Bolted Joints between 3D printed Parts

[MarcoC501]

Dear all,
I performed a series of FEM simulations considering a 3D printed frame made up of several components fabricated with additive manufacturing. For the analyses I considered their behaviour much similar to the one of the steel structures (aware of the uncertainties this procedure is causing).
When I simulated the bolted connections between bodies, I simply assumed "bonded-type contact". I did not find in the literature any example dealing with frictional joints between AM parts, so I went for the simplest option.
Do you have suggestions on papers I could read which deal with similar problems? In particular I am interested in the stiffness and damping of steel joints for 3D printed material, how they influence the connection and how I should take this into account in the FEM simulations. Moreover, it would be good to have an idea on how reliable is the approach I used (in terms of uncertainty and underestimation of the stress level).
Thanks a lot for your help and suggestions

 

[drawoh]

MarcoC501,

My assumption about FEA is that it does an excellent job of predicting strain. When you have something like a bolted assembly, the behaviour of joints can we weird and complicated, especially if a bolt's tightening force gets exceeded. This is beyond the capabilities of a lot of FEA packages.

On a metal structure, my rude rule of thumb would be that the weak points are the bolted and screwed joints. I would do statics, and then analyze each joint by hand.

In the case of a frame of 3D printed parts, you might have to model this thing joint by joint.

--
JHG

 

[jhardy1]

A key issue with most additive 3D Printing technologies is that the part itself is not homogeneous, and the effective mechanical properties of the 3D printed material are not the same as the "raw" material when cast or extruded.

You are likely to have a laminar micro-structure, as the part is built up in layers, so may be considerably weaker in tension in the vertical direction than in the two horizontal directions. (I.e. both the effective tensile modulus AND the effective tensile strength, are likely to be significantly less than the figures you will get from a tension test of a sample of the raw feedstock.) You may also have a discernable micro-structure within each layer, if the part has been laid down by a moving nozzle or laser tracing out a 2D path, rather than each layer being printed as an instantaneous homogeneous plane. These artefacts of manufacturing can act as significant stress concentrators, so may have an impact on fatigue behaviour of the part. 3D printing also offers the opportunity of creating honeycomb cores and / or leaving internal voids to minimise material consumption (reduce the effective density) - but of course, these internal voids also reduce the effective bulk mechanical properties of the part (modulus and strength), and must be factored into any analysis and testing.

There are post treatments for some additive manufacturing technologies which can address these sorts of issues, but you still need to have a good understanding of the whole manufacturing process and the real effective mechanical properties, to be able to undertake meaningful stress / strain analysis.

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