ASNT NDT Level II flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Skin depth formula

Depth below the surface where eddy current density falls to about 37% (1/e) of its surface value, given by δ = 1/√(π f μ σ).

Lift-off effect

Change in coil-to-part spacing that alters impedance magnitude strongly; it must be held constant, compensated, or phase-rotated out to avoid false indications.

Fill factor

Fraction of the coil's internal cross-section occupied by the test part. A higher fill factor concentrates the magnetic field and increases sensitivity, especially on small-diameter tubing.

Phase angle in eddy current

Angular position of the impedance-plane signal. It shifts with defect depth and material changes, letting defect signals be separated from lift-off or support plates.

Remote field testing

Low-frequency through-wall eddy current technique for ferromagnetic tubing, where the field travels along the wall and is detected by an internal receiver spaced from the exciter.

Circular magnetization

Magnetic field running circumferentially around the current path, produced by prods or a central conductor, and used to detect longitudinal defects parallel to the part axis.

Longitudinal magnetization

Magnetic field running along the part axis, produced by a coil or head shot, used mainly to detect circumferential defects transverse to the axis.

Prod current rule

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

Demagnetization requirement

Performed after testing when residual magnetism could interfere with machining, welding, or service, using a reversing alternating field reduced gradually to zero.

Continuous versus residual method

In the continuous method current flows while particles are applied and sensitivity is higher; the residual method relies on retained magnetism and needs high-retentivity material.

Capillary action in penetrant

Force that draws penetrant into tight surface openings; it requires low surface tension and good wetting so the liquid enters narrow discontinuities.

Post-emulsifiable penetrant

Penetrant removed by applying an emulsifier before washing. It gives better excess-removal control on rough surfaces and reduces over-washing of indications.

Developer function

Forms a thin uniform film that draws penetrant out of discontinuities by capillarity and provides a contrasting background so indications become visible.

Penetrant dwell time

Time the penetrant stays on the surface, typically 10–15 minutes, with longer times for tight or shallow discontinuities and for lower temperatures.

Penetrant temperature limits

Standard penetrant testing requires surface temperatures between 50 and 125 °F (10–52 °C); outside this range special penetrants or technique qualification are needed.

Geometric unsharpness formula

Ug = F·t/d, where F is source size, t is object-to-film distance, and d is source-to-object distance. Smaller focal spots or greater distances sharpen the image.

Inverse square law

Radiation intensity varies inversely with the square of distance from the source, so doubling source-to-film distance reduces intensity to one quarter.

Half-value layer

Material thickness that reduces radiation intensity by half. It is used to estimate exposure times and shielding for a given radiation energy and material.

Penetrameter (IQI)

Image quality indicator of wires or steps placed on the source or film side; the required designation depends on material thickness and specified sensitivity.

Film density range

Photographic darkness of a radiograph, typically accepted between 1.8 and 4.0; densities below the code minimum lower contrast and require re-exposure.

Iridium-192 source

Gamma source with a 74-day half-life used for steel roughly 0.5 to 3 in thick; Co-60 is chosen for thicker sections.

Acoustic impedance

Product of density and sound velocity (Z = ρc). The impedance mismatch at an interface determines how much ultrasonic energy is reflected.

Near field length

N = D²f/(4c). Inside this distance the beam is complex and non-uniform, so defect sizing and evaluation are normally done beyond it.

Snell's law in ultrasonic testing

sin θ1/c1 = sin θ2/c2. It governs refraction at the wedge-to-part interface and determines the incident angle needed for a desired refracted angle.

Distance amplitude correction curve

Plot of echo height versus distance from a reference reflector that compensates for beam spread and attenuation when evaluating and sizing indications.

First critical angle

Incident angle at which the refracted longitudinal wave reaches 90 degrees and vanishes; beyond it only refracted shear waves travel in the part.

Six dB drop method

Sizing technique in which defect length is bounded by the points where the echo amplitude falls 6 dB from its maximum value.

Illumination for visual testing

Generally at least 100 foot-candles (about 1000 lux) at the inspection surface, with higher levels for small or low-contrast indications.

Visual acuity requirement

Near vision corrected to at least 20/20, demonstrated with a Jaeger J-1 or equivalent chart at 12–14 in, and re-verified at least annually.

Direct visual testing

Examination with the eye no more than 24 in (600 mm) from the surface and a viewing angle of at least 30 degrees, preferably 45 degrees.

Remote visual testing

Inspection using borescopes, fiberscopes, or cameras where direct access is impossible; resolution and illumination must be adequate at the remote location.

Contrast in visual testing

Luminance difference between an indication and its background; clean surfaces and oblique lighting raise contrast and improve detection reliability.