PCN NDT Ultrasonic Testing Level 2 Exam cheat sheet

Ultrasonic Testing Level 2

• Snell's law: sin θᵢ / sin θᵣ = V₁ / V₂; first critical angle θ₁ = sin⁻¹(V₁/V₂) when refracted longitudinal wave is 90°; second critical angle when refracted shear wave is 90°.

• Acoustic impedance Z = ρV; reflection coefficient at normal incidence R = (Z₂ − Z₁)/(Z₂ + Z₁); transmission coefficient T = 2Z₂/(Z₂ + Z₁).

• Near field length N = D²/(4λ) for a circular piston transducer; beam spread half-angle sin θ = 1.22 λ/D (to first null) or sin θ = 0.51 λ/D (to −6 dB).

• Wavelength λ = V/f; in steel (V_L ≈ 5900 m/s) at 5 MHz, λ ≈ 1.18 mm; in water (V ≈ 1480 m/s) at 5 MHz, λ ≈ 0.296 mm.

• Calibration blocks: IIW/V1 for angle beam index point, refracted angle, and probe resolution; V2 (DIN 54-122) for setting time base and sensitivity; ASME basic cal block for distance and area linearity.

• 6 dB drop method for flaw sizing: move probe until echo amplitude drops 6 dB (half amplitude) from maximum; for a planar reflector larger than beam, this approximates the beam width or flaw length.

• DAC (distance amplitude correction) curve compensates for material attenuation and beam spread; reference reflector typically a side-drilled hole (SDH) or flat-bottom hole (FBH); TCG (time-corrected gain) applies gain as a function of time to flatten DAC.

• Angle beam inspection: refracted angle determined by Snell's law from wedge angle and velocities; skip distance = 2T tan θ (T = thickness); surface distance = T tan θ for first leg.

• Acceptance criteria often based on amplitude relative to DAC (e.g., 50% DAC recording, 100% DAC rejection) and length; compare indication length to allowable per code; classify as linear or rounded.