tofd-testing

name	tofd-testing
description	Time-of-flight diffraction (TOFD) — diffraction mechanism (Huygens principle), probe separation (PCS), lateral wave and backwall signal, flaw depth measurement (time-of-flight formula), sizing accuracy (±1 mm), near-surface dead zone, TOFD for crack growth monitoring, combination with PAUT, signal interpretation (parabolic arcs, phase reversal), ASME Code Case 2235 application, and calibration procedures per ISO 10863.
metadata	{"priority":7,"promptSignals":{"phrases":["TOFD","time of flight diffraction","TOFD inspection","TOFD crack sizing","diffraction ultrasonic","ASME Code Case 2235"],"minScore":3}}

Time-of-Flight Diffraction (TOFD) — Complete Skill

TOFD Operating Principle

Diffraction Mechanism

TOFD principle: based on Huygens' principle — tip of any discontinuity acts as point diffractor of ultrasonic energy Transmitter fires: broad beam illuminates entire weld cross-section At flaw tip: tiny fraction of energy diffracted in all directions → diffracted signal detected by receiver Signal arrives earlier or later than geometric reflection depending on tip position → time-of-flight → depth

Key signals on TOFD A-scan (in time order):

1. Lateral wave: surface-creeping wave; travels at top of component; arrives first; used as timing reference
2. Diffracted signals from flaw tips: arrive between lateral wave and backwall; small amplitude
3. Backwall echo: reflected from opposite surface; arrives last; large amplitude; phase-reversed relative to lateral wave

Time-of-flight geometry: For transmitter T and receiver R separated by probe center separation PCS = 2S (symmetric about weld centerline): t_lat = PCS / v_L = 2S / v_L [lateral wave; travels along surface] For flaw tip at depth d below surface: t_tip = (1/v_L) × √((2S)² + 4d² + 4S×d×sin(θ) ... ) [exact geometry]

Simplified formula (for small probe separation relative to thickness): t_tip = (2/v_L) × √(S² + d²) [transmit-receive symmetry; S = half PCS] Flaw depth: d = √((v_L × t_tip / 2)² − S²)

Depth of upper tip (d_u) and lower tip (d_l) → through-wall height: H = d_l − d_u [through-wall flaw height; sizing accuracy ≈ ±0.5–2 mm]

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Probe Selection and Setup

Probe Center Separation (PCS)

Optimum PCS: Optimal: beam intersection at mid-wall depth; angle at focal zone = 60–70° Rule: PCS = 2 × t × tan(θ) [t = wall thickness; θ ≈ 60° for best sensitivity] Or: PCS ≈ 2 × t/1.5 ≈ 1.3 × t [approximate; works for t = 15–100 mm]

Example (t = 25 mm wall): PCS = 2 × 25 × tan(60°) = 2 × 25 × 1.732 = 86.6 mm → use 80–90 mm PCS

Probe angle (θ_probe): Selected so that refracted angle = 60°–70° for optimum coverage Typically 60° shear wave probes in wedge; frequency 5–10 MHz for welds

Frequency: Lower frequency (2.25–5 MHz): thicker welds (> 25 mm); less attenuation; lower resolution Higher frequency (5–15 MHz): thin welds (< 20 mm); better resolution; more attenuation Resolution limit: Δd_min ≈ v × pulse_width / 4 ≈ λ/2 at best

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TOFD Signal Interpretation

D-scan (TOFD Image)

D-scan: grayscale image; x-axis = scan position along weld; y-axis = time (depth) Lateral wave: bright top line (early time); backwall: bright bottom line (late time) Flaw tip signals: appear between these reference signals as bright arcs

Phase relationship: Lateral wave: positive phase (white-black-white in RF display) Backwall: phase-reversed (black-white-black) due to reflection from rigid boundary Upper flaw tip diffraction: phase as lateral wave (positive) Lower flaw tip diffraction: phase-reversed (as backwall) — criterion for upper vs. lower tip identification Phase reversal is critical interpretation skill: distinguishes upper from lower flaw tip

Parabolic arc geometry: When scanning laterally: flaw tip traces parabolic arc in D-scan image Apex of parabola = closest approach of scan to tip (tip directly below scan line) Parabola curvature ∝ 1/d (shallower tips = sharper parabola; deeper = flatter)

Indications: Through-crack: two hyperbolic arcs (upper and lower tips); H = t_lower − t_upper converted to depth SDH: single bright arc (no through-wall height) Smooth reflector (lack of fusion): strong reflected signal; TOFD less effective than PAUT for planar reflectors

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Depth Measurement Accuracy

Sizing Performance

TOFD depth accuracy: Tip-to-tip sizing: ±0.5–1.5 mm (best practice; dependent on frequency, wave speed knowledge) Through-wall height: ±1–2 mm for production scanning Primary source of error: wave speed uncertainty; Δv/v = 0.5% → Δd = ±0.5% of depth → ±0.25 mm at 50 mm depth

Reference study (TWI benchmark): TOFD consistently outperforms all other UT methods for height sizing accuracy Root mean square error (RMSE) for TOFD: 1.0–1.5 mm vs. PAUT 1.5–3 mm; RT (X-ray) 3–5 mm

Depth sizing formula expanded: d = √((v_L × t_flaw / 2)² − S²) − S × correction_for_angle Reference: measure lateral wave time t_lat first; apply offset correction

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Dead Zones

Near-Surface and Far-Surface Dead Zones

Near-surface dead zone: Below lateral wave: ring-down + probe recovery time masks signals Dead zone depth: typically 3–10 mm from inspection surface (at 5 MHz; deeper at 2.25 MHz) Consequence: near-surface flaws (surface-breaking or within dead zone) NOT detectable by TOFD Solution: combine TOFD with PAUT or pulse-echo for near-surface coverage

Far-surface dead zone: Region masked by backwall ring-down; depth: last 1–3 mm before opposite surface Solution: scan from both sides if root region cracks needed

Combined TOFD + PAUT strategy: PAUT: detects and locates (position, orientation); excellent for near-surface; calibrated amplitude TOFD: sizes through-wall height with best accuracy; real-time monitoring Both required for full weld volume coverage per ASME Code Case 2235 intent

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ASME Code Case 2235

Flaw Sizing Acceptance

Code Case 2235 (Use of TOFD for examination of welds): Alternative to radiographic testing (RT) for volumetric examination of welds Requirements:

• Inspection qualified per ASME Section V Appendix VIII Supplement 14
• Demonstrated ability: detection and sizing PDI (Performance Demonstration Initiative) qualification
• Through-wall height sizing: ±3 mm (95% confidence); TOFD typically achieves ±1.5 mm

Acceptance criteria (ASME Code Case 2235 + B31.3 Level 1): For pressure piping: through-wall height H ≤ 0.3t (t = wall thickness; depth-dependent allowable) Reject if: H > allowable; or indication length > allowable; or near-surface (within dead zone) without supplemental inspection

Procedure qualification: Written procedure per ASME Section V Article 14 Specimen: production-representative mock-up welds with artificial flaws; blind testing of sizing accuracy Personnel certification: ASNT Level II or III; additional qualification per Code Case

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Crack Growth Monitoring

Periodic TOFD Monitoring

TOFD for in-service crack monitoring: Initial baseline scan → repeated scans over time → compare tip depths Growth rate: Δd = d_new − d_old over time interval Δt Advantage: highly sensitive to depth changes (±0.5 mm per scan); non-destructive; can track slow fatigue crack growth

Applications: Pipeline girth weld cracks: monitor depth growth over inspection intervals; predict remaining life Pressure vessel nozzle cracks: baseline during construction; monitor during service Nuclear RPV reactor pressure welds: TOFD preferred by ASME XI for critical welds

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Standards and References

Standard	Scope
ISO 10863	TOFD technique — procedures and calibration
ASME Code Case 2235	TOFD as alternative to RT for weld volumetric examination
ASTM E2373	Standard practice for TOFD
EN 583-6	Ultrasonic examination — TOFD technique
ASME Section V Article 14	TOFD examination procedure
AWS D1.1 Annex Q	TOFD provisions for structural welds

Output

Provide: application (weld type; material; t [mm]; inspection code: ASME/AWS/ISO; monitoring: one-time or periodic), probe setup (PCS = 1.3×t [mm]; frequency f [MHz] for resolution ΔR = λ/2; probe angle [°]; wedge material), timing reference (lateral wave t_lat = PCS/v_L [μs]; v_L [m/s] = [material value]; backwall t_bw = 2t/v_L + PCS²/(4t×v_L) [μs]), depth formula (d_tip = √((v_L×t_tip/2)²−S²) [mm]; calibrate v_L with known-depth SDH), sizing (d_upper [mm]; d_lower [mm]; H = d_lower − d_upper [mm]; sizing accuracy ±[mm]; compare to t [%]), dead zones (near-surface: [mm] from surface at f=[MHz]; mitigation: PAUT supplement; far-surface: [mm]; covered by opposite-side scan?), signal interpretation (lateral wave phase: positive; backwall phase: reversed; upper tip: positive → lower tip: reversed; arc curvature confirms depth), code compliance (Code Case 2235: H ≤ 0.3t; qualification per Appendix VIII Sup 14; personnel: ASNT Level II + TOFD qualification), and applicable standard (ISO 10863; ASME CC 2235; ASTM E2373).