Test pieces help to learn and get used to the production process, and each new one gets better and better. However, a test piece is only half worth when not break-tested or at least loaded within its boundaries. Having invested a couple of hours of work and getting in love of that neat piece, it takes some dicipline to go further and break it. But hey, don’t forget why you built it in the first place: you wanted to gain information on its strength and stiffness. So, today was the right time to break that nice aramid honeycomb test piece I produced last week!
I started with loading the piece within its boundaries:
I measured the deflection to roughly estimate Young’s modulus E and the flexural rigidity D, important factors in designing shells/plates. The load was increased until the deflection in the center was roughly 10 mm. It looks already quite intimidating, though the theoretical maximum stress at the skin was far from being reached:
The behaviour corresponds very well to what is expected from theory, namely a linear relationship between deflection and load:
The steepness is related to the stiffness of the plate, where steeper means less stiffer. Because the skin layout is not symmetic, the stiffness depends on which side the load is applied. This behaviour was already mentioned by Herbert Funke in his dissertation (see PDF at R&G’s wiki).
Based on the usual beam theory and a second order of innertia of I = 805 mm⁴, I estimated the modulus of the skin to be E = 10.8 GPa . Herbert Funke’s assumed value for glass fibers was E = 15 GPa . This means that the usage of flax reduced somewhat the modulus. However, some of the discrepancy might be attributed to not considering all effects, such as shear deformation and Poisson’s contraction.
After measuring the stiffness, I continued with an impact test. I expect flax fibers to add some energy absorption and to keep the other fibers together in case of rupture. Well at least that’s how a manufacturer of flax fiber fabrics promotes their products. You can achieve similar properties by using aramid, but its handling is difficult. The fibers are extremely energy absorving and difficult to cut/sand. The charm of flax is that it is renewable and very easy in handling, even easier than glass or carbon fabrics. My idea is to use flax to increase skin thickness and add some damping, and in very critical sections to add aramid for some extra protection of the pilot.
Following video resumes the impact test: