Steel Solutions Center
9.1. Non-Destructructive Examination (NDE) Methods
The most commonly used NDE method in structural steel fabrication is visual (VT). VT includes observation before, during, and after welding. Other examination methods are also used: dye penetrant (PT), magnetic particle (MT), radiographic (RT), and ultrasonic (UT). The selection of the method to be used is based on consideration of the defects anticipated in a particular type of weld as well as the defect identification capability and relative cost of each method. AWS D1.1/D1.1M Clause 8 provides information about the inspection methods and acceptance criteria. AISC Specification Chapter N provides a QA/QC plan with inspection method selections and frequencies.
9.1.2. What non-destructive examination (NDE) inspection beyond visual should be specified? What acceptance criteria should apply?
AWS D1.1/D1.1M requires that all welds be visually inspected. AISC Specification Chapter N further breaks visual inspection into tasks conducted before, during, and after welding. Specification Chapter N and Seismic Provisions Chapter J also require UT and MT of certain joints.
When specified by the Structural Engineer of Record (SER), visual (VT), magnetic particle (MT) and dye penetrant (PT) inspection imply that internal soundness adequate for the service conditions will be provided by adherence to the requirements of AWS D1.1/D1.1M. Rework required to correct profile, size, undercut or overlap, and/or excessive pinholes or cracks is considered to be part of the contract requirements. However, because these are essentially surface or near-surface inspection methods that do not describe the internal condition, rework required by the owner to correct internal discontinuities if found by other means, is considered to be a change in contract requirements.
When specified by the SER, radiographic (RT) and ultrasonic (UT inspection imply that the total internal soundness of the weld is important to the structural integrity and must meet the established standard of acceptance. Any rework required to meet this standard of acceptance is considered to be part of the contract requirement. It should be understood that there are practical limitations to the effectiveness of RT and UT, such as geometry and thickness of the joined pieces.
The different NDE methods find different defects, and they have different costs.
PT is good for making visual displays of surface breaking defects. Clearly visible red indications shine through white backgrounds in PT specimens. But it is slow. Plus, the specimen needs to cleaned properly before applying the developer. MT is faster for surface breaking flaws. But it can be dependent on preparation. Ridges in the profile can trap media and make it look like a flaw. The magnetic fields have to be oriented to find the anticipated defects and spaced closely enough to obtain continuous inspection.
RT finds volumetric inclusions. But planar separations parallel to the surface are invisible. Due to potential health hazards, RT requires specialized training and licensing. UT avoids the potential health hazards that come with using RT. UT finds planar defects parallel to the surface, but it may well miss planar flaws perpendicular to the surface. An NDE technician typically selects the method based in part on what defects they think they need to find in the weld they are examining.
9.1.5. Non-destructive inspection is specified for base metal. What acceptance criteria are appropriate?
Occasionally, severe service conditions may necessitate non-destructive examination (NDE) to verify a high degree of soundness of the parent material. The acceptance criteria in ASTM A435/A435M Specification for Straight-Beam Ultrasonic Examination of Steel Plates is available for plate. ASTM A898/A898M Standard Specification for Straight Beam Ultrasonic Examination of Rolled Steel Structural Shapes can be ordered for shapes. These standards provide scanning procedures and acceptance criteria. These examinations can be conducted at the mill or at the fabrication shop. Conducting the examinations at the shop may offer the advantage of scanning only areas of interest (e.g. connection areas of columns) and examination after fabrication processes that can make acceptable indications into unacceptable defects.