MTS Series 609 Alignment Fixture - Manual (Page 15)

Misalignment
Series 609 Alignment Fixture Product Information Introduction 15
Misalignment
In uniaxial testing, perfectly aligned grips produce uniform axial tensile strains in
a specimen. For grips to be perfectly aligned, their loading axes must be
concentric.
Perfectly Aligned Grips Produce Uniform Axial Strains
Misalignment between the grips produces nonuniform axial strains in a
specimen. Some areas will have higher than average strains; other areas lower
than average strains. Bending strain is the difference between the average strain
and areas with higher or lower than average strains.
Many ASTM procedures limit maximum bending strains because they cause
specimens to exhibit much lower strengths than if all axial strains were uniform.
The alignment fixture lets you reduce bending strains by improving concentric
and angular alignment between the upper and lower grips.
Grips can have concentric and angular misalignments. Both can occur together
and have a combined effect on the bending strains that appear in the specimen.
About Concentric Misalignment
Concentric misalignment shifts the vertical axes of the grips laterally away from
each other. This puts an “S” shaped bend in the specimen.
A specimen with an “S” bend has a zero bending strain in the middle of its gage
section. It has higher than average bending strains at the top and bottom of its
Mounting Surface
Mounting Surface
Equal Strain Equal Strain
Loading Axis
Loading Axis