Corrosion 6: Magnetic Particle Inspection (MPI) C6

Magnetic Particle Inspection (MPI)

• MPI is a method used for detecting invisible cracks and other defects in ferromagnetic materials like iron and steel.
• It is reliable for the rapid detection of defects located on or near the surface, outlining their location, approximate size, and shape.
• Consists of magnetizing the part and then applying ferromagnetic particles to the surface area to be inspected.
• Ferromagnetic particles, the indicating medium, can be held in suspension in a liquid, either flushed over or immersed in the part.
• The particles can also be in dry powder form, dusted over the surface.
• Particles are coloured or black, with fluorescent particles used in a process called Magnaglo Inspection.
• AC magnetism detects surface cracks.
• DC current detects subsurface cracks, with better penetrating properties.
• North and south magnetic poles form across a fault or crack, attracting ferromagnetic particles between the poles.
• Subsurface faults might not produce sufficiently sharp magnetic poles to attract particles.
• Magnetic lines of force should pass approximately perpendicular to the defect for detection.
• Inducing magnetic flux in more than one direction is necessary as defects may exist at any angle.
• Longitudinal or circular magnetization can reveal defects at 45 degrees to the magnetic field.
• Parts should be magnetized both longitudinally and circularly to ensure all flaws are identified.
• Circular current passing through a coil produces a Longitudinal Magnetic field as per the right-hand rule, with magnetic lines of force parallel to the coil's axis.
• Current passed directly through the component creates a magnetic field perpendicular to it, detecting defects running parallel to the electric current, also following the right-hand rule.
• Tubular parts can be inspected using the Circular Magnetism Central Conductor Method. They are slipped over a conductive rod and placed between the heads of a magnetizing machine, a common testing method for nuts, pipes, and similar small parts.

Demagnetizing

• After MPI, residual magnetism must be removed if the part is to be returned to service via a demagnetization operation
• A convenient procedure involves subjecting the part to a magnetizing force that continually reverses in direction and gradually decreases in strength.
• As the decreasing magnetizing force is applied in alternating directions, the magnetization of the part decreases.
• When using AC, the domains will alternate their polarity, and if the part is slowly removed from the field while the current is still flowing, the reversing action will progressively weaken and the domains will be left with random orientation, demagnetizing the part.
• Residual magnetism after MPI can be detected or verified as removed by using a Magnetic Field Strength Meter or a Gauss Meter.

Types of Discontinuities

• Cracks
• Laps
• Seams
• Cold shuts are folds formed during casting or forging that interrupt continuity.
• Inclusions are foreign material formed by impurities during metal processing.
• Splits
• Tears
• Pipes
• Voids
• These can affect the reliability of parts in service.
• Cracks, splits, bursts, tears, seams, voids, and pipes are formed by an actual parting or rupture of the solid metal.

Eddy Current Inspection

• Composed of free electrons under the influence of an induced electromagnetic field drift through metal.
• Used to detect: Surface cracks, pits, subsurface cracks, corrosion on inner surfaces, and to determine heat treat condition.
• Can be used on ferrous and non-ferrous metals to inspect castings, stampings, machine parts, forgings, extrusions, turbine shafts and vanes, wing skin, wheels, bolt holes, and spark plug bores for cracks.
• Can be performed without removing surface coatings like primer, paint, and anodized films.
• Can be effective detecting surface and subsurface corrosion, pits, and heat treat conditions.

How Eddy Current Works

• Alternating current (AC) through a coil develops a magnetic field. This induces a voltage of opposite polarity in the coil, opposing the original current's flow. If the coil is placed so the magnetic field passes through an electrically conducting specimen, eddy currents are induced.
• Flaws or thickness changes influence eddy current flow and change the impedance of the coil.
• Instruments display impedance changes via impedance plane plots or needle deflection.
• Calibration of eddy current test equipment requires a reference standard made from the same material as the tested item.

Eddy Current Instrumentation

• Eddy current inspection determines the ease at which a material accepts induced current (eddy currents).
• The AC is measured (impedance) to find characteristics of the material
• Conductivity, permeability, mass, and any voids or faults determine the ease at which a material accepts current.
• Conductivity varies with grain size, alloy type, degree of heat treatment, and tensile strength.
• Eddy current test instruments perform three basic functions: Generating, receiving, and displaying.
  • The - generating portion provides alternating current to the test coil.
  • The - receiving section processes the signal from the test coil into the required form and amplitude for display.
  • Instrument outputs or displays include visual, audible, storage, or transfer techniques utilizing meters, video displays, chart recorders, and alarms.

Ultrasonic Inspection

• An NDT technique using sound energy moving through a test specimen to detect flaws.
• Ultrasonic equipment can locate defects in all types of materials.
• Increasing application in aircraft bonded structure and repair uses sound energy moving through the test specimen to detect flaws.

General Information

• Minute cracks to small for x-ray may be detected and one side of the test piece needs to be accessible (pulse echo method). Angle or straight beam testing methods are other options

Reference Standards

• Used to calibrate the ultrasonic instrument.
• They serve two purposes, providing an ultrasonic response pattern that is related to the part being inspected and establishing the required inspection sensitivity.
• Must be made of the same material as the tested part and must have contain a knows flaw.

Two Ultrasonic Inspection Procedures

• Immersion Testing: the part under examination and the search unit are completely immersed in a liquid couplant, such as water.
• Contact Testing: Adapts readily to field use and the part under examination and the search unit are coupled with a viscous material, liquid, or a paste called a couplant.

Three Ultrasonic Inspection Methods

• Pulse Echo
• Through Transmission
• Resonance

Pulse Echo

• Flaws are detected by measuring the amplitude of reflected signals and the time required for these signals to travel between specific surfaces and the discontinuity.

Display Patterns

• "A" scan
• "B" scan
• "C" scan
• Each provides a different view of the specimen being tested.
• Pulse-echo instruments may be used with angle beam testing to detect flaws not directly underneath the probe. It differs from straight beam testing only in how the ultrasonic waves traverse the material.

Through Transmission

• Uses two transducers: one to generate the pulse and another on the opposite surface to receive it.
• A disruption in the sound path indicates a flaw on the instrument screen.
• It is less sensitive to small defects than the pulse-echo method.

Resonance

• Amplification of a periodically applied force is in harmonic proportion to a natural frequency of the system and the material.
• Used mainly for thickness measurements when the two sides of the material being tested are smooth and parallel, and the backside is inaccessible.
• The point where the frequency matches the resonance point of the material is the thickness determining factor.
• Additionally, it is used in areas of potential corrosion such as tubing, tanks, and aircraft wing skins, to identify corrosion inside a tube or on the backside of an aircraft skin.
• Resonance Frequency: The frequency at which a body vibrates freely after being set in motion by some outside force.

Radiographic Inspection

• X and gamma radiations are applied to inspect metal fabrications and nonmetallic products.
• Penetrating radiation is projected through the inspected part, producing an invisible or latent image on the film.
• When processed, the film turns into a radiograph or shadow picture of the object.

General Radiographic Inspection Facts

• This inspection medium and portable unit provides a fast and reliable means for checking the integrity of airframe structures and engines.
• Techniques locate defects or flaws in airframe structures or engines with little or no disassembly and This is done in contrast to other nondestructive testing that usually require removal, disassembly, and stripping of paint from the suspected part

Limitations of Radiographic Inspections

• The provisions of the appendix apply only to NDT using liquid penetrant, magnetic particle, ultrasonic and eddy current methods.
• X-ray inspections can also be performed on Bonded Structures

Caution: Radiation Hazards

• Radiation from x-ray units and radioisotope sources is destructive to living tissue.
• Adequate protection must be provided and personnel must keep outside the primary x-ray at all times.
• Signage posted on the entrance indicating an X-ray inspection is happening in the building means Do not enter.