Mechanical Properties of Bolts Test —— Tensile Load Test of Bolts

Bolts play a vital role in the mechanical connection, and their tightening ability is directly related to the safety and reliability of the equipment. In order to ensure that the bolt does not fail prematurely when under axial tension, tensile load test (Tensile Load Test) is a key means to detect its mechanical properties. This paper will introduce the principle of bolt tensile load test and its application in the industry.

What is the tensile load test of the bolt?

The tensile load test of the bolt is an axial tensile test used to assess the tensile resistance and fracture characteristics of the bolt to ensure the requirements of GB / T 3098.1, ISO 898-1, ASTM F606, etc.

test objective:

• Deof the maximum load capacity of the bolt at tensile tension.

• Assess the plastic deformation ability and fracture mode of the bolts.

• Ensure that the bolts do not fail in practical application due to tensile load overload.

• 

Test method for the bolt tensile load test

1. Sample preparation

• Select the bolts that meet the standard requirements to ensure that there are no visible defects, such as cracks, burrs, etc.

• Select the suitable test equipment according to the nominal diameter of the bolt (M10, M12, M16, etc.).

2. Test the equipment

• Universal material tester (Universal Testing Machine, UTM)

• High-precision fixture (Tensile Testing Fixture)

• Computer data acquisition system

3. Test steps

(1) Fixed bolts

• Install the bolts in the fixture of the tensile tester to ensure that the force direction is consistent with the bolt axis.

• If the bolt is fully threaded, nut fixture.

(2) Apply a tensile force

• The test machine applies an axial tensile load to the bolts at a constant rate (e. g. 1~10 mm / min).

• Record the elongation of the bolt at different loads and the force value at the final break.

(3) Analyze the fracture situation

• If the fracture occurs at the bolt rod and the fracture is plastic extension, the material has good toughness.

• If the fracture occurs at the bolt head or the thread root, there may be manufacturing

• defects or material quality problems.

Factors affecting the results of the bolt tensile load test

1. Material quality

• The chemical composition of carbon steel and alloy steel can directly affect the tensile strength of the bolt.

• For example, grade 10.9 and 12.9 high-strength bolts usually use medium carbon alloy steel (42 CrMo, 35 CrMo) to improve their tensile properties

• 

• after heat treatment.

2. Production process

• Cold heading process helps to improve the strength of the bolt head and reduce the stress concentration.

• Compared with cutting thread, rolling thread can reduce stress concentration and improve tensile performance.

3. Heat treatment process

• Proper quenching + tempering can optimize the mechanical properties of the bolts and improve the tensile strength.

• Excessive quenching may cause brittle fracture, making the bolt difficult to pass the tensile load test.

Industrial application of bolt tensile load test

1. Aerospace

• The bolts in the aircraft structure must meet the strict tensile strength requirements to withstand the high load vibration environment.

2. Rail transit

• The high-speed rail connection bolts need to undergo a strict tension test to ensure that they do not fail under high-frequency vibration.

3. Wind power generation

• The connecting bolts of the fan blades and tower are subject to long-term tensile load to ensure structural stability.