ASNT NDT III UT cheat sheet

Acoustic impedance Z = density × sound velocity (ρc); reflection coefficient at normal incidence R = (Z2 − Z1)/(Z2 + Z1).

In a given mode and material the sound velocity is fixed; wavelength λ = c/f, so higher frequency means shorter wavelength and smaller detectable reflector size.

Snell's law: sin(θi)/c1 = sin(θr)/c2; the first critical angle occurs when the refracted longitudinal wave reaches 90 degrees.

In the second critical angle condition only a refracted shear wave remains in the second medium; beyond it, surface (creeping) waves can form.

Near field length for a circular piston transducer: N = D²/(4λ); beyond N the beam enters the far field and diverges with half-angle sin θ = 1.22λ/D.

Beam spread, near-field length, and lateral resolution all depend on transducer diameter, frequency, and material velocity.

Attenuation combines absorption and scattering and increases with frequency; it is expressed in dB per unit length.

Mode conversion, beam spread, and interface losses explain why amplitude alone is not a reliable measure of reflector size.

A-scan displays amplitude versus time (distance); B-scan and C-scan present cross-sectional and plan views, respectively.

Vertical linearity verifies that displayed amplitude responds proportionally to input signal over the usable screen height.

Horizontal linearity verifies that echo position corresponds accurately to time or distance across the full sweep.

Resolution is split into near-surface (dead zone) resolution, lateral resolution, and axial (range) resolution.

Calibration blocks (e.g., step-wedge or IIW-type blocks) establish velocity, delay, and angle references; reference reflectors set sensitivity.

DAC/TCG curves compensate for material attenuation and beam spread so equal reflectors give comparable amplitudes at different depths.

Couplant selection affects transmission; couplant must match surface condition and be compatible with the material being tested.

Transducer frequency, element size, and damping determine pulse length, bandwidth, and resulting sensitivity and resolution trade-offs.

A written procedure must state the standard referenced, material, thickness range, surface condition, and acceptance criteria.

Technique selection covers wave mode, transducer frequency and size, incidence angle, couplant, and scan pattern (raster, circumferential, or longitudinal).

Scan overlap and index increment must ensure full coverage; the increment is typically tied to beam width or element size.

Calibration and reference reflectors (flat-bottom holes, side-drilled holes, notches) must be defined with size, depth, and orientation.

Sensitivity setting procedure (e.g., reference gain plus transfer correction) and scanning gain must be explicitly documented.

Recording thresholds and reporting requirements must be defined so that all rejectable and recordable indications are captured.

Personnel qualification level, equipment identification, and calibration interval belong in the procedure and the test record.

Indication location is derived from sound path and angle using trigonometric relationships; depth and surface distance follow from the beam geometry.

Amplitude comparison against a DAC or reference curve classifies indications but does not by itself define true reflector size.

Sizing methods include the 6 dB drop, 20 dB drop, maximum amplitude technique, and tip-diffraction or time-of-flight approaches.

Indication shape and orientation affect amplitude; a favorably oriented small flaw can return more energy than a larger misoriented one.

Evaluation compares recorded indications against acceptance criteria in the applicable code; interpretation assigns probable cause and type.

Reports must identify equipment, calibration, technique, indication locations and sizes, and the accept/reject disposition.

Spurious indications from geometry, mode conversion, or couplant can mimic flaws and must be distinguished from real reflectors.

A written practice defines how the employer implements the governing standard for qualification and certification of personnel.

Calibration intervals for equipment and reference blocks must be documented, traceable, and met; out-of-calibration equipment invalidates results.

Codes specify acceptance criteria, examination coverage, and reporting; specifications add project-specific requirements.

Records must be retained per the governing standard and must be traceable to equipment, personnel, and procedure used.

Audits and surveillance verify that procedures, calibration, and personnel qualifications match the standard in force.

When code and specification conflict, the more restrictive requirement generally governs unless the specification states otherwise.

Document control ensures only current revisions of procedures, standards, and calibration records are in use.

Certification levels: Level I performs calibrated tests under supervision; Level II sets up, calibrates, interprets, and reports; Level III develops procedures and supervises.

Qualification is the demonstrated ability; certification is the written attestation by the certifying body or employer.

Eyesight requirements, including near-vision acuity and color perception, must be verified at the interval set by the standard.

Training hours and experience requirements vary by method and level and must be documented before examination.

Examinations typically include general, specific, and practical components; passing criteria are defined by the certification scheme.

Level III duties include procedure development, technique review, and interpretation of codes and standards for the method.

Recertification requires evidence of continued satisfactory performance and may require reexamination at defined intervals.