Chapter 5: Study Guide Summary

Question 1

What is the construction material of a pie gage?

Answer 1

Very low-retentivity ferrous material

It is shaped like pie slices and fabricated from eight pieces.

Question 2

True or false: The probe must be positioned at any angle to measure the magnetic field accurately.

Answer 2

FALSE

The probe must be perpendicular to the magnetic flux for accurate measurements.

Question 3

Name other devices that can be used to measure external fields.

Answer 3

• Shim configurations
• Magnetometers
• Flux meters

These devices are similarly configured but are not commonly used.

Question 4

How should the Hall Effect probe be positioned on the part?

Answer 4

Perpendicular to the part surface

This positioning is crucial for accurate measurements.

Question 5

The face of the probe must be ______ to the field being measured.

Answer 5

perpendicular

Positioning is critical for accurate measurements.

Question 6

What is the primary purpose of a pie gage?

Answer 6

To indicate the direction of a magnetic field

It is not a good quantitative indicator of field strength.

Question 7

What are shims used for in magnetization?

Answer 7

• Indicators of field strength
• Indicators of direction

Shims are thin, low-retentivity ferromagnetic indicators that can be used on any part configuration.

Question 8

Quick Break System – MPI Bench

Answer 8

Purpose:
Enhances detection of surface cracks by rapidly collapsing the magnetic field after magnetization.

How It Works:
Applies high current to magnetize the part.
Quickly interrupts current flow (via capacitor or switch).
Causes a sharp magnetic field collapse.
Leakage fields at cracks attract magnetic particles more strongly.

Benefits:
Improves visibility of fine surface defects.
Increases inspection sensitivity.
Reduces residual magnetism in the part.

Used In:
Direct magnetization setups.
Wet fluorescent MPI.
Inspections requiring high surface sensitivity.

Question 9

What device is based on the Hall Effect for measuring magnetic fields?

Answer 9

Tesla meter

Previously known as a gauss meter.

Question 10

What are the two important factors in determining if discontinuities can be detected in a magnetic field?

Answer 10

• Direction
• Intensity

If the field intensity is too low or the direction is incorrect, indications will not be formed.

Question 11

What do field indicators measure?

Answer 11

The external residual field after demagnetization

They read in tesla (gauss) and are accurate to 0.002 T (20 G).

Question 12

Care must be exercised to ensure that the probe is on the surface of the part and perpendicular to the ______.

Answer 12

magnetic flux

Proper orientation is essential for accurate readings.

Question 13

What is the purpose of pie gages in magnetization?

Answer 13

To observe indications formed at the junction between the pie sections

The field direction is roughly 90° to the strongest indications.

Question 14

Why is it necessary to evaluate the features of the external magnetic field?

Answer 14

There is no practical way to measure the actual strength or direction of the internal magnetic field

Any device inserted into the part to measure the internal field produces a discontinuity.

Question 15

What types of devices are used to evaluate external fields produced by a magnetized part?

Answer 15

• Field indicators
• Pie gages
• Shims
• Hall effect meters
• Other devices

These devices are affected solely by the external field surrounding the magnetized object.

Question 16

Which device comes closest to measuring the actual internal magnetic field?

Answer 16

Hall effect device

It is used to evaluate the external field but provides the best approximation of the internal field.

Question 17

What does the brazed section of the pie gage indicate?

Answer 17

The strongest indication is the one most perpendicular to the magnetic flux

Indications are formed on the copper plating or shim when placed against a part being magnetized.

Question 18

What happens if the field intensity is too high?

Answer 18

Nonrelevant indications may be formed

This could mask relevant indications and increase examination time.

Question 19

What must be done to the pie gage after each use?

Answer 19

Demagnetized

This is necessary to ensure accurate measurements in subsequent uses.

Question 20

Why cant MPI be used on non-magnetic metals?

Answer 20

Non-magnetic metals don’t retain magnetic fields
Metals like aluminum, copper, titanium, stainless steel (austenitic grades), and magnesium are non-ferromagnetic. Even if you pass current through them and generate a magnetic field, they:

Don’t concentrate the field internally
Don’t create leakage fields at defects
Don’t attract magnetic particles

So, while electromagnetic fields can exist around any conductor, MPI depends on the material itself becoming magnetized, which non-magnetic metals can’t do effectively.

Question 21

True or false: The residual field in the pie gage is a measure of the residual field in the part.

Answer 21

FALSE

The residual field in the pie gage is not a measure of the part’s field intensity.

Question 22

When the current is not flowing in nonferromagnetic parts, what is the magnetic field?

Answer 22

0

The pie gage will still show an indication.

Question 23

What is the Hall Effect?

Answer 23

The production of an electromotive force within a conductor or semiconductor when there is a strong transverse magnetic field

Discovered by Edwin Hall in the early 1900s.

Question 24

What does the Hall device measure?

Answer 24

Only the external field that passes through the element

It is effective for measuring tangential or normal field direction and intensity.