MAE 160 – Mechanical Behavior of Materials Homework 3 Due February 10
th
(Saturday) at midnight. 1. In a laboratory experiment for potential creep deformation, it was found that the creep rate (
) of an alloy is 0.5% per hour at 780°C and 2.8 ´
10
-2
% per hour at 650°C. (a) What is the activation energy for creep in the given temperature range? (b) At a temperature of 550°C, what is the estimated creep rate? 2. For a certain high-temperature alloy, failure was reported after 4100 hrs at 680°C when subjected to a stress level of 270 MPa. If the same stress were applied at 725°C, how long would the sample be expected to last? (Assume Larson-Miller constant 20 hr) 3. The following creep data were taken on an aluminum alloy at 400°C and a constant stress of 25 MPa. Plot the data as strain versus time, then determine the steady-state creep rate. Note: The initial and instantaneous strain is minimal and can be ignored. Time (min) Strain 0 0.000 2 0.025 4 0.043 6 0.065 8 0.078 10 0.092 12 0.109 14 0.120 16 0.135 18 0.153 20 0.172 22 0.193 24 0.218 26 0.255 28 0.307 30 0.368 4. Calculate the softening temperature for a soda-lime-silica glass at which the viscosity is equal to 10
7
Pa•s if the activation energy for viscous flow is 250 kJ/mol and the viscosity at 1,000°C is 10
3
Pa•s. 5. The viscosity of a SiO
2
glass is 10
14
P at 1,000°C and 10
11
P at 1,300°C. What is the activation energy for viscous flow in this glass? Note: 1 P = 0.1 Pa•s. 6. Determine the activation energy for deformation for the three glasses from the following data. (Hint: plot ln viscosity vs. 1/T.) Temperature (°C) viscosity High-lead glass 400 1E+14
ε
