Lightweighting - Part III

In this the third and final installment of our series on lightweight construction for fuel efficiency, I shall focus on two overlapping classes of structural materials, porous or cellular metals, also known as metallic foams, and micro-truss structural panels. I shall also have a bit to say about structural aerogels, a class of materials presumed to offer the highest stiffness to mass ratio of all.

At present, neither porous metals nor micro-trusses are used to any significant degree in any vehicular application, while structural aerogels are still entirely experimental. But such are the advantages of these materials technologies that they may yet find a place in commercial products.

We have already encountered cellular materials in our discussions of wood and of advanced composites having wood or foam plastic cores. All wood is cellular in structure, while most advanced composite constructions utilize cores fabricated from manmade structural materials exhibiting cellular structures as well. Unfortunately, neither wood nor advanced composites appear to lend themselves to low cost mass production of the sort that characterizes the automobile industry.

In this light porous metals present themselves as a possible alternative for lightweighting, a class of materials which can achieve the same impressive strength to mass and stiffness to mass ratios of advanced composites while not excluding the low cost manufacturing techniques characteristic of sheet metal fabrication.

Porous Metals

I prefer the term porous metal to the alternative, cellular metal because it is more appropriate to an entire range of constructions having real possibilities in the transportation systems of the future. Cellular metal is properly a subset of porous metal though the terms are often used interchangeably in technical discussions. The definitions that follow will explain why.

Porous metal configurations are mostly confined to sheet metal constructions, in other words, the materials are intended for skins or body panels. Such materials may be used in frames as well, but their advantages in such applications are less impressive.

Porous metal sheets differ from conventional sheet metal in that the latter is solid through and through whereas the porous constructions are riddled with small air cavities. The metal walls of the individual air cavities resist deformation and give the resulting hollowed sheets a degree of structural strength that is only slightly inferior to a solid sheet of equivalent thickness while providing weight reductions as much as 80%. Even at thicknesses as little as 5 millimeters these materials show impressive bending resistance.

In the following sections I will consider the various forms and configurations of porous metal structural components and the ways in which they are fabricated.

Honeycomb Sheets and their Kin

One of the easiest ways to make a high performance porous metal sheet is to utilize a honeycomb construction where a hexagonal vertical grid structure resembling a real honeycomb in appearance is inserted between two very thin surface sheets or skins. This type of metal construction has been used for decades, particularly in aircraft and marine construction, with thin gauge sheet aluminum being the metal of choice.

Honeycomb does not really lend itself to curved surfaces, and so it tends to be confined to flat panels. Furthermore, it cannot be worked like conventional sheet metal, and crimping tools will crush the internal walls and destroy the integrity of the panels.

Honeycomb metal panels made of aluminum or magnesium alloys are widely available today and are considered to be the commercial state of the art in cellular metal products. They are used to a limited extent in at least one high performance sportscar, the new Chevrolet Corvette. They are not, however, the true state of the art.

Rather resembling honeycomb but easier to fabricate and not so strong is corrugated sheet metal where a single corrugated sheet is bonded to two flat sheets comprising the outer skins. It is also possible to use rectangular sheet metal grid structures for internal reinforcement. Each of these variants has slightly different structural properties, but all are strong and light.

Strongest of all are what is known as hierarchical structures which are fractal in nature and, incidentally, which are difficult and expensive to fabricate. An example would be an internal reinforcing corrugated structure which itself is made up ultra-thin corrugated sheets, replicating the overall structure in the individual structural components.

Foamed Metals

Foamed metals are just what the name implies, metallic structures full of tiny bubbles of air or some other gas. They may take the form of either sheets or castings of various shapes. All of the commercial products within this category utilize aluminum though numerous unsuccessful attempts have been made to develop a cost effective production process for foaming magnesium.

Foaming may be accomplished by any of several methods including injecting gas into the molten metal, introducing various chemicals which release gas upon contact with the metal, and by adding solid spheres or particles which gasify on contact, leaving pockets. There is no one dominant production technology at present simply because the category itself is not well established as a structural material.

Foaming can reduce mass and internal volume by as much as 80% with fairly minimal reductions in strength and stiffness. Foamed sheets have the further advantage of being able to absorb the energy of collision very effectively, crumpling without rebounding as each individual cell collapses. Foamed metal body panels for car provide maximal crash protection for occupants as well as reducing curb weight very substantially. Moreover, such materials can be formed and shaped with ordinary sheet metal fabrication tools.

Finally, the foaming processes, being simple in nature and not requiring a lot of specialized machine tools, as is the case with the production techniques for honeycomb structures, permit the manufacturing of sheet materials at much lower costs than have been achieved for advanced composites using fiber re-enforced plastics.

I believe that foamed aluminum is a much better candidate for widespread adoption by the automotive industry than is carbon fiber lay-up in the spite of the fact that the latter has already found a market of sorts. Lay-up is poorly suited to mass production and requires molds for the fabrication of complex curved shapes. Furthermore, as we have seen, such shapes, which are the norm in modern automobiles, do not exploit the beneficial structural properties of carbon fiber.

Auto manufacturers are in fact looking at foamed metal panels as substitutes for conventional sheets, but to date such products have not appeared in commercial vehicles.

Micro Trusses

Micro truss supported metal panels represent the ultimate currently available structural material for manufacturing the skins and interior surfaces of high performance vehicles. Whether we will be seeing them in the automobiles of the future cannot be said with certainty, however.

Micro trusses are just what the name implies, support frameworks consisting of tiny rods joined together in a repetitive sequence of interlocking triangles. On the macro scale trusses are used in bridges, light weight girders, and, most obviously, in bicycle frames where a single triangle is the key structural element in the frameset. Truss structures have been utilized for well over a thousand years, probably originating in China.

Micro truss panels consist of two very thin pieces of sheet metal joined together with an internal three dimensional assemblage of trusses formed from wire or thin gauge stamped sheet metal. In the most advanced structures hollow wire will be used to form the truss structure.

Methods have been devised for the automated assembly of such panels, but since more steps are involved than in ordinary sheet metal forming, and due to the likelihood of a fairly high reject rate reflecting the delicacy of the internal structures, such panels are very expensive. A tiny market for them currently exists in aerospace, but even there aircraft manufacturers hesitate to pay the steep premium attached to this ultimate achievement in stiffness per unit of mass. Or perhaps not the ultimate. Recently a company in San Diego called ESLI devised a nonmetallic micro truss that exceeds even the most sophisticated metal micro structure in strength per unit of mass.

This particular micro truss consists of filament thin strands of carbon fiber a tenth the thickness of a human hair and is more akin a fabric in appearance than like an internal framework. In fact it looks surprisingly like steel wool. A square meter of the stuff weighs a mere seven grams. The material was developed for a very specific purpose, the creation of what is known as a solar sail. This is proposed propulsion system for a space craft which might cover thousands of square meters and which uses the force of photons falling on the surface to drive the craft forward. Because of the absence of friction in interplanetary space the craft would be continuously accelerated until it reached a velocity amounting to a sizable fraction of the speed of light.

Provided with a skin of paper-thin Spectra ballistic fiber such a mesh would provide unrivaled strength per unit of mass.

Structural Aerogels

Aerogels are materials made from gels. The liquid in the gel is replaced by gas, resulting in a mass of almost microscopic interlocking filaments. First developed in 1931, aerogels have since been fabricated out of many different materials, some carbon based, and some aluminum compounds, as well as a great variety of other chemicals. Aerogels are translucent, indeed, nearly transparent, and have a blue tint, almost like frozen smoke. They are unbelievably lightweight.

Aerogels, liked foamed metal, collapse with no rebound under impact and have been used experimentally for things like helmet liners. Mostly, however, they are used as insulating layers, for which purpose they are extraordinarily effective.

In absolute terms aerogels are not extraordinarily strong. Proponents see them being used as cores in composite structures with skins made of some high tensile strength fiber such as Kevlar or fiberglass. To date, however, structural aerogels have not found any significant markets. I have seen aerogels used to make the diaphragms of very exotic high performance loudspeaker tweeters, but that's about the only application today. Aerogel composites simply aren't on the market. I suspect a major problem is bonding them to skins. They contain so little material that there is simply not much surface on which adhesion can take place.

Six years ago researchers at the University of Missouri-Rolla announced the development strong aerogel nanocomposites consisting a combination of silica and plastics, but to date that product has never been commercialized.

Looking Forward

Figure on continued progress in the development of strong, lightweight structural materials. The aerospace industry whose willingness to pay inflated prices for incremental improvements continues to drive research and development and in the fairly near future concerns regarding fuel efficiency in automobiles will exert powerful effects as well. The difference will be that automobile industry will insist upon materials that are cheap to produce and easy to work. And that may well take the advanced structural materials industry in a decidedly different direction than it has followed in the past.