Testing the High Tensile Strength of Steel

Compared to general structural steels, high tensile strength steel has increased strength making it ideal for the automotive industry where environmental problems and demands for collision safety are high.

In order to combat global warming, improvements in fuel economy are needed and auto body weight reduction is one way to achieve this. This has led to an increase in the adoption of high tensile strength steel as it provides the same strength as general steel materials but with thinner gauge products.

Tensile strength and elastic modulus are two evaluation items for metallic materials. The material testing standards ISO 6892-1 and JIS Z 2241 dictate how these mechanical properties are measured. This article describes a tensile test of high tensile strength steel using strain rate control in accordance with JIS Z 2241 Annex JB. In order to test the high tensile strength steel without it slipping in the grips, hydraulic flat grips with high gripping force were used.

Measurement System

A Shimadzu AGX-V Series precision universal testing machine was used for this test. A Shimadzu SSG- H Series strain gauge extensometer was also used to measure the elastic modulus as it allows for highly accurate measurement of the elongation in the gauge length of the specimen. Table 1 details the specification of the chosen test system and Figure 1 shows the condition of this test.

Table 1. Specification of Test System

Figure 1. Condition of Test

Measurement Conditions

The test is carried out at a strain rate of 0.00007 s^-1 or 0.00025 s^-1 in accordance with JIS Z 2241 Annex JB until the appearance of the upper yield point or the completion of the proof strength measurement. The test needs to be performed at a predicted strain rate of either 0.00025 s^-1 or 0.002 s^-1 until the end of discontinuous yielding in case the upper yield point appears.

One of the predicted strain rates of 0.00025 s^-1, 0.002 s^-1, or 0.0067 s^-1 is selected after the required yield strength or proof strength has been measured. In this test, the strain rate was set at 0.00007 s^-1 until the proof strength measurement was completed.  The predicted strain rate after the completion of proof strength measurement was set at 0.002 s^-1. The test conditions are detailed in Table 2.

The strain rate must be controlled within ±20 % according to JIS Z 2241 Annex JB.  The software TRAPEZIUM X-V has an added function compared to conventional software that can graph the test speed during the test, which made it easy to confirm that the test speed satisfied the test standard.

Table 2. Test Conditions

Measurement Results

The actual strain rate is shown in Figure 2 where the gray part is the ±20 % permissible error region. It was shown that testing can be performed with extremely high accuracy indicated by the strain rate error of ±10% or less.

Figure 2. Example of Strain Rate in Test

Figure 3 shows the stress-strain curve and Table 3 details the test results. The average values of tensile strength, proof strength, and the elastic modulus are 982 MPa, 718 MPa, and 199 GPa, respectively. The elastic modulus is the same as that of general steel. However, the distinctive feature of high tensile strength steel is displayed in the tested material.

Figure 3. Stress-Strain Curve

Table 3. Test Results

Conclusion

This article discussed a tensile test of high tensile strength steel using strain rate control, performed in accordance with JIS Z 2241 Annex JB. Testing of high tensile strength steel is difficult with manual grips due to their inadequate gripping force but satisfactory testing until rupture was possible here by using hydraulic flat grips, which have higher gripping force.

Furthermore, excellent strain rate control was demonstrated with accuracy greatly exceeding the accuracy of ±20 % allowed by the standard. These results demonstrated that the AGX-V precision universal testing machine, hydraulic flat grips, and extensometer facilitate the satisfactory tensile testing of high tensile strength steel using strain rate control.

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.