The elongation and area reduction of metallic materials can provide specific information about the characteristics of the materials.
Elongation is a broad concept covering various parameters such as maximum force total elongation (uniform elongation), post-fracture elongation, yield point elongation, maximum force plastic elongation, total fracture elongation, etc.
For example, elongation after fracture considers the uniform plastic deformation capacity and local deformation (necking) capacity of metallic materials, while maximum force total elongation (uniform elongation) more accurately reflects the uniform plastic deformation capacity of metallic materials with strain hardening characteristics than elongation after fracture.
Thus, elongation is widely used as a crucial index to measure the mechanical properties of metallic materials.
This article describes the main aspects of using common strain gauges in testing tensile properties. It highlights the differences and effects of different instruments during the testing process and examines methods for improving the accuracy of metal elongation testing while adhering to the standard's accuracy requirements for test results.
1. Test stretching accuracy
Elongation is a conventional test item for metallic materials in third-party testing agencies. Tensile testing for metallic materials is conducted using a microcomputer-controlled universal testing machine. The elongation of materials is determined by measuring the permanent deformation of the material at fracture.
However, there are several methods for measuring strain.
To ensure the accuracy of the testing process and obtain correct test results, attention should be paid to the following aspects.
1. Select the appropriate test method according to the type of material tested.
The materials to be tested can be broadly divided into metals and non-metals, and the conditions produced by these materials during the tensile process are different.
Although some non-metallic materials exhibit good toughness and produce minimal vibrations during fracture, thus allowing the use of a clamping extensometer to ensure test accuracy, some metallic materials produce significant vibrations after fracture. This vibration can result in uncontrollable displacement of the attached extensometer, leading to inaccuracies in test results.
To measure the elongation after fracture of metallic materials, the fractured samples are joined together and a caliper is used to measure the final deformation. However, due to the irregular deformation process of metal fracture, seam gaps may occur, resulting in an approximate value close to the actual deformation.
Ignoring the influence of irregular deformation and splicing gaps can significantly affect test accuracy. Furthermore, the surveyor may not be able to ensure accurate positioning of the caliper and reference parameter due to the limitation of human eye resolution, and the location of the fracture will have a significant impact on the accuracy of the test.
Therefore, the elongation accuracy after fracture testing is typically much lower than that of the caliper. Laboratories often use a caliper accurate to 0.02 mm to mitigate measurement errors.
Other parameters such as yield point elongation, maximum force, total elongation, etc. can be tested with a clamping extensometer.
2. Select the appropriate extensometer.
In testing the mechanical properties of metallic materials, strain gauges are often used to measure the deformation of the material.
Currently, there are three common types of strain gauges: mechanical, optical and electromagnetic. Of these, mechanical extensometers are the most prevalent and can be categorized into large strain, metallic and fully automatic extensometers.
To measure strain, these strain gauges must be attached to the surface of the sample. Metal strain gauges require an elastic band for installation and fixation, and the following three points must be remembered:
Firstly, the elastic must have moderate elasticity and be stretched to approximately 80% of its maximum length or approximately four times its original length before use.
Second, the winding must be straight and the force resulting from the winding must be perpendicular to the sample and in a straight line with the knife edge. Otherwise, there will be a component force pulling on the extensometer arm and causing unexpected rotation, which will directly impact the direction of the test curve.
Thirdly, the measured distance must be accurate. During installation, ensure that both strain gauge arms are open to the positioning rod without pressure, leaving a gap of about 1mm.
Generally, large strain strain gauges are specifically designed for non-metallic materials with high elasticity, such as rubber. These strain gauges can measure deformations of up to 80 cm with an accuracy of 0.008 mm.
The maximum deformation that can be measured with metal strain gauges is generally 25 mm, with a test accuracy of 1.5 μm.
These two types of strain gauges are the most used today, thanks to their high cost-benefit ratio and wide range of applications.
However, they also have certain limitations:
- The range and reference length of the metal extensometer are poorly adjustable, limiting its use to a small range or specific reference length. The range of the extensometer is relatively fixed.
- The sample fracture needs to be removed before measuring its elongation, which cannot be measured directly.
- If the extensometer is not securely connected to the tested specimen, it may slip.
- The extensometer may face difficulty in adapting to special testing environments, such as measuring the sample deformation in a measuring environment box.
- Although the large strain extensometer has a large measuring range, its measuring accuracy is relatively low. During testing, slight slippage may occur with changes in sample thickness.
3. The problems of using mechanical extensometers listed above can be avoided by using grid extensometers.
The grid extensometer has fast, high-resolution accuracy, capable of reaching 1 μm. It is currently considered the best tool for measuring stretching.
However, the relatively high price of such strain gauges means that they are not typically included as standard equipment in general measuring devices.
2. Stretch test results
The extensometer is a fundamental device that measures axial and radial deformation between reference lengths of specimens.
Typical parameters measured with an extensometer include elongation, Poisson's ratio, tensile hardening index, plastic strain index, among others.
Using appropriate tools and methods during the measurement process can minimize measurement errors. It is important to not only prioritize the accuracy of testing instruments, but also follow standardized testing methods during the actual testing process.
Fig. 1 Appearance of two types of common strain gauges
Currently, two types of strain gauges are commonly used in laboratories: large strain strain gauges and metallic strain gauges, as illustrated in Figure 1.
Metal extensometers have a reference length (original reference length) of 25, 50 and 100 mm and can withstand a deformation of 10 mm.
When measuring elongation after a metal fracture, it is not advisable to use an extensometer as the fracture produces a lot of vibration, which can affect the accuracy of the extensometer. Extensometers should only be used to measure elongation before fracture, such as yield point elongation.
The large deformation extensometer has a maximum deformation of 80 cm, and the original length of the gauge can be adjusted as needed. This type is ideal for measuring the elongation of plastics, rubber and tapes. However, thin samples such as plastic wrap are unsuitable due to the dead weight of the extensometer, which can damage the sample when clamped.
Manual measurement with calipers has an accuracy of 0.02 mm, while the accuracy of extensometer measurement is 0.0015 mm, which is more than ten times higher.
However, in GB/T 228.1-2010, the test results require rounding the yield point elongation to 0.1% and other elongations, including post-fracture elongation, to 0.5%. The final results of both methods are almost identical after rounding.
Therefore, it is evident that when the accuracy of the equipment meets the necessary requirements, the test accuracy will be affected by the accuracy of the strain gauge.
Conclusion
Currently, there are an increasing number of strain gauges available on the market and their accuracy is improving.
However, each strain gauge has its own limitations and uses, which are quite obvious.
Therefore, it is essential to understand the measuring characteristics of the extensometer used in testing, reasonably utilize its advantages, avoid its limitations, and choose suitable measuring instruments based on different test items.
The extensometer has a significant impact on the metal elongation measurement results. However, it is not the high-precision extensometer that can guarantee accurate test results.
It is necessary to choose appropriate measuring instruments based on different test materials to obtain accurate test results.
