ENG1001 - Lighter Faster Stronger Project: Materials Testing, Selection and Design - Engineering Assessment Answer

Assessment Task:

ENG1001 - Lighter Faster Stronger Project: Materials Testing, Selection and Design Assessment Answer

Part 1: Answer all the following questions:

Question 1

(a) Plot the engineering stress-strain curves obtained by tensile testing the three aluminium specimens. These should be presented together on a single graph.

Question 2

(a) By generating lines of best fit for the elastic region of the graphs, determine the elastic modulus of the two Al1050 samples that were tested. Show your fitted curves on a new graph.

(b) The literature value of the elastic modulus of Al1050 H19 is 70 ± 3 GPa (CES Edupack database). On an atomic scale, what determines the modulus of a material? According to theory would you expect the modulus of the Al1050 O (annealed) sample to be higher, lower or the same as the as-rolled aluminium? What about the Al 6063 sample?

(c) Do your measurements of elastic modulus match your predictions and the literature values in Question 2(b)? Describe some of the sources of uncertainty in the measurements of both stress and strain, and comment on how these affect how your modulus measurements should be interpreted.

Question 3

What is a dislocation? Describe an edge dislocation with the aid of a diagram showing how the atoms are arranged and describe how these can be introduced into the metallic lattice. Explain why dislocations are important in determining the mechanical properties of different types of aluminium, and which properties they affect.

Explain the effects of cold-working and annealing processes on the number of dislocations in a material, and how this affects the sample mechanical properties.

Question 4

Present a table with the 0.2% proof stress, ultimate tensile strength, uniform elongation and fracture strain for the three materials. Demonstrate how you found these values for the cold-rolled sample on a new graph.

Al 6063 is an alloy which forms precipitates during heat treatment. Explain how precipitates form, and describe the effect precipitates have on dislocation movement and material strength in aluminium alloys.

Question 5
The yield stress, UTS and ductility (elongation) for Aluminium 1050 H19 and Aluminium 1050 O can be found in Table 1. These correspond to fully work-hardened and annealed aluminium respectively.

Prepare tables that compare the yield stress, UTS and ductility of your cold rolled sample with Al1050 H19. Discuss the similarities and differences between the theoretical values and the ones you measured. Repeat for your annealed sample compared with Al1050 O. Discuss what these results might imply about the amount of work hardening in the specimens that you tested.

Part 2: Materials design project:

Question 1

Firstly, design an appropriate member using the literature properties of work-hardened Al1050 from Part 1, Q2 and Q5. This will act as your baseline with which to compare your other designs. You may assume that Al 1050 has σy = 165 MPa, E = 70 GPa, a density of 2700 kg/m3, costs $2.25/kg and emits 12 kg CO2 per kg of material produced. For this question, assume a solid square cross-section. Show all steps of your process in your report.

(a) Find the minimum cross-sectional area required to prevent yielding in member AC

(b) Find the minimum second moment of area required to prevent buckling in member AC

(c) Using a square cross-section, determine the minimum cross-sectional dimensions for member AC

(d) Determine the final volume and mass of the members (e) Determine the cost and carbon footprint of the materials

Question 2

Design appropriate members for each of the available materials in Table 2, choosing an appropriate cross-sectional shape and dimensions. Justify all your results with calculations or discussion. You should at least:

• Show the full process with example calculations for at least one material, as an example.

• Discuss what cross-sectional shapes are appropriate for use in the designs. You will need to consider the loading of the member (hint: buckling can potentially occur in more than one direction) as well as any potential constraints caused by the processing of different materials.

• Demonstrate that your beams comply with all required elements of the design brief. State the final properties of each of your final beams (ideally in a table or similar format that is easy to read), including the final dimensions, cross-sectional area, second moment of area, volume, mass, cost and carbon footprint.

Question 3

Recommend two different options for the beam design, based on the requirements in the design brief. Note that you will need to balance different requirements; there is no single, optimal solution. Justify your recommendation.

Question 4

Apart from those properties that you have already considered, name 3 additional considerations that might be important in choosing materials for the bridge. Provide a short explanation for each of your responses.

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