3. Chap1_cont.pdf

Question 1

What type of deformation occurs at the crack tip?

Answer 1

A plastic zone forms around a crack tip due to very high stresses.

Question 2

What is the equation for displacement of a crack surface when the body it is in is under tensile stress?

Answer 2

δ = K_I/Eσ_y = G/σ_y

Question 3

What are five reasons for conducting mechanical testing to evaluate materials resistance to fracture? State which are comparitive and which are specific.

Answer 3

1. To determine whether there is a strong probability that catastrophic failure will occur in a particular material under a given loading condition. (comparitive)
2. Choose between potential materials for a given application. (comparitive)
3. Quality control tests by the materials producers. (comparitive)
4. Analysis of failures that have occured in service. (specific)
5. Obtain data which can be directly used for design purposes. (specific)

Question 4

What is the most popular fracture test? Why?

Answer 4

the Charpy V-Notch Impact test. It is fast and inexpensive.

Question 5

What is the Charpy V-notch test?

Answer 5

Hitting a sample containing a notch with a swining pendulum.

The height of the pendulum after the swing indicates the energy absorbed during fracture.

Results usually given as a function of temperature.

Question 6

What are 3 macroscopic modes of fracture?

Answer 6

1. Flat fibrous mode: stable form of normal rupture parallel to the plane of the notch.
2. Flat crystalline mode: indicates cleavage (brittle fracture)
3. Shear lip zone at 45 degrees to the notch plane: fracture occurs by shear rupture.

Question 7

List the criteria for evaluating a material:

Answer 7

1. The average energy criterion: temp. corresponding to an energy that is one half of the difference between the max and min energies.
2. The ductility transition temperature: The temp corresponding to an arbitrary low value of impact energy such as 15 ft lbs
3. Propagation Transition temperature: the highest temp at which cleavage appears on the fracture surface
4. The 50% crystalline temp.
5. The 70% crystalline temp.
6. Initiation Transition Temp.: the lowest temp where fracture initiates by normal fracture.
7. Nil Ductility Temp.: the highest temp at which the fracture is 100% crystalline.

Question 8

What is the drop weight tear test?

Answer 8

Similar to Charpy, except a large plate with dimensions 1x5x18” with a 2” deep embeded weld as a notch starter.

Question 9

What is the notch tensile test?

When is it useful?

Answer 9

Useful in evaluation of the tougness of sheet metal specimens whic are too thin to be tested in a Charpy or drop weight tear test.

Consists of a 60 degree notch with a 0.0001 rad in a cylindrical or flat specimen such that the notched cross section is less or equal to 1/2 the load bearing area in the gage length.

Question 10

Define Notch Strength Factor.

Answer 10

NSR = σ_net (for notched specimen at max load)

              UTS of un-notched specimen

For ductile materials and low stress concentration factors:

NST > 1.00

Question 11

What is the Robertson Test?

Answer 11

Define the crack arrest temperature (CAT): the highest temp at which unstable crack propagation can occur. CAT depends on the applied stress and plate thickness.

Question 12

What is the Pellini Drop Weight Test?

Answer 12

Brittle weld bead is deposited on a surface.

A small notch (1/4” long, 1/2” deep) is introduced into the bead

The plate is immersed in a constant temp bath.

If the plate is restricted in the middle such that it deforms elastically, the bending stresses will not exceed the yield stress.

Crack propagation along the tensile face will be occuring at the yield stress σ_Y.

We degine the NDT as the highest temp at which a small (1/4” or less) crack can initiate fracture at the yield stress.

The temp is varied until a temp is reached where propagation across the tension face no longer occurs (NDT Temp.)