Fasteners are critical components in mechanical connections, and their quality directly impacts the safety and reliability of equipment. To ensure that fasteners meet applicable standards, they must be comprehensively evaluated through a systematic inspection process.
Appearance inspection is the primary step. Inspectors visually or through a magnifying glass inspect the fastener surface for defects such as cracks, scratches, rust, and burrs. They focus on checking that the threads are intact and undamaged, and that the transition between the head and shank is smooth, ensuring there are no obvious manufacturing defects.
Dimensional measurement utilizes precision measuring tools such as calipers, micrometers, and thread gauges to quantitatively verify key fastener parameters such as diameter, length, and thread size. Inspections must strictly comply with design drawings or industry standards (such as ISO, DIN, and GB) to ensure dimensional tolerances are within acceptable limits. For special-purpose fasteners, additional parameters such as head thickness and groove depth must also be measured.
Mechanical property testing is a core step. A tensile testing machine is used to test the fastener's tensile strength and yield strength to verify that its load-bearing capacity meets the material grade requirements. Hardness tests (such as Rockwell or Brinell hardness) are also performed to ensure that the material heat treatment process meets standards. For fasteners such as bolts and nuts, torque-preload tests or screw-on tests are also required to evaluate their assembly performance.
Non-destructive testing is suitable for demanding applications. Magnetic particle testing (MT) or penetrant testing (PT) are used to detect surface or near-surface cracks; ultrasonic testing (UT) or radiographic testing (RT) are used to detect internal defects, especially for high-strength or critical fasteners.
Surface treatment testing includes coating thickness measurement (such as galvanizing), salt spray testing (corrosion resistance), and analysis of environmental indicators (such as hexavalent chromium content) to ensure corrosion resistance and compliance.
Finally, all test data is compiled into a report and signed and confirmed by quality inspectors. Defective products are isolated and the cause is traced. Only qualified products can enter packaging or assembly. This standardized process effectively ensures the quality consistency and engineering safety of fasteners.
