Overview
ISO 3800:1993 - Threaded fasteners: Axial load fatigue testing - Test methods and evaluation of results specifies standardized methods for determining the axial fatigue behaviour of threaded fasteners. It defines test conditions (unless agreed otherwise, fluctuating tension at room temperature), minimum requirements to reduce test-condition influence, and procedures for calibration and centring control of test apparatus. The standard is intended to produce comparable S/N (Wöhler) curves and fatigue-strength data for quality control, product specification and design verification.
Key topics and technical requirements
- Test type and scope: Fluctuating axial tension tests with loading centric along the fastener axis; compliance of clamped parts is not taken into account. Failure is normally defined as complete separation of the fastener unless otherwise specified.
- Apparatus and accuracy:
- Testing machines must maintain loads within ±2% of required values and record cycle counts.
- Frequency range of testing: 4.2 Hz to 250 Hz; sinusoidal loading required.
- Machines must not automatically restart after power interruption.
- Fixtures and alignment:
- Detailed requirements for test fixtures, inserts and threaded adapters to ensure concentricity and perpendicularity.
- Alignment checks using a load verification stud with strain gauges; parallel part length of the stud = four times its diameter.
- Self-aligning devices are not recommended (if used, alignment must be verified).
- Specimen preparation:
- Test nuts generally used once; threaded adapters may be reused if undamaged.
- Test washers/chamfers may be used to clear head fillets; washer hardness should match fixtures.
- Specimens cleaned and lightly oiled prior to testing.
- Test procedures and statistical evaluation:
- Tests are performed in the finite-life range and the transition (infinite-life) range to determine S/N curves.
- Finite-life testing: minimum of six specimens per stress level; sample sizes often increased by ~10% to allow for contingencies.
- Transition-range evaluation typically requires ~15–20 specimens to estimate fatigue strength within about ±5%.
- Statistical methods include Gaussian probability plotting, staircase (stepwise) and boundary/arc-sine approaches to estimate median fatigue strength (N50) and confidence limits.
- Environmental and monitoring:
- Tests conducted at room temperature; specimen temperature rise should not exceed 50 °C (measured at first engaged thread).
- Atmospheric conditions (e.g., humidity) should be controlled/recorded where possible.
Applications and who uses it
ISO 3800 is used by:
- Fastener manufacturers for product qualification and production quality control
- Test laboratories performing axial fatigue characterization of bolts, screws and studs
- Automotive, aerospace, heavy machinery and civil engineers specifying fastener fatigue strength for design and safety evaluations
- Procurement and quality assurance teams to verify conformity to fatigue performance claims
Keywords: ISO 3800:1993, axial load fatigue testing, threaded fasteners, S/N curve, fatigue strength, test methods, testing machine calibration, test fixtures, statistical evaluation.
Related standards
Referenced normative standards include several ISO fastener and testing documents such as ISO 273, ISO 554, ISO 885, ISO 4032/4033, ISO 8673/8674 (as listed in ISO 3800).