Does Alignment Affect My Testing?
The answer is most certainly yes...
It would be wrong to think that just because your materials testing
system is regularly calibrated for force, strain and displacement that
you are assured correct and reliable materials testing data.
Load frame alignment can change for a number of reasons, including:
- Changing grips
- Fitting new or replacement fixtures
- Repositioning the fixed cross head
- Wear or damage to fixtures or load frame components
As a consequence, the importance of accurate alignment is being recognized more and more by:
- Accreditation bodies
- Major aerospace corporations
There is a growing requirement in the aerospace industry to
demonstrate that your systems meet the alignment requirements specified
in many ASTM Test standards that reference tolerances for either bending
stresses or alignment.
What is Alignment?
Specimen Alignment is affected by two factors:
- Angularity (parallelism)
Concentricity is how well the center of the top of
the specimen coincides with the center of the bottom of the specimen
when viewed down on the specimen in an aerial view. Angularity is how parallel the specimen's gripping faces (top & bottom) are to each other.
Why is Good Alignment Necessary?
The easiest way to put unwanted stresses into a test piece is to bend
it. The easiest way to bend it is to misalign it initially and/or load
it non-uniformly by:
- Application of an angular offset - C type bending
- Application of a concentricity offset - S type bending
Many testing standards specify alignment requirements in terms of % bending (e.g.Less than 5% of nominal strain or of strain amplitude).
How do you Determine What Your Alignment is?
Specimen bending is the key parameter in determining alignment. You can:
- Use a strain gauged alignment cell for verification of machine and load string alignment, or
- Use a strain gauged typical test piece
Then you need to carry out a series of tests followed by some
detailed calculations. ASTM has produced ASTM E1012, which outlines the
requirements and calculations. This standard is frequently quoted as an
acceptable method for checking and quantifying materials testing machine
Instron markets special Alignment fixturing, strain gauged specimens
and alignment software. This is very useful where customers are
regularly changing their testing configurations. However, where a
customer has a reasonably fixed testing configuration Instron Service
can also provide a one off Alignment verification service to provide
confidence and evidence of the testing set up.
The Instron Alignment Measuring Service
Instron can provide verification following the guidelines and
calculations detailed in ASTM E 1012, Nadcap AC7101<, 7122, 7109, NASM
1312, ISO 3800 etc. and issue a certificate. These certificates have
been used as objective evidence by many of our customers to external
assessment organizations, such as NADCAP, that their machines have been
recently checked for alignment.
Please contact your local Instron Service office for further information on how we can help you in this area.
Related Literature: IPS Nadcap Alignment and Support Services