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Materials selection and design

For a typical gasoline passenger car, when the energy consumed over the life cycle from material production to disposal is compared stage by stage, which stage is usually the largest, and what materials selection policy follows from it?

  1. AThe material production stage; using as much material with low embodied energy as possible is most effective
  2. BThe use (driving) stage; choosing materials that lighten the body and reduce fuel consumption is effective
  3. CThe disposal stage; it is enough to consider only reducing incineration and landfill
Show answer and explanation

Answer: B. The use (driving) stage; choosing materials that lighten the body and reduce fuel consumption is effective

In life cycle assessment (LCA), the use-stage energy from fuel consumed while driving usually accounts for most of the total for a gasoline passenger car. So even aluminum alloys and CFRP, which take a lot of energy to produce, can pay off overall if they lighten the body and improve fuel economy in service. The production stage dominates for products that use no energy in service, such as building materials. The disposal stage generally accounts for a small share of energy, so considering it alone is insufficient. Which stage dominates varies by product, so the basic approach is first to identify it and then choose materials that reduce that stage. The proportions change with the assumptions; in electric vehicles, for example, battery production takes a larger share.

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More Materials selection and design questions

  1. Take the production energy of aluminum alloy as 200 MJ per kg for primary metal made from ore and 15 MJ per kg for secondary (recycled) metal made by remelting scrap. Which value is closest to the production energy per kg of a material containing 60% secondary metal by mass?
  2. When steel scrap is remelted in an electric arc furnace for recycling, which elements tend to accumulate in the steel with repeated recycling and cause problems by degrading quality (tramp elements)?
  3. Which is an appropriate materials selection and design policy for improving the recyclability of a product?
  4. You want to minimize two objectives at once, the mass and the cost of a part, but lighter materials are more expensive and no material is best on both. Which is an appropriate way to choose a material in this case?
  5. In Ashby's method of materials selection, the design requirements are organized into function, constraints, objective and free variables. In selecting the material for a bicycle frame, which of these is the objective?