Influence of Abnormity of Contact Extensometer on Measurement of Plastic Strain Ratio

The plastic strain ratio (r value) reflects the ability of the metal sheet to resist thinning and fracture during tensile loading, and the r value measured in different sampling directions can reflect the anisotropy of the material itself.

Under the action of uniaxial tensile force, the specimen is stretched to the stage of uniform plastic deformation.

When the specified engineering strain level is reached, the length and width changes of the specimen are tested and calculated, and the r value is calculated by using the formula derived from the principle of volume invariance before and after plastic deformation.

Common factors affecting r value test results include:

Different test methods and evaluation methods, specimen (bending, torsion, bow and other abnormalities) centering, specimen preparation (specimen surface and edge quality), laboratory conditions, longitudinal and transverse extensometer tests, non-uniform deformation of specimens, etc.

The longitudinal and transverse extensometers directly contact with the specimen, which is the most critical factor.

Researchers from the Quality Inspection Center of Wuhan Iron and Steel Co., Ltd. selected the cases of r value test result deviation caused by longitudinal and transverse extensometer jitter, poor clamping, sagging, inaccurate positioning and other abnormalities, and conducted in-depth analysis on them, and found some treatment measures for the reference of industry insiders.

1. Test materials and test conditions

The test materials are:

Cold rolled super deep drawing steel; GB-P6 sample (gauge length 80mm, width 20mm), JIS-5 sample (gauge length 50mm, width 25mm);

Z050 and Z100 full-automatic tensile testing machines (0.5 accuracy) and contact type longitudinal and transverse high-resolution digital extensometers (0.5 accuracy) are used;

The standard GB/T 228.1-2010 Metallic Materials – Tensile Testing – Part 1: Test Methods at Room Temperature shall be adopted for testing;

The equivalent strain rate of beam displacement control is 0.00025 s-1, and the strain rate after yielding is 0.0067s-1.

2. Test cases

2.1 Case 1 (lateral extensometer jitter)

On the automatic tensile testing machine, a standard P6 sample of DC06 grade is automatically tested.

The test parameter settings meet the requirements of the standard Metallic Materials Tensile Testing Part 1: Room Temperature Test Method (GB/T 228.1-2010).

The r value test result is 8.463, while the r value of this type of steel is generally 2.6~3.0, which is significantly higher.

Influence of Abnormity of Contact Extensometer on Measurement of Plastic Strain Ratio 1

Fig. 1 Positive deviation of y-axis of plastic deformation in width direction

R value stress relation curve (case 1)

The r-value stress relationship curve with positive deviation of y-axis of plastic deformation in the width direction is shown in Fig. 1, and the r-value stress relationship curve with obvious mutation of plastic deformation in the width direction is shown in Fig. 2.

Influence of Abnormity of Contact Extensometer on Measurement of Plastic Strain Ratio 2

Fig. 2 R-Stress Relationship Curve with Obvious Mutation of Plastic Deformation in Width Direction (Case 1)

Through analysis, it is found that:

When the plastic strain is – 0.6%~0.6%, the y-axis of plastic deformation in the width direction has a positive deviation;

When the plastic strain is – 10%~10%, when the stress reaches 158MPa, the plastic deformation in the width direction has obvious mutation.

Influence of Abnormity of Contact Extensometer on Measurement of Plastic Strain Ratio 3

Fig. 3 Elongation width relationship curve (Case 1)

It can be seen from the elongation width change curve (see Fig. 3) that there are obvious abrupt points in the curve, indicating that the lateral extensometer shakes.

Through investigation, it was found that the fixing screws of a group of contact columns of the transverse extensometer were slightly loosened, and abnormal shaking occurred when the contact column and the sample slid during the test, which led to the tracking error of the transverse extensometer.

2.2 Case 2 (poor clamping of extensometer)

On the automatic tensile testing machine, a group of WEDQ standard No. 5 samples are automatically tested.

The test parameters meet the requirements of the standard GB/T 228.1-2010. The r value test result is 0 or more than 100.

However, the r test value of this type of steel is generally 2.8~3.2, and the test result is obviously abnormal.

Influence of Abnormity of Contact Extensometer on Measurement of Plastic Strain Ratio 4

Fig. 4 Test Time Width Relationship Curve (Case 2)

The test time width relationship curve is shown in Fig. 4.

Through analysis, it is found that:

When the test time is 3s, the tracking curves of the longitudinal and transverse extensometers show abnormal jitter.

According to the investigation, this is because when the transverse extensometer initially clamps the sample, the clamping position is outside the clamping column.

During the tensile process, the transverse extensometer shakes and the longitudinal extensometer shakes.

2.3 Case 3 (lateral extensometer sagging)

On the automatic tensile testing machine, a WDQ standard sample No. 5 is automatically tested. The test parameter settings meet the requirements of the standard GB/T 228.1-2010, and the r value test result is 3.35.

However, the r value of this type of steel is generally 2.7~3.2, and the test result is high.

Influence of Abnormity of Contact Extensometer on Measurement of Plastic Strain Ratio 5

Fig. 5 Relationship Curve between r Value and Stress of Plastic Deformation in Width and Length Direction (Case 3)

The r value stress relationship curve of plastic deformation in width and length directions is shown in Figure 5.

It is found that the r value increases with the increase of strain.

Through investigation, it was found that the width extensometer was slightly sagging, and the gauge length of sample 5 was only 50mm.

During the stretching process, the width extensometer contacted the clamping arm of the longitudinal extensometer, which led to poor tracking of the longitudinal extensometer.

2.4 Case 4 (inaccurate positioning of longitudinal extensometer)

On the automatic tensile testing machine, a standard P6 sample of DC06 grade is automatically tested.

The test parameter settings meet the requirements, and the r value test result is 3.25, while the r value of this type of steel is generally 2.6~3.0, and the test result is abnormal.

Influence of Abnormity of Contact Extensometer on Measurement of Plastic Strain Ratio 6

Fig. 6 r value plastic strain and stress plastic strain relationship curves of plastic deformation in width and length directions (Case 4)

The r value plastic strain and stress plastic strain relationship curves of plastic deformation in width and length directions are shown in Fig. 6.

Through analysis, it is found that the r value has a large attenuation range.

Through investigation, it is found that the original gauge distance positioning of the longitudinal extensometer is obviously smaller than the set value.

3 Theoretical analysis of abnormal data

When testing the r value, use equation (1) for calculation.

Since the evaluation of r value is only for the plastic deformation stage, the influence of the elastic stage is not considered when the tester collects the thickness, width, gauge length and other data.

Use equations (2)~(3) for calculation.

Influence of Abnormity of Contact Extensometer on Measurement of Plastic Strain Ratio 7

Where:

  • B0 is the initial specimen width;
  • B1 is the average width of the specimen after strain and unloading;
  • △bplast is the transverse plastic strain of the specimen at the reference position;
  • L0 is the test length of the extensometer;
  • L1 is the gauge length of the specimen after the agreed strain and unloading;
  • △L is the plastic elongation of the specimen at the reference position;
  • △b is the test width of the extensometer;
  • V is Poisson’s ratio, generally 0.3 for steel materials;
  • E is the elastic modulus;
  • σ is the tensile strength of the specimen at the reference position.

The state of the extensometer directly affects the detection result of r value, such as the difference between the true deformation in the width direction of the sample and the value measured by the extensometer, the length of the extensometer, the clamping position and form (closer to the transition arc or in the middle of the gauge distance), the relative sliding between the tool edge of the extensometer and the surface of the sample, the clamping effect of the width testing device, etc.

Su Daxiong and others found that the test position of gauge length and width would affect the detection accuracy of r value.

In case 1, at the initial stage of the test, due to the poor clamping of the transverse extensometer, the extensometer signal shows some positive strains, and the r value decreases, while the Δb reduction is small, which has little impact on the r value;

When the engineering stress reaches 158MPa, the transverse extensometer shakes abnormally, causing the width Δb to be abnormally high, thus the r value is abnormally high.

In case 2, the abnormal jitter of the width extensometer caused an abnormally high Δb, resulting in an abnormally high r value.

In case 3, the transverse extensometer hinders the tracking deformation of the longitudinal extensometer, and the test of the longitudinal extensometer lags behind that of the width extensometer, which makes the Δb test value larger and the r value larger.

In Case 4, the test length L0 of the longitudinal extensometer is less than the set original gauge length, and the actual plastic elongation of the sample during the test is greater than the theoretical value, resulting in an increase in the sample width Δb and an abnormally high r value.

The above cases further illustrate that the accuracy of the r value test is related to the accuracy of the longitudinal and transverse extensometers test, and also put forward a very high requirement for the detection synchronization, that is, the essence of the r value test deviation is that the longitudinal and transverse extensometers test feedback is not synchronized.

4. Treatment measures

In view of the diversity and complexity of materials, the non-uniformity of specimen deformation after stress and the change of test conditions, various graphs can be fully used to investigate the causes when handling the abnormal r value test.

The operation of the extensometer can be confirmed by the deformation time curve and width time relationship curve of the longitudinal extensometer.

Check whether the corresponding relationship between the two-way extensometer during the test is normal by the width and length plastic deformation stress graph.

Track whether the r value test result is abnormal by the instantaneous r value and time correlation curve.

Analyze and troubleshoot item by item to find out the cause of the abnormality accurately.

During routine maintenance, pay attention to the routine maintenance of mechanical parts such as extensometer and test bench;

Generally, only the tracking process change value of the extensometer is confirmed during the test, but the original gauge length of the longitudinal extensometer will produce cumulative offset due to continuous positioning.

Note that if it is not possible to directly test and calibrate, the original gauge length can be indirectly calibrated by the distance of the impression left on the sample by the test blade of the extensometer chuck coated with inkpad.

There are many factors influencing the r value test of metal sheet and strip, and the test can also be done in the following ways:

Regularly test the standard samples and track the results by means of control charts;

Set the r value of different materials, and use the testing machine or management means for real-time monitoring.

5. Conclusions and Suggestions

In the case, the deviation of r value test results is caused by the asynchronous test feedback of longitudinal and transverse extensometers.

The shaking of the transverse extensometer, the deformation and wear of the contact angle column, the failure of the closing spring, the poor clamping, the small positioning of the longitudinal original gauge distance, and the clamping at the transition arc of the sample all cause the abnormal large Δb, which destroys its corresponding relationship with ΔL, leading to the larger r value, and vice versa.

The daily maintenance of extensometer, test bench and other mechanical parts is very important.

It is recommended that the equipment manufacturer upgrade the software function first, which should be able to display the test channels during the tensile test, especially the relationship curves between the deformation of longitudinal and transverse extensometers and time and load, so as to monitor the working state of extensometers;

Then upgrade the hardware functions to solve the problems such as the original gauge distance positioning offset of the longitudinal extensometer and the lack of direct verification means, so as to obtain reliable test results.

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