Q: What does Magnetic Particle Inspection (MPI) detect?
A: MPI detects surface and near-surface defects in ferromagnetic materials by using magnetic fields and particles.
Q: What types of materials are tested by MPI?
A: Only ferromagnetic materials such as iron, nickel, and cobalt.
Q: Name three industries where MPI is commonly used.
A: Automotive, oil & gas, aerospace (also: structural steel, power generation).
Q: What is a “leakage field” in MPI?
A: A leakage field is a distorted magnetic field at a discontinuity that causes magnetic particles to gather, revealing defects.
Q: Why is MPI preferred over Liquid Penetrant Inspection (LPI) for ferrous materials?
A: MPI is faster, requires less surface preparation, and can sometimes detect subsurface flaws
Q: List three advantages of MPI.
A: Fast, detects fine/cracks, has few limitations on part size/shape.
Q: List two disadvantages of MPI.
A: Material must be ferromagnetic, correct field orientation is critical.
Q: What is meant by “magnetic permeability”?
A: Magnetic permeability is the ease with which a material can be magnetized.
Q: Define “retentivity” in magnetic materials.
A: Retentivity is a material’s ability to retain magnetism after the magnetizing force is removed.
Q: What are paramagnetic materials? Give examples.
A: Materials weakly attracted to magnets, with no retained magnetism after field is removed; e.g., aluminum, platinum.
Q: What are ferromagnetic materials? Give examples
A: Materials strongly attracted to magnets, which retain magnetism; e.g., iron, nickel, cobalt.
Q: What happens to magnetic domains during magnetization?
A: Domains line up to make the material magnetized, creating a strong magnetic field.
Q: Name two main types of magnetic fields used in MPI.
A: Circular (current through/around part) and longitudinal (current from coil or magnet).
Q: When is multi-directional magnetization used?
A: To ensure detection of discontinuities in any orientation
Q: Name two methods of inducing magnetism during MPI.
A: Direct magnetization (current through part) and indirect magnetization (part in coil or solenoid).
Q: Which type of electrical current is most common in MPI and why?
A: Alternating Current (AC); readily available and enhances surface particle mobility.
Q: What kind of current locates subsurface defects best?
A: Direct Current (DC), or rectified AC.
Q: What is the “right hand rule” in MPI?
A: It helps determine the direction of the magnetic field created by current flow.
Q: What is the purpose of demagnetization after MPI?
A: To remove residual magnetism, which is especially important for aircraft or sensitive components.
Q: What is a hysteresis loop, and what does it show?
A: A graph showing how a material’s magnetic induction responds to changing magnetizing force.
Q: What wavelengths are used in UV-A fluorescent MPI?
A: Wavelength around 365 nm, emitting visible green-yellow at about 550 nm.
Q: Why are fluorescent particles used in MPI?
A: Because the human eye is most sensitive to green-yellow light in near darkness, improving defect visibility.
Q: Name three types of UV light used for NDT inspection.
A: Mercury vapor arc lamps, LED lamps, Metal halide lamps
Q: What is the minimum UV-A intensity required for inspection under NRCan standards?
A: 1000 µW/cm² measured at the inspection surface, typically at 15 inches from the lamp face.
