Sunday, March 6, 2011

The Value of Mass in Material Products

Strength versus density for a range of materials.  Source: University of Cambridge.
For a great number of material products, their mass is inextricably linked to their attractiveness and commercial success.  Mass is also a key parameter targeted in the constant development of new materials, part of the mantra "Lighter, Faster, Stronger."  For example, steel has been progressively replaced with materials such as aluminum (~35% as dense), magnesium (~22% as dense), and in some cases polymer or carbon fiber (~10-15% as dense).

The value of weight reduction is an interesting topic of discussion; it is also a topic which will constantly evolve with the development of new markets, products, as well as new materials and their properties.  The latter point is of particular focus in materials science.  For example, aluminum alloys possess a wide range of strengths, depending primarily on a) particular element additions to the aluminum base, and b) amount of mechanical working introduced.  Improvements to either can enable aluminum alloys to be applied to new products, e.g. in which a minimum strength is required and minimum density increases the product value.

However, not all commercial products and markets value (or perceive) weight reduction equally.  As described by Ashby [1], exchange constants provide the value or 'utility' of a unit change in mass.  The perceived value of these constants can be determined by graphing cost versus mass for a range of products in a given market sector.  For example, Ashby [1] has performed an exchange constant analysis for bicycles; he found that the constants in this market range from $20/kg (plain steel) to $2,000/kg (carbon fiber).  On the other hand, engineering values of these constants can be determined based on engineering criteria.  In the transport systems sector, for example, the constants are "...derived from the value of the fuel saved or of the increased payload, evaluated over the life of the system."  Namely [1]:
  • Family car: $1-2/kg;
  • Truck: $5-20/kg;
  • Civil aircraft: $100-500/kg
  • Military aircraft: $500-1,000/kg
  • Space vehicle: $3,000-$10,000/kg
In other words, it doesn't make a lot of engineering sense to produce family cars from extremely light-weight materials because weight savings aren't valued highly in that market.  (But based on the perceived value, there may be a small market for carbon fiber minivans.)

The comparison of product prices per unit weight between market sectors can also offer interesting insight into the value of mass.  From Ashby [1]:
  • Buildings (car parks to high-tech buildings): ~$0.10-$2/kg;
  • Packaging (paper to metal foil): ~$1-10/kg;
  • Marine and Offshore (bridge to luxury yacht): ~$1-100/kg;
  • Automotive (subcompact to Ferrari): ~$7-$300/kg;
  • Appliances (refrigerator to portable computer): ~$9-$1,000/kg;
  • Sports Equipment (skis to fly-fishing rod): ~$100-$10,000/kg;
  • Aerospace (light plane to space craft): ~$200-$60,000/kg; and
  • Biomedical (toothbrush to contact lens): ~$300-$100,000/kg
While there are a number of surprizes in this list - for example, that a toothbrush costs as much as a Ferrari on a weight basis - the range is a testament to the importance of market knowledge to materials engineering and weight-centric design.  As mentioned above, this list is ever-changing - responding to new markets, new materials, and new properties; but it is also closely linked with environmental changes such as volatility of materials, changes in consumer perception, and evolving political landscapes.  Because of the time required to get new materials to market, materials engineers and designers must therefore remain equally dynamic and flexible to meet the needs of consumers.

[1] M.F. Ashby.  Materials Selection in Mechanical Design, 4th Edition.  Butterworth-Heinemann (2010), pp. 208-210, 482-483.

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