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The Magnetic Leakage Field of Surface Crack under Geomagnetic Excitation LI LUMING, HUANG SONGLING, WANG LAIFU, LIU SHIFENG NDT CENTER, DEPARTMENT OF MECHANICAL ENGINEERING, TSINGHUA UNIVERSITY, BEIJING 100084 CHINA. Contact

Abstract:

Typically, magnetic flux leakage testing (MFL) is accomplished based on the outside artificial strong magnetic field generated by magnet or current. It is very interesting for the topic that it is possible to inspect the surface defects using the geomagnetic field that is existing and even anywhere, particularly the technology for weak magnetic field testing is becoming not very inconvenient recently.

The magnetic flux leakage field under geomagnetic field is calculated in this paper using finite element method aimed to study the possibility to measure the field and inspect the defects. 0.20mm deep and 0.10mm wide slot is used for the analysis. The analyzed result is discussed and the leakage field distribution is given. They all show a very constructive conclusion.

Keywords: magnetic flux leakage testing, geomagnetic field, finite element method

Introduction

Geomagnetic field is a great gift that the nature gives us. There are many applications of geomagnetic, such as geomagnetic compass. For the NDT area, a new progress of geomagnetic application is to evaluate the stress concentration within ferromagnetic materials by testing the difference of magnetic field above the specimen[1].

Magnetic flux leakage testing (MFL) is a very important method for ferromagnetic materials surface inspection, particularly in pipe industry. Traditionally, MFL is accomplished based on the outside artificial strong magnetic field generated by magnet or current. It is a very interesting and challenging topic that it is possible to inspect the surface defects using the geomagnetic field (geomagnetic MFL, G-MFL), a magnetic source which is even, existing everywhere and without any cost and any pollution additionally.

As known, the geomagnetic field is very weak comparing with the exciting one for MFL. (typical value for the similar magnetic particle inspection is 0.8T, however the geomagnetic field is only about 5E-5T). One topic would be raised that how weak if there is a leakage field under the geomagnetic field. The following one is that it is possible to measure the field and inspect the defect.

Undoubtedly, the crack at ferromagnetic surface will disturb the magnetic field distribution. To answer the questions above, the leakage field under geomagnetic field was discussed in this paper by finite element method.

FEM. analysis of G-MFL

There are two distinct advantages in using the FEM. to evaluate MFL. First, the random factors of natural cracks and manufacturing defects in the material could be minimized. Second, a satisfactory analysis can be made of any size crack no matter how small by applying magnetic theory and making adjustments as necessary.

0.20mm deep, 0.10mm wide crack was analyzed using FEM. The specimen used in this model is made of annealed 45# steel. The B-H curve used during the analysis was determined in the National Key Magnetic Materials Laboratory, China. Fig.1 is the normal component of leakage field distribution under geomagnetic field. It showed the same characteristics as the crack magnetized with strong magnetic field. The lift-off is 0.05mm. Fig.2 showed the normal component values, very similar testing along with surface inspected. However, it quickly deceased as the increase of lift-off.

Fig 1: leakage field distribution.

Fig 2: leakage field value at lift-off 0.05mm.

Fig.3 is the value at lift-off of 0.50mm, the peak-peak value was decrease to 8E-6T. And there was a slight change on the gradient of normal component when the lift-off is 2.00mm.

Fig 3: leakage field value at lift-off 0.50mm.

Fig 4: Leakage field value at lift-off 2.00mm.

Questions in G-MFL

Although the analysis results showed lots of characteristics same as normal MFL, there are many issues need to discuss carefully.

1. B-H curve for FEM. analysis


As known, B-H curve used for analysis will affect the result. Though the B-H cure used in the analysis above were one from laboratory, the value used also could be suspected: it is very difficult to obtain the exact data at so weak field. It is necessary to testing the material magnetic property at geomagnetic field.


2. Sensors for G-MFL application


Typically, the leakage field of normal MFL is about mT grade. However, the value changed to E-6 T grade. The leakage field under geomagnetic will challenge the common sensors for MFL, such as coil or Hall probe, particularly not only high sensitive but also having high space resolution.


3. The geomagnetic direction


The field which is perpendicular to the crack(parallel to inspected surface) was used for calculation. Geomagnetic has its own orientation. If the geomagnetic is upright to inspected surface, how about the result? In another words, the surface defect will also affect the field distribution at this situation, it is possible to be used for inspection.


4. Specimen geometry


The specimen used here is 150mm long and parallel to magnetic field. Therefore, the specimen will generate normal component gradient in geomagnetic. The gradient will affect the leakage field. This could be solved by applying differential processing using the difference gradient of defect and from specimen.

Potential research will be done next

It is a new idea to inspect surface defects applying geomagnetic field. Potential research will be done at our NDT center are as following:

1. FEM. analysis:
   The magnetic property will be carefully tested with emphasis on the value at weak magnetic field in order to understand the material properties under geomagnetic. The model will be established not only field orientation perpendicular to cracks but also parallel to cracks. The difference to normal MFL will be carefully concerned.

2. Sensor study:
   As the requirement of G-MFL, new kind of sensor, unlike typical one, will be considered. Great resistance type is one choice as its high sensitivity. One problem of resistance sensor is that the space resolution is not very nice. A lot of works need to do on this point.
   Coil with special cone is another potential sensor satisfies the requirement. Special signal processing need also to do.[2]

3. Stress concentration testing
   Stress concentration will cause the material property change. This change disturbs the geomagnetic field above material surface. It is interesting and challenge to testing the stress concentration, particularly during structure or material fatigue. Stress concentration testing will combine to G-MFL. Experimental instrument was established and typical stress concentration specimens are under going.

Acknowledgement

This work has been sponsored by the National Natural Science Foundation of China (Grant No.50001006).

Reference:

1. Dubov, A.A, Study of Metal Properties Using Magnetic Memory Technique, Metallovedenie i Termicheskaya Obrabotka Metallov , Sept 1997 p 35-39
2. Li Luming Zhang Jiajun, A New Kind Of Amorphous Alloy Magnetic Sensor Used For Magnetic Testing , TRENDS IN NDE SCIENCE AND TECHNOLOGY - PROCEEDINGS OF THE 14TH WORLD CONFERENCE ON NDT (14TH WCNDT), VOLS 1-5, 1996, pp 1593-1596

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