ndt-methods

name	ndt-methods
description	Non-Destructive Testing — UT (A/B/C-scan, PAUT, TOFD), RT, MT, PT, ET, VT, AE, thermography. Flaw detection limits, code requirements (AWS D1.1, ASME V, API 1104).
metadata	{"priority":7,"promptSignals":{"phrases":["NDT","non-destructive testing","ultrasonic testing","radiography","magnetic particle","liquid penetrant","phased array","TOFD","eddy current"],"minScore":3}}

Non-Destructive Testing (NDT) — Complete Skill

Ultrasonic Testing (UT)

Principle

Piezoelectric transducer generates ultrasonic pulse → travels through material → reflects from flaws or back wall → time-of-flight → flaw location

Sound velocity: c_steel_long = 5,900 m/s (longitudinal), c_steel_shear = 3,230 m/s (shear) c = √(E/ρ) for longitudinal; c_s = √(G/ρ) for shear

A-scan

Single waveform: amplitude vs. time (echo location) Displays: initial pulse, back-wall echo, any flaw echoes between them Flaw depth: d = t × c/2 (pulse-echo, two-way travel)

B-scan

2D cross-section image: position along scan line vs. depth Shows: flaw depth profile, embedded flaws

C-scan

Plan view: scanned area with color-coded amplitude or depth Shows: flaw extent, distribution (used in aircraft/composites inspection)

Phased Array UT (PAUT)

Multiple elements fire with time delays → electronic beam steering + focusing Advantages: faster scanning, multiple angles without moving probe, better coverage S-scan (sector scan): sweeps beam angles (e.g., 30°-70°) in one pass Quantitative flaw sizing: use DAC (Distance Amplitude Correction) + ASME V Article 4

TOFD (Time of Flight Diffraction)

Transmitter + receiver probes, angled beams Detects: diffracted waves from flaw tips Flaw height sizing from tip diffraction times: h = |d_bottom - d_top| Better for: planar flaws (cracks), sizing accuracy ±1 mm (vs. ±3 mm for A-scan pulse-echo) Combined TOFD + PAUT: covers volumetric + planar flaws

Calibration

Reference block with side-drilled holes (SDH) or flat-bottom holes (FBH) DAC curve: distance amplitude correction for spherical wave spreading AWS D1.1: 20% DAC reference sensitivity; report all indications ≥ 20% DAC

Acceptance Criteria

ASME BPVC Sec VIII: no planar flaws > acceptance; rounded flaw length limits per Appendix 4 AWS D1.1 Table 8.3: reject if indication > class limits per joint category and weld size API 1104 Sec. 9: weld quality requirements for pipeline girth welds

Radiographic Testing (RT)

Principle

X-ray or γ-ray passes through material → attenuated by thickness/density → detected on film/digital detector Film: radiograph = projected 2D image Digital: computed radiography (CR) or digital detector array (DDA)

Beer-Lambert: I = I₀ × exp(-μ × t) (μ = linear attenuation coefficient) Sensitivity: 2% of thickness minimum (IQI/penetrameter wire)

Sources

X-ray (accelerator or tube): 150 kV–15 MeV; variable energy; directable beam Ir-192: 600 keV γ (most common isotope); half-life = 73.8 days Co-60: 1.17/1.33 MeV γ; stronger penetration; half-life = 5.3 years; less portable Se-75: lower energy; thinner material

Image Quality Indicators (IQI)

Wire IQI (ASTM E747): smallest visible wire → sensitivity % Hole IQI (ASTM E1025/plaque): smallest visible hole ASME requires: sensitivity ≤ 2% of weld thickness

Acceptance (AWS D1.1 Clause 8.12)

Cracks: not acceptable Porosity: ≤ aggregate size limits (1/4" max isolated, frequency limits) Slag: ≤ 2/3 of weld size in length Incomplete fusion/penetration: not acceptable for critical joints

Magnetic Particle Testing (MT)

Principle

Magnetic field applied → flux leaks at surface/near-surface discontinuity → particles attracted to leak field Only works on: ferromagnetic materials (carbon steel, ferritic SS, some cast iron) Not for: austenitic SS, Al, Cu, Ti

Magnetization Methods

Circular magnetization: current through part → detect longitudinal flaws Longitudinal magnetization: coil or yoke → detect transverse flaws Yoke (portable): AC or DC; most common field method

AC current: best for surface flaws (skin effect concentrates B near surface) DC current: better for subsurface (deeper penetration of flux)

Media

Wet fluorescent: most sensitive (UV light inspection); specification MIL-STD-1949 Dry powder: outdoor, rough surfaces Color contrast: black or red particles on white background

Acceptance (ASME BPVC Sec V, Article 7; AWS D1.1 Sec 8)

Linear indications > 1/16" length: reject Rounded > 3/16" (weld root area < 1/4"): reject per ASME Must remove indication and retest after repair

Liquid Penetrant Testing (PT)

Process (ASTM E165, ASME V Article 6)

1. Surface cleaning (solvent, detergent)
2. Apply penetrant → dwell time 5-60 min (type/temperature dependent)
3. Remove excess penetrant
4. Apply developer → draws penetrant from flaw → visible indication
5. Interpret within 10-60 min after developer

Types: Type I — Fluorescent: UV light inspection, most sensitive, for clean shop environments Type II — Visible dye: red dye + white developer, no UV light needed

Methods: Method A — Water washable: fastest, can miss broad/shallow defects Method C — Solvent removable: best for field/spot inspection Method D — Post-emulsifiable: most sensitive, complex process

Limitations

Surface-open flaws only (no subsurface detection) Not for: porous materials, plastics (absorb penetrant) Temperature: 4-52°C (standard range)

Acceptance

Any relevant linear indication > 1/16": evaluate per code Relevant rounded indication > 3/16": evaluate Must clean and retest after repair

Eddy Current Testing (ET)

Principle

AC in probe coil → eddy currents induced in conductive material → flaw disrupts currents → impedance change in probe Detection: surface and near-surface flaws Materials: any electrically conductive (not limited to magnetic)

Typical Applications

Aircraft skin inspection, tubing inspection, coating thickness, conductivity measurement Heat exchanger tube inspection: ET from inside (internal rotary probe) Standard depth of penetration: δ = √(1/(πfμσ)) [skin depth] f = frequency; μ = permeability; σ = conductivity Higher f → shallower penetration; lower f → deeper

Advantages/Limits

Fast scanning; no couplant needed; good for thin materials Cannot inspect welds volumetrically; limited penetration depth (few mm)

Visual Testing (VT)

Types

Direct VT: eye within 600mm, angle ≥ 30°, ≥ 50 foot-candles illumination Remote VT: camera, borescope, drone Enhanced VT: magnification, UV (for PT), dye enhanced

Requirements

AWS D1.1 Clause 6.9: all welds VT first ASME BPVC: 100% VT required before other NDE Acceptance: no cracks, incomplete fusion, overlap; size limits for porosity, undercut

Acoustic Emission (AE)

Principle

Crack growth, plastic deformation, corrosion → stress waves → detected by piezo sensors Passive (no external excitation) — detects ACTIVE defects Entire structure monitored simultaneously from multiple sensors Source location: triangulation from arrival time differences

Applications

Pressure vessel proof testing (during pressurization) Bridge monitoring (crack growth during traffic) Composite materials (delamination during loading)

Thermographic Testing (IT/IRT)

Active Thermography

Heat applied → thermal camera records cooling/heating Subsurface voids/delaminations: different thermal diffusivity → hot/cold spots Flash thermography: pulsed heat, captures thermal wave propagation

Passive IR

Temperature variations from electrical issues, friction, blocked flow No external heating needed — steady-state abnormalities

NDT Method Selection Matrix

Flaw Type	Best Methods
Surface cracks (steel)	MT, PT
Surface cracks (any conductive)	ET, PT
Subsurface cracks/voids	UT (PAUT), RT
Weld volumetric flaws	RT, UT
Weld planar flaws	PAUT + TOFD
Wall thickness measurement	UT (pulse-echo)
Corrosion mapping	PAUT C-scan, UT
Delamination (composites)	UT C-scan, thermography
Leak detection	PT, AE, helium mass spectrometer

Output

Provide: NDT method selected, code standard (ASME V/AWS D1.1/API 1104), sensitivity/acceptance criteria, flaw size detection limit, required personnel qualification (ASNT Level II/III).