4. impact_fracture.pdf

Question 1

What is the equation estimating the plastic zone size for plane stress?

Answer 1

r_y = (K_I/σ_y)²/2π

Question 2

What is the equation estimating the plastic zone size for plane strain?

Answer 2

r_y = (K_I/σ_y)²/6π

Question 3

What are the dimensional restrictions imposed on specimens?

Answer 3

a >= 2.5 (K_IC/σ_y)²

B >= 2.5 (K_IC/σ_y)²

w >= 5(K_IC/σ_y)²

B/r_y ~= 47

Question 4

What are the 2 conditions specimen dimensions must obey?

Answer 4

1. The test specimen is large enough that linear-elastic behavior of the material being tested occurs and that the plastic zone size is negligable.
2. There is a triaxial tensile stress field present and a condition of plane strain.

Question 5

What is the procedure for determining K_IC?

Answer 5

1. Determine critical specimen dimensions (a, B, w)
2. Select a specimen geometry and make the specimens to specification. [a (initial) ~= 0.45 w]
3. Fatigue-crack the test specimen to obtain a sharp crack, extend a to 0.5w. Be sure that the load during cracking corresponds to K lower then K_Q (usually 15 ksi in^0.5) otherwise we have a large plastic zone.
4. Use fixture for holding specimen and lip gauge displacement measureing the crack opening displacement.
5. Perform test: obtain a record of load vs COD displacement.

Question 6

Review derivations in “4. impact+fracture.pdf”

Answer 6

c’mon son

Question 7

What is the dynamic fracture toughness?

Answer 7

K_Id

The relation between KIC and loading rate ˙K (ksi in^0.5/sec), usually obtained from impact tests on Charpy size specimens.

Question 8

How does the yield stress of steel change with temperature?

How is the plastic zone effected?

Answer 8

Yield stress increases sharply with decreasing temp, increasing the plastic zone considerably?

Question 9

Plot the critical fracture toughness and kinimatic fracture toughness VS temperature

Answer 9

Question 10

Plot (K_IC/σ_y)² VS Temp.

At what temp. can no valid value of K_IC be obtained?

Answer 10

Above a certain temperature the magnitude of plastic deformation increases asymptotically.

Question 11

How does fracture toughness behave with increase temperature?

Answer 11

Question 12

What factors affect K_IC at high temperatures?

Answer 12

Trick question mofo!

At high enough temps, K_IC can undgergo changes independent of any other factors.

This is due to a transition in plane strain.

Question 13

As material thickness increases, what causes a decrease in fracture toughness?

Answer 13

A triaxial state of stress

Question 14

Plot Load VS Displacement for plane strain, elastic plastic, and plastic crack tips.

Question 15

Describe the change in fracturing of a brittle material as its temperature is increased.

Answer 15

Before temp. increase, a brittle material will fracture because of forming microvoids that develop into microcracks and cause rapid (unstable) crack growth.

As temp. increases, the metal becomes more ductile so that slip occurs before microcracking, resulting in a larger plastic zone.

Question 16

What is meant by plain strain transition?

Answer 16

Where individual grains begin to undergo large amounts of plasticity (microscopic) while the rest of the specimen is in plane strain.

Question 17

How does crack toughness behave with increasing temperature?

increasing loading rate?

Answer 17

K_IC increases with Temp, decreases with loading rate

Question 18

True or False:

K_IC and K_ID can be measured at the service temperature.

K_IC and K_ID can be measured at the service loading rates.

Answer 18

False

False

Question 19

Which test is conducted for materials used at lower loading rates?

Answer 19

Charpy V-Notch Test (CVN test)

Question 20

What does NDT stand for?

Which test determines NDT

Answer 20

nil-ductility transition temperature

It is determined by the Pellini Test

Question 21

What does DT stand for? What is another name and acronym for it?

Answer 21

Drop Test, aka

Drop Weight Tear Tester (DWTT)

Question 22

What test is used when the specimen is too large for a Charpy test?

Why?

Answer 22

DWTT, because of thickness, and higher observed transition temps

Question 23

What are two different types of CVN data? What values can they be used to solve for?

Answer 23

Slow Bend CVN Data is used to predict K_IC

CVN Impact Data is used to predict K_Id

Question 24

List the steps in appoximating the entire K_IC - T curve from CVN data

Answer 24

1. obtain complete CVN impact curve and fracture appearance.
2. Obtain tensile properties corresponding both the upper and lower Charpy shelf values.
3. At the highest temp of 100% brittlle fracture appearance

K_IC = 0.45σ_ys in^0.5

4. At the 100% ductile fracture temp:

(K_IC/σ_ys)² = 5(CVN/σ_ys - 0.05)

5. At the 50% fracture appearnce transition temp. (FATT) place K_IC at 1/2 the sum of upper and lower shelf values

6.