Direct Answer
Non-destructive testing (NDT) evaluates the integrity of a weld without damaging the structure. Common methods for structural steel: Ultrasonic Testing (UT), Magnetic Particle Testing (MT), Penetrant Testing (PT), and Radiographic Testing (RT). NDT is required for Demand Critical welds in seismic moment frames, CJP welds in high-stress applications, and wherever the project specification mandates it.
Full Explanation
UT (ultrasonic) uses sound waves to detect internal weld defects — porosity, incomplete fusion, cracks. It is the most common NDT method for structural welds and is required by AWS D1.8 for Demand Critical welds. MT (magnetic particle) detects surface and near-surface defects in ferromagnetic materials by applying a magnetic field and iron particles. It is fast and effective for fillet welds. PT (dye penetrant) detects surface cracks only. RT (radiographic) uses X-ray or gamma radiation to image internal weld quality — rarely used for structural steel, more common for pressure vessels.
What This Means for Your Shop
NDT inspection adds time and cost. Plan for NDT hold points in your welding schedule — UT inspection of a CJP weld must be completed and accepted before the joint is covered. Failing UT results require weld repair and re-test, which can significantly impact your schedule.
Common Mistakes
Not scheduling NDT hold points in the production schedule. If UT inspection is a surprise at the end of fabrication, the queue of welds needing testing can delay shipping by weeks.
Recommended Process
Identify all NDT-required welds at the start of the project (Demand Critical designations in the structural drawings). Build NDT hold points into your production schedule. Engage your NDT contractor early to confirm availability and turnaround times.