F2020_HW8-Questions

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Oregon State University, Corvallis *

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Mechanical Engineering

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Feb 20, 2024

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1/5 MATS 322 Homework #8 Due December 1 st (Tue) at 8 pm (Submit PDF through Canvas-gradescope ) Name: ___________________________________ Student ID #: _____________________________________ From lecture 1. The following creep data were taken on an aluminum alloy at 480°C (900°F) and a constant stress of 2.75 MPa (400 psi). Plot the data as strain versus time, then determine the steady-state or minimum creep rate. Note: The initial and instantaneous strain is not included. Time (min) Strain Time (min) Strain 0 0.00 18 0.82 2 0.22 20 0.88 4 0.34 22 0.95 6 0.41 24 1.03 8 0.48 26 1.12 10 0.55 28 1.22 12 0.62 30 1.36 14 0.68 32 1.53 16 0.75 34 1.77

2/5 2. (a) Steady-state creep data taken for an iron at a stress level of 140 MPa are given here: ε ̇ (h –1 ) T (K) 6.6 × 10 –4 1090 8.8 × 10 –2 1200 If it is known that the value of the stress exponent n for this alloy is 8.5, compute the steady-state creep rate at 1300 K and a stress level of 83 MPa. (b) Steady-state creep rate data are given in the following table for a nickel alloy at 538 ° C (811 K): ε ̇ (h –1 ) σ (MPa) 10 -7 22.0 10 -6 36.1 Compute the stress at which the steady-state creep is 10 -5 h − 1 (also at 538 ° C).

3/5 From lab (Week 7) 1. Briefly describe the materials used in the SEM fractography demo. Describe the preparation of the materials for SEM and describe the imaging mode used. (week 8 lab demo) 2. (a) Figure 1 is an SEM image from the furnace-cooled 1018 steel Charpy bar that was tested at room temperature. Does the fracture surface suggest that the brittle or ductile fracture occurred? To support your answer, describe identifying characteristics in the fracture surface. (Maximum 3 sentences) (b) Figure 2 is an SEM image from the furnace-cooled 1018 steel Charpy bar that was tested after cooling in liquid nitrogen. Does the fracture surface suggest that brittle or ductile fracture occurred? To support your answer, describe identifying characteristics in the fracture surface. (Maximum 3 sentences) Figure 1 Figure 2

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4/5 (Week 8) NOTE: The impulse excitation (Buzz-o-sonic) data and the Young’s modulus measured from tensile tests of 1018 steel (from another quarter than what you analyzed previously) are posted in the Canvas files along with ASTM standard E1876. 1. Describe the materials and the step-by-step procedures for use of the impulse excitation technique (ping test) to determine Young’s (elastic) modulus, shear modulus, and Poisson’s ratio of 1018 steel. This is shown in the week 9 demo video which follows ASTM standard E1876. In a journal paper, one could refer to the standard to describe the procedure in brief, but for this question give detail about what was measured and the four excitations needed to find the elastic properties of the steel. 2. Plot a bar chart of the Young modulus for 1018 steel obtained from the impact excitation test and the set of tensile test results for 1018 steel that is posted with the Buzz-o-sonic results. Your bar chart should have vertical standard deviation error bars. Use the three values from the Buzz-o-sonic measurement to calculate the standard deviation. Your plot should include axis labels with units, a figure number, title, and descriptive caption.

5/5 3. First consider the Young’s modulus from the tensile tests. Do you expect the modulus for the different processing conditions to be similar within the error of the measurements. Why or why not? 4. When you compare the Young’s modulus found for the different heat treatments from the tensile tests, do they conform to your expectations? If they do not, briefly describe possible reasons why. 5. Why are the error bars so large on the tensile test data? Why are they relatively small using the impact excitation test? In your answer consider both (1) the physical nature of the test method and (2) the number of separate specimens tested with each technique.