Which inspection technique might be used to uncover defects not visible from the surface?

Nondestructive testing (NDT) is a crucial inspection technique used to uncover defects that are not visible from the surface of a bridge. NDT methods allow for an in-depth examination of structures without causing any damage, which makes it especially valuable for assessing the integrity of critical components like welds, joints, and internal structures.

Common NDT techniques include ultrasonic testing, magnetic particle testing, radiographic testing, and acoustic emission testing, all of which are designed to detect subsurface defects such as cracks, disbonds, or corrosion that may compromise the structural safety of a bridge. These methods provide quantitative data about the condition of the material and are essential for informed decision-making regarding maintenance and repair strategies.

In contrast, visual inspection relies solely on what can be seen with the naked eye and is limited to surface conditions. FLIR thermography detects thermal patterns on surfaces, which can indicate certain issues but may not reveal deeper structural flaws. Weight testing assesses a bridge's performance under load but does not evaluate internal defects. Thus, NDT is the most suitable option for identifying hidden defects effectively.