If a specimen bends during a tensile test, strain peaks occur, which occur under increasing load near the fixing points. The result is an apparent premature failure with a break point near the specimen grips. Break points like these are indicators of misalignment of the testing machine.
Since these strain and stress peaks are not directly visible in the testing machine, the material is characterized to indicate that there was homogeneous strain distribution across the entire cross-section. This leads to a misinterpretation of the measured force and strain values, which has an impact on the measured tensile modulus and strength values.
The testing machine must first be equipped with suitable specimen grips. They must be able to grip the specimen axially in a reproducible way each time the test is run and may not cause any significant rotating or tiling movement when under load.
The specimen grips' suspension assembly may not have a tiltable universal joint or freely moveable supports, since these are not held in a defined initial position and they transfer impermissible movements in the specimen during the test through the alignment.
Parallel gripping specimens are suitable for use. They should be mounted on a fixed but adjustable (in all directions) tensile axis. This makes it possible to align the entire tensile axis prior to the test so it remains under constant load in this alignment.
The theoretical foundation for measuring alignment is defined in ASTM E1012. The degree of misalignment is one factor that depends on the specimen geometry. Therefore it is also important that alignment is always determined in relation to the specimen shape to be tested.
To simplify the task, a reference specimen is defined in the Nadcap Audit Criteria AC 7122 for composites. Its dimensions are very similar to typical tensile specimens. This type of specimen is usually used for alignment measurements.
This precisely straight metal alignment bar has a total of 12 strain gauges distributed across three planes. The strain gauges are instrumented and connected to a measurement amplifier. A special test sequence in testXpert II reads the measured values and displays in real time the test axis' bending and offset values. The software setting prompts help the user align the test axis with precision and create an alignment behavior measurement report.