Which of the following is true of magnetic particle inspection?
a. The magnetic field is always at right angles to the direction of current flow.
b. Once a part has been magnetized it can no longer be used for its intended purpose.
c. Protective eyewear should be worn to prevent injury from ferromagnetic particles.
d. Discontinuities that are perpendicular to the source may not be seen.
The correct answer and explanation is :
The correct answer is:
c. Protective eyewear should be worn to prevent injury from ferromagnetic particles.
Explanation:
Magnetic Particle Inspection (MPI) is a widely used nondestructive testing (NDT) technique for detecting surface and near-surface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. This technique is especially effective for identifying cracks, seams, laps, voids, or other structural flaws.
MPI works by inducing a magnetic field into a component. When a magnetic field encounters a discontinuity such as a crack, the field is distorted or “leaked” at the defect. Finely divided ferromagnetic particles, either dry or suspended in a liquid, are then applied to the surface. These particles are attracted to areas of flux leakage, gathering at the defect and forming a visible indication under appropriate lighting conditions or ultraviolet light (if fluorescent particles are used).
Let’s examine the options:
- Option A is incorrect. The magnetic field direction depends on how magnetization is applied—whether via direct or indirect magnetization—and can vary. It is not always at right angles to the current flow.
- Option B is false. A part can be demagnetized after testing and reused for its intended purpose. Magnetization is temporary and can be removed using a demagnetizer.
- Option C is correct. Protective eyewear is important during MPI, especially when using dry magnetic particles or air-propelled particle applicators, as these can be inhaled or enter the eyes, causing irritation or injury. Safety measures, including proper PPE (personal protective equipment), help protect technicians from exposure to fine particles and chemicals.
- Option D is misleading. Discontinuities parallel (not perpendicular) to the magnetic field may not be easily detected. Cracks or flaws perpendicular to the magnetic field produce strong flux leakage and are therefore more visible.
Conclusion:
Option C is correct because it emphasizes the importance of safety in MPI, highlighting the risk of exposure to ferromagnetic particles, which can harm the eyes without protection.