ASNT NDT III RT flashcards

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

Inverse square law

Radiation intensity from a point source decreases with the square of distance. Doubling source-to-film distance quarters intensity, so exposure time must increase fourfold to maintain density.

Geometric unsharpness formula

Ug = F*t/d, where F is focal-spot size, t is object-to-detector distance, and d is source-to-object distance. Smaller focal spot or larger d reduces blur.

Film density definition

Density D = log10(I0/It), the logarithm of incident-to-transmitted light ratio. Density 2.0 transmits one percent of incident light through the film.

Half-value layer

The absorber thickness that reduces radiation intensity by half. After n half-value layers, remaining intensity equals I0 times (1/2)^n.

Linear attenuation coefficient

The constant mu in I = I0*e^(-mu*x) describing how strongly a material attenuates radiation per unit thickness; divided by density it becomes the mass attenuation coefficient.

Ir-192 half-life

Iridium-192 has a half-life of about 74 days, so source strength decays noticeably within months and exposure charts must be corrected for decay.

Co-60 half-life

Cobalt-60 has a half-life of about 5.3 years and emits high-energy gamma rays, giving greater penetration than Ir-192 for thick steel sections.

Photoelectric absorption

A low-energy interaction where a photon ejects an inner-shell electron. It dominates at low kV and produces high subject contrast in thin or low-density materials.

Compton scattering

Photon interaction with an outer electron that reduces energy and changes direction. It dominates at higher energies and increases scattered radiation, lowering image contrast.

Film latitude

The range of exposures producing acceptable density. High-latitude films show gradual density change and tolerate exposure variation but require longer exposures.

Image quality indicator

A penetrameter with wires or holes of known size placed on the part to verify radiographic sensitivity; the required wire or hole must be visible at the specified density.

Lead screens purpose

Lead foil screens absorb scattered radiation and emit secondary electrons that enhance film exposure, improving contrast and reducing exposure time.

Source-to-film distance rule

Increasing source-to-film distance reduces geometric unsharpness and increases the irradiated area, but requires longer exposure due to the inverse square law.

Film characteristic curve

A plot of density versus log exposure. Its slope (film gamma) indicates contrast; steeper slopes give higher contrast but narrower latitude.

Scattered radiation control

Collimation, lead screens, and backscatter shielding reduce scattered radiation reaching the film, improving contrast and image quality.

Se-75 half-life

Selenium-75 has a half-life of about 120 days and is used for thin-section radiography where lower energy improves contrast compared with Ir-192.

Discontinuity versus defect

A discontinuity is any interruption in material homogeneity; it becomes a defect only when it exceeds the acceptance criteria of the applicable code.

Sensitivity versus resolution

Sensitivity is the smallest discontinuity detectable; resolution is the ability to distinguish two closely spaced features as separate indications.

Probability of detection

The likelihood that a discontinuity of a given size is detected. POD increases with discontinuity size and depends on method, procedure, and inspector.

SNT-TC-1A certification

An employer-based recommended practice for qualifying and certifying NDT personnel in Levels I, II, and III; the employer's written practice governs certification.

CP-189 certification

ANSI/ASNT CP-189 is a standard requiring an accredited certification body; it is more stringent than SNT-TC-1A and includes additional requirements.

Level III responsibilities

Level III personnel develop and approve procedures, interpret codes and standards, establish acceptance criteria, and may certify other personnel.

ALARA principle

As Low As Reasonably Achievable: radiation exposure is minimized through time, distance, and shielding while still accomplishing the inspection.

Occupational dose limit

In the US, the annual whole-body occupational dose limit is typically 5 rem (50 mSv); dosimeters such as TLDs monitor cumulative exposure.

Calibration versus standardization

Calibration compares instrument response to a known reference traceable to a national standard; standardization checks response against a reference before use.

UT physical principle

Ultrasonic testing relies on reflection of high-frequency sound at interfaces where acoustic impedance differs, detecting internal discontinuities.

MT physical principle

Magnetic particle testing uses magnetic flux leakage at surface or near-surface discontinuities in ferromagnetic materials, revealed by applied particles.

PT physical principle

Penetrant testing relies on capillary action drawing penetrant into surface-breaking discontinuities, then bleed-out after developer application.

ET physical principle

Eddy current testing uses electromagnetic induction to generate currents in conductive materials; discontinuities alter coil impedance and signal.

Method selection factors

Choice among RT, UT, MT, PT, ET, VT depends on material, geometry, discontinuity type and orientation, and code requirements.

Traceability requirement

Measurement and calibration references must be traceable to national standards so that inspection results are reliable and comparable over time.