Acoustic Emission Inspection

Acoustic Emission Inspection (manufacturing-quality/ndt/acoustic-emission-inspection)

Use when the task is passive acoustic emission testing of aerospace parts and structures: thresholding recorded hits, grouping hits into events, computing signal energy, locating emission sources from sensor arrival times, and judging damage progression with the Kaiser effect and Felicity ratio. This leaf is the passive-listening counterpart of ultrasonic-inspection (which sends pulses and reads echoes) and complements ndt-method-selection, which picks among the NDT methods before any instrument is deployed.

Domain quick reference

• AE sources: sudden localized deformation releases elastic waves; typical aerospace sources are crack growth, fiber breakage, delamination, matrix cracking, and fretting.
• Sensors: resonant piezoelectric transducers are the common pick, with typical bands of 100-500 kHz (a 150 kHz resonant sensor is a common default); broadband sensors are used when spectral content matters. Couplant, mounting, and sensor spacing set the coverage.
• Hit vs event: a hit is one sensor channel crossing the amplitude threshold; an event is the emission of one source, assembled from hits whose arrival times fall inside the hit definition time (HDT) window.
• Threshold and energy: a fixed amplitude threshold in dB (40 dB is a common start) separates signal from noise; amplitude is reported in dBae; signal energy is the measured area under the rectified signal envelope (MARSE-style) or the deterministic proxy dt * sum of squared amplitudes.
• Kaiser effect: on reload, no emission is expected until the previous maximum load is exceeded. Felicity ratio = load at which emission resumes / previous maximum load. A ratio at or above 1 means the Kaiser effect holds and damage is stable; a ratio below 1 (0.95 is a common threshold for composites) signals the Felicity effect and progressing damage.
• Source location: linear location uses two sensors on a line and the arrival-time difference; planar location triangulates from three or more sensors by solving the hyperbolic time-difference system with an iterative least-squares scheme. Sources far outside the array are unreliable and are rejected.
• Applications: composite damage assessment, pressure vessel hydro and proof testing, structural load monitoring. Personnel qualification follows NAS-410 (and the SNT-TC-1A context) level certification; AS9100 frames NDT as a special process. Standards are referenced by name only per standards-map.yaml.

Workflow

1. Set the amplitude threshold and filter the record with hit_threshold_check to get the hits and counts.
2. Group the hits into events with group_hits_to_events using the HDT window.
3. Compute the energy of each hit or event with signal_energy.
4. For a sensor pair on a line, call source_location_linear with the gauge distance, the two arrival times, and the wave speed.
5. For an array, call source_location_planar with the sensor positions, the arrival times, and the wave speed; the function raises ValueError on impossible times, inconsistent time sets, and out-of-array solutions.
6. On reload data, call felicity_ratio and kaiser_effect_check to get the ratio and the damage verdict.
7. Confirm the deterministic behavior with the contract test scripts/test_acoustic_emission_inspection.py.

Worked example: planar source location

A square array at (0, 0), (1, 0), (0, 1), (1, 1) meters on a composite panel with wave speed 3000 m/s. A source near (0.4, 0.6) m gives arrival times of about 240.4, 282.8, 188.6, and 240.4 microseconds at the four sensors. source_location_planar returns the source at (0.4, 0.6) m within 1e-6 m with a near-zero residual; the same call with a corrupted arrival time raises ValueError instead of reporting a wrong point.

Related leaves

• ndt-method-selection: picks among NDT methods by defect and material class before the AE system is deployed.
• ultrasonic-inspection: active echo-based cousin of passive AE listening.
• thermography: surface and near-surface thermal technique for the same defect classes.
• visual-inspection: baseline surface screening that precedes instrumented NDT.

Pitfalls

• Reading the Felicity ratio backwards: a ratio at or above 1 means the Kaiser effect holds and damage is stable, while a ratio below 1 (0.95 is the common composites threshold) signals the Felicity effect and progressing damage - treating a sub-1 ratio as a pass misses the very damage progression AE is deployed to catch.
• Counting hits as events: a hit is one sensor channel crossing the amplitude threshold, and hits only become one source event when their arrival times fall inside the HDT window - grouping must run before hit counts are quoted as emissions.
• Trusting a located source without checking the array geometry: sources far outside the array are unreliable, and the planar solver raises ValueError on impossible arrival times, inconsistent time sets and out-of-array solutions instead of returning a wrong point.
• Mixing dB scales: amplitudes are reported in dBae against a fixed dB threshold (40 dB is a common start), so thresholds and hit amplitudes from different scales cannot be compared directly.
• Ignoring the coverage chain: couplant, mounting and sensor spacing set the coverage, and resonant sensors are band-limited (typical 100-500 kHz), so a gap in coupling or an out-of-band emission silently produces no hits.
• Treating AE as a general inspection without the qualification context: personnel certification follows NAS-410 (SNT-TC-1A context) level certification and AS9100 frames NDT as a special process, so an AE verdict outside that qualification frame has no disposition standing.

Behavior contract (gate 3)

Run the deterministic contract test (stdlib unittest, offline):

python3 scripts/test_acoustic_emission_inspection.py

The test covers threshold crossing including a zero threshold, event grouping with the HDT window, energy weighting, linear location including impossible arrival times, planar location including inconsistent time sets and out-of-grid sources, and the Kaiser/Felicity verdicts.

Compliance

• Standards referenced, not reproduced: as9100 and nas-410 resolve in standards-map.yaml, both reference-only; the method content is common AE methodology, summarized only.
• compliance: STANDARDS-REF, gated: false.