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Alternating Current Field Measurement (ACFM) - [SNT-TC-1A course  as per Company Written Practice]

The Alternating Current Field Measurement (ACFM) technique is a non-contact electromagnetic technique capable of both detecting and sizing (length and depth) defects in metals. The basis of the technique is that an alternating current flows in a thin skin near the surface of any conductor. By introducing a remote uniform current into an area of the component under test, when there are no defects present, the electrical current will be undisturbed. If a crack is present the current flows around the ends and down the faces of the crack.
The current flowing in the surface has an associated magnetic field above the surface and this magnetic field will also be disturbed if the current is disturbed by a defect. The ACFM method involves the measurement of this magnetic field. The magnitude of any disturbances in the field can then be related back to the size of defect causing them by the use of mathematical models. In its simplest form, ACFM involves the use of a single hand-held probe which contains the field induction and the field measurement sensors. This is then connected to an ACFM instrument which is, in turn, controlled by a PC which provides data displays and recording.

As the technique requires no electrical contact with the surface, it can be used to inspect through paint and coatings. The technique is widely used for weld and thread inspection and for subsea inspection of offshore platforms. It can also be used on both magnetic and non-magnetic components
and at elevated temperatures in excess of 600°C.
Array probes containing large numbers of sensors can be deployed either manually or robotically, typically in situations where larger areas need to be inspected or where ‘pick and place’ deployment is preferable to probe scanning. ACFM array systems have been incorporated into automated inspection systems to give simple PASS/FAIL reporting, thus avoiding the need for skilled operators.

The ACFM method should only be applied to surface-breaking defects when used on carbon steels but is suitable for subsurface flaws in some non-magnetic materials

Alternating Current Field Measurement (ACFM) Level I

  • 1.0 Introduction to Electromagnetic Testing
    1.1 Brief history of testing
    1.2 Basic principles of NDT testing

    Electromagnetic Theory

    2.1 Eddy current theory
    2.1.1 Generation of eddy currents by means of an AC
    2.1.2 Effects of fields created by eddy currents
    2.1.3 Properties of eddy currents Travel in circular direction Eddy current distribution Effects of lift off and geometiy Relationship of magnetic field in relation
    to a current in a coil Effects of permeability variations in
    magnetic materials Effect of discontinuities Relationship between frequency and depth
    of penetration Standard depths of penetration

    2.2 Flux leakage theory
    2.2.1 Terminology and units
    2.2.2 Principles of magnetization B-H curve Magnetic properties
    2.2.23 Magnetic fields Magnetic permeability Factors affecting magnetic permeability

    2.3 Basic electrical theory
    2.3.1 Basic units of electrical measurement
    2.3.2 Direct current circuits
    2.3.3 Ohm’s law
    2.3.4 Faraday’s law
    2.3.5 Resistance
    2.3.6 Inductance
    2.3.7 Magnetic effect of electrical currents

    Technique Course
    1.0 Alternating Current Field Measurement Theory
    1.1 Production of uniform fields
    1.2 Current flow, Bx, Bz, and By relationships
    1.3 Relationship of the Bx, Bz, and butterfly plots
    1.4 Other sources that influence the signals

    2.0 Types of Probes
    2.1 Coil arrangements
    2.1.1 Primary induction coil
    2.1.2 Bx and By sensor coils
    2.2 Coil factors (lift off)
    2.3 Theory of operation
    2.4 Applications
    2.5 Limitations
    2.6 Probe markings

    3.0 Probe Software
    3.1 Probe software versions and compatibility
    3.2 Manufacturers’ sensitivity settings
    3.2.1 Gain
    3.2.2 Scalings
    3.2.3 Relationship between gain and current settings
    3.3 Sensitivity checks

    4.0 Factors Affecting the Choice of Probes
    4.1 Type of part to be inspected
    4.2 Type of discontinuity to be inspected
    4.3 Speed of testing required
    4.4 Probable location of discontinuity

    5.0 Types of Hardware and Operating Software Applications
    5.1 Choice of systems for specific applications
    5.2 Choice of software for specific applications
    5.2.1 Depth and length sizing capabilities
    5.2.2 Probe resolution
    5.2.3 Coating thickness

    6.0 Scanning for Detection
    6.1 Initial set up
    6.2 Setting position indicators
    63 Probe orientation
    6.4 Scanning speed
    6.5 Scanning pattern for tubulars and pipes
    6.6 Scanning pattern for linear sections
    6.7 Scanning for transverse cracks

    7.0 Signal Interpretation
    7.1 Review of display format
    7.2 Detection and examination procedure
    7.3 Crack signals - linear cracks, angled cracks, line contacts
    and multiple cracks, transverse cracks
    7.4 Other signal sources - lift-off, geometry, materials,
    magnetism, edges, and corners

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