First off, great primer Jeff H. I have taken courses in Composites that were not as clearly presented as your primer.
I realized that I am way late getting in on this topic but it sure has brought up a lot of old memories. I am not a marine architect but I was an aerospace stress analyst in a past life. And I can surely tell you that sandwich panel construction is a love hate relationship. The benefits that Jeff H states are accurate but the maintenance
, inspectability and durability issues are also a concern.
It was interesting reading Jeff’s fiberglass
primer coming from an aerospace background. I have worked extensively with Fiberglass
, Graphite and Kevlar with epoxy resin. Kevlar would have to be my least favorite. The primary reason that I would use a sandwich (cored) panel was to stiffen the panel for bending or panel instability (compressive or shear buckling). I have found that it is hard to hide Kevlar from the compressive loads in a sandwich panel. One of my first hard learned lessons as an engineer
dealing with composites is, that if a ply of a certain type or orientation wants to fail under loading remove it. It is much harder to add plies to reduce the strain that is causing it to fail. In the case of Kevlar that would be a compressive strain which it just doesn’t handle well. I also have a vague recollection that the compressive fatigue properties weren’t great either.
I struggled with this when we were building a remotely piloted vehicle for a foreign customer who thought it would be tougher and harder to shot down if it was completely constructed from Kevlar. Off course the top bosses agreed. I have never worked so hard to substantiate a design in my life. In the end we had to use fiberglass anyway and the Kevlar was just window dressing. Now if you can find an application where Kevlar is entirely in tension and durability is required I would willingly use it. But cored panels
should never be subjected to pure axial loads or they are being misused.
I hope Jeff H can jump in here a bit because I can see how some people might think that cored panels
are stronger for flat panels but weaker for curved ones. I think this stems for the fact that cored panels are often used to prevent buckling and curved panels can have a higher resistance to buckling than flat panels. As I stated I am not a marine engineer
, but a boat looks structurally to me like a closed section subjected to bending loads, shear loads and torsion loads. A well designed structure would have these global loads being reacted in the plane of skins, either solid or cored. I cannot see how a boat would be stiffer with a core hull unless the in plane loads would cause buckling of the hull panels. Also since sandwich panel skins tend to have a combined thickness that is less that a solid panel the overall axial & shear stiffness would be less. I do see how a cored hull or deck would be stiffer to the out of plane loads.
Another issue we had with bonded sandwich panels was manufacturing. I also did some analysis on sandwich paneled engine
Inlet Cowls and Fan Cowls. We did it two ways co-cured and pre-cured. In co-cured we laid up the outer skin plies into the mold
then put down the core and then the inner skin plies. Bagged and cured in the autoclave (we were using prepreg epoxy). In pre-cured we laid up and cured the skins separately and then assembled and bonded the parts
together. Neither way was particularly fun. In co-curing we needed a sufficiently dense core with adequate compressive strength to allow the skins to cure without dimpling. In pre-curing we had great skins but the core fit was problematic and we were assured to have some area of disbonding. Murphy had a big hand in all of this as all of these areas of disbond seemed to happen near important fittings and the parts were too expensive to toss. In either case we often had detailed ultrasonic mapping of the core bonding interface. If I was buying
a boat with a cored hull I don’t think that I would want to see an ultrasound of these interfaces because I might not be happy with what I saw.
From my experience I would be surprised if cored hull and deck panels arrived from the manufacture with out disbonds of some sort. I would worry that if these areas are subjected to out of plane loads (which is why they are there in the first place) the core skin bond area surrounding the disbanded area would be subjected to peel stress which is not a desirable load path for resins or adhesives.
Thanks Jeff, you sure got me thinking on this one.
Lopez Island, WA