Hi,

The neck of the Harp that I made is twisting. This is not surprising as the pull of the strings is 660Lbs or 300Kg. To put this into perspective, that is a 1300cc Suzuki Hayabusa and rider hung 15mm away from the side. The wooden harp that this is based on twists slightly worse than this.

Would putting your 250gsm unidirectional carbon fibre on the side that is under tension help with this? The side that is under compression has become a bit lumpy, a bit like fish scales.

Any other suggestions for improving this?

Did I read somewhere that there is a problem with infusing Unidirectional fibre?

Brian.

This is how the neck bends.



This is a reminder of what the harp looks like.

Hi, it sounds like you've got compression failure of at the base of the arm which is causing most of the deflection.

If you've got 300kg of tension that is approx 3kN of force, lever = 15+(arm thickness / 2) say by eye 30mm so you have about 3kN of tension or compression in each respective face. which tbh is next to nothing.

I'm not sure what layup you have at that point or in the rest of the arm so cannot comment on it. Also is the internal space in the arm a void or does it have a filler? If it's a void then it's likely the compression face has buckled so remaking the arm with the centre filled with an expanding epoxy foam will help prevent it happening again

Post curing the part might help...but not sure!

Matthieu Libeert
Founder MAT2 Composites X Sports
website: www.mat2composites.com

as an additional comment, looking more closely at the picture, has it sheared at the joint between the 2 halves at the bottom or is that a trick of light in the picture?

The shearing is a trick of the light.

The layup for each side of the neck is:-

200g 2x2 twill carbon 0deg
200g 2x2 twill carbon 90deg
450g 2x2 twill carbon 45deg
3mm PVC foam
450g 2x2 twill carbon -45deg
200g 2x2 twill carbon 90deg
200g 2x2 twill carbon 0deg

The top section where the tuning pins are has another layer of foam and carbon.
I wanted to make sure the tuning pins would grip and the bridge pins would screw in.

The 2 halves are then bonded together with PT 326 so the core is hollow.

Brian.

Matthieu makes a very good point. What temperature was the part cured at and was it post cured. Most epoxy resins need post curing at around 60 degrees to achieve their maximum mechanical properties.
You could also infuse some lengths of channel section, bond them together to form an "I" section and then bond them between the two halves. Or you could also add some lengths of 10mm half round dowel to the mould. This would give a corrugated effect to the part once infused adding a lot of strength. Picture attached to hopefully explain what I mean

Warren

Carbon Copies Ltd

It was cured at about 28deg and only post cured at about 40deg.
I was thinking along similar lines of the corrugated effect. But using the PVC foam running from the foot of the neck to the top. If I can get the height of the ridges correct I can bond them together when I glue the 2 sides.

Brian.

What resin ratio did you use and which resin?

It was IN2 Epoxy resin. Not sure of the resin ratio. I stopped the infusion feed as the resin reached the vacuum line.

Brian.

Your description of fish scales or puckering would seem to me that the resin is soft and has not cured fully.
Matt hit it on the head for you.
When a panel is not cured fully you can tap it with a hard object like a box wrench and the sound will be almost like hitting a sheet of wax. Ask me how I know!! I then took the same panel many days later and brought it up to a reasonably high temperature for a day. Now the panel rings like a glass plate. The fish scales are probably the resin deforming like play-doo.
These are only my opinions and I hope it helps.
Fred