Magnetic particle inspection modified

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MAGNETIC PARTICLE INSPECTION (MPI)

MAGNETIC PARTICLE INSPECTION (MPI)

2 INTRODUCTION TO MPI

MPI is a non-destructive testing method used for the inspection of ferromagnetic materials.

Magnetic Particle inspection detects surface flaws and slightly subsurface discontinuities present in the material

It uses magnetic fields and magnetic particles for detecting flaws.

It is popular ,fast , accurate, easy to apply and low cost method.

MPI inspect a variety of products including forging , casting, wire, rod, sheet, bar stock, blooms, slabs, billets, gears, cylinder, discs and weldment parts.Industries that uses MPI are automotive , aerospace, petrochemical , and structural steel.

3 Magnetism is a force of

attraction or repulsion and this is due to electric currents and magnetic moments which gives rise to magnetic field

Ferromagnetic materials show a long-range of ordering  phenomenon at the atomic level which causes the unpaired electron spins to line up parallel with each other in a region called a domain.

4Example of ferromagnetic materials

Elements1. Iron2. Nickel3. Cobalt

Alloys1. Carbon Steel2. Cast Iron

5working principle

First step is to magnetize the component which is under inspection Secondly iron particles in dry or wet suspended form applied to

surface of magnetize part The magnetic field is spread out when it encounters with small air

gap that is created by the crack making magnetic flux leakage Iron particle are attracted and cluster at flux leakage field making

a visible crack indication

6PROCEDURE

The basic step involved are

Pre-cleaningIntroduction of the magnetic

field

Application of magnetic particles

Interpretation of magnetic particle

indication

7PRE-CLEANING

The component must be cleaned and dry before starting the inspection

Oil or grease presence disturb the process and prevent the attraction of magnetic particle to leakage field

Contaminants also interfere with interpretation of indications

8Magnetic field can be introduced by different ways:1. Flowing an electric current through specimen(Direct Magnetization)2. By using permanent magnet or electromagnet(Indirect Magnetization)3. Using a coil for production of magnetic field(Indirect Magnetization)

9Direction of magnetic field

Since flaws can occur in various directions ,two type of magnetic fields are there

Detection of crack is determined when magnetic lines of force are at right angle to the longest dimension of the defect

Longitudinal magnetic field has lines of forces that run parallel to longest axis of part longitudinal field is set up by using coil or solenoid or permanent magnet

Circular magnetic fields have lines of forces that run circumferentially along the perimeter of the part and attain by passing current through the part

Flux Leakage

No Flux Leakage

10Types of current used in MPI

Both types of current can be used in the process (i.e. A.C and D.C) Dc current is more desirable when detecting surface defects because

DC generate a magnetic field that penetrate deeper into the material Change in magnetic field produces eddy currents in the test object The eddy current produces a magnetic field that opposes the primary

field , reducing the net flux below the surface

11Portable equipment

Permanent magnets

Prods

Electromagnetic yoke

Portable coil and conductive cables

Portable power supplies

Stationary equipment

Electromagnetic yoke

Prods

Portable coil and conductive cables

Portable power supplies

Stationary equipment

12Application of magnetic particles

There are two methods for the application of magnetic media1. Dry method - it is a more portable2. Wet method - sensitive as liquid carrier particles give extra mobility Dry magnetic particles usually have number of colours so that a high

degree of contrast must be obtained between dry particles and part which is required to be inspected

Wet particle are provided in a wet suspension of water or oil Wet particles are supplied as fluorescent particles

13ADVANTAGES OF MPI: Can detect both surface and sub-surface flaws. Can inspect parts having intricate, irregular shapes easily. Pre cleaning of components is not much critical as it is for some other

inspection techniques. Most of the contaminants present in a flaw will not hinder flaw detectability.

Fast method of inspection and indications are quite visible on the specimen surface.

Equipments costs are relatively low. High sensitive (can detect small flaws).

14LIMITATIONS OF MPI:Cannot inspect non-ferrous materials such as aluminum,

magnesium, copper, lead, zinc, tin etc.

Large currents are needed for large parts.

Paints and other coverings adversely affects sensitivity of process.

Requires relatively smooth surface.

Post cleaning is often necessary.

Only ferromagnetic materials can be inspected.

15 Research Article

AUTOMATED SYSTEM FOR MAGNETIC PARTICLE INSPECTION OF RAILWAY WHEELS• It is an advanced technique used for

flaw detection. The main goal was to develop the highly reliable system that gives a warning of surface flaws to prevent possible future accidents. By this technique, cracks are visible by using high resolution digital camera.

• Desired wheel diameter is between 500-1300 mm. The minimal size of detected flaw was determined to 1 mm in length and 0.3 mm in thickness.

• Then the coil is used for the magnetization of the inspected wheel. The whole volume of railway wheel must have to be magnetized.

• So, in order to ensure complete magnetization, instead of one, two coils are used.

• For proper configuration of coils and location of railway wheel, the first measurements were performed.

16• For measurement, six measured points

was determined. , the minimal intensity was reached near to the center (locations 3 and 4) of railway wheel. On the other hand, the maximal intensity was reached in place 5 .

• Then, wheel is normally coated with liquid containing ferrous fluorescent particles. In the presence of these particles, flaws are very easily visible. During this step we had to find out at which phase the liquid needed to be spout on the inspected wheel. The best time we found was the time during magnetization.

• Defects and cracks were found using ultraviolet lights. For automated operation, digital camera is normally used. Scanning is done by digital camera.

• Adjoining figure shows whole assembly of the process.

17Defect Detection and Classification: Noise Reduction: Recorded picture shows high noise level and affects the detection process. In order to suppress noise, space pixel

averaging method is used. Each pixel records amplitude level. Places with high amplitude represents flaws and random amplitude represents noise, it means places with random amplitude used to be suppressed.

Defect Detection and Classification: Signal processing technique is used for flaw detection and separates them noise. Flaw shape can also be determined by this method. REFERENCE: Stanislav Starman and Vaclav Matz, STARMANS electronics s.r.o., V Zahradac 24,180 00 Prague 8, Czech Rebublic, [email protected].

Stanislav Starman and Vaclav Matz, STARMANS electronics s.r.o., V Zahradac 24,180 00 Prague 8, Czech Rublic, [email protected]

18 Conclusion

IT IS AN INNOVATIVE METHOD HAVING THE ADVANTAGES OF VERY HIGH-EFFICIENCY, HIGH-ACCURACY AND CONVENIENCE. THIS TECHNIQUE CAN FULFIL THE SPECIAL INSPECTION TASKS OF TESTING PIPELINES, RAILWAY WHEELS AND OTHER STRUCTURAL COMPONENTS ETC WHICH ARE NECESSARY AND RELIABLE SOURCES OF OPERATIONS IN DAILY LIFE.

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