Discussion on the influencing factors of bolt tension test
The tensile strength of the bolt is the maximum tensile force that the bolt can bear before breaking in the axial direction. It is the critical value of the transition from the uniform plastic deformation to the local concentrated plastic deformation. It is also the maximum bearing capacity under the static tensile condition. The yield strength of bolt refers to the phenomenon that when the stress reaches a certain value, it will not increase or fluctuate in a small range, but the strain will increase sharply.
Generally, the national standard for testing the mechanical properties of bolts is GB/t3098.1-2010 mechanical properties of fasteners bolts, screws and studs. The standard stipulates that the tensile test shall be carried out according to the requirements of GB/t228. The current effective standard is GB/t228.1-2010 metallic materials tensile test Part 1: room temperature test method. The tests discussed in this paper are conducted for FF test series (i.e. finished bolt) specified in GB / t3098.1-2010, excluding MP series (i.e. fastener material test).
Inspector’s quality is a very important factor, which permeates in every link. Including the selection and understanding of the standard, the operation of the test process, the calculation and processing of the inspection data after the test and so on. Any mistake in any of these links will affect the accuracy of the test results. Therefore, we must improve the quality of inspectors and reduce the human impact.
Impact of equipment
Regular verification of inspection equipment
The tensile testing machine must be verified regularly and can only be used after it is qualified. Generally, the verification period is one year. If the equipment moves, moves, repairs, transforms or changes the existing state of the equipment, the equipment shall be verified after the equipment is stabilized and before use. GB / t3098.1-2010 has made requirements for the equipment, that is, the tensile testing machine shall comply with the provisions of GB/t16825.1.
Applicability of equipment range
Generally, when the inspection data is about 2 / 3 of the full range of the tensile testing machine, the data accuracy is relatively the highest. Therefore, the appropriate equipment should be selected according to the tensile limit force value of the inspected bolt. If the range of the equipment is not properly selected, it is easy to cause certain deviation, which will affect the accuracy of the data.
Before the test, the equipment shall be preheated and run in no-load for several times to make the equipment in a better state to be inspected, so as to reduce the error.
GB/t3098.1-2010 has made requirements for clamps. Fixture of tensile testing machine: the hardness shall be no less than 45hrc. Matching thread clamp: the hardness shall be no less than 45hrc. Due to the different surface treatment of fasteners, the thread accuracy of internal thread fixture is different. The thread accuracy of internal thread fixture should reach 6h, 6az and 6AX. If the hardness of the fixture is too low, during the tensile test, the bolts cannot be effectively fixed, and the fixture is easy to be damaged; if the accuracy of the internal thread is low, the phenomenon that the bolts cannot be screwed into the fixture well may occur, affecting the test.
- (1) In the process of inspection, the samples after the basic size, thread accuracy and other items that do not damage the samples shall be tested.
- (2) The bolts without surface defects should be selected for the test. If the surface has quality defects, the results of tensile test will be seriously affected.
Effect of test method
Correctness of selection criteria
If the inspection standard can not be selected correctly, then the follow-up work is wrong. The inspection must be carried out according to the express standard of the product, which can not be taken for granted. To confirm the validity of the standard. Bolt products have their own particularity. Some bolts used in old equipment or for other reasons must be produced according to the expired and obsolete standards. At this time, the inspection shall be carried out in accordance with the express standards of the product, otherwise the judgment of the inspection results will be affected.
Cognition and understanding of standards
It is necessary to read the standard carefully and understand the requirements in the standard accurately. Otherwise, the test cannot be carried out in full accordance with the standard requirements, and the operation not in accordance with the standard procedures will affect the test results.
Standardization of test process operation
The first is to select the internal thread clamp with the same specification as the bolt to be inspected. Different types of bolts have different requirements when they are installed on the fixture. Too short screwing length will cause the specimen to loosen or break away from the fixture during the tensile process; too short non screwing length will affect the effective fracture position of the specimen and directly affect the test results. GB/t3098.1-2010 has specific regulations on how to mount the sample into the clamp, as follows:
- Bolt: the bolt to be inspected shall be screwed into the internal thread clamp, the effective screwing length shall be ≥ 1D, and the non screwing length shall be ≥ 1D;
- Stud: both ends shall be screwed into the internal thread fixture to ensure the effective Screwing Length ≥ 1D and the non Screwing Length ≥ 1D;
- For bolts with short thread bolting structure, the length not screwed in shall be less than 1D (D is the nominal diameter of the bolt).
The second is to clamp the internal thread fixture in the fixture of the tensile testing machine. It should be noted that the chuck heads of the upper and lower internal thread clamps should be vertically clamped in the fixture of the tensile testing machine and kept in the coaxial state. Otherwise, the eccentric force will be generated on the test piece, resulting in the axial deviation between the test force and the test piece and affecting the test results. The collet of the internal thread fixture is generally cylindrical, so the V-type fixture should be selected for the fixture of the tensile testing machine, and its size should also match the collet size of the internal thread fixture. At the same time, the fixture of the tensile machine shall not produce bending stress on the test piece, otherwise the test results will be affected.
The third is the stretching speed. The movement speed of the collet of the tensile testing machine shall be ≤ 25 mm / min until it is broken. For the bolts that can be tested for yield strength, too much speed will easily lead to a larger yield strength result, sometimes it will also lead to failure to test the yield strength, affecting the test results.
Accuracy of calculation of test results
The tensile strength and yield strength are calculated by the force value obtained from the test. The formula specified in GB / t3098.1-2010 is as follows:
Where: RM — tensile strength, MPa; FM — ultimate load, kn; as — Nominal area of bolt, mm2.
Where: rel – lower yield strength, MPa; FEL – ultimate load, kn; as – nominal area of bolt, mm2 (available in GB / t3098.1-2010).
The records of FM and Fel values shall be accurate, otherwise, the calculation results will be wrong; and the calculation results will also be wrong if the as check table is wrong.
GB / t3098.1-2010 and GB / t228.1-2010 both stipulate that the test is generally carried out in the laboratory at 10 ℃ ~ 35 ℃. If the ambient temperature is too low, the yield strength of BCC metal bolts will increase with the decrease of temperature, while that of BCC metal bolts will not change much; if the ambient temperature is too high, the yield strength of bolts will decrease with the increase of temperature. Therefore, the laboratory should keep the temperature appropriate to avoid the impact on the test results.
To sum up, in the process of bolt tension test, we must try to avoid the above factors, in order to reduce the error of test results and improve the accuracy.
Source: China Fasteners Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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