Summary of Non-Destructive Inspection Methods
Non-Destructive Inspection Methods: Your Guide to NDI Techniques
Introduction
Nondestructive inspection (NDI) techniques allow technicians to evaluate components and structures without causing damage. These methods are essential in aviation, manufacturing, and maintenance to find defects early, ensure safety, and extend service life. This material summarizes common NDI methods (excluding liquid penetrant inspection, which is covered elsewhere), explains key principles, and shows how to apply them in real-world scenarios.
Definition: Nondestructive inspection (NDI) is the set of techniques used to detect, characterize, and size defects in materials and components without impairing their future usefulness.
Overview of Common NDI Methods
We focus on methods referenced in the source content: magnetic particle inspection, eddy current inspection, and radiographic (X-ray) inspection, plus general practices such as tool calibration and inspection program references.
Magnetic Particle Inspection (MPI)
- Purpose: Detect surface and near-surface discontinuities in ferromagnetic materials (iron, steel, some nickel alloys).
- How it works: The part is magnetized; ferromagnetic particles (dry or suspended in liquid) are applied. Particles collect at flux leakage fields created by defects, forming visible indications.
Definition: Magnetic particle inspection (MPI) is an NDI method that uses magnetic fields and ferromagnetic particles to reveal surface and near-surface discontinuities.
Steps (high-level):
- Clean the part thoroughly to remove grease, oil, dirt, and scale.
- Magnetize the part using an appropriate method (direct, indirect, yoke, or coil).
- Apply magnetic particles to the area being inspected.
- Observe particle patterns under proper lighting (visible or UV if fluorescent particles are used).
- Demagnetize the part completely after inspection if required.
- Clean the part and document findings.
Why magnetize both circularly and longitudinally:
- Circular magnetization detects discontinuities that run lengthwise relative to the part (axial cracks). Longitudinal magnetization detects discontinuities that run across the part (circumferential or transverse cracks). Using both methods ensures defects in any orientation are revealed.
Practical tips:
- Use the right particle type (visible or fluorescent) for contrast against the part.
- Ensure adequate field strength and direction; weak magnetization can miss defects.
- Demagnetize engine components to prevent attraction of wear particles during operation.
Eddy Current Inspection
- Purpose: Inspect surface and near-surface defects in conductive materials, especially nonferrous metals like aluminum. Also used to detect intergranular corrosion and measure coating thickness.
Definition: Eddy current inspection uses induced alternating currents in a conductive material to detect changes in electrical conductivity or magnetic permeability caused by defects or material changes.
How it works:
- A probe carrying an alternating current generates a changing magnetic field that induces eddy currents in the test material.
- Defects, corrosion, or material changes alter the eddy current flow, which the instrument detects as changes in impedance.
Applications:
- Detecting intergranular corrosion in nonferrous parts (aluminum castings and forgings).
- Inspecting fastener holes, surface cracks, and thin structures such as wing skins.
Practical tips:
- Surface preparation and lift-off control are important for repeatable results.
- Calibration standards with known defects help quantify indications.
Radiographic (X-ray) Inspection
- Purpose: Reveal internal structure and volumetric defects such as corrosion in inaccessible internal areas (e.g., wing internal structure).
Definition: Radiographic inspection uses X-rays or gamma rays to create an image of a pa
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Nondestructive Inspection Techniques
Klíčové pojmy: MPI works on ferromagnetic materials only, Use both circular and longitudinal magnetization in MPI to detect across and lengthwise cracks, Eddy current inspection is best for detecting intergranular corrosion in nonferrous metals, Radiography (X-ray) is suitable for internal/volumetric defects such as internal wing corrosion, Thorough cleaning is required before MPI and eddy current inspections, Demagnetize engine parts after MPI to avoid attraction of wear particles, Torque wrenches should be calibrated every 6 to 12 months depending on usage, Approved aircraft inspection programs are in the manufacturer maintenance manual and ICA, Micrometer types: outside, inside, depth, thread micrometers, Combine NDI methods when necessary for more reliable detection