Influence of sealant on assembly process

Hi, this is Saint Petersburg and SPbPU again! My name is Artem Eliseev and in this post I would like to tell you about a new emerging direction of collaboration between SPbPU and Airbus SAS.

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Our university has a long-term and fruitful relationship with this company – my colleagues have already written about it in this blog, see the links below (Our collaboration with Airbus, Fasteners number optimization). One of the main results of our collaboration is the fact that SPbPU became the first university to introduce it’s own developed computer software in Airbus – our product called ASRP (Assembly Simulation of Riveting Process) has received such an honour (we also wrote about it in ECMI blog: SPbPolytech – first university to introduce computational computer code for Airbus).

ASRP is a software complex which is capable to simulate aircraft assembly process taking into account several complicated phenomena strongly connected with it. For example, one of the core problems in assembling is to find the gap between parts of an aircraft after riveting (indeed, this problem gave ASRP its name). From mechanical point of view, it is a non-linear contact problem and ASRP after almost 15 years of development is capable to solve it in fractions of a second even on fine meshes.

As any other software ASRP has some directions to advance and one of such directions is to include the influence of sealant on riveting process. Indeed, sealant is always put between jointed parts in order to shim and seal the joints. Corresponding contact problem has a different level of complexity compared to the previous one since contact between sealant and jointed parts is a fluid-structure interaction problem. Moreover, practically used sealants demonstrate quite sophisticated behaviour under loading – to be more precise, both viscous and elastic properties are noticed. This fact forces us to use not Newtonian but one of the viscoelastic rheological models for sealant modelling and it complicates the already complicated problem again.

Nevertheless, we faced this problem and I would like to present some of the intermediate results that we have received up to date. First of all, we have considered one of the widely used viscoelastic models – upper-convected Maxwell model – and derived a new modified Reynolds equation for it taking elastic effects into an account. This equation describes pressure distribution in sealant and it arises in thin layer approximation, which seems to be appropriate here since the gap between parts is about several millimetres and it is quite small compared to the length of parts (several meters usually).

Then, a prototype computer code has been developed which is capable to resolve the mentioned fluid-structure interaction process. An approach used there has not been implemented in ASRP yet, but we hope to do it if it is approved by our colleagues form Airbus. Below you can see some results of the simulations – the first picture demonstrates the gap between two parts, where lower one is fixed and the upper one is bent after three fasteners are installed there. The corresponding sealant pressure field is presented in the second picture – notice the three peaks near points of fastener installation.

In previous simulations we considered sealant to fill the whole gap between parts but our colleagues clarified for us that is not true. That means that we are now supposed to solve a free surface problem for sealant (life is tough, isn’t it? :)) Nevertheless, we already have something in mind about it and look forward to implementing and testing it. Another thing for us to consider is the fact that sealant solidifies during the riveting (complex chemical reaction called curing takes place) changing from fluid state to almost a solid one.

We hope to return with new results soon, see you!