Mechanical Properties Test of Bolts —— Wedge Negative Load Test

In mechanical connections, the bolts need to withstand a variety of complex stress environments, including axial tension, shear force, and bending stress. To ensure the reliability of bolts in actual operating conditions, the wedge negative load test (Wedge Load Test) is widely used to assess the bending and shear resistance of bolts, especially for high strength bolts. This paper will analyze the principle, test method and significance of wedge negative load test and its significance in engineering application.

What is the wedge-negative load test?

Wedge negative load test is a special tensile test, mainly used to detect the tensile strength and ductility of bolts, and to evaluate their failure under inclined installation or bending condition. Unlike ordinary tensile tests, the wedge negative load test adds an angular deviation during the test to simulate the force situation of the bolts under non-ideal installation conditions.

test objective:

To assess the tensile strength of the bolt under bent conditions.

Ensure the strength of the bolt head and the rod to prevent brittle fracture.

Test the toughness of the bolt material to reduce the risk of failure in practical application.

Test method for the wedge negative load test

1. Sample preparation

Select the bolt samples that meet the standard requirements (generally according to ISO 898-1, ASTM F606, etc.).

The test sample shall not have obvious defects, such as cracks, burrs, etc.

Determine the angle of the wedge (usually 5,10 or 15) based on the nominal diameter of the bolt.

2. Test the equipment

Universal material tester (Universal Testing Machine, UTM)

Special Wedge Jig (Wedge Plate)

Measurement equipment (such as vernier caliper, microscope)

3. Test steps

Place the sample

Place the bolt on the wedge fixture to ensure that the bolt head is in good contact with the wedge.

Adjust the fixture so that the bolt bar portion deviates from the tensile direction.

Apply pull

Use a tensile tester to stretch the bolts until the bolt breaks or reaches the specified load.

Record the maximum load value and observe the fracture situation.

Analyze the test results

The fracture occurs in the bolt rod part, and the fracture has a plastic deformation feature, indicating that the bolt has good ductility and toughness.

If the fracture occurs at the head of the bolt or at the junction with the rod, there may be material defects or heat treatment problems.

Compare the standard requirements to judge whether the bolts meet the design requirements.

Influactors of wedge negative load test

Bolt material

Low steel bolts (e. g. grade 4.8) are usually flexible and easy to pass the wedge negative load test.

High strength bolts (e. g., grade 10.9,12.9) may be more prone to brittle fracture at the head or rod junction due to the stiffer material.

Heat treatment process

Proper quenching + tempering can optimize the mechanical properties of the bolt and improve its bending resistance.

Excessive quenching may lead to brittlement of the material and reduce the test pass rate.

fabrication process

The cold heading process helps to improve the toughness of the bolt head and reduce the risk of brittle fracture.

Rolling thread has better surface quality than cutting thread, reducing stress concentration and improves bearing capacity.

Application value of the wedge negative load test

Improve product quality

Through wedge negative load test, bolts with poor head quality or insufficient material strength can be effectively screened to ensure high quality products.

Adapt to complex working conditions

The bolt installation surface of many equipment is not completely flat, such as rail transit, wind power generation equipment, aerospace structure, the bolt may withstand a certain bending moment. Therefore, the wedge negative load test can better simulate these real working conditions and improve the reliability of the bolts.

Comply with industry standards

In aviation, rail transit and other high demand industries, wedge negative load test has become a necessary means of high strength bolts.