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Introduction to FE Analysis

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Introduction to FE Analysis

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represent a cleat pull out load. The stresses I see are far bigger than the

deck material can take. From my experience with manufacturing these

boats it cannot be that bad?

The model is probably set up with a point force applied to the shell mesh of the deck. The

reason that you are getting very high local stresses is that the load is trying to act in the FE

model as a force at a finite point. The finite point is of indeterminate size, but in FE terms it

is very, very small. It is rather like a lady passenger walking on your deck with a stiletto heel.

You probably wouldn’t allow that!

This results in very high stresses as the elements immediately adjacent to the loaded point

load try to transmit the stresses out to their neighbours. The stress gradient – that is the rate

at which the stress drops off as you move away from the point, is very high. This means that

the results will be very inaccurate.

As an interesting exercise, if you refine the mesh at the loaded point the stresses will keep

getting higher and higher – you are in fact chasing a mathematical singularity. The area of

load application gets smaller and smaller and the pressure applied to maintain the net

force goes up and up. Or in other words, that lady passenger just keeps getting sharper

and sharper heels. Her weight stays the same, but the local stresses she causes are going

to just get bigger and bigger.

So how do you deal with this? The modeling I mean – not the passenger!

reason that you are getting very high local stresses is that the load is trying to act in the FE

model as a force at a finite point. The finite point is of indeterminate size, but in FE terms it

is very, very small. It is rather like a lady passenger walking on your deck with a stiletto heel.

You probably wouldn’t allow that!

This results in very high stresses as the elements immediately adjacent to the loaded point

load try to transmit the stresses out to their neighbours. The stress gradient – that is the rate

at which the stress drops off as you move away from the point, is very high. This means that

the results will be very inaccurate.

As an interesting exercise, if you refine the mesh at the loaded point the stresses will keep

getting higher and higher – you are in fact chasing a mathematical singularity. The area of

load application gets smaller and smaller and the pressure applied to maintain the net

force goes up and up. Or in other words, that lady passenger just keeps getting sharper

and sharper heels. Her weight stays the same, but the local stresses she causes are going

to just get bigger and bigger.

So how do you deal with this? The modeling I mean – not the passenger!

In practice the cleat will be attached to the deck at

several bolting points and be well tightened down. That

creates a much bigger footprint (excuse the pun) over

which the load will act.

Here is an example of a cleat, it happens to have two

pads that attach to the deck. Notice the footprints

shown below the cleat image.

If we can manage to mesh the deck structure so that

we map the footprint of the cleats reasonably well, then

we can apply a distributed load over the mesh.

several bolting points and be well tightened down. That

creates a much bigger footprint (excuse the pun) over

which the load will act.

Here is an example of a cleat, it happens to have two

pads that attach to the deck. Notice the footprints

shown below the cleat image.

If we can manage to mesh the deck structure so that

we map the footprint of the cleats reasonably well, then

we can apply a distributed load over the mesh.

Don't forget we said the cleats would be well tightened down, it would also help if they

were bonded to the deck.

If the cleats are not well tightened then we would have to consider the action of the bolts

as they connect to the deck, but even then there would be probably be washers or load

spreaders under the decking.

There are several other methods to distribute load, such as using rigid spider type

elements. This link will take you to descriptions of this type of element.

were bonded to the deck.

If the cleats are not well tightened then we would have to consider the action of the bolts

as they connect to the deck, but even then there would be probably be washers or load

spreaders under the decking.

There are several other methods to distribute load, such as using rigid spider type

elements. This link will take you to descriptions of this type of element.

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