Delamination is the most common source of failure, it happens between the outer carbon fiber skin and the Nomex core. The epoxy adhesive film we currently use doesn’t work as well as we would like since the epoxy we use may not be strong enough and the bonding area may be minimal since Nomex has a honeycomb shape, we can’t get as much epoxy on it as we would like. The following research will be on trying to find ways to fix this.
Shear-key insertion
Taken from the research paper: Improving delamination resistance capacity of sandwich composite columns with initial face/core debond.
This research paper was attempting to find new ways to combat the issue with delamination, they outlined how a popular method for doing this is by Z-pinning which is a very “expensive” method although I don’t know what they classify as “very expensive”. They were trying to find a more cost-efficient way to do this, it’s called shear-key insertion.
This method involves the “insertion of shear-keys in grooves within the PVC foam core”. Although this experiment was done using PVC foam as a core and not honeycomb, I believe it won’t be too different? (correct me if I’m wrong) The following are pictures of the actual inserts and what the sandwich layup will look like:
The shear keys can be made out of virtually any material and they can be made of any shape and size (this is all conceptual). They used pre-manufactured semi-circular shear-keys made out of glass-fiber and epoxy resin. The way they made them in this research paper was “woven roving strands were placed in a mold which was designed based on the shape and size of the foam core groove. If we were to pursue this option, it would be worth making this mold out of something that may be used many times such as aluminum, not a one-off. The shear-keys pictured above were the roving strands vacuum infused with epoxy resin and hardener followed by curing at room temperature. This method can also be combined with other methods to make ultra-strong sandwich layups!
Stitching
As you can possibly suspect, this method is done by literally stitching the sandwich panels together using Kevlar thread. I think this method is super cool and interesting.
There are many different types of stitching that people have used but the most popular stitch style is a “modified lock stitch in which the knots linking needle and bobbin threads are formed at one surface of the laminate to minimize in-plane fibre distortion”. It is pictured below:
Although this method is used on a foam core, I don’t see why it wouldn’t work for a honeycomb core since we just have to thread the yarn through the holes in the Nomex? One issue I saw while reading is that they say “in order to minimize possible damage to composite sandwich panels, through-the-thickness stitching was performed on the sandwich panels before the resin application” as we use prepeg, this may be an issue since this type of stitching was used on a wet layup/resin infusion? I don’t see why it wouldn’t exactly work on a prepreg but I may be wrong in thinking that.
They used Kevlar 129 to stitch the layers together which I imagine won’t be cheap since they say Kevlar 129 is specifically strong? so it may be more expensive? idk. I genuinely have no idea how much Kevlar yarn actually costs.
The following is a picture of the machine they used to sew the yarn in:
Another issue that arises is that We will have to most likely make our own machine? Since conventional sewing machines cannot pierce through the sandwich layup.