Overview
ISO 23788:2012 - "Metallic materials - Verification of the alignment of fatigue testing machines" specifies a strain-gauged method to verify the geometric alignment of fatigue test machines. The standard applies to dynamic uniaxial tension/compression, pure torsion and combined tension/compression plus torsion fatigue testing machines for metallic materials. The methodology is generic and can also be applied to static testing machines and to non‑metallic materials testing.
Key topics and technical requirements
- Purpose: Measure and quantify machine misalignment that produces unwanted bending strains in specimens or an alignment measuring device (alignment cell).
- Measurement principle: Use a carefully machined, strain‑gauged alignment cell (measuring device) to detect bending strain superimposed on axial or torsional loading. Rotate the device 180° to separate machine misalignment from inherent device imperfections.
- Applicable test types: Dynamic uniaxial tension/compression, pure torsion, combined tension plus torsion; extendable to static tests and non‑metallic materials.
- Instrumentation & metrology: Requires a force‑measuring system compliant with ISO 7500‑1. Strain gauge placement, measurement planes and orientations (0°, 90°, 180°, 270°) are defined to capture average axial strain and bending strain vectors.
- Design of alignment cell: Should match overall specimen length, fit grips without special adaptors, and for cylindrical cells use recommended diameters (commonly 5 mm, 7.5 mm or 10 mm; diameter d ≤ 10 mm or specimen diameter, whichever is greater). Material selection guidance favors stable, high‑strength metals (e.g., fully tempered steels; high‑strength aluminium alloys as alternatives).
- Calculations & classification: Procedures for computing maximum bending strain, percentage bending (bending strain relative to average axial strain), measurement uncertainty and classification of machine alignment are specified. Normative and informative annexes cover uncertainty evaluation, lateral stiffness measurement, gauge configurations and worked examples.
- Reporting: Essential and special reporting items are defined so results are reproducible and auditable.
Practical applications and users
Who uses ISO 23788:
- Test laboratories performing fatigue testing and certification
- Manufacturers and service providers of fatigue testing machines
- Quality and metrology engineers ensuring valid fatigue data
- Research institutions studying fatigue behavior
Practical benefits:
- Detect and quantify misalignment that would bias fatigue life results
- Improve test repeatability and comparability across machines and labs
- Provide a documented alignment verification procedure for quality systems and accreditation
- Inform corrective actions (mechanical adjustments, fixture redesign or machine maintenance)
Related standards
- ISO 7500‑1 - verification/calibration requirements for force‑measuring systems used in tension/compression testing (normative reference in ISO 23788).
Keywords: ISO 23788, alignment of fatigue testing machines, alignment cell, strain gauges, fatigue testing, machine alignment verification, metallic materials, percentage bending, test machine calibration.
