ASNT NDT Level II cheat sheet

Skin depth δ = 1/√(π f μ σ); it decreases as frequency, conductivity, or permeability increases, and increases with resistivity.

Approximate field/density at depth: 1δ retains about 37% of surface density, 2δ about 14%, 3δ about 5%.

Lower test frequency penetrates deeper but lowers resolution; raise frequency when only surface and near-surface defects matter.

The impedance plane plots resistance against inductive reactance; defect depth is read from phase angle, while amplitude alone is unreliable.

Fill factor = (coil inside area − part area) / coil inside area; a higher fill factor concentrates the field and raises sensitivity on small-diameter parts.

Lift-off shifts the signal mainly along one impedance-plane direction; hold it constant or use phase rotation to null it out.

Ferromagnetic materials restrict ET to surface and near-surface inspection because permeability dominates; remote field testing is used for ferromagnetic tubing.

Reference standards must match the part's alloy, temper, and heat treatment, because conductivity and permeability control the eddy current response.

Detection is best when the magnetic field is roughly perpendicular to the defect; a defect parallel to the flux produces little particle accumulation.

Circular magnetization (prod, central conductor, contact) finds longitudinal defects; longitudinal magnetization (coil, head shot) finds circumferential defects.

Coil magnetization current: I = 45,000 / (L/D) amperes, valid for parts with L/D of 3 or more; low L/D parts self-demagnetize and need more current.

Prod spacing is typically 3–8 in (75–200 mm) with about 100 A per inch of spacing (commonly 90–125 A/in).

The continuous method (current on while particles are applied) is more sensitive than the residual method, which requires a material with high retentivity.

Demagnetize after testing whenever residual magnetism could affect machining, welding, or service; use a reversing field of decreasing amplitude.

Fluorescent MT requires a minimum black light intensity of about 1000 µW/cm² at the inspection surface, about 5 min dark adaptation, and a darkened area.

Wet fluorescent particles in suspension give the best sensitivity for fine defects; dry powder suits high-temperature work and rough castings.

Scale, oil, and coatings can bridge or trap particles and produce false or missed indications, so surfaces are cleaned before magnetization.

Standard sequence: pre-clean, apply penetrant and dwell, remove excess, dry, apply developer and dwell, inspect, post-clean.

Penetrant dwell is typically 10–15 min and developer dwell 10–30 min; tight or shallow discontinuities need the longer times.

Standard penetrant testing is limited to surface temperatures of 50–125 °F (10–52 °C); outside that range special penetrants or qualification are required.

Type I penetrant is fluorescent and needs black light; Type II is visible and needs adequate white light; sensitivity levels run 1/2 through 4.

Methods are A (solvent removable), B (water washable), C (post-emulsifiable, lipophilic), and D (post-emulsifiable, hydrophilic).

Water-washable penetrants require a water-tolerance test; post-emulsifiable systems are preferred on rough or as-cast surfaces.

Over-washing leaches penetrant from discontinuities and produces faint or missed indications, so wash time and pressure must be controlled.

Developer forms a thin uniform film, draws penetrant out of the discontinuity by capillarity, and increases visible contrast.

Blasting, grinding, or machining can smear or peen metal over discontinuities and mask them; chemical cleaning is preferred before PT.

Geometric unsharpness Ug = F·t/d, where F is source size, t is object-to-film distance, and d is source-to-object distance; reduce it with a smaller source or larger d.

Attenuation follows I = I0 e^(−μx); the half-value layer halves intensity and the tenth-value layer reduces it to one tenth.

Inverse square law: intensity varies as 1/d², so doubling source-to-film distance requires about four times the exposure.

Typical acceptable film density is 1.8–4.0; density below the code minimum reduces contrast and requires re-radiography.

A source-side IQI (penetrameter) gives the required sensitivity; film-side placement demands a smaller, harder-to-see IQI for the same sensitivity.

Ir-192 has a 74-day half-life and serves steel about 0.5–3 in thick; Co-60 covers thicker sections; Se-75 and Yb-169 suit thinner material.

Lead intensifying screens cut exposure time and filter low-energy scatter, while fluorescent screens trade image resolution for speed.

Scatter control: collimate the beam, mask the part, back the cassette with lead, and use a backscatter shield checked with the Pb 'B' test.

Film processing time, temperature, and chemistry must be controlled, or density and contrast are not reproducible between exposures.

Wavelength λ = c/f; higher frequency gives shorter wavelength, better resolution and sensitivity, but greater attenuation.

Acoustic impedance Z = ρc, and the reflection coefficient at normal incidence is R = (Z2 − Z1)/(Z2 + Z1).

Near field length N = D²f/(4c); flaw sizing and evaluation are normally performed beyond N, where the beam is more uniform.

Beam divergence half-angle for a circular transducer: sin θ = 1.22 λ/D; larger diameter and higher frequency reduce spread.

Snell's law sin θ1/c1 = sin θ2/c2 sets the wedge angle; the first critical angle eliminates refracted longitudinal waves and the second eliminates shear waves.

Common refracted shear angles are 45°, 60°, and 70°; 70° suits near-surface and weld cap inspection, 45° suits thicker sections.

Distance amplitude correction (DAC) curves compensate for beam spread and attenuation; record indication height as dB relative to the DAC.

Couplant must wet both surfaces and exclude air; glycerin, gel, oil, and water are selected for surface finish and material compatibility.

Typical steel velocities: longitudinal about 0.232 in/µs (5900 m/s) and shear about 0.127 in/µs (3230 m/s).

Minimum illumination at the inspection surface is generally 100 foot-candles (about 1000 lux), with higher levels for small or low-contrast features.

Near vision must be corrected to 20/20 (Jaeger J-1 or equivalent) at 12–14 in (300–350 mm) and re-verified at least annually.

Direct visual testing requires the eye within 24 in (600 mm) of the surface with a viewing angle of at least 30°, preferably 45°.

Remote visual testing uses borescopes, fiberscopes, or cameras; resolution and illumination must be demonstrated at the remote inspection location.

Contrast is the luminance difference between indication and background; oblique lighting improves detection of surface-breaking discontinuities.

Common aids include magnifiers (2× to 10×), mirrors, weld gauges for undercut, fillet, and hi-lo, and surface comparators.

Surfaces must be clean and free of scale, spatter, and coatings that can hide discontinuities or create false indications.

Weld examination looks for undercut, porosity, cracks, incomplete fusion, misalignment, and spatter, with size and location recorded.