Standard Fastener Testing

Standard fastener inspection mainly conducts scientific testing and analysis on the mechanical property testing, dimension and geometric precision testing, surface defect testing, metallographic structure testing, surface coating testing and torque-clamping force relationship testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Mechanical property testing: Mainly including the tensile strengths, yield strengths, elongations after fracture, reduction of area and hardness testing. These items are the core indicators of evaluating the load-bearing capabilities, plastic deformation capabilities and local pressure resistance of the fasteners, directly related to their safety and reliability during the service.

Dimension and geometric precision testing: Covering the thread precision (such as the pitch diameters, major diameters, minor diameters, pitches and thread profile angles), head dimensions (width across flats, width across corners and head heights), shank diameters, flange dimensions and overall lengths. The precise dimensions are the basis of ensuring the assemblability and interchangeability of the fasteners.

Surface defect testing: Checking whether the fastener surfaces have the defects such as the cracks, laps, indentations, rust and burrs, as well as the decarburization or overburning caused by the heat treatment. The surface defects are often the stress concentration points, which will significantly reduce the fatigue lives and cause the early failures.

Metallographic structure testing: Through the metallographic microscopes, the micro-structures of the material are observed, such as whether the grain sizes, banded structures, decarburized layer depths, non-metallic inclusion grades and the structures after the heat treatment (such as the tempered sorbite) meet the requirements. The structures determine the properties and are the key of the internal cause control.

Surface coating testing: For the fasteners with the surface treatments such as the electroplating, dacromet and hot-dip galvanizing, the coating thicknesses, adhesion, corrosion resistance (such as the neutral salt spray testing) and hydrogen embrittlement tendency need to be tested. The coating qualities affect the anti-corrosion capabilities and assembly torques.

Torque-clamping force relationship testing: Simulating the actual assembly working conditions, measuring the relationship between the applied tightening torques and the generated axial clamping forces. This testing is crucial for ensuring the preload consistency of the connection pairs and preventing the loosening or overload breakage.

Fatigue property and stress relaxation testing: Evaluating the durability limits (fatigue lives) of the fasteners under the alternating loads and the clamping force decay conditions under the long-term static loads (stress relaxation). These two items are the advanced testing items of evaluating their long-term service properties.

Testing Scope

Bolts, studs and screws: Covering the hexagon head bolts, socket head cap screws, studs, set screws and self-tapping screws. The testing emphases are the thread precision, head strengths, shank mechanical properties and overall dimension conformity.

Nuts: Including the hexagon nuts, flange nuts, lock nuts and cap nuts. The main testing items are the thread precision, proof loads, hardnesses, nut heights, widths across flats and the engagement properties with the matching bolts.

Washers and retaining rings: Such as the flat washers, spring washers, toothed lock washers and hole/shaft elastic retaining rings. Their hardnesses, elasticity (for the spring washers), surface flatness, inner and outer diameter dimensions and hydrogen embrittlement resistance (if electroplated) are tested.

Pins and rivets: Including the cylindrical pins, taper pins, cotter pins and solid/hollow rivets. The testing scope involves the diameter tolerances, hardnesses, shear strengths, surface roughnesses, as well as the shear and tensile properties after the assembly.

Assemblies and special fasteners: Such as the bolt-washer assemblies, weld studs and eye bolts. In addition to the testing of the individual part properties, the overall functions after the combination also need to be evaluated, such as the torque consistency of the pre-applied adhesives and the welding strengths.

Fasteners of the different materials and processes: Covering the carbon steel, alloy steel, stainless steel, non-ferrous metals (such as the copper, aluminum and titanium alloys), as well as the fasteners undergoing the different heat treatments such as the carburizing, quenching and tempering and hardening and tempering. The testing needs to be adjusted according to their material characteristics and process requirements.

Full dimensions and service environments: The testing scope needs to cover the full dimension spectrum from the micro electronic fasteners to the large high-strength bolts for the wind power and bridges, and consider their applicability testing in the special environments such as the high temperatures, low temperatures and corrosive media.

Testing Methods

Tensile testing: According to the standards such as GB/T 3098.1 and ISO 898-1, the universal material testing machines are used to carry out the axial tensile testing on the bolts, studs and screws until the fracture, to determine the key mechanical parameters such as their tensile strengths, yield strengths and elongations after fracture.

Hardness testing: The common methods include the Rockwell hardness (HRC, HRB), Vickers hardness (HV) and Brinell hardness (HBW). The appropriate methods are selected according to the part sizes and hardness ranges. The testing is usually carried out at the bolt head ends or shank cross-sections, to verify the heat treatment effects.

Proof load testing: Mainly used for the nuts. The nuts are screwed on the dedicated test mandrels, and the standard-specified proof loads are applied and held for a certain time. After the unloading, whether the nut threads have the stripping or fracture is checked, to verify their load-bearing capabilities.

Wedge load testing: For the bolts and studs. During the tensile testing, a wedge pad of the specified angle is applied below the bolt heads, simulating the eccentric load working conditions of the bolt heads, testing their tensile strengths under the eccentric forces. It is required that the fracture must occur in the shanks or thread parts, not at the head-shank junctions.

Thread go/no-go gauge testing: The metrologically calibrated thread go gauges (GO) and no-go gauges (NO GO) are used for the fast and large-batch functional inspection of the fastener threads. The go gauges should be able to be smoothly screwed through, and the screwing amount of the no-go gauges should not exceed two turns.

Metallographic analysis method: After the sampling, mounting, grinding, polishing and etching, the micro-structures are observed and photographed under the metallographic microscopes. According to the standards such as GB/T 6394 and GB/T 10561, the grain sizes, non-metallic inclusion grades and decarburized layer depths are evaluated.

Non-destructive testing: Including the magnetic particle flaw detection (suitable for the surface and near-surface defects of the ferromagnetic materials), penetrant flaw detection (suitable for the surface open defects of the non-ferromagnetic materials) and ultrasonic flaw detection (for the internal defect testing). These methods carry out the defect screening under the premise of not destroying the parts.

Testing Standards

Domestic Standards:

1. GB/T 3098.1, Mechanical properties of fasteners — Bolts, screws and studs

2. GB/T 3077, Alloy structure steels

3. GB/T 1220, Stainless steel bars

4. GB/T 6394, Determination of estimating the average grain size of metal

5. GB/T 10561, Determination of content of nonmetallic inclusions in steel — Standard rating chart micrographic method

6. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

7. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. ISO 898-1, Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs with specified property classes — Coarse thread and fine pitch thread

2. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Universal material testing machine: The core equipment, used for the mechanical property testing such as the tensile, compression, bending and shear. Equipped with the high-precision force transducers and displacement extensometers, it can realize the precise acquisition of the load-displacement curves, and the computer control systems automatically calculate the various performance indicators.

Hardness tester series: Including the Rockwell hardness testers, Vickers hardness testers, Brinell hardness testers and portable Leeb hardness testers. Need to be regularly calibrated with the standard hardness blocks, ensuring the accuracy and traceability of the measurement results, suitable for the laboratory and on-site fast testing.

Thread comprehensive measuring instrument: The high-precision instrument which can quickly measure the multiple parameters such as the pitch diameters, major diameters, minor diameters, pitches and thread profile angles of the threads, and automatically carry out the qualification judgment, with the efficiency far higher than the traditional gauges, especially suitable for the occasions with the high precision requirements or batch testing.

Metallographic microscope and image analysis system: Consisting of the optical microscopes, digital cameras and dedicated image analysis software. Used to observe and analyse the micro-structures of the material, and can automatically measure the grain sizes, phase ratios and decarburized layer depths, realizing the quantitative metallographic analysis.

Torque tester and axial force transducer: Used for the torque-clamping force relationship testing. The fasteners are installed on the dedicated fixtures, the input torques are controlled through the torque transducers, and at the same time, the generated clamping forces are measured with the axial force transducers, drawing and analysing the torque-clamping force-rotation angle curves.

Salt spray test chamber: Used to evaluate the corrosion resistance of the fastener surface coatings. Through the creation of the constant temperature, constant humidity, continuous or intermittent spraying sodium chloride solution environments, the accelerated corrosion conditions are simulated. According to the standards (such as GB/T 10125), the time of the white rust and red rust appearance is evaluated.

Profile projector and coordinate measuring machine: Used for the precision testing of the complex geometric dimensions and geometric tolerances. The profile projectors (two-dimensional) are suitable for the fast comparative measurement of the two-dimensional profiles; the coordinate measuring machines (CMM) can realize the high-precision scanning and evaluation of the three-dimensional spatial dimensions.

Spectroscopic analyser: Such as the direct-reading spectrometers or X-ray fluorescence spectrometers, used for the fast and accurate analysis of the chemical compositions of the fastener materials. Ensuring that the raw material compositions meet the requirements of the relevant standards (such as GB/T 3077 and GB/T 1220), controlling the material qualities from the sources.