> ??? Copper and iron have very different material properties. That is why
> engine blocks are made of iron, and not copper.

Iron is chosen for engine blocks because it's cheap, it wears well with iron piston rings to get a nice, tight seal, and it has good thermal expansion and heat transfer properties. Any material is stiff if it's formed into an big lump like an engine block. It's apples-and-oranges again. Tensile-test a strip of iron and a strip of copper and you'll find they're more similar than dissimilar.

> However, a brick dropping on a pvc pipe will most likely simply bounce
> off. The copper pipe will be dented, resulting in a flow restriction, a
> hardened brittle spot, and cosmetic marring. A solder joint nearby will
> probably be damaged to failure by the impact, while a pvc fusion joint
> nearby will most likely not be affected by the impact.

> Impact loading also varies tremendously. There is quite a difference
> between bellying up on a smooth boulder and impaling the boat on a spike.
> That's why spears are pointy and knives are sharp.

Right, failure modes differ and have to be considered separately. Landing on a flat rock will result in bending stresses in the material, landing on a pointed rock will lead to a combination of both bending and shear stresses.

> If the object was to not yield, the material has failed. I really don't
> think you're going to want a kayak that yields readily in use. Even if it
> were to retain watertight integrity, the paddling characteristics would be
> most adversely affected.

Not necessarily. The yielding may only result in a white spot where the cloth separates from the wood and the wood underneath is crushed, but the kayak is still intact and readily paddled. Failure is when your hull has been punctured or fractured and water enters. I would rather paddle a kayak with a dent than a kayak with a bow hatch full of water.

> Ok. It was looking to me like you were saying it as an absolute. Remember
> too with the latter point, you can make the entire body as a whole
> stronger by weaking selected areas (I beams as an example), but you cannot
> make the entire body stronger by weakening it throughout. All over
> variables held constant of course.

You can make it stronger by making it less stiff throughout.