Chapter 8: Study Guide Summary

Question 1

What happens when a bar magnet is broken?

Answer 1

• Two complete bar magnets are formed, each with a north and south pole
• This process can continue until multiple magnets are created

Each broken piece retains its magnetic properties.

Question 2

What are leakage fields in magnetic particle testing?

Answer 2

Fields set up at cracks or physical discontinuities in a magnetized part

Leakage fields are crucial for detecting defects in materials.

Question 3

How does the strength of the magnetic field at a crack vary?

Answer 3

It is a function of the depth and width of the crack

The magnetic field strength differs from that of fully broken pieces.

Question 4

What is an indication in magnetic particle testing?

Answer 4

A bridge of particles visually detected at a crack

An indication is not a defect unless it exceeds specifications.

Question 5

What factors control the leakage field strength?

Answer 5

• Material permeability
• Depth and width of the discontinuity
• Relative strength of the applied magnetic flux

These factors influence the detection of defects.

Question 6

What is the primary application of dry particles in magnetic particle testing?

Answer 6

• Detecting subsurface discontinuities
• Inspecting unground welds
• Inspecting raw sand castings

Dry particles provide greater sensitivity for rough surfaces.

Question 7

What is a disadvantage of using dry particles outdoors?

Answer 7

Breezes can cause the powder to blow around

This can affect the accuracy of the inspection.

Question 8

What are the advantages of using wet suspension in magnetic particle testing?

Answer 8

• Can be applied on any surface
• Easy to determine coverage
• Best for detecting very small discontinuities

Wet suspensions hold particles on the surface and allow them to float to small leakage fields.

Question 9

What is magnetic rubber used for?

Answer 9

• Detecting very fine cracks in critical steel aircraft components
• Providing a permanent record of the inspection

It allows for examination under ideal conditions and magnification.

Question 10

What is the residual magnetization technique?

Answer 10

Application of current followed by magnetic particles

It is effective for high carbon steels with high retentivity.

Question 11

What is the continuous magnetization technique?

Answer 11

Current is applied while the suspension is on the part

This method provides the most sensitivity during inspection.

Question 12

True or false: Magnetic particle testing can only be performed on uncoated surfaces.

Answer 12

FALSE

Some parts can be inspected after coating, while others must be inspected before.

Question 13

What is critical for the successful development of a magnetic particle testing technique?

Answer 13

Determining the proper direction for the magnetic field

The flux direction is important for reliably detecting discontinuities within ±45° to the magnetic field.

Question 14

Complete magnetic particle inspection coverage requires at least two directions of the magnetic field oriented at what angles?

Answer 14

Approximately 0° and 90° apart

This ensures comprehensive detection of discontinuities.

Question 15

What types of discontinuities is circular magnetization used to detect?

Answer 15

• Stringers
• Seams
• Longitudinal discontinuities

It is also used to determine the cleanliness of steel forging stock.

Question 16

What is the primary technique used for detecting fatigue cracks?

Answer 16

Longitudinal magnetization

Fatigue cracks are usually formed transverse to the centerline of a part.

Question 17

In longitudinal magnetization, the magnetic field is directed parallel to what?

Answer 17

The axis of the coil

This orientation allows for reliable detection of cracks within ±45° of the coil’s axis.

Question 18

What factors control the flux density resulting from longitudinal magnetization?

Answer 18

• Current
• Number of turns in the coil
• Air gap between the coil and the part

Procedures should specify the orientation and position of the part with respect to the coil.

Question 19

What is the purpose of multidirectional units in magnetic particle testing?

Answer 19

To combine circular and longitudinal magnetism

They continuously vary the direction of the magnetic field while magnetizing a part.

Question 20

What device can be used to determine the flux direction?

Answer 20

Pie gage

It consists of triangular segments of soft iron and indicates the strongest direction of flux.

Question 21

Why should pie gages not be used as flux density indicators?

Answer 21

They are affected by external fluxes

Pie gages are influenced not only by the fluxes in the metal being inspected but also by external factors.

Question 22

What do magnetic field indicators measure?

Answer 22

• Relative measure of field strength
• Flux direction

Magnetic field indicators can provide a crude measure of field strength near the surface during magnetization.

Question 23

What is the ideal technique for measuring flux density?

Answer 23

Measurement within the metal

Any technique for internal measurements produces variations in permeability affecting the relationship of H and B during magnetization.

Question 24

What is the primary limitation of permanent magnets in magnetic particle testing?

Answer 24

Induced field strength is relatively low

Permanent magnets are portable and do not require external power, but their applications are limited.

Question 25

What are the two types of **yokes** used in magnetic particle testing?

Answer 25

* Electromagnetic yokes * Permanent magnets ## Footnote Yokes can provide both deep penetration and demagnetization capabilities.

Question 26

What is the output range of **portable power supplies** for magnetic particle testing?

Answer 26

100 A to 1000 A ## Footnote They typically have an input of 120V single-phase alternating current.

Question 27

What is the purpose of **demagnetization**?

Answer 27

Reorientation of magnetic domains ## Footnote Reducing the strength of the field to an acceptable level is crucial.

Question 28

What is a common technique for **demagnetization**?

Answer 28

Using an alternating current coil ## Footnote This technique helps to randomize the orientation of magnetic domains.

Question 29

Why is **demagnetization** often needed before final assembly?

Answer 29

To prevent induced fields in adjacent parts ## Footnote Residual magnetism can cause malfunctions, such as attracting metal particles into bearings.

Question 30

What is the most common method of **demagnetization**?

Answer 30

Using an alternating current coil ## Footnote This method involves removing the part from the coil while the power is still on.

Question 31

What should the initial demagnetizing field be compared to the magnetizing current?

Answer 31

Higher than the highest magnetizing current ## Footnote This ensures effective demagnetization.

Question 32

What can affect the **sensitivity** of magnetic particle testing?

Answer 32

* Surface coatings * Paint * Cladding ## Footnote Thicker coatings may require additional amperage for equivalent results.

Question 33

What must be removed from areas where electrical contact is made for **magnetization**?

Answer 33

Paint ## Footnote Paint is not a conductor and can interfere with the magnetization process.

Question 34

What should be removed from the surface before magnetic particle testing?

Answer 34

* Oil * Grease * Scale * Rust * Paint chips ## Footnote These contaminants can prevent particles from moving and affect test results.