Original question

Assumption:

• i made several swords with different cross sections (lenticular, single broad fuller as in viking swords, diamond, hollow ground diamond)
• the blades are made using the same steel and have no taper
• i chose the width for each blade in such a way that all my swords have equal stiffness (producing equal degree of bending requires equal force for each sword)

Question:

• which sword would be less likely to break on impact along the blade's edge?
• which sword would be less likely to break on impact on the flat of the blade?

Edit:

I found an article on quora which suggests that the "broad fuller" sword blade design is similar to I-beam used in construction: Link

This is actually very helpful because comparing sword geometries can be replaced with comparing different beam shapes which is a lot more widespread topic with a lot of information available

Basically the question boils down to:

• you have an I-beam and a rectangular beam of same mass
• deforming both beams along the weak axis (horizontal on the photo) requires equal amounts of strength

• which beam would require more stress to break it along the strong axis?
• which beam would require more stress to break it along weak axis?

i assume the second question it still valid despite both beams having same strength along the weak axis because their yield points might still be different

• You might just want the largest thickness along the impact direction, but I'm not 100% sure on this. Commented Oct 13, 2018 at 13:39
• I think it might actually be the opposite. When you bend a thick blade it requires more force but puts a lot of stress on the outer side. A thinner blade would be easier to bend but endure a greater deformation before snapping. Commented Oct 13, 2018 at 13:50
• Assuming the picture "common blade cross-sections" has horizontal x-axis and vertical y-axis i think if you want the blade to endure a strike on the flat it should be thin along x-axis and the stiffness would be achieved by making it taller (along y-axis). The opposite is true for the strike on the edge Commented Oct 13, 2018 at 13:53
• This however does not take into account that the blade can relieve some stress from the impact on the edge by twisting. Also the "broad fuller" shape might compress along the y-axis a bit because a thin fuller might be able to deform and act as a spring. Commented Oct 13, 2018 at 13:54
• sorry that's what I meant. Commented Oct 13, 2018 at 14:04

In short:

The fuller does act like an I-beam and thus adding a fuller (almost) does not reduce the sword's strength against the impact on the edge but it does noticeably reduce the sword's strength against the impact on the flat.

The trade you're getting here is strength along the weak axis vs mass

With proper edge alignment (which demands good technique from the swordsman) loosing a bit of strength along the weak axis is less of an issue while reducing the sword's mass provides more speed which is very advantageous in a fight.

So another way to view this trade is that by adding a fuller you get a sword that is less forgiving to bad technique but can move faster.

Regarding the original question about the swords with same strength along the weak axis:

the main difference would be that a sword with a fuller will be heavier and thicker (because trying to fully compensate for the loss of strength along the weak axis by making the sword wider would produce a sword that is too wide to even be called a sword).

of course the sword with a fuller that is heavier and thicker would be a LOT more durable against the strike on the edge but that comes at the cost of being heavier and most swords are unlikely to bend on the strong axis anyway