AEROSPACE MATERIALS LAB

(ASE 224 L)

Spring 1992Midsemester Quiz2-4 p.m.WRW 102

Please attempt all four questions. The points assigned to each question are shown in ( ). This is a closed notes/homework/book exam. Space for your answers is provided in this booklet.

NAME: ______

Lab section meets (circle one)TW

1.The stress strain behavior of a metal is shown schematically below. Following yield, the stress has a dependence on the total strain,, which is given by

oon .

Where o and o are the yield stress and strain, respectively; o= 80 ksi and o=0.2%.

a.) Find the Young’s modulus of the material. (2)

b.) The following data were taken in the plastic range:

(in/in) (ksi)

0.005110

0.020127

0.040145


0.100174

Find the exponent n . (5)

c.) The metal is to be used in an application that requires a yield strength of 140 ksi. How would you obtain this? (3)

d.) What property of the material was employed in (c)? Explain how it occurs, based on dislocation models. (5)

2.9.54 kg of aluminum and 0.46 kg of copper are melted in a crucible. The mixture is stirred thoroughly and slowly cooled.

a.) Using the phase diagram below, identify and estimate when solidification starts and is complete. (2)

b.) At 600o C, identify which phases are present, their compositions and their amounts (masses). (6)

c.) What are the and  phases that appear at low temperatures? Where does the phase precipitate? (4)

d.) How would you eliminate the phase? How does strengthening then occur? (3)


3.A material is being considered for use as a turbine blade in a jet engine. The service temperature is expected to exceed 0.5 Tm , the melting point of the material. The maximum stress due to centrifugal forces is expected to be well below the yield strength of the material. Nonetheless, two failure modes are anticipated: (i) interference between the blade tip and the fan shroud or (ii) rupture.

a.) Explain why these failure modes are being considered. (2)

b.) What are primary, secondary and tertiary creep, and how are they explained in terms of dislocation models? (6 )

c.) The secondary creep rate of a material s is given by

s= A (/)n exp { -H/RT}.

For a given stress level, explain how this behavior can be used to predict rupture times at various temperatures by obtaining and examining the Larson Miller parameter. (5)

d.) How are different stress levels accounted for when using the Larson Miller parameter? (2)

4.You are given a cube of a polymeric material, a temperature cabinet and an instrument that can measure the change in length of one side of the cube.

a.) How would you determine if the polymer were completely crystalline or amorphous? (2)

b.) How does crystallization occur in polymers? (3)

c.) What is stress relaxation and how does it occur in amorphous polymers? (3)

d.) The combination of springs and dashpots shown below is to be used to model the creep behavior of a polymer. Show that the differential stress/strain law is

d  = 1 d + 1  . (4)

d t E d t 

e.) Find the creep compliance of the material. (3)