't represent the expected deformation in reality and you get highly stressed areas.

Things also flop a lot more when you don't constrain them, so if you're referring to a frequency analysis, in general, it's exactly the opposite. Deflection drives bending stress.

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MrJatt•

If you're referring to modal analysis, I've primarly used that for extracting natural frequencies. I'm not familar with stress analysis baked into modal analysis. To my understanding the objective is to extract natural frequencies in order to avoid load input frequency near them or designing the fn away from input frequency.

Realistic constraints still are ideal, otherwise you might end up with higher natural frequencies than they actually are. So you might approach your fn much earlier than you might be anticipating during testing.

As for transient response, deformation should be based on actual constraints, you wouldn't model a pinned beam as cantilever just because it gives you lower deflection, if the beams are pinned they ought to be modeled as pinned.

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billsil•

I'm referring to a frequency analysis. The modes are an input.

Modes are weird. They have no units, but you can think of them as deflections. If you have that shape, you can scale the deflections and the stress and that's your stress (quite useful for limit cycle oscillation). You can also think of them as velocities and accelerations, in which case it takes on a different meaning. I did a static trim analysis using modes a few years back, so M*xdd + K*x = F. I ended up using some modes in both the x and xdd terms.

You wouldn't model a pinned beam as cantilever just because it gives you lower deflection

I think you mean it gives you greater deflection (e.g., centerpoint deflection for a fixed-free beam vs. a fixed-fixed or a fixed-pinned beam). Regardless, I agree on the point, but it was a meant to show how ridiculous the idea is that a more rigid constraint is more conservative. Also, don't forget about deflection constraints.

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MrJatt•

Ahh I haven't worked on anything where modes are an input. I'm not too familiar with that methodology. But yes I agree, an overarching assumption like that is questionable.

I envy you actually, most of what I've worked on is based on load input vs deflection input. Wrestling with contacts is a nightmare for me, but it's not as much of a problem with deflection based input.

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ProjectWheeeOP•

Thank you. Do you have any suggestions for reference material or topics I should look up to help me get more familiar with this? I don't really have anyone at work who will give me the time of day, so I need to find other sources.

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MrJatt•

Best resourse for me was statics, trying to understand how all the forces are balanced in statics problems is a big help.

For example in statics pinned joints only react 2 Ways and a spherical connection reacts 3. The challenge is to discretize your structure into these elements. Statics is a pretty solid reference for that.

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billsil•

It depends on the structure, but it can be true. However, if a one support is really soft and another is hard, guess what's going to take the load? In the limiting case, all the load goes through only one of the reactions.

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ProjectWheeeOP•

Thank you. Do you mind elaborating a bit, or at least giving me some topics to look up to learn more about this?

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HairyPrick•

Fixed joints to ground/all 6dof fixed supports to ground vs flexible supports or some support dof "released" ? Or fixed body body joints vs revolute joints or other body-body joints with some stiff dof?

Probably need more clarifications but I'd say yes these distinctions can be pretty important either way depending on specific usage. In some cases fixing all 6dof may be more conservative or having some stiffness or free dof may be more conservative.

Often if following a design code an acceptable support config may be given or assumptions explained e.g. Lifting loads assumed split between 3 out of 6 points etc.

In general I would be making the distinction between "braced" supports or joints (large internal forces as a result of support config) and "compliant" joints/supports (small internal forces). The difference is easily seen by probing your reaction forces and a sense check on whether they are too large for wherever joint/support/body they represent.

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doodler_daru•

There are no hard and fast rules for constraints. Generally, you leave at least one translation free, for example, a cylindrical heat exchanger should be allowed to expand in it's radial direction with respect to it's mounting pads.

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