Visual Inspection and Dye Penetrant Testing
Visual inspection (VT) is the first, fastest, and most cost-effective weld inspection method. It is the baseline inspection for all structural welds before any other NDE method is applied — if a weld fails visual inspection, there is no purpose to performing more expensive tests. AWS D1.1 (Structural Welding Code) provides specific acceptance criteria for visual inspection. Measured parameters: weld size (fillet leg size and throat dimension) measured with a weld gauge; weld profile (convexity and concavity of the weld face — excessive convexity creates a notch at the weld toe, excessive concavity indicates insufficient fill); undercut (maximum 1/32" for statically loaded, no undercut for dynamically loaded); porosity (maximum allowable cluster and linear porosity counts); cracks (none permitted under any circumstance); overlap (weld metal that extends beyond the toe without fusing to the base metal — must be removed); weld length and location compliance with drawing requirements. A properly trained visual inspector also examines the weld orientation, sequence compliance (multi-pass welds welded in the specified sequence), and inter-pass cleanliness. Dye Penetrant Testing (DPT/PT): a surface examination method that detects cracks and porosity open to the surface that are too small to see visually. Process: 1) Clean the surface (solvent or detergent — all paint, oil, rust, and loose scale removed). 2) Apply penetrant (red dye aerosol or brush) liberally, allow dwell time (10–30 minutes for standard penetrant) so the dye wicks into any surface-breaking discontinuities by capillary action. 3) Remove excess penetrant from the surface with a lint-free cloth dampened with remover — never rinse directly with water or remover as this flushes penetrant from the discontinuities. 4) Apply developer (white chalk-like spray) — the developer draws the trapped penetrant out of discontinuities by reverse capillary action, creating red indications visible against the white developer background. 5) Interpret: a linear indication (length > 3× width) is a crack or linear discontinuity — typically rejectable. A rounded indication is a pore — assessed against the relevant code's acceptance criteria.