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 standards that reference tolerances for either bending stresses or alignment.
What is Alignment?
Specimen Alignment is affected by two factors:
- Concentricity: 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 (parallelism): 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 alignment.
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 etc. and issue a certificate. These certificates have been used as objective evidence by many of our customers to external assessment organisations, 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.