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What kind of materials fracture?

Many things can fracture. Everything from baseball bats to windows to bones.

Many structures are made of steel and/or concrete, and both of these materials can fracture.

Civil engineers use both steel and concrete, so they must be concerned with fracture.

In steel, fracture is often ductile and may be due to fatigue.

A fatigue crack is caused when a cyclic load (*) is applied. You are applying a cyclic load to a soda can tab when you push it back and forth to break the tab off of the soda can.

"Ductile fracture" means that the part of the material which is cracking is also deforming (its shape is changing). Think of pulling a piece of silly putty into two pieces: as you pull it apart, it deforms until you have created two new pieces.

photo courtesy of Jim Hanson (#)

On the other hand, concrete fracture is brittle. Brittle fracture means there is no deformation around the crack; the very grains of the material are separating, or cracking themselves.

Concrete is the material of choice for many, many structures: buildings, bridges, dams, to name a few. Concrete offers many advantages: low cost, good weather and fire resistance, good compressive strength (*), and excellent formablility (*). Concrete is bad in tension and therefore prone to cracking. Also, it's behavior is not so predictable.

Many dams are made of concrete and have suffered from cracking, which is caused by many different forces.

But what is concrete?

Everyday cracks Concrete properties