Steel pipe flaw detection and reliability analysis

Discussing the reliability of MFT,ET,UT and EMAT several nondestruction detection methods in flaw detection, Based on the comparison of the above mentioned several methods of reliability is can be seen,EMAT has advantages and broad prospect that the other nondestruction detection methods have not.

1.General

Steel pipe is the most widely used varieties of steel. Its quality directly affects the economic benefits and the safety of personnel. All countries in the world have paid much attention to  pipe quality detection ,used a variety of nondestructive testing ( NDT ) methods to inspect steel pipe strictly.

2.The reliability of MFL（Magnetic Flux Leakage) method

A number of factors affect MFL detection and characterization accuracy. These factors can be grouped in five areas:

1)Magnetization: the relationships between the magnetization system, the pipe material, and the applied flux field.

2)Leakage: the relationships between the applied flux field, the anomaly, and the leakage field.

3)Measurement: the relationships between the leakage field, the sensor, and the measured signal.

4)Recording and display: the impact of recording resolution and playback methodologies.

5)Analysis: the process used to classify anomaly types and characterize metal-loss geometries from the measured signal.

The output or results of each area affects the input and results of the next area. In addition, all five of these areas have theoretical capabilities and limitations. In designing commercial MFL inspection systems, inspection tool designers try to reach these limits within economic constraints.

3.The reliability of ET(Eddy Current Testing) method

One of the major advantages of eddy current as an NDT tool is the variety of inspections and measurements that can be performed. In the proper circumstances, eddy currents can be used for:

1)Crack detection

2)Material thickness measurements

3)Coating thickness measurements

4)Conductivity measurements for:

5)Material identification

6)Heat damage detection

7)Case depth determination

8)Heat treatment monitoring

Some of the advantages of eddy current inspection include:

1)Sensitive to small cracks and other defects

2)Detects surface and near surface defects

3)Inspection gives immediate results

4)Equipment is very portable

5)Method can be used for much more than flaw detection

6)Minimum part preparation is required

7)Test probe does not need to contact the part

8)Inspects complex shapes and sizes of conductive materials

Some of the limitations of eddy current inspection include:

1)Only conductive materials can be inspected

2)Surface must be accessible to the probe

3)Skill and training required is more extensive than other techniques

4)Surface finish and and roughness may interfere

5)Reference standards needed for setup

6)Depth of penetration is limited

7)Flaws such as delaminations that lie parallel to the probe coil winding and probe scan direction are undetectable

4.The reliability of UT(Ultrasonic Testing) method

Electromagnetic acoustic transducer(EMAT) is a transducer for non-contact sound generation and reception using electromagnetic mechanisms. EMAT is an ultrasonic nondestructive testing (NDT)method where couplant is not needed since the sound is directly generated in the material underneath the transducer. Due to this couplant free feature, EMAT is particularly useful for the NDT applications of automated inspection, hot and cold environments. EMAT is an ideal transducer to generate Shear Horizontal (SH) bulk wave mode, Surface Wave, Lamb Wave and all sorts of other guided wave modes in metallic and/or ferromagnetic materials. As an emerging UT technique, EMAT can be used for both thickness measurement,flaw detection, and material property characterization. After decades of research and development, EMAT has found its applications in many industries such as primary metal manufacturing and processing, automotive, rail road, pipeline, boilers and pressure vessel industries.

But the disadvantage of EMAT can be summaries as follows:

1)Low transduction efficiency. EMAT transducers typically produce lower raw signal than piezoelectric transducers. As a result, more sophisticated signal processing techniques are needed to isolate signal from noise.

2)Limit to metallic or magnetic products. NDT of plastic and ceramic material is not suitable or at least not convenient using EMAT.

3)Size constraints. Although there are EMAT transducers as small as a penny, commonly used transducers are large in size. Low profile EMAT problems are still under research and development. Due to the size constraints, EMAT phased array is also difficult to be made from very small elements.

4)Caution must be taken when handling magnets around steel products.

5.Conclusion

As an Ultrasonic Testing (UT) method, EMAT has all the advantages of UT compared with other NDT methods. The same as piezoelectric UT, EMAT can also be arranged into pulse echo, pitch catch, and through transmission configurations. EMAT phased array probe is also constructed to improve its focusing and beam steering capability.

Eddy current inspection (ET) is an effective tool for a variety of applications in Non Destructive Testing.  Many different uses for eddy current equipment exist, not just for detecting surface and near-surface flaws, the ability to determine the thickness of a coating or used for sorting evaluations can be utilised by means of an eddy current testing technique.