Hi all,
So thought I’d kick off my first post with a question for you all. I’m afraid it’s a long one!
As part of my final year (BSc Marine and Composites Technology degree) I have to design, manufacture and test a composite hatch cover for a merchant vessel. Small hatches designed for access to spaces below deck are a common requirement on most vessels. They must be capable of being closed weather-tight or watertight, as applicable. Their opening is normally 2.5 square metres or less.
The IACS rules for steel hatches of dimensions 630mm x 630mm require a minimum cover plate thickness of 8 mm. Such a hatch including fittings would weigh approximately 35kg. The objective of my design assignment is to save weight while meeting the performance specification. The design requirement is thus for a small composite square hatch cover of dimensions 630mm x 630mm for the after part of a vessel and capable of supporting a pressure of 30.6 kPa before failure. The cover is to demonstrate a maximum deflection of no more than 10 mm at the working load. Consideration is also to be given to reduce the weight by more than 50%.
So far I have manufactured and tested samples consisting of a plastic honeycomb core and aluminium honeycomb core both with 2mm E-glass/epoxy resin skins either side. After basic impact testing, the plastic honeycomb core appeared to outperform the aluminium core. There was little/no delamination but the plastic honeycomb had deformed plasticly. The aluminium core was completely crushed. I then manufactured and tested some infused balsa and PU foam samples. Neither of these performed well under impact testing either. The balsa sample actually transmitted sufficient energy through the core to cause some pretty serious delamination on the underside of the sample. Meanwhile the top side cracked. The foam held up slightly better with a crack leading from the point of impact to one edge of the sample with little deformation visible. However, the crack propagated to the open edge (the other was closed) and I feel that if both were open it would have cracked clean across the whole sample.
So as it stands, the plastic honeycomb core is the clear winner. Although the core deformed plasticly slightly it would still meet IACS regulations. Further more if cracked, the plastic honeycomb would only fill the affected individual cells with water and would not result in a leak. In other cases such as the balsa and honeycomb, there would not be a great deal stopping a pretty major leak.
So now I’m trying to explore ways of enhancing the performance of the plastic honeycomb core and monolithic skins. I’m keen to steer clear of reinforcement ribs as that will then require a way of dispersing away the stress concentrations associated with ribs, at the edge of the panel. This would add complication, weight and be unnecessary if there are other solutions. Other thoughts I have had include; creating a thicker top skin to improve it’s mechanical properties in compression, fitting a kevlar skin on the bottom to improve it’s mechanical properties in tension and maybe trying to find a thicker core. The hatch cover will only be exposed to impacts from sea water from above and so the design can be optimised to enhance the performance of the relevant mechanical properties of the separate skins and core.
So, if you haven’t all been bored to death by reading that, what do you reckon? Am I barking up the wrong tree or have I got the right idea?? This is work in progress obviously and will need refinement as and when.
Any thoughts welcomed with open arms 
Thanks in advance.
Jonny.