Non-Destructive Testing Methods

## Why NDT Matters Destructive tests—tensile, hardness, Charpy—consume the specimen. For production quality control and in-service inspection, NDT methods detect defects in the actual component that will go into service. NDT is required by most structural codes (ASME, AWS D1.1, ASTM) for welds, pressure vessels, aircraft structures, and other safety-critical components. The choice of NDT method depends on: the type of defect expected (surface cracks, volumetric porosity, internal delaminations), the material (ferromagnetic, non-ferromagnetic, conductive), component geometry, access limitations, and the required detection sensitivity. ## Liquid Penetrant Testing (PT) Liquid penetrant testing (PT, also called dye penetrant testing) detects surface-breaking defects in any non-porous material: 1. Clean the surface thoroughly (degreasing, abrasive cleaning) 2. Apply a low-viscosity penetrant (dye in liquid) and allow dwell time (10–60 minutes) for the liquid to enter defects by capillary action 3. Remove excess penetrant from the surface without washing the penetrant out of defects 4. Apply a developer (fine absorbent powder or suspension) that draws penetrant out of defects to form visible indications 5. Inspect under visible or UV light, depending on penetrant type (color contrast or fluorescent) PT is simple, inexpensive, and applicable to metals, ceramics, and plastics. It detects cracks, seams, laps, cold shuts, and porosity open to the surface. It cannot detect subsurface defects. Minimum detectable crack width is approximately 1 μm for high-sensitivity fluorescent penetrants. ## Magnetic Particle Testing (MT) MT detects surface and near-surface defects in ferromagnetic materials (carbon steel, alloy steel, cast iron) by magnetizing the component and applying iron oxide particles (dry or in suspension). Magnetic flux leaks at discontinuities, attracting particles to form visible indications. **Wet fluorescent MT** (WFMT) is the most sensitive variant, detecting cracks 1–2 mm deep subsurface and very fine surface cracks. It is the standard inspection method for aerospace forgings, aircraft landing gear, and high-strength steel components. MT is not applicable to austenitic stainless steels, aluminum, copper, or titanium (non-ferromagnetic materials). ## Ultrasonic Testing (UT) UT sends high-frequency sound waves (1–25 MHz for metal inspection) into the component and analyzes the echoes to locate and characterize internal discontinuities: **Pulse-echo UT**: a single transducer transmits a pulse and receives the echo. Defect depth is determined from the time of flight. Used for volumetric inspection of plate, forgings, and welds. **Phased array UT (PAUT)**: an array of elements steers and focuses the beam electronically to cover a wider area in a single scan, with better sensitivity and visualization than conventional UT. Increasingly replacing radiography for weld inspection because it leaves no radiation hazard and provides real-time imaging. **Time-of-flight diffraction (TOFD)**: uses diffracted signals from defect tips to accurately size defects in welds. Highly accurate for sizing but requires interpretation expertise. UT can inspect thick sections (up to several meters in large steel forgings) and detect internal porosity, inclusions, cracks, and laminations. Sensitivity for steel: detects flat-bottom holes equivalent to 1 mm diameter (ASTM E428 reference standard). ## Radiographic Testing (RT) RT uses X-rays or gamma rays to produce a shadow image of the component on film or digital detector. Dense material absorbs more radiation; voids and pores transmit more, appearing as dark areas on the radiograph. **Film radiography** (X-ray RT) provides a permanent record and is the reference method for weld inspection in pressure vessel codes (ASME Section V, Article 2). Minimum detectable porosity or pore diameter: approximately 1–2% of the wall thickness (sensitivity expressed as 2-2T or 1-1T per ASTM E1025). **Digital radiography (DR) and computed tomography (CT)**: DR replaces film with flat-panel detectors for real-time imaging. Industrial CT (Computed Tomography) reconstructs a full 3D volumetric image from hundreds of projections, allowing internal geometry and defect mapping without sectioning. CT is used for complex casting inspection, additive manufactured parts, and composite structures. Limitations of RT: radiation safety requirements (exclusion zones, permits), access needed on both sides of the component, limited sensitivity to tight planar cracks oriented parallel to the beam. ## Eddy Current Testing (ET) ET induces a circulating electrical current (eddy current) in the test object using an alternating magnetic field from a coil. Defects disrupt the eddy current pattern, changing the impedance of the probe coil. ET is applicable to any conductive material. Applications: - Surface crack detection in non-ferromagnetic alloys (aluminum, titanium, austenitic stainless) - Aircraft fuselage and wing skin inspection for fatigue cracks beneath fastener heads - Tube inspection in heat exchangers (probes pass through the bore of each tube at high speed) - Coating thickness measurement - Sorting alloys by conductivity (different alloys give different eddy current signatures) ET has limited depth penetration (skin depth effect), typically detecting defects within 5–10 mm of the surface in most metals. For steel, depth is limited to 1–2 mm due to low conductivity and ferromagnetic permeability. ## Selecting the Right NDT Method | Defect Type | Material | Preferred Method | |-------------|----------|----------------| | Surface cracks | Any non-porous | PT | | Surface/near-surface cracks | Ferromagnetic steel | MT (WFMT) | | Surface cracks | Aluminum, Ti, austenitic stainless | ET | | Internal volumetric defects | Any | UT or RT | | Internal planar cracks in welds | Any | PAUT or TOFD | | Complex 3D internal inspection | Any | CT | | Tube bore inspection | Conductive | ET |