Magnetic Testing Fundamentals

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Questions and Answers

What direction is the force in relation to the magnetizing current according to the right hand rule?

At 45 degrees

At right angles (correct)

Opposite

Parallel

Which magnetization method involves passing current directly through the part?

Indirect magnetization

Thermal magnetization

Direct magnetization (correct)

Electromagnetic induction

When magnetizing an object, what are the electrons in materials compared to?

Neutrally charged atoms

Tiny electrical circuits

Massless particles

Little magnets (correct)

In magnetic field theory, what must defects be in relation to the lines of force?

Perpendicular (B) Signup and view all the answers

What is the smallest permanent magnet in magnetic field theory known as?

Domain (A) Signup and view all the answers

Which phenomenon describes the lag between magnetization and the external magnetic field?

Hysteresis (A) Signup and view all the answers

What defines the ease with which a material can be magnetized?

Permeability (B) Signup and view all the answers

What is the term for the field remaining in a ferromagnetic material after the magnetizing force is reduced to zero?

Residual field (C) Signup and view all the answers

Which property differentiates high residual field materials from low residual field materials?

Coercive force (D) Signup and view all the answers

Which of the following has a strong external magnetic field at its ends?

Permanent magnet (A) Signup and view all the answers

Which method is used to specify the direction of magnetic fields?

Right Hand Rule (A) Signup and view all the answers

What technique should be used to verify adequate field distribution in a given location?

Use a QQI (C) Signup and view all the answers

Which materials exhibit the strongest magnetic properties?

Ferromagnetic (B) Signup and view all the answers

What is the correct relationship between permeability and other magnetic properties?

Permeability is compared with other properties in magnetic field theory. (D) Signup and view all the answers

What effect does alternating current (AC) have during surface examinations?

Stays only on the surface (B) Signup and view all the answers

What is the purpose of demagnetization in the MT process?

To eliminate residual magnetic fields (A) Signup and view all the answers

What type of current is carried by a solid conductor of a magnetic material in image 38?

Alternating current (A) Signup and view all the answers

What happens to lines of force in a magnet?

They travel from north pole to south pole. (D) Signup and view all the answers

Which current type is known as Full Wave Direct Current (FWDC)?

FWDC (B) Signup and view all the answers

Which process involves aligning the domains in a magnetic material?

Magnetizing an object (A) Signup and view all the answers

How is the magnetic field distributed in a hollow conductor of a magnetic material carrying alternating current?

In and around the conductor (B) Signup and view all the answers

Which characteristic is unique to a nonmagnetic hollow conductor carrying direct current as described in the content?

Direct current with field in and around (C) Signup and view all the answers

Which kind of field distribution involves aligning magnetic forces across the width of a piece?

Transverse field (A) Signup and view all the answers

Which of the following is a characteristic of skin effect in alternating current (AC)?

Restricts AC to the surface (D) Signup and view all the answers

What does the CBC DC conductor carry inside a hollow nonmagnetic central conductor?

Direct current (C) Signup and view all the answers

What happens to particles in an alternating current (AC) magnetic field?

Are given mobility to move (B) Signup and view all the answers

Which term describes the conductor that has a magnetic material with alternating current?

Magnetic hollow (B) Signup and view all the answers

What is the primary use of magnetizing current in magnetic testing?

To create a magnetic field for detecting defects (A) Signup and view all the answers

What type of conductor has a direct current with a solid nonmagnetic material as detailed in the content?

Nonmagnetic solid (D) Signup and view all the answers

In which scenario is the term 'magnetic hollow AC' applicable based on the content?

Magnetic material with alternating current (C) Signup and view all the answers

How is magnetic distribution categorized in a magnetic solid under direct current?

In and around the conductor (A) Signup and view all the answers

What method is used to eliminate existing magnetization in a part?

Applying a suitably intense magnetic field in the opposite direction (C) Signup and view all the answers

What is required for a DC step-down demagnetization process?

Direct current reversing direction completely (A) Signup and view all the answers

What factor should be considered when orienting a part for demagnetization?

The orientation of the part's long axis in an east/west direction (D) Signup and view all the answers

How long does the DC step-down reversing demagnetization process typically take?

About 30 seconds (C) Signup and view all the answers

What is one benefit of using DC step-down demag?

It offers very effective and complete penetration (B) Signup and view all the answers

What is a notable disadvantage of the DC step-down demag method?

It needs large, heavy equipment (A) Signup and view all the answers

Which technique is suggested for yoke demagnetization?

Placing yoke in AC (A) Signup and view all the answers

What is the main purpose of passing a part through an energized coil?

To demagnetize the part (A) Signup and view all the answers

In Half Wave Direct Current, what happens to the alternating current flow?

It is blocked (D) Signup and view all the answers

What is a characteristic of the magnetic field in Half Wave DC?

Flux density is zero at part center (D) Signup and view all the answers

What is a consequence of the pulsating effect in Half Wave DC?

Enhanced particle mobility (C) Signup and view all the answers

What depth of subsurface is difficult to detect unless it is larger than a specific size?

0.060 inches (D) Signup and view all the answers

How does Full Wave DC differ from Half Wave DC in terms of AC flow?

Negative portions are inverted (D) Signup and view all the answers

What design feature of Three Phase FWDC enhances indication formation?

Quick break circuit (A) Signup and view all the answers

What is the goal of demagnetization in magnetic testing?

Reducing magnetism (B) Signup and view all the answers

What method is used in demagnetization to reverse the magnetic field?

Field reversal (A) Signup and view all the answers

Study Notes

Fundamentals of Magnetic Testing (MT)

Section Agenda: Right Hand Rule, Magnetization Methods, Magnetic Field Theory, Magnetizing an Object, Domains, Hysteresis/Hysteresis Loops, Permeability vs. other properties

Right Hand Rule

The direction of magnetic lines of force is always perpendicular to the magnetizing current.
Defects need to be oriented perpendicular to the lines of force to be detected.

Magnetization Methods

Direct Magnetization: Passing a current directly through a part being tested.
Indirect Magnetization: Bringing the part under the influence of an external magnetic field.

Magnetic Field Theory

Domains: Smallest permanent magnets composed of quadrillions of atoms and roughly the size of a pinhead.
Domains align in the direction of the magnetic field when it is applied, causing a part to become magnetized.
In unmagnetized materials, domains are randomized, neutralizing each other's effects.
Applying a magnetic force causes domains to align, producing magnetization.

Hysteresis/Hysteresis Loops

A hysteresis loop shows the relationship between induced magnetic flux density (B) and magnetizing force (H).
Hysteresis: The lagging magnetic effect when the magnetizing force changes.
Coercive Force: The reverse magnetizing force needed to reduce the residual field to zero.
Residual Field: The field remaining in a ferromagnetic material after the magnetizing force is reduced to zero.

Permeability vs. Other Properties

Permeability: A measure of how easily a material can be magnetized.
Low permeability materials are difficult to magnetize.
High permeability materials are easily magnetized.
High/Low permeability relates to high/low reluctance and high/low retentivity respectively.

Magnetic Poles

Magnetic Poles: Regions of strongest magnetic field at both ends of a magnet (North and South).
Magnetic field lines travel from north pole to south pole.

Permanent Magnets

Permanent magnets are made from hard ferromagnetic materials. These include alnico and ferrite.
These materials are magnetized during fabrication and are difficult to demagnetize.

Material Classification

Ferromagnetic: High permeability, strongly attracted by magnets (Iron, Nickel, Cobalt).
Paramagnetic: Slightly attracted by magnets (Aluminum).
Diamagnetic: Slightly repelled by magnets (Gold, Silver, Copper).

Magnetic Fields

Circular Field: Magnetic field surrounding an electrical conductor when a current is passed through it. Not easily detected with usual instruments.
Longitudinal Field: Magnetic field with flux lines running parallel to the part’s axis. Detected with instruments without difficulty.

Defect Orientation

Defects need to be positioned perpendicular to the magnetic field lines, otherwise may be missed.

Permeability

Provides a measure of how a material responds to a magnetic field.
Permeability is relative, as compared to the permeability of a vacuum.
Table of Relative Permeability for various materials.

Section Review

Current types: AC, HWDC, FWDC, Three-Phase FWDC
Magnetizing Current: The current used to magnetize a part during a magnetic test.

Rectification in MT

Method to convert AC to DC, used in half-wave and full-wave rectifiers to obtain direct current.

Demagnetization

Process for reducing or eliminating existing magnetization in ferromagnetic material.

Curie Point

Temperature where a material loses its magnetic properties.

Depth of Penetration

Depth into which a magnetic field penetrates a material. Subsurface defects are harder to identify with MT if the depth of penetration is too shallow.

Yoke Field Distribution and Coil Field Distribution

Characteristics of magnetic fields and flux distribution in various magnetic coil arrangements.

Transverse Field

Field direction perpendicular to the part's axis.

Circular Field versus Longitudinal

Features differentiating circular and longitudinal magnetic fields.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

Explore the key concepts of magnetic testing, including the Right Hand Rule, magnetization methods, and the underlying magnetic field theory. This quiz will cover essential topics like domain alignment, hysteresis, and permeability in the context of magnetic materials. Enhance your understanding of how magnetic fields are utilized in testing applications.