ASNT NDT III RT cheat sheet

Inverse square law: intensity varies as 1/d^2, so doubling source-to-film distance reduces exposure by a factor of four; exposure time must be quadrupled to hold density.

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

Film density D = log10(I0/It); a density of 2.0 means 1/100 of incident light is transmitted. Standard density range for X-ray viewing is typically 1.8-4.0 depending on code.

Half-value layer (HVL) is the absorber thickness that halves intensity; after n HVLs the remaining intensity is I0*(1/2)^n.

Attenuation: I = I0e^(-mux), where mu is the linear attenuation coefficient; mass attenuation coefficient = mu/rho.

Radioactive source half-lives: Ir-192 about 74 days, Co-60 about 5.3 years, Se-75 about 120 days, Cs-137 about 30 years.

Contrast improves with lower kV (more photoelectric absorption) and with lower scattered radiation; lead screens and collimation reduce scatter.

Film latitude is the range of exposure over which density change is acceptable; higher-latitude (finer grain, slower) film tolerates more exposure variation but needs longer exposure.

Image quality indicators (penetrameters) verify sensitivity; essential wire or hole must be visible at the required density per the governing code (e.g., ASTM E1025 / E747).

NDT methods include RT, UT, MT, PT, ET, VT, and LT; selection depends on material, geometry, defect type, and code requirements.

Discontinuity vs defect: a discontinuity is any interruption in homogeneity; it is a defect only if it exceeds acceptance criteria.

Sensitivity is the smallest detectable discontinuity; resolution is the ability to distinguish closely spaced features.

Probability of detection (POD) increases with discontinuity size; reliability studies show POD curves vary by method, inspector, and procedure.

Calibration and standardization verify equipment response against a known reference; they must be traceable to a national standard.

Personnel certification per ASNT SNT-TC-1A (employer-based) or ANSI/ASNT CP-189 (accreditation-based); Level III may develop procedures and interpret codes.

Radiation safety: dose limits (e.g., 5 rem/yr occupational whole-body in US), ALARA principle, use of TLD/dosimeters and survey meters.

Physical principles: RT relies on differential attenuation; UT on acoustic impedance mismatch; MT on magnetic flux leakage; PT on capillary action; ET on electromagnetic induction.