Question

The non-destructive testing technique(s) for detecting internal defects in a steel component is/are

• A. X-ray tomography
• B. Ultrasonic technique
• C. Gamma radiography
  (D) Dye penetrant technique
• D. Dye penetrant technique

Hint:

A simple way to categorize NDT methods is by what they can "see": - {Internal (Volumetric):} Radiography (X-ray, Gamma), Ultrasonics. - {Surface-Breaking:} Dye Penetrant, Magnetic Particle Inspection.

Answer 1

The Correct Option is A, B, C

Step 1: Understanding the Concept:
Non-destructive testing (NDT) involves inspecting a component for flaws without damaging it. The question asks to identify techniques capable of finding defects located *inside* the material (internal or subsurface defects), as opposed to those only on the surface.
Step 2: Detailed Analysis of Each Technique:
- (A) X-ray tomography (CT): This technique uses X-rays to create cross-sectional and 3D images of an object. Differences in material density or thickness (caused by defects like porosity, voids, or inclusions) show up as variations in brightness on the image. It is a powerful method for visualizing the size, shape, and location of internal defects.
- (B) Ultrasonic technique (UT): This method uses high-frequency sound waves transmitted into the material. The waves travel through the material and are reflected by interfaces, such as the back wall of the component or an internal defect (e.g., crack, void, delamination). By analyzing the reflected signals (echoes), the presence, size, and location of the defects can be determined.
- (C) Gamma radiography: This method is similar to conventional X-ray radiography but uses gamma rays from a radioactive isotope as the radiation source. Gamma rays can penetrate very thick sections of steel. The radiation passes through the component and exposes a film or detector on the other side. Internal defects like voids or cracks are less dense than the surrounding metal and allow more radiation to pass through, creating a darker indication on the film.
- (D) Dye penetrant technique (DPT): This is a method for detecting surface-breaking defects only. A low-viscosity dye is applied to the surface and seeps into any open cracks or pores. After the excess dye is removed, a developer is applied which draws the trapped dye out, revealing the flaw. This method cannot detect defects that are entirely below the surface.
Step 3: Why This is Correct:
X-ray tomography, ultrasonic testing, and gamma radiography are all "volumetric" inspection methods, meaning they are capable of interrogating the entire volume of a component to find internal flaws. Dye penetrant testing is strictly a surface inspection method. Therefore, (A), (B), and (C) are the correct choices.