5: Discontinuities

Question 1

What should you do with “discontinuities” producing a weak or questionable indication?

Answer 1

Test with another method

Question 2

What is a butt joint?

Answer 2

2 pieces joined together on SAME plane

Question 3

What is a t-joint?

Answer 3

2 pieces joined that are 90 degrees to each other

Question 4

What is a lap joint?

Answer 4

2 pieces joined by overlapping and welding seam between them

Question 5

What are corner joints?

Answer 5

2 pieces joined that are 90 degrees, forming an “L”

Question 6

Elongated porosity is called what?

Answer 6

Wormholes

Question 7

Where does lack of fusion occur?

Answer 7

Between weld & parent metal OR between weld metal & and weld metal

Question 8

Slag inclusions are found in welds produced by a process that uses what? (SMAW, SAW, FCAW)

Answer 8

Flux

Question 9

Oxide inclusions produce similar indications to what?

Answer 9

Porosity

Question 10

What type of inclusions are virtually undetectable using MPI?

Answer 10

Tungsten

Question 11

What type of cracking may appear as longitudinal, traverse or star shaped?

Answer 11

Hot cracking

Question 12

What is a common cause of hot cracking?

Answer 12

Shrinkage stresses during solidification

Question 13

What can occur immediately on cooling or after a period of hours/days?

Answer 13

Cold/under-bead cracking

Question 14

Where does lamellar tearing occur?

Answer 14

Base metal of structural welded shapes

Found at plate edges/areas of cut cross section

Question 15

What are the 3 main causes of service discontinuities?

Answer 15

1. Over-stress
2. Fatigue stress
3. Corrosion

Question 16

80% of failures involving non-stationary parts are attributable to what?

Answer 16

Fatigue

Question 17

Fatigue properties are usually related to what?

Answer 17

Yield point

Question 18

What type of fatigue can result in fracture?

Answer 18

Thermal fatigue

Question 19

Describe creep:

Answer 19

A solid material that tends to move slowly/deform under mechanical stress

Question 20

What are the 2 key contributors to creep?

Answer 20

1. Cyclic stresses

2. High temp

Question 21

What is cyclic stress?

Answer 21

Distribution of forces that change over time in a repetitive fashion

Question 22

What type of fatigue can lead to rapid failure and is important to detect ASAP?

Answer 22

Corrosion fatigue

Question 23

If a part contains internal stresses, what will appear on surface where stresses are the highest?

Answer 23

A pit or notch (corrosion or oxidation)

Question 24

What is stress corrosion cracking?

Answer 24

Constant tensile stresses in a corrosive environment

Question 25

What is the difference between corrosion fatigue cracking and stress corrosion cracking?

Answer 25

Corrosion fatigue can start from 1 PIT but stress corrosion requires a larger area

Question 26

Stress and corrosion combined leads to crack growth along what?

Answer 26

Grain boundaries

Question 27

What is erosion corrosion?

Answer 27

Degrading of material by abrasion

Question 28

What are the 2 types of erosion corrosion? Describe them

Answer 28

1. Fretting corrosion: 2 components rubbing together causing small particles on surface to bond together, then torn away by continued rubbing 2. Cavitation corrosion: pitting accelerated due to constant flow of liquid against surface

Question 29

What’s the easiest way to minimize fretting corrosion?

Answer 29

Less movement and a looser fit

Question 30

What is hydrogen cracking?

Answer 30

Fracturing that occurs due to corrosive environments produced by hydrogen media while exposed to applied tensile/residual stress **follows grain boundaries

Question 31

How can hydrogen be introduced?

Answer 31

Electroplating, pickling or welding in a moist area

Question 32

What is the first step in distinguishing relevant and non relevant indications?

Answer 32

Know whether the part was in service or not

Question 33

What does magnetic writing result from?

Answer 33

One ferromagnetic part rubbing against another, both will contain some residual magnetism, magnetic poles will be created, revealing themselves in lines or “writing”

Question 34

How can longitudinal magnetism in a coil or yoke legs create false indications?

Answer 34

Magnetic poles will be created at the ends where lines of force are most densely packed so particles will accumulate there

Question 35

How can a part that was subjected to cold working cause false indications?

Answer 35

The change in permeability to that area will attract magnetic particles when magnetized

Question 36

Non relevant indications where there is an abrupt change of section will appear how?

Answer 36

More fuzzy and broad

Question 37

How do cracks appear in areas with abrupt changes of section?

Answer 37

They flow with less uniformity and are more jagged

Question 38

If subsurface discontinuities are not a concern, what should be used whenever possible? And why?

Answer 38

AC will reduce non relevant indications and improve sensitivity

Question 39

When are magnetic rubber inspections used?

Answer 39

Surface/subsurface discontinuities with poor visual access

Question 40

What do magnetic rubber inspections form?

Answer 40

A cast replica

Question 41

What does Alginate impression compounds involve?

Answer 41

“Lifting” magnetic particles from inaccessible areas which can’t be accurately viewed using black light

Question 42

Where are flash line found and what are they caused by?

Answer 42

Cracks that may be found on a forging sheet to improper trimming of flash

Question 43

What type of defects are seams and what are they caused by?

Answer 43

Processing defects caused by surface cracks that are stretch out and lengthened during rolling operations of a bar

Question 44

What type of discontinuity is segregation?

Answer 44

Inherent

Question 45

What is pipe caused by?

Answer 45

failure to remove a shrink cavity that forms at top of ingot

Question 46

Where is a toe crack found?

Answer 46

Where face of weld joins base metal

Question 47

When do hot cracks develop?

Answer 47

During cooling process of weld due to shrinkage