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Alignment Measurement—Verifying Test Axis Alignment

Rising quality requirements have caused the traceability of the geometrical alignment of materials testing machines to increase in importance. Precise alignment prevents the risk of false measurements.

ZwickRoell offers alignment as well as alignment measurement, including reporting, for your materials testing machine.

What is the alignment about?

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.

How can misalignment be avoided?

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. 

Verifying Test Axis Alignment as per Nadcap

Nadcap stands for "National Aerospace and Defense Contractors Accreditation Program" and was originally a global accreditation program for suppliers to the aerospace and defense industries. Nadcap's goal is to generate economical approaches to special processes and products and promote continual improvement within accredited companies.

In contrast to certification of quality management systems as per ISO 9001 and EN 9100, Nadcap accreditation includes both detailed examination of special technical processes and an audit with regard to the observance of defined specifications and guidelines. Aerospace industry manufacturers and suppliers in particular define extremely demanding requirements for the technical and staff qualifications of testing laboratories. These requirements are specified in standards, directives, and audit criteria, and are checked through Nadcap audits allowing maximum process stability to be attained and standardization of quality guaranteed.

The standardized option for alignment measurement is described in ASTM E1012 and includes a recorded inspection of test axis alignment.

Alignment measurement is a component of Nadcap accreditation and is based on ASTM E1012, which stipulates compliance with defined tolerances. This is reflected in Nadcap-defined audit criteria AC 7101 and AC 7122, which define the conditions for the measuring procedure, that is the verification of test axis alignment. In addition to aerospace manufacturers and service providers, an increasing number of companies in other sectors are affected by alignment measurement as part of Nadcap accreditation requirements, for example in the mechanical and plant engineering industry and in the metals industry.

The standardized option for alignment measurement is described in ASTM E1012 and includes a recorded inspection of test axis alignment. The electronic measurement of bending influences that can arise as a result of the smallest angular errors or offset in the test axis is required.

Erroneous Alignments of Test Axes Cause Non-Uniform Force Application to the Specimen

These so-called alignment errors lead to an increased risk of breakage near the clamping area. Alignment errors can be especially common with brittle materials. Stiff and brittle materials break with seeming low tensile stress and strain in the area of the largest bending. This is where ZwickRoell alignment services come into play: a highly developed procedure enables visualization of the specific bending strain with the help of strain gauges and reporting to initiate required measures for alignment.

Angular alignment errors

Representation of non-uniform force application on the specimen through angle offset.

Concentric alignment errors

Representation of non-uniform force application on the specimen through axial offset.

Our standardized strain-gauged alignment transducer works with a large percentage of the materials testing machines to be tested.

  • Based on ASTM E1012, strain-gauged alignment transducers, which preferably conform to the geometry of the specimen to be tested at a later time, are required. An experienced ZwickRoell service technician positions the strain-gauged alignment transducer in the test axis of your testing system and it is then loaded in the elastic range.
  • In the event of non-uniform force application to the specimen, the applied strain gauges will register different strains. These are digitalized through a connected electrical measurement amplifier and transferred to a special ZwickRoell testXpert software test program. Material properties are determined and logged in testXpert according to standard specifications and specimen geometries. A simple, structured evaluation in testXpert enables a ZwickRoell service technician to identify errors in the test axis on the basis of characteristic measurement results. If necessary, measures can be taken using components specifically developed for alignments to resolve alignment errors. In addition, testXpert provides adjustment recommendations for the alignment unit option, allowing detected alignment errors to be corrected easily and conveniently.
  • Our standardized strain-gauged alignment transducer works with a large percentage of the materials testing machines to be tested. Furthermore, defined geometry data can be used with strain-gauged alignment transducer according to your specific requirements.

Benefits of Choosing ZwickRoell

  • By using standardized strain-gauged alignment transducers, ZwickRoell guarantees a high degree of comparability with customer-specific specimen dimensions (adaptation)
  • Individual geometry data can be converted used with the strain-gauged alignment transducer
  • Alignment measurement as part of inspection and calibration is possible, which means no additional travel costs are incurred
  • Alignment measurement is quick thanks to ZwickRoell's standard-compliant strain-gauged alignment transducers
  • Convenient testXpert software with adjustment diagrams directly display the individual components of the alignment error and adjustment instructions for the operator
  • Continuous graphic and numerical display of test results of bending and percentage bending
  • Predefined test sequences consist of zero-measurement, measurement clamped without axial strain, measurement under load, zero-measurement repetition, measurements at a 0°/90°/180°/270° transducer mounting position, and top-down measurements
  • Precise measurement results via measurement of inverted positioning
  • Complete, detailed recording of measurement results as required by the standards and beyond (incorporation of images possible)
  • Factory test report stating mechanical and electrical characteristics of the ZwickRoell alignment transducer
  • Complete integration of the HBM measurement amplifier in the testXpert testing software
  • Everything from a single source—from consultation to completion
  • Correction of alignment errors through adjustment with the optionally available alignment fixture
  • Alignment measurement is also possible on machines from other manufacturers
  • Qualified ZwickRoell experts guarantee professional and individualized support for questions and individual requirements.
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  • Product information: Alignment measurement – verifying test axis alignment PDF 638 KB

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