Maintenance & Reliability · SOP

Assembly Glue Penetration Depth Check

Standardised procedure to measure glue penetration depth in bonded assemblies using fluorescent dye penetrant inspection. Ensures bond integrity and compliance with manufacturing specifications.

Estimated Time

15 min

Difficulty

Intermediate

Regulation

ASTM E1417

Version

1.0

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Purpose

This procedure verifies that the adhesive fully wets both substrates and penetrates the joint to the required depth. Incomplete penetration reduces bond strength and can lead to premature failure. Regular checks reduce rework, warranty claims, and field failures.

Scope

Applies to all structural adhesive assemblies in the production line, including metal-to-metal and composite-to-metal bonds. This SOP covers the fluorescent dye penetrant (Type I, Method C) inspection of glue joints with a nominal thickness up to 3 mm. Operators must be certified to Level I NDT per ASNT SNT-TC-1A.

Safety Precautions

LOTOLockout/Tagout any pneumatic or UV curing equipment before sample cutting. Do not bypass interlocks.
PPEWear nitrile gloves, safety glasses, and lab coat. Penetrant and developer are mild irritants — avoid skin contact.
UVUV-A (365 nm) sources can damage eyes. Use UV‑blocking safety glasses. Limit exposure to open skin.
CHEMWork in a well‑ventilated area. Penetrant solvents are flammable — no open flames or sparks.

Required Tools & Materials

Fluorescent dye penetrant (Type I, Method C — solvent‑removable)
Non‑aqueous white developer (aerosol or brush)
Cleaner/remover solvent (isopropyl alcohol or manufacturer‑specified)
Lint‑free cloths (white, low‑lint)
UV‑A inspection lamp (365 nm, minimum 1000 µW/cm² at 15 in)
Optical comparator or digital calliper with depth rod (0.01 mm resolution)
Trimming saw or abrasive cutting wheel for sample preparation
Calibration block with known penetration depth (for daily verification)

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Procedure: Step-by-Step

1 Prepare test sample: Cut a representative cross‑section of the bonded assembly (approximately 25 mm × 20 mm) using a straight, cool-cutting saw. Deburr edges. Wipe clean with lint‑free cloth and solvent.

2 Apply fluorescent dye penetrant: Spray or brush a thin, even layer over the cross‑section surface, ensuring the bond line is completely covered. Allow dwell time of 10 minutes (or per penetrant manufacturer data sheet).

3 Remove excess penetrant: Wipe the surface with a dry lint‑free cloth. Then lightly moisten a fresh cloth with solvent and wipe in one direction only — do not saturate. Avoid drawing penetrant out of the bond line.

4 Apply developer: Shake developer can thoroughly. Hold 150–200 mm from surface and apply a thin, uniform white coating. Do not blow dust onto the surface. Wait 7–10 minutes for the developer to draw any residual penetrant out.

5 Examine under UV light: Turn off ambient lights. Position UV lamp 150–200 mm from the surface. Look for yellow‑green fluorescent indications along the bond line. Measure the maximum perpendicular distance from the bond entrance to the deepest fluorescent indication using an optical comparator or digital calliper with depth attachment.

6 Record results: Log the penetration depth in mm, the assembly serial number, operator initials, and date. Compare against the acceptance criterion (typically ≥ 1.5 mm for structural bonds). If below limit, flag the assembly for engineering review.

Assembly Glue Penetration Depth Check

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