ASNT NDT III ET cheat sheet

ASNT SNT-TC-1A is a recommended practice; a written practice issued by the employer is the controlling document for personnel qualification and certification.

A Level III is responsible for method selection, technique development, procedure approval, and interpretation of codes and standards for the method certified.

Certification to Level II requires passing a general examination, a method-specific examination, and a practical examination, plus documented experience and vision acuity.

Near vision acuity is typically verified at 12 in (30 cm) with Jaeger J1 or equivalent, and color perception must be demonstrated per the written practice.

Probability of detection increases with discontinuity size, favorable orientation to the inspection direction, and higher signal-to-noise ratio; it is never 100 percent.

A discontinuity is any interruption in material continuity; a defect is a discontinuity that exceeds acceptance criteria or impairs serviceability.

Reference standards establish the baseline response; they must be traceable, of the same material family as the part when possible, and verified before and after inspection.

The examination re-certification interval is defined by the employer's written practice; typical intervals are 3 to 5 years with periodic performance checks.

Eddy current testing depends on Faraday induction: a time-varying coil current induces eddy currents in the conductor, and the secondary field from those currents is sensed by the coil as a change in impedance.

Standard depth of penetration is delta = 1/sqrt(pifmu*sigma); at one delta the current density is 36.8 percent of the surface value, and effective penetration is about three delta.

For a nonmagnetic conductor, delta (mm) is approximately 503*sqrt(rho/f) with resistivity in microhm-cm and frequency in Hz; higher frequency or higher conductivity reduces depth.

Fill factor is the ratio of test object cross-sectional area to coil cross-sectional area; higher fill factor increases coupling and signal amplitude.

Lift-off changes coil impedance and appears as a phase shift; it is minimized by close coupling or suppressed by choosing a test frequency and phase that separates lift-off from the flaw signal.

Absolute coils respond to gradual property changes and drift; differential coils reject common-mode effects such as temperature and lift-off but do not detect gradual changes.

Phase analysis separates variables: discontinuity depth, conductivity, permeability, and geometry each produce characteristic phase angles on the impedance plane.

Magnetic particle and flux leakage methods apply to ferromagnetic materials; eddy current testing is most effective on nonmagnetic conductors unless permeability is controlled.

Narrow-band filtering and phase discrimination improve signal-to-noise ratio, but filtering must not distort or delay the flaw indication beyond acceptable limits.