During compression testing, friction between the test piece and the platens restricts the radial expansion of the specimen's ends. This causes the metal to bulge near the contact surfaces rather than uniformly in the center, resulting in a shape resembling a maritime bollard. It skews the stress-strain data by introducing complex shear stresses rather than pure uniaxial compression.
We need to lubricate the platens better on the next run, we're getting too much bollarding on these aluminum cylinders.
Using a Teflon film or glass lubricant between the specimen and the compression platens minimizes the friction that causes bollarding.
Related terms
Cold flow
The slow, permanent deformation of a solid material under continuous mechanical stress at ambient temperatures.
Ductile fracture
A mode of metal failure characterized by extensive plastic deformation before the material finally tears apart.
Vickers hardness
A measure of a metal's resistance to indentation using a diamond pyramid.