Dimensional Stability of Materials

[shipwrightParticipant @shipwright]

I have built a couple of scale boats during the past 12 months. Both of these models use GRP, HIPS (High Impact Polystyrene sheet) and wood as materials.

I have experienced problems with differential expansion and wondered whether others can advise.

In particular, where I have used a large area of HIPS for a detachable deck and attached the deck to longitudinal wood strip for support using epoxy adhesive, the deck tends to curve with the wood expanding more than the HIPS so that you get a concave curvature with the ends of the deck higher than the centre. I have sealed the wood with varnish. In one model the wood used is balsa and in the other the wood support strips are spruce.

I have done a quick search of the internet and found that the thermal coefficient of expansion for HIPS is of the order of 1 part in 100000 per degree change in temperature. The expansion/contraction of wood is more complex because it is affected by humidity as well as temperature and also the expansion/contraction varies according to direction of the grain.

Ian

[shipwrightParticipant @shipwright]

[Paul TParticipant @pault84577]

Ian

Try introducing saw cuts into the wood strips at 150mm intervals, these will act as expansion joints and help reduce the tension on the HIPS.

On future builds instead of using wood as a strengthener try using built up laminations of HIPS. In the same way that plywood works the layers of HIPS and glue produces a very stiff section that wont expand at a different rate to the surrounding materials.

Hope this helps

Paul

[shipwrightParticipant @shipwright]

Paul,

Thanks – I like your idea of making a HIPS laminate beam.

Ian

[Paul TParticipant @pault84577]

Ian

One advantage of making a laminate beam is that the piece can be manufactured to follow curves as well as being simple beams, ideal for ribs and stringers.

Paul

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