Mechanical Property Test for Fasteners

[Abstract]:Elaborate on hardness, tensile, proof load and impact test methods to scientifically assess fastener mechanical performances.

Mechanical properties directly determine fastener load capacity and safety, covering strength, plasticity and toughness. To comply with GB/T, ISO, ASTM specifications, Shenzhen Yongjing Precision Technology Co., Ltd. implements standardized mechanical inspection procedures.

1. Hardness Test

As the most efficient conventional inspection method, hardness testing characterizes resistance to localized plastic indentation.

Rockwell Hardness: Preferred for finished fastener rapid inspection with HRC for high hardness and HRB for low hardness. Test proceeds on unthreaded end faces of bolt heads or shanks. It features easy operation, high throughput and minor workpiece surface damage, yet only reflects localized surface performance and fails to detect uneven metallurgical structure.
Vickers Hardness: Adopts diamond square pyramid indenter with full-range measuring accuracy. Ideal for case depth gradient inspection of carburized/decarburized layers and tiny zones such as thread flanks, despite more complicated operation.
Brinell Hardness: Generates large indentations to represent bulk average material properties with relaxed surface roughness requirements, widely used for raw stock and heat-treated blanks.

2. Tensile Test

Fundamental for mechanical performance evaluation to acquire tensile strength, yield strength, elongation after fracture and reduction of area.

Test Specimens

Full-size Fastener Tensile Test: Tests finished bolts and screws to verify strength at the minimum cross-section of threaded portions, reflecting comprehensive performance of raw material, thread forming and heat treatment.
Machined Coupon Tensile Test: Standard tensile bars are processed from batch-matched wire or furnace-coupon blanks to test intrinsic material properties excluding thread geometry interference.

Core Indicators

Tensile Strength: Maximum tensile stress before specimen rupture.
Yield Strength: Stress triggering evident plastic deformation; 0.2% offset yield strength applies for materials without distinct yield point.
Elongation & Reduction of Area: Plasticity metrics; higher values mean superior plastic deformation absorption and enhanced operational safety.

3. Proof Load & Wedge Load Test

Exclusive inspection protocols engineered for bolt qualification.

Proof Load Test: Apply specified non-yield ultimate tensile load for 15 seconds then unload and check permanent elongation. Products pass if elongation and thread stripping stay within standard limits, verifying resistance to long-term static load yielding and relaxation.
Wedge Load Test: Stretch bolts with angled wedge shim under bolt head until fracture. The test validates bulk strength plus toughness at stress-concentrated zones including head-shank fillet and thread root; valid rupture shall occur on shank or thread instead of head or head-shank junction.

4. Impact Test

Evaluates toughness against brittle fracture under dynamic shock load, essential for fasteners serving at low temperature or cyclic loading.

Charpy V-notch impact test dominates: Pendulum fractures notched standard samples to measure absorbed impact energy. Mandatory for Grade 12.9+ high-strength bolts and wind power/construction machinery fasteners requiring low-temperature toughness.

5. Tightening Torque Test

Dominant mechanical verification for tapping screws and wood screws via driving performance assessment.

Driving Torque Test: Measure torque needed to drive screws into benchmark panels to predefined insertion depth simulating practical assembly.
Breakaway Torque Test: Record torque value at head fracture during screwing to assess torsional resistance capacity.

Yongjing Precision owns full-spectrum inspection apparatus including universal material testers, Rockwell/Vickers/Brinell hardness testers, impact machines and torque gauges. All inspections follow statutory standards and customized client demands. Systematic mechanical testing ensures outgoing product quality and supplies reliable data for product design and process refinement.