Question

State four failure modes of materials. Describe typical examples of each mode.

Answer 1

1) Fracture: When cracks appear

*I guess that everyone saw some cracks in a bridge or in a wall… well, that’s a sign that the structure has a problem!

*When a new cracks appear or existing cracks are extended, you are basically looking at a fracture mechanism.

*Not every part fractures in the same way:

*Some parts fracture slowly and deform plastically before seeing a sign of a crack… that’s because those parts are built in a ductile material.

*Ductile fracture with characteristic distortion and shear lip

*Steel is basically a ductile material. It extends and deforms before failing.

*That’s why the stress-strain curve is like this:

*Now… you have also brittle materials!

*Those are much more « sneaky » if I dare say

*Those materials tend to fail suddenly and abruptly without showing any sign of plastic deformation.

*Concrete and glass are 2 examples of brittle materials.

*Note that you have also materials which a ductile up to a point and then become brittle.

*Some materials like bone can be ductile or brittle in function of the strain rate ( Source)

*Here’s a picture that shows the difference between brittle, ductile and semi-ductile materials:

2.) Yielding: When a body experience stresses

*Yielding is probably the most common type of failure analyzed with FEA

*It describes a body which experiences stress in excess of the yield stress.

*Yielding is only called yielding when it actually compromises the integrity or function of the part that yields.

*Why do I need to precise that?

*Sometimes you may observe a « localized » yielding that cause strains near stresses concentrations in your part, but then… the stress in redistributed and the yielding actually stops.

3) Insufficient Stiffness (Deflection): When flexibility causes failure

*Whatever the mechanical system you build, it must be stiff enough to resist the loads.

*Seems pretty basic, but that’s really a notion at the heart of mechanical engineering.

*It was also probably one of the first lessons I learned when I used FEA for the first time in my life.

4.) Buckling: When a loss of stability creates big damages…

*In a nutshell, buckling a kind of failure that happens to certain types of slender geometries because of the inner instabilities that occur in the loading.

*In it very dangerous because buckling failure can happen much before the material failure (Yielding).

5.) Fatigue: When time causes a loss of strength

*When you hear a story in your family that they were driving the same car they drive for more than 10 years and suddenly a part failed and they didn’t know why…

*That’s probably because of Fatigue