Which two NDT methods are commonly used to detect hidden cracks in steel members?

• A. Ultrasonic testing and Magnetic Particle testing
• B. Visual inspection and Thermography
• C. Radiography and Acoustic Emission
• D. Liquid penetrant and Eddy current

Answer: (A)

The ability to detect cracks that aren’t visible on the surface relies on methods that can look inside or at the near-surface of steel. Ultrasonic testing uses high-frequency sound waves that travel through the metal and reflect back from flaws like cracks. When a crack is present, the reflected signal changes in a way that reveals its location, giving information about internal cracks deep inside the member. Magnetic Particle Testing magnetizes the steel and sprinkles iron particles on it; cracks disrupt the magnetic field, so the particles gather at the flaw and create a visible indication on the surface or just beneath it. This makes it especially effective for revealing cracks that are open to the surface or close to it, which is common in steel components.

Using these two methods together covers both hidden, internal flaws and surface-adjacent cracks, providing a practical and efficient approach for inspecting steel structures. Other methods exist for internal imaging or surface screening, but this pairing is a widely used, complementary combination for detecting hidden cracks in steel members.