Chapter 4: Study Guide Summary

Question 1

What is required to produce longitudinal magnetization in a ring?

Answer 1

Use a coil

This method can be applied to nuts, bearing races, washers, etc.

Question 2

What is the general rule of thumb for circular magnetization using direct current?

Answer 2

800 A to 1000 A per 25 mm (1 in.) of diameter

This is a recommended value for effective magnetization.

Question 3

How can a ring be magnetized?

Answer 3

• In the circular direction
• In the longitudinal direction

A ring can be viewed as a bar bent into a circle or as a slice cut from a hollow cylinder.

Question 4

If the radius of the bar is R and the field at the surface is B, what will the field be at a distance 2R from the center?

Answer 4

B/2

This illustrates how the field strength decreases with distance from the center.

Question 5

Fill in the blank: There is no field inside of the _______ conductor.

Answer 5

hollow

The external magnetic field is greatest at the outer surface and decays with distance from the surface.

Question 6

For complex configurations, what is required to estimate the magnetic field intensity?

Answer 6

Mechanical or electronic devices

Equations can be used for simple shapes.

Question 7

What is circular magnetization produced by?

Answer 7

• Passing current directly through the part (direct magnetization)
• Passing current through a central conductor (indirect magnetization)

Circular magnetization creates a field around the conductor.

Question 8

What type of field flows out into the upset portion of a magnetized bar in a coil?

Answer 8

Longitudinal field

The field tends to become radial along the surface B, C.

Question 9

The field strength is much greater within a magnetic conductor carrying current compared to a _______ conductor.

Answer 9

nonmagnetic

The field strength outside the conductor is the same for both solid and hollow cylinders.

Question 10

Regardless of the technique used for establishing the magnetic field, it is essential to know the intensity of the magnetic field in the area of interest. True or False?

Answer 10

TRUE

This is particularly important for parts with complex shapes.

Question 11

What is the recommended current for circular magnetization using alternating current?

Answer 11

500 A to 600 A per 25 mm (1 in.)

This value is lower than that for direct current.

Question 12

In nonmagnetic conductors, the external field must be calculated from the _______.

Answer 12

center of the cylinder

The external field will be the same for both solid and hollow conductors.

Question 13

What is longitudinal magnetization performed in?

Answer 13

a coil

The strongest magnetic field is near the inside surface of the coil.

Question 14

Equation used to determine ampere
turns needed for longitudinal magnetization using an encircling coil with the part adjacent to the inside surface of the coil.

Answer 14

Question 15

What happens to the field strength if the central conductor is placed adjacent to the inner circumference of a hollow cylinder?

Answer 15

Field strength is much stronger at the cylinder wall nearest to the conductor

Proper placement of the conductor is crucial for effective magnetization.

Question 16

The magnetic field when passing current through a part is at _______ to the direction of the current path.

Answer 16

90°

The length of the part does not affect the strength of the circular field.

Question 17

In a central conductor carrying direct current, the magnetic field rises from 0 at the center to a maximum at the _______.

Answer 17

outer surface

The field will drop slightly in the space between the central conductor and the part.

Question 18

What is the field strength inside the hollow area of the conductor?

Answer 18

No field

There is no magnetic field present inside the hollow area.

Question 19

The magnetic field set up by a yoke is essentially longitudinal to what?

Answer 19

A line drawn between the contact points

This orientation is crucial for effective weld testing.

Question 20

In circular magnetization, the magnetic field is perpendicular to what?

Answer 20

Current flow

This applies to simple parts like rods, nuts, and bolts.

Question 21

Why are permanent magnets rarely used?

Answer 21

Because the magnetic field cannot be turned off and on

This makes them difficult to properly place and remove, especially when high-strength magnets are used.

Question 22

How much current is required to magnetize a 50 mm (2 in.) round bar?

Answer 22

2000 A

The round bar has a diagonal equal to its diameter.

Question 23

When magnetizing a square bar, what is required compared to a round bar?

Answer 23

More amperage

The magnetic field in the square bar is greatest along the center of the face.

Question 24

How can a longitudinal field be induced into a large ring?

Answer 24

Wrap a cable coil around the ring

This allows for a high flux density to be created in the ring.

Question 25

True or false: The direction of a **magnetic field** is unpredictable for symmetrical shapes.

Answer 25

FALSE ## Footnote The direction is predictable as long as the item being magnetized is constant and symmetrical.

Question 26

What is the primary use of **alternating current** in magnetic testing?

Answer 26

To detect surface discontinuities ## Footnote The depth of penetration of the magnetic field is very shallow.

Question 27

If the radius of the bar is **R** and the field at the surface is **B**, what is the field at a distance **2R** from the center?

Answer 27

B/2 ## Footnote This demonstrates how the magnetic field strength decreases with distance.

Question 28

What is the field strength at the **center of the solid conductor**?

Answer 28

0 ## Footnote The field strength is also 0 at the inside surface of the hollow conductor.

Question 29

What happens to the **magnetic field strength** at the outer surface of a conductor?

Answer 29

It is about the same for both hollow and solid conductors ## Footnote The field strength rises sharply from the inside of the metal to near the surface.

Question 30

What happens to the magnetic field if a part surrounds the **central conductor**?

Answer 30

Rises sharply to a maximum at the inside surface of the part ## Footnote The field then drops through the part to an intensity of f.

Question 31

In a **central conductor** carrying direct current, the magnetic field rises from 0 at the center to a maximum at what point?

Answer 31

Outer surface of the central conductor ## Footnote The field drops slightly in the space between the central conductor and the surrounding part.

Question 32

What is a disadvantage of the **direct contact technique** for magnetizing a ring?

Answer 32

It is very susceptible to burns ## Footnote This technique requires two steps for magnetization.

Question 33

How much current is required to magnetize a **50 mm (2 in.) square bar** with a 71 mm diagonal?

Answer 33

2820 A. Square bar diagonal is taken to be the diameter of a bar. ## Footnote This is based on the 1000 A per 25 mm (1 in.) rule.

Question 34

What types of current can be used for magnetization techniques?

Answer 34

* Alternating current * Direct current ## Footnote The choice depends on various factors discussed in Chapter 3.

Question 35

What happens to the magnetic field strength at the surface of a conductor as the **diameter** increases?

Answer 35

Decreases ## Footnote Higher current must be used to maintain the same flux density at the surface.

Question 36

When calculating amperages for coils, which factors must be considered? List them.

Answer 36

* Number of turns in the coil * Location of the part (edge or center) * Length and diameter of the part * Radius of the coil * Fill-factor constant ## Footnote These factors influence the magnetization process.

Question 37

What is an alternate technique for obtaining a **longitudinal field** in a large ring?

Answer 37

Place the ring in a coil perpendicular to the plane of the coil ## Footnote The field will flow along two sides of the ring in a longitudinal direction.

Question 38

What are **prods or yokes** used for in magnetization?

Answer 38

* Secure high-strength fields * Avoid extremely high currents ## Footnote They are effective for testing without needing to magnetize the whole weldment.

Question 39

As the size of the part being magnetized increases, the simple rules of thumb governing magnetization often become __________.

Answer 39

impractical ## Footnote Rules of thumb should only be used on objects of uniform cylindrical shape.

Question 40

What is the procedure for magnetizing a **single part with multiple sections** of different diameters?

Answer 40

* Magnetize and evaluate the section requiring the least amperage first * Proceed to the next highest amperage section * Continue this process until all sections are magnetized ## Footnote If the highest amperage section is magnetized first, smaller sections may be overmagnetized, leading to false indications.

Question 41

The **1000 A per 25 mm (1 in.) rule** is impractical when the cross section of a part varies greatly. True or False?

Answer 41

TRUE ## Footnote Using this rule can lead to unrealistically high current requirements.

Question 42

What are the two forms of **magnetization** discussed?

Answer 42

* Circular magnetization * Longitudinal magnetization ## Footnote These methods can be achieved using electric current.

Question 43

What should always be considered when **magnetizing** an irregularly shaped part?

Answer 43

The probable path ## Footnote This consideration is crucial for effective magnetization.

Question 44

Where is the **flux density** strongest in a coil?

Answer 44

near the inside surface of the coil ## Footnote The flux density decreases toward the center of the coil.

Question 45

What technique allows a ring to be magnetized in the **circular direction**?

Answer 45

Induced magnetism using a toroidal magnetic field ## Footnote This method can magnetize the complete ring in one operation.

Question 46

What may be necessary to evaluate the **magnetization** of a part depending on its diameter?

Answer 46

Rotating the part and evaluating several sections of the length ## Footnote This is per ASTM E 1444 / E 1444 M standards.

Question 47

What can be assumed about the **flux density** in a part that is relatively uniform in permeability and cross section?

Answer 47

it will be uniform over the cross section except at each end ## Footnote For complex parts, the flux density will not be uniform.

Question 48

The external field for solid and hollow **nonmagnetic conductors** is calculated from what point?

Answer 48

Center of the cylinder ## Footnote The external field is the same for both solid and hollow conductors.

Question 49

What is the **cross-sectional area** requirement for low fill-factor coils?

Answer 49

10 or more times the cross-sectional area of the part being inspected ## Footnote This requirement ensures effective inspection using low fill-factor coils.

Question 50

What feature was designed to help compensate for the decreased flux density at the ends of the part?

Answer 50

**quick-break feature** ## Footnote This feature addresses the non-uniform flux density in complex parts.

Question 51

To determine the current needed for magnetizing irregularly shaped bars, what should be compared?

Answer 51

Perimeter of the cross section of the bar with a bar having the same diameter ## Footnote This helps estimate the required current based on surface area.

Question 52

What is the field strength at the center of a **solid conductor**?

Answer 52

0 ## Footnote There is no field inside the hollow area of the conductor.

Question 53

The current density J in a conductor is equal to the amperage I divided by the _______.

Answer 53

cross-sectional area ## Footnote If the cross-sectional area is increased, current density decreases and magnetic flux density decreases.

Question 54

What is the **direct magnetizing method** for a ring?

Answer 54

Flow current through the ring as a conductor ## Footnote For complete magnetization, the ring should be rotated at least 90°.

Question 55

What is the **diagonal dimension** of a square bar that is 50 mm per side?

Answer 55

71 mm (2.82 in.) ## Footnote This is calculated using the Pythagorean theorem.

Question 56

How should a ring be rotated to evaluate the other two quadrants after magnetization?

Answer 56

Rotate 90° ## Footnote A 60° rotation and three separate inspections are preferred.

Question 57

What happens to the magnetic field when a part surrounds the **central conductor**?

Answer 57

Field rises sharply to a maximum at the inside surface of the part ## Footnote The field then drops through the part to an intensity of f.

Question 58

What must be done if the **largest section** of a part must be magnetized first?

Answer 58

Demagnetize the part between magnetizing operations ## Footnote This process can be time-consuming and may cause problems in other areas.

Question 59

What is a disadvantage of using **permanent magnets** compared to electromagnets?

Answer 59

They would be very heavy and awkward to handle to obtain equivalent field strength ## Footnote The placement and use of permanent magnets is similar to that for alternating current or direct current yokes.

Question 60

For proper current density, each **13mm (0.5 in.)** cross-sectional element would require somewhat more than how many amperes?

Answer 60

500A ## Footnote This leads to a total of somewhat more than 2000 A for the web.

Question 61

To maintain a constant flux density, what must be increased as the cross-sectional area is increased?

Answer 61

Amperage ## Footnote This is necessary because current density decreases with an increase in cross-sectional area.

Question 62

Why is **alternating current power** excellent for locating fatigue cracks?

Answer 62

Because fatigue cracks are always open to the metal’s surface ## Footnote This allows for effective detection using alternating current.

Question 63

Equation used to calculate the current required for direct contact magnetization of oddly shaped uniform cross sections:

Answer 63

Question 64

What is the **cross-sectional area** requirement for a coil to be considered a **Cable Wrap** or **High Fill-Factor Coil**?

Answer 64

Less than twice the cross-sectional area ## Footnote This specification is important for the design and efficiency of the coil.