By Pass

18th World Conference on Non destructive Testing, 16-20 April 2012, Durban, South Africa

Magnetic Particle Inspection: Characterisation of the magnetic field for various magnetization techniques Ephraim MOTUKISI1
1

Southern African Institute of Welding; Johannesburg, Gauteng, South Africa motukisie@saiw.co.za; www.saiw.co.za

Abstract Magnetic particle inspection is one of the mostcommonly used non destructive testing methods for detecting surface and limited subsurface indications in ferromagnetic materials. The magnetic field provides the means by which indications are formed and detected. It is thus essential that the magnetic field be optimized, not only in magnitude, but also in direction when detection of specific indications is required. For example, too low a fieldstrength would not result in adequate particle build-up and would thus reduce the probability of detection for small indications. This paper discusses the magnitude, direction and special distribution of the magnetic field for various different magnetization techniques utilized. Special attention is given to the correct usage of various field detection instruments. Keywords: Magnetic Particle Testing(MPI/MPT), field strength, field direction, Nondestructive Test (NDT)

Measurement of magnetic field characteristics for MPI

1. Introduction
Magnetic Particle Inspection, a surface method used to inspect Ferromagnetic Materials, is one of the most commonly used methods in NDT. In order for the test method to be used effectively, it is essential that the strength of the magnetic field beappropriate for the task. If the strength of magnetic field is too low, the particle build-up identifying a defect may be inadequate and a defect may go undetected. If the magnetic field is excessive there can be a heavy accumulation of particles around superficial irregularities and real indications may be masked (furring), especially those indications that stem from in-service failure mechanisms,which result in tight cracks. This paper discusses some of the factors, which influence the quality of test results including:1. The importance of determining the direction of the induced magnetic field when affected by residual stresses. 2. The use of instruments correctly to measure magnetic field characteristics. 3. Using reference samples for optimizing test parameters.

2. Determining thedirection of induced magnetic field

2.1 Equipment commonly used to verify magnetic field in MPI 1. 2. 3. 4. Gauss/Tesla Meter (Field Strength Meter) Pie Gages Shims Gages Reference samples

2.2 Common errors 1. Incorrect placement of Field Strength Meter Probe 2. Using instruments incorrectly to verify magnetic field 3. Making assumptions that the magnetic field is in the desired direction2.3 Tesla meter
Fig. 2 Tesla meter Probes

Fig. 1 Tesla meter connected with a Tangential Probe

[1]

It has become a norm to place the instrument’s probe on the surface of the material to be inspected and record the readings. The direction of the probe and area of placement, are important things to consider when verifying magnetic field.

Fig. 3 Probe output versus Flux Angle

[2]

AB

C

The plane of the probe must be perpendicular to the surface of the part at the location of measurement to within 5°. See figure 3A. The direction and magnitude of the tangential field on the part surface can be determined by placing the Hall-Effect Tangential-Field Probe on the part surface in the area of interest. The direction of the field can be determined during the application ofthe magnetizing field by rotating tangential field probe while in contact with the part until the highest field reading is obtained on the instrument. The orientation of the probe, when the highest field is obtained, will indicate the field direction at that point. Various specifications call for the use of different Field Strength Meter values in particular applications. [3]

Fig. 4 A...