Dem bones gotta walk around...
CSC 297 Robot Construction: Materials
Dem bones gotta walk around...
Robots inhabit the physical world, and various engineering materials
are employed to provide shape, strength and durability, and to support
non-structural components.
Metals, plastics, and composites tend to dominate the structural
elements, but other materials are occasionally used.
The following segments summarizes some design-relevant properties.
Most of the discussion concerns rigid materials, which constitute
most structural elements in current robots.
The final segments contain brief descriptions of
elastomers and flexible tensile elements, mechanically distinct
material classes with useful properties.
He had a heel of steel and a head of lead. His tongue was depleted uranium.
- Sasha Derebrovska, "Metal Men".
Metals are strong, rigid, hard, tough, heat resistant, and isotropic (their properties have no directional dependence). This combination of characteristics is unmatched by any other common class of materials. They also tend to be heavy, and moderately to extremely difficult to fabricate. Metals are typically shaped by forging, bending, machining (sawing, drilling, milling turning), and grinding. Some can also be cast, though typically some additional machining is required. Metals in their pure form are often surprisingly soft, so engineering materials are typically alloys with other elements. The amount required is often remarkably small: less than 1% carbon for steel, a few percent copper or magnesium for aluminum can produce nearly an order of magnitude increase in yield strength over the pure material.
Metals are basically unmatched in their combination of strength (hardness) and toughness. Other very hard and strong materials tend to be brittle, which makes employing their underlying strength a challenge. What gives metals this property is the fact that they deform before they break. The stress at which a metal deforms is known as its yield point, and represents a practical upper limit to the forces that cane be applied to a metal structural element. However, this deformation greatly reduces the stress concentration at the tip of an incipient fracture, and prevents brittle propagation of the defect. Some plastics also have this property, but with much lower yield points.
The following metals are some of those most commonly encountered as structural materials.
Making steel.
Image source
Steel is the most common and least expensive metal, and also one of the strongest. Unhardened mild steel yields at 30,000-50,00 psi. For structural purposes, it can be hardened easily to 100,000 psi, and for tooling, can be strengthened to nearly 300,000 psi. It has a density of approximately 8 times the density of water (7.9 gm/cc), and a melting point around 1400 degrees C (pure iron 1530 C), which looks white hot.
Ordinary, or low alloy steel is mostly iron, with a small amount of carbon (less than 1%) serving to harden it. It often also contains 1% or so manganese, and small amounts of other elements such as silicon. A 4-digit AISI/SAE code such as 1015 or 1040, or an analogous 5-digit UNS number (10150, 10400) is often used to describe steel alloys. The first two AISI digits specify a broad type; the initial 10 in 1015 means ordinary low-allow carbon steel. The last two digits are the carbon in hundredths of a percent.
In general, the more carbon a steel has, the more it can be hardened through heat treatment, which is the process of bringing it to orange heat, and then cooling it rapidly (usually in water or oil) sometimes followed by a milder re-heating or "tempering" process, which makes the hardened material less brittle. "Annealing" is a process where the steel is brought to red or orange heat, and then cooled very slowly (over a period of hours or even days), which returns it to a (comparatively) soft state. Above one percent carbon, it becomes increasingly difficult to control brittleness, and steels grade into the material known as cast iron
Steel stock comes in hot-rolled and cold-rolled forms. Hot roll is typically the cheapest and has heavy, black "mill scale" oxidation on the surface. It may also have variable surface hardness due to rapid cooling from contact with the roll. These properties can be tough on tooling, causing more rapid wear or breakage than expected. Cold roll is worked in a cooled state, and has a more finished surface and more uniform consistency. It tends to be a bit harder and stronger than hot roll due to work-hardening effects, but generally can still be machined.
For our purposes, mild or annealed steel is the only form which can be worked. It can be machined with moderate effort. Machinists tables say that it can be cut with high-speed-steel (HSS) tools at up to 100 surface feet per minute (that's the rate at which the tool moves across the metal surface) but this is a bit of a maximum with adequate cooling, lubrication, and appropriate feed rates. As beginners, if we want to avoid damaging our tools, 25 SFM is a good point to aim for. It looks slow, but it will get the job done.
Steel is easily joined by welding (if you have a welder), and the resulting joints can be as strong as the underlying material. It can also be joined by a lower temperature process known as brazing, but you still generally need a welding torch. Tinned sheet steel (the material of the old tinsmiths) can be joined by low-temperature soldering, but it takes some practice to master and much higher-power equipment than is used for soldering electrical components.
For our robots steel will be useful for fasteners, bearings, springs, and other small parts, but is probably overkill for most of the structural components.
Designation of steel alloys from Wikipedia
Designation of Steel alloys from Engineers Edge
Properties of steel from SteelConstruction.info
Table: Mechanical properties of steels 1015-1340 from Engineers Edge
Table: Mechanical properties of steels 3140-9310 from Engineers Edge
Aluminum is roughly one third as heavy, and one third as strong as steel (density 2.7 gm/cc, yield point 10,000-40,000 psi). It generally melts around 600 C (pure aluminum 660 C), which is just above red heat. It is generally malleable and ductile, and takes a high polish. Engineering aluminum is usually an alloy containing a few percent other metals which provide strength and hardness. Common additives are copper, magnesium, silicon, and manganese.
Aluminum does not rust, but sometimes corrodes in wet environments, especially if there are acidic or basic impurities in the water. Surface treatments generically referred to as "anodization" can provide a high degree of protection. Chemically, aluminum is unstable both with respect to air and to water, but self-protects with a very tough, microscopically thin layer of aluminum oxide. In pure oxygen, it will burn intensely, but fortunately it is very difficult to ignite in air, even in fine shavings.
Aluminum is MUCH easier to machine than steel, which makes it the structural metal of choice for our robots. It cuts easily with HSS tools at 100 SFM. Aluminum has a tendency to "gall", producing a rough surface if the tooling is not sharp. It also sometimes sticks or "melts" to the cutting edge of the tool. These accumulations can usually be snapped off a cooled tool using a piece of hard plastic, soft metal, or even a fingernail, but you need to be careful not to chip the cutting edge or cut yourself in the process. Aluminum cannot be joined by soldering, and can be welded only with special techniques. Components are hence generally assembled using threaded fasteners.
For small components, aluminum is about twice the cost of steel. For large components, the difference is greater. However, the easier machining often more than makes up for the difference in the cost of the bulk material.
Aluminum is non-magnetic, and an excellent electrical and thermal conductor. Its use as an electrical conductor is usually limited to applications where very large conductors are needed, due to the difficulty of making good connections (it cannot be soldered, and contact surfaces sometimes become resistive and heat up). It is the material of choice in heat sinks and heat exchangers except in applications where corrosion resistance is needed or temperatures above 300 C will be encountered.
Designation of aluminum alloys from Engineers Edge
Table: Properties of wrought aluminum alloys from Engineers Edge
Table: Properties of aluminum casting alloys from Engineers Edge
Brass and bronze are copper alloys occasionally used for structual purposes. Brass refers to an alloy of copper and zinc, bronze to copper and other elements, most commonly tin, but sometimes, aluminum, silicon, or other metals. They have about the same density as steel, and generally half to two-thirds its strength (density 8.4 gm/cc, yield point 25,000-60,000 psi). Both melt at around 900 C, a good red-orange heat. Pure copper melts at 1080 C, bright orange.
Copper alloys are non-magnetic, non-sparking, and some of them are very corrosion resistant. They are used instead of steel when these properties are important enough to justify the cost, which is 4-5 times that of normal carbon steel in small amounts and more in large pieces. Brass is sometimes used in bearings as it has self-lubricating properties both against itself, and against steel. An unlubricated steel-on-steel bearing will rapidly self destruct, as will aluminum (which is generally not hard enough for bearing purposes in any case).
Most common brasses machine easily. The machinability of bronzes varies from easy to difficult. Many copper alloys can be joined by soldering.
Pure copper is rarely used structurally as it is very soft, but it is used nearly universally as an electrical conductor, and sometimes in heat exchangers in applications where aluminum would corrode. It is used in plumbing due to the combination of corrosion resistance, strength and heat resistance (compared to plastics), and easy solderability.
Technically stainless is a variety of steel since iron is the largest component. However, it contains relatively large amounts (10%-20%) of both nickle and chromium, and sometimes other elements such as molybdenum or vanadium. These provide a high degree of corrosion resistance. Its strength characteristics, density, and melting point are similary to carbon steel. Stainless can be magnetic or non magnetic. Non-magnetic stock may become magnetic when bent or deformed, which should be remembered if you are counting on non-magnetic properties. It is a relatively poor conductor of heat and electricity compared to copper and aluminum, and even to regular carbon steel. (But it is still a really good conductor compared to the human body or insulating materials, so take the same care around electricity you would with any other metal)
Stainless is used as a structural material mainly when corrosion resistance needs to be combined with high strength and temperature resistance. Machined stainless has an attractive look and will take a mirror polish. It is scratch and chemically resistant and easy to clean. Because of these properties it is sometimes used where appearance is important. Stainless typically costs 2-3 times as much as carbon steel for small components, more for large ones.
Stainless steel is notoriously difficult to machine. Even when annealed it work-hardens rapidly when cut. It has a tendency to break or chip tooling. If you must machine it, use carbide tooling if available, and run at 1/4-1/3 the speed you would cut carbon steel of equivalent hardness. Stainless cannot be soldered or brazed, and needs special techniques for welding. Correctly made, welded joints are of high quality.
Designation of stainless alloys from Engineers Edge
Table: Mechanical properties of stainless alloys from Engineers Edge
Titanium is an exotic metal sometimes found in high-tech applications. It is about half the density of steel (4.3 gm/cc) but some alloys can be made almost as strong (yield 150,00 + psi). It is high-melting (1670 C), extremely corrosion resistant, and biologically inert. It is also very expensive, difficult to machine (more so than stainless), and can only be cast, forged, or welded with very specialized equipment. Thin shavings will burn in air if ignited, so turnings are a fire hazard. It will also burn in carbon dioxide and even pure nitrogen, which makes extinguishing such a fire tricky (dry sand will work).
Titanium is generally used only when biological inertness is essential, or the application requires an unusual combination of light weight and high strength (e.g. some aerospace applications). We will probably not have occasion to use it in this course.
Magnesium
has properties similar to aluminum, but is lighter
(density 1.7 gm/cc), more expensive, more corrosion prone,
and its shavings will burn intensely in air (machining it is a fire hazard).
It is used only when the low weight outweighs all the negatives.
We probably won't use it.
Wikipedia on magnesium
Zinc
is sometimes used for low-cost
metal components in cars, locks, toys, and low-quality machinery.
The advantage is that
components can be quickly die-cast at low temperature.
The down side is that the material is weak and not very heat resistant
for a metal, and nearly as heavy as steel (density 7.1 gm/cc).
Wikipedia on zinc alloys
Lead
is also easy to cast at low temperature,
and some of its alloys have
better mechanical properties than zinc. It is also traditionally used
to make weights due to its high density (11.3 gm/cc).
Some of the best low-temperature solders contain lead.
Unfortunately some of its compounds are toxic if eaten or inhaled,
and can cause cumulative brain damage in children at low dosages.
The extensive use of lead carbonate as a white pigment in house paint
in the 19th and early 20th centuries resulted in widespread exposure
as deteriorated paint chipped and peeled directly into living spaces.
Lead thus became widely known as a very bad thing, and most uses
have been discontinued.
Some electrical solder still contains lead as it is easier to use than
any of the lead-free alternatives so far discovered.
Large amounts are still used in automotive batteries.
Use lead solder only in well ventilated areas.
Don't eat solder or batteries, and wash your hands after handling them.
Wikipedia on lead
Mercury
(not a structural metal!) is
primarily interesting because it is
liquid at ordinary temperatures. It was widely used in household
thermometers and in mercury switches, which provide a very nice spark-free,
position-sensitive electrical contact.
Mercury is significantly more toxic than lead, and the free element
is a hazard because it evaporates slowly into the air and can be inhaled.
If you drop a mercury switch, the glass tube can shatter and then little
droplets of mercury will race all over the floor and into every crack and
corner.
UR would have to send in a decontamination team, and we
would probably be closed down forever.
So I'm not putting out the old mercury switches we got once as robot
orientation sensors.
Interesting factoid: mercury is denser than lead (13.5 gm/cc).
Wikipedia on mercury
Depleted uranium
is even better for weights than
lead or mercury (density 19.1 gm/cc).
It is used to ballast the keels of some high-performance sailboats and
in military armour-piercing bullets.
Unfortunately it is slightly radioactive and can cause lung cancer if
inhaled as dust, and probably other cancers
if eaten in a bio-available form.
Fine shavings will burn in air if ignited, producing uranium oxide
as finely divided, inhalable smoke.
We would need to get special permission to use it in a robot.
Wikipedia on depleted uranium
Plastics are mainly 20th century products of applied chemistry, and they provide materials with an amazing array of properties. No existing plastic approaches metal in absolute strength and heat resistance, but otherwise they can be produced with practically any mechanical characteristics seen in other materials.
Chemically, plastics are polymers: long chains of repeating subunits. Most often these units are organic compounds, but there are some plastics based on silicon and other unusual chemistry. The chemicals representing constituent subunits are known as monomers. The properties of a polymer however, are most closely associated with attributes of the chain, especially its length, its local flexibility, and whether chains are bonded to each other - an attribute known as crosslinking.
Plastics as a group tend to be waterproof, corrosion proof, and resistant to chemical attack particularly by acids and bases. Some can be softened or dissolved by organic solvents. Many plastics can be made in transparent form, and these form a useful class of optical materials. In general they are excellent electrical insulators and relatively poor conductors of heat.
Most plastics are subject to photo-degradation by ultraviolet radiation, which has sufficient energy to disrupt carbon-carbon bonds and thus break the polymeric chains. Exposed to outdoor light they cloud, yellow, weaken, and eventually disintegrate over a period of weeks to years. Plastics intended for outdoor use have ultraviolet inhibitors added to slow the process. Some of these are good for a decade or more in the sun. Long term however, the only way to prevent eventual degradation is to block exposure to UV light.
Most plastics soften at relatively low temperatures; many lose all or most of their structural strength above 100 C. A plastic that retains useful mechanical properties above 300 C is a rarity.
Plastics are widely used structurally as rigid bulk material. The best engineering plastics are approximately an order of magnitude lighter, and an order of magnitude weaker than mild steel (yield strength 3,000-12,000 psi). In weight-limited applications, this makes them an attractive alternative. They also find application as flexible films and fibers. In this form their strength to weight ratio can greatly exceed that of steel. Polymers are also the basis for most elastomers and a large class of semi-rigid materials, which we will discuss separately.
Following are some of the plastics most frequently used as rigid structural materials.
Polystyrene is mostly included here as a plastic NOT to use in robotics. It is cheap, rigid, and easily injection molded. It is widely used in cheap toys and plastic model kits, and as expanded "styrofoam". Chemically, it is relatively inert, but can be dissolved in acetone and some other organic solvents. This permits easy and permanent bonding with plastic model cement. However, styrene is relatively weak as rigid plastics go (yield point about 3,500 psi), and particularly prone to brittle fracture. It can be machined with modest care. Drilling must be done slowly to prevent melting or fracture. It has a tendency to fracture when tapped or if screws are overtightened. The main application of polystyrene in robotics is as a remarkably inexpensive source of detailed and complex pre-made parts scavanged from toys or model kits. The spaceship models in the first "Star Wars" movie were decorated with parts from all sorts of mass-market hobby kits. Styrofoam materials are sometimes useful structurally when extremely light components are needed. If constructed with a tensile skin, such parts can be remarkably strong for their weight.
Also known as acrylic, and polymethylmethacrylate, plexiglass is widely available as sheets intended as a non-shattering replacement for glass windows. Although certainly less prone to impact fracture than sheet glass, plexiglass is not a particularly robust engineering plastic. Its absolute tensile strength is quite high (yield point around 10,000 psi), but like styrene, it is very brittle. A hard blow will cause fracture, as will strong localized pressure e.g., from a threaded fastener. Also like styrene, it has a tendency to fracture or melt if drilled too fast. It is a satisfactory material for small to moderate sized transparent panels that are not expected to sustain strong impact. Plexiglass is a relatively inexpensive plastic, less than the cost of aluminum in bar form, and considerably less in large commodity sheets designed for windows.
PVC is short for polyvinylchloride. It is most familiar as the soft, flexible plastic that covers wires, and from which cheap raincoats, inflatable toys, and imitation leather is made. Without plasticizers however, it is a strong, rigid material. Plastic plumbing components are often made from PVC. Its yield strength is around 7,000 psi, and it is considerably more impact resistant than plexiglass. It also deforms 10-20% before breaking. These properties make it a reasonable structural material. It machines well, and can be permanently glued together using solvent welding cements. PVC is the least expensive of the structurally useful plastics, less than half the price of aluminum stock in small pieces, and very much cheaper in large amounts of commodity material. It can be obtained in rod or sheet form in large sizes. PVC pipes are readily available at low cost in the plumbing section of hardware stores, and can often be used as structural components.
Short for Acrylonitrile-Butadiene-Styrene, ABS is a copolymer formed from three different monomers. It is an example of an engineered plastic where monomers whose homo-polymers have different characteristics are combined to produce a material with new, desireable properties. In this case a sythetic rubber, polybutadiene becomes interlaced with a rigid styrene/acrylonitrile copolymer. The product has the rigidity of polystyrene but is not brittle, and polar attraction from triple-bonded nitrogens in acrylonitrile provides increased inter- (and intra-) molecular bonding. The resulting plastic is strong (yield around 5,000 psi), and very impact resistant. ABS exhibits considerable ductile deformation, stretching 20% or more before breaking. It is what LEGO bricks are made of.
ABS machines smoothly and easily. It can be tapped for threaded fasteners, and holds them well. It can be effectively glued or sovent welded. It retains its strength and impact resistance at temperatures down to -40 C. On the negative side, it loses its strength above 80 C, so is not good for hot locations. It is also not readily available in transparent form. In small amounts, ABS costs about the same as aluminum. In large commodity forms (e.g some plumbing pipe) it is considerably cheaper.
Also known by the trade name "Lexan", polycarbonate is the classic "indestructible" miracle plastic. It is very strong (yield point around 10,000 psi), extremely impact resistant, and deforms up to 100% before breaking. Sheet material is sometimes formed by cold bending, just like a metal. It can be made in crystal-clear form, and is popular for "bulletproof" windows. It is resistant to acids, bases, alcohols, and most oils, but can be softened by acetone and some other solvents.
Polycarbonate machines reasonably well. It has some tendency to melt when drilled, so drilling should be done at low speed, and the tool withdrawn frequently so that chips do not become trapped between the drill and the hole wall where they turn instantly into a gooey mess. It can be glued, though not as robustly as PVC. Polycarbonate is one of the more expensive plastics, 1.5+ times the cost of aluminum in small amounts. Unlike PVC, plexiglass, and ABS it is not commonly used in household plumbing or other high volume consumer applications, so cheaper commodity forms are not generally available.
Also known by the trade name Delrin, acetal is effectively polyformaldehyde - a simple backbone of alternating carbon and oxygen atoms. It is strong (yield 10,000 psi), relatively ductile (up to 60% deformation) and very chemically inert. It is unaffected by acids, bases, and all common solvents. It is abrasion resistant and has a low coefficient of friction, which makes it useful for low-pressure bearings and sliding parts. It is somewhat susceptible to impact fracture, but resists brittle fracture under gradually applied forces.
Acetal machines easily, can be tapped, and holds screws reasonably well. In applications where screws might tend to loosen, the low coefficient of friction could be a complicating issue. Due to its slippery nature, it is less problematic to drill than stickier plastics, but it is still possible to melt or burn a hole by too high a drill speed. Because of its unreactive nature, acetal cannot be securely glued, though some flexible adhesives may provide a low-strength bond. Acetal is relatively inexpensive - a bit less than aluminum in small amounts. It is not a household commodity.
Polyethylene (PE) and polypropylene (PP) are common, inexpensive plastics with similar properties. Both are relatively soft, with considerable ductility and ultimate bulk strength around 5000 psi. They are slippery, non-gluable, chemically inert, and unaffected by all common solvents. Because of these properties they are commonly used as containers for all types of chemical reagants from strong acids to paint thinner. Small non-polar organic molecules will slowly diffuse through both plastics, so they are not suitable for extremely toxic liquids. Huge amounts of both plastics are used as disposable food and beverage containers. Large slabs of both PP and PE can sometimes be found as consumer products in the form of cutting boards.
Both plastics have enough mechanical strength to be useful structurally, though they are considerably softer and more flexible than the other engineering plastics we have considered. They can be machined with reasonable care. Some precautions are needed because the softness and flexibility of the material. Tools must be very sharp, and material must be well supported so that it does not bend away from the tool. Neither material can be glued, and even flexible adhesives often hold only weakly. PE and PP are both inexpensive, only slightly pricier than PVC.
Polyethylene actually comes in three forms. Low density PE (LDPE) is rather soft and susceptible to both impact fracture and fatigue when employed in bulk. This limits its usefulness as a structural material. High density PE (HDPE) is much stiffer and more impact resistant. It is often used for large plastic containers such as garbage cans and chemical drums, and is quite durable. Ultra High Molecular Weight PE (UHMWPE) is a relatively recent development. It is one of the most impact and abrasion resistant plastics known. It has been used for iceless skating rinks and joint replacement implants.
UHMWPE can be drawn into a fiber known by the trade names Spectra and Dyneema. The ultra-long molecules, when brought into alignment give this fiber almost incredible properties. Its absolute tensile strength can exceed 350,000 psi, which is higher than very high strength steel cable, and has been exceeded by metals only in laboratory settings. In terms of strength to weight ratio, it is 10 times as strong as steel cable. The only down side is that the fiber loses its strength above 100 C. It is used in bullet-proof clothing and other military applications. Amazingly, you can buy this material at relatively low cost in your local sporting-goods store -- as fishing line.
Nylon is the original DuPont trade name for its revolutionary fiber. The term is now used generically for synthetic polyamide plastics. Although the word is usually associated with fibers and fabric, nylon is also used in bulk solid form to make structural components. It can be very strong, with a tensile strength as high as 12,000 psi, and it undergoes considerable plastic deformation before breaking. It does not have the impact resistance of ABS or polycarbonate, and when repeatedly stressed to near the elastic limit, eventually fatigues and breaks. Kept well under the elastic limit, it can be repeatedly stressed almost indefinitely. It has a low coefficient of friction (i.e. it's slippery), resists abrasion, and can be easily injection molded into highly precise forms. This combination of properties make it an ideal material for plastic gears and other moving parts. It is far less expensive to produce nylon gears than metal ones. Most of the gear trains and sliding components in modern consumer devices are made of molded nylon.
Nylon can be machined drilled at tapped with the usual precautions against melting needed with plastics. It cannot be glued. It is relatively inexpensive, slightly less than the cost of aluminum in small amounts.
PETE is an abbreviation for PolyEthyleneTErephthalate, a polyester plastic. It is best known as the superstrong stuff from which soda bottles are made, but it is also the basis for Dacron polyester fiber, and Mylar film. It is sometimes used in bulk form, but is of most interest to robotics as a very strong sheet and film material. The strength is produced by stretching the amorphous plastic in both "x" and "y" directions. This stretches out the polymer molecules so they are aligned with the sheet surface. Any further deformation has to break the attraction of long molecules along their entire length, rather than just locally as would be the case if they were randomly coiled up like a pot of spaghetti. The result is great strength and stiffness in the two extended directions of the film. PETE can be produced in a form that withstands temperatures above 200 C. This material has been used to make plastic food containers that can be placed directly in an oven for heating.
Because of its strength, durability, and chemical inertness, PETE film has been used in a huge number of applications from packaging, to magnetic tape substrates, to balloons. The familiar shiny surface of Mylar is produced by vacuum deposition of aluminum vapor. With appropriate adhesives, the film can be securely glued for assembly.
Sortable table: Properties of plastics from Boedeker.com
Table: Properties of plastics from Plastics International
Table: Plastics mechanical properties from Curbell
Technically, a composite is a structural material composed of two or more simpler substances, often having very different properties, combined (in macroscopic manner) to yield a material with combinations of properties not present in any of the components in isolation. For example, very strong, but brittle glass is combined with relatively weak, but more flexible plastic resins to yield fiberglass which is both exceptionally strong and extremely fracture resistant.
By the above definition, most engineering metals are technically composites. Carefully polished and etched, and under modest magnification, alloys can be seen to consist of different crystal phases interlocked and cemented together. It is largely the combination of the different mechanical properties of these components that gives alloys their desirable characteristics. We will use the term in its more generic sense for materials made by combining more obviously disparate elements.
A characteristic of many composites is non-isotropy in mechanical properties such as strength. A carbon-composite fishing rod can endure lunges from a hooked marlin that would bend steel in the same form, but split if stepped on. This is due to a common composition technique where a strong but brittle material is made into thin fibers which are bonded together along their length with a weaker, more flexible "glue". Brittle fracture propagates because of the mechanical concentration of stress at the tip of a fracture supported by the stiff bulk of material behind it. Individual fibers can easily tolerate the (minor) distortion of a stressed bulk element. Any concentrated forces cause local yielding of the softer material, spreading the stress along the length of the fiber and avoiding the bulk mechanical leverage that propages brittle fracture. Along the fibers, the composite has most of the strength of the fiber material, since the weak glue holds the fibers together edge-to-edge over a long distance. Pulled apart across the fibers however, the composite has only the strength of the glue.
The engineering of modern composites is an entire field in itself. In building our robots, we are likely to encounter only a few, including wood (yes, wood), fiberglass, possibly pre-made carbon fiber elements, and glass-filled plastics.
Wood on the hoof
Wood might not seem like an appropriate material for shiny modern robots, but it has its advantages. Chief among them are that wood is inexpensive, widely available, and easily formed with commonly available tools. But wood itself is a remarkable structural material. It is a natural composite (cellulose fibers glued together with a substance called lignin) having a stiffness to weight ratio that matches steel or aluminum. This is important in structures that are sufficiently extended that they deflect under their own weight. Wood is stronger in absolute terms than many plastics, (generally several thousand psi along the grain) and tougher than most. More importantly, once dried, it has little tendency to "creep", a slow deformation under stress that plagues many plastics. Wood can be securely joined together in more ways than practically any other material. It can be nailed, screwed, glued, bolted, wedged, doweled and dovetailed. There are dry assembly methods that are as secure as anything using external fasteners. And it can be finished to have practically any appearance desired.
Wood does have its problems. It is strong only in one direction - along the grain. It is typically only 1/100 as strong in tension across the grain, and is usually significantly weaker in compression as well. For structural purposes, wood should be considered to have effectively zero tensile strength across the grain. Plywood, which is made by gluing thin sheets together with the grain at different angles has good two-dimensional strength. A strong isotropic wood-based material is not currently available. Particle board is somewhat isotropic, but it is weak in all directions. Theoretically fibers could interlace in all three directions, but trees have not managed it on a bulk scale. Certain "unsplittable" woods such as elm are known, but they are really not even close to isotropic.
Wood has knots, splits, defects, uneven grain, and variable strength and density - even within the same board. And it moves when the humidity changes. A tight fit on a dry day may buckle on a humid one. More commonly gaps open in originally well-fitting assemblies. Originally flat boards bend or twist. The movement is uneven. Along the grain wood changes little in size. Across the grain shrinkage of 5 percent or even more may occur, especially if it was slightly green to begin with. Much of the art of woodworking is involved with ensuring that articles remain functional in the presence of wood's natural movement.
For our robots the most useful forms of wood are likely to be small bars and dowels, which make reasonable lightweight beams, and some of the thinner plywoods, which are useful as mounting substrates and simple enclosures.
Table: Mechanical properties of US hardwoods from Engineers Edge
Table: Mechanical properties of US softwoods (1) from Engineers Edge
Table: Mechanical properties of US softwoods (2) from Engineers Edge
Fiberglass is made by impregnating a matrix of oriented flass filaments with liquid plastic resin, usually an epoxy, which then sets, binding the whole into a strong and rigid material. The glass filaments may be longitudinal which yields an material with primarily one-dimensional strength like wood, or made into cloth or a felted mat, which gives two-dimensional strength. Three-dimensional weaves are possible, but rarely encountered, and usually only in expensive, exotic applications. Cured fiberglass composite can be as strong as medium steel in its strong directions, and is considerably lighter (density around 2 gm/cc).
Fiberglass is a reasonable do-it-yourself material. Glass fabric and epoxy resins are readily available at automobile parts stores, craft and hobby dealers, and even some hardware centers. It is one of the easiest ways to make curved custom parts. The hazards associated with the materials are manageable: the resins can be irritating, and glass fibers can sometimes penetrate the skin causing a rash and itching. Both need to be absolutely kept out of the eyes, and dust from sanding or sawing should not be inhaled or left where it could be disturbed and become airborne. If cured fiberglass must be abraded, wet-sanding methods should be used.
Carbon composite, sometimes called "graphite" is similar to fiberglass except that ultra-strong carbon fibers made by pyrolizing certain oriented polymers in a furnace are used instead of glass. It is both stronger and more expensive. Carbon fiber is more hazardous to work with than glass. The stiff fibers penetrate skin easily, and unlike glass, which can (very slowly) dissolve in the body, they NEVER go away. Inhaled carbon fiber is a possible carcinogen though most industrial fibers are in the 6-10 micron diameter range, which is significantly larger than the .5 micron diameter which is most damaging to the lungs. In any case, precautions should be taken. Pre-made carbon composite parts should never be sawed or sanded without respiratory protection and containment systems that prevent residues from escaping into the environment.
Plastic resins are sometimes mixed with (relatively short) fibers of harder or stronger materials to increase their yield strength. The most frequently used filler is chopped glass fiber, but other ceramics, metals, and even other plastic fibers are sometimes used. Because the fibers are short and generally do not have coordinated orientation, such materials are not as strong as engineered composites using the same materials (e.g. fiberglass). However, glass-filled plastics can easily have twice the strength of the un-modified resin. This is usually at the expense of ductility and sometimes impact resistance, although for brittle resins, glass fill can increase impact resistance by "bridging the gap" and impeding fracture propagation.
Ceramics can be thought of as artificial igneous rocks - that is, heat-fused refractory, fully oxidized materials. They are typically made by heating specially compounded mixtures of minerals to high temperature in a furnace, whereupon they stick together and remain firmly stuck after the material cools. The mixtures may or may not fully melt, and the process may also involve high pressures or special atmospheres. The canonical example is pottery, which has been known since antiquity.
Ceramics were not traditionally thought of as structural engineering materials. Except for bricks, which were not usually thought of as ceramics. This started to change in the mid 20th century when architects began using glass as a load-bearing material. Today high-tech ceramics are important structural materials in applications ranging from electrical insulators, to bearings, to gas turbine blades. The brittleness associated with glass and porcelain has been greatly reduced in many of these materials. Although ceramics do not yet match good metals in toughness, they can greatly exceed metals in strength, hardness, and wear resistance, especially at elevated temperatures.
Because of their hardness, ceramics are difficult to work. They are often directly manufactured in their final form, or are subject only to a final polishing step. We will probably not have much reason to use ceramics as structural materials in our robots outside of the occasional glass window.
Glass is most easily available in ordinary "window glass" form, which has familiar fragile properties including the tendency to shatter into large, extremely sharp pieces. Glass is intrinsically a very strong material. Its fragility is due to brittle fracture where a crack, once started, mechanically concentrates stress at its tip. Since glass has effectively zero ductility, the stress cannot be relieved by plastic deformation, and the fracture propagates rapidly. Glass surfaces generally contain microscopic scratches, and when that surface is placed under tension these scratches serve as initiation sites for fractures.
The fragility of glass can be reduced by various "tempering" procedures. Essentially, tempering places the outermost layer under compression, balanced by tension on the inside. This means that the glass can be stressed without placing the surface under tension, so microscopic scratches have no tendency to grow into fractures. The flip side is that if a chip or scratch penetrates the compressed surface layer, the pre-existing tension in the interior causes the entire volume to rapidly fracture into small pieces, as is observed in automobile side windows. In general such small pieces are less of a hazard to people than large sharp shards.
Tempering was originally accomplished by rapidly cooling of the surface of glass in a hot, plastic state. The surface layer freezes, with shrinkage being locally accomodated by the still-plastic interior. Then as the interior cools, its shrinkage pulls the surface layer into compression. This technique really only works well for simple shapes such as sheets and smooth curves.
More recently, methods of "chemical" tempering have been introduced that can be used to treat arbitrary shapes. Here, glass in its solid state is placed in a chemical bath that replaces certain mobile elements in the glass matrix (e.g. sodium in soda-lime glass) with larger analogous atoms (e.g. potassium or cesium). Since these take up a bit more space, the surface layer is placed under compression. Usually this is done at elevated temperatures to increase the mobility of the atoms, but not high enough to soften the glass. One of the simplest methods is to place pieces of soda-lime glass to be tempered in a bath of molten potasium nitrate (about 350 C = 660 F) for 8-24 hours. You could almost do this in your home oven, but don't. Molten KNO3 can react violently with organics and metals, and it decomposes with evolution of oxygen and nitrogen only slightly above its melting point (at 400 C). Your home oven has hot spots that will cause a disaster.
Extreme tempering has recently been used to produce products such as "gorilla glass" (used in smart phone screens and other applications), thin sheets of which can be bent double without shattering, and basically don't seem like glass at all.
Untempered glass sheet (e.g. window glass) can be fairly easily "cut" by scoring and breaking using an inexpensive glass cutter. Larger pieces can shaped into more complex forms, such as highly precise optical components by abrasive grinding and polishing, but this needs special wet-process equipment. Minor grinding can be done on an ordinary abrasive wheel or a stationary belt sander if the abrasive can be kept wet without creating an electrical hazard or damaging the equipment. Dry machine grinding of glass typically causes fracture from thermal stress. Sharp edges can be relieved using a hand sanding block and fine (>= 220 grit) sandpaper. Special carbide and diamond drill bits are available that, with some care, can be used to put round holes in glass sheet. The diamond hole saws can also produce a useful disk. They are always used with water or other liquid coolant. Tempered glass typically cannot be cut, and can be only lightly ground and polished. It must be shaped before tempering.
Window glass can be bought inexpensively at home centers and hardware stores. Specialty commercial and auto glass stores can provide plate glass in different thicknesses and colors. They can also order tempered glass sheets made to size and with holes and polished edges at extra cost. Optical houses such as Edmund Optics can supply high-quality glass lenses, ports and filters at very high prices (and at even higher price, some components made from non-glass materials such as sapphire and silicon).
Elastomers are rubbery materials. They have unique mechanical characteristics, the most useful of which is their ability to undergo extreme deformation under modest stress and then rapidly recover their original form when the stress is removed. In a bulk sense elastomers, like liquids, have a relatively high compressive modulus. Under uniform (isostatic) pressure, elastomers change very little in volume. Their interesting properties can be attributed to comparatively low shear moduli that effectively allow them to flow under stress. Elastomers are sometimes modeled as liquids with extremely high surface tension. This is a reasonably good model for a spherical rubber ball. Certain modifications need to be made to accomodate other shapes.
Elastomers are typically polymers, often cross-linked assemblages of loosely coiled chains. They are generally waterproof and can be made highly resistant to chemical attack. Their combination of elastic and conforming properties makes them ideal materials for seals and gaskets. Rubber O-rings can reliably seal a moving shaft against pressures of several thousand psi. They make hydraulic pistons possible. Elastomers are also used in shock and vibration reducing mounts, flexible connectors, and for surfaces requiring good grip, high friction, and a soft touch. Some elastomeric components can be deformed millions of times and still retain their original shape and resiliancy (think tires).
Cured elastomers are extremely difficult to shape. Because they deform under low stress, they cannot be cut precisely. They tend to be abrasion resistant, and abrasive machining, if possible, often produces an unacceptably rough surface. Thus most elastomeric parts are manufactured in their final form. Some elastomers have an uncured form that can be melted and molded (e.g. unvulcanized rubber), others start as liquid resins that are polymerized directly in the mold.
In robots, elastomers are used to provide traction against the ground or friction on grippers. They are used as soft contacts to reduce impact forces and prevent damage to and by parts that contact the environment. They provide passive compliance in joints and manipulators, which is often essential for robust (and damage-free) interaction with objects. And elastomers find use as seals when dust or liquids need to be kept out or in.
The word rubber is often used in a generic sense to refer to any elastomeric material. More specifically, the term refers to carbon-backbone polymeric elastomers, and even more specifically, to those derived from the natural material. Natural rubber is made from a milky sap called latex produced by a variety of plants. The commercial source is the tropical rubber tree (Hevea brasiliensis), a kind of euphorbia, but many other species produce rubbery latex including milkweed and dandelions. Its natural role appears to be to permanently gum up the mouthparts of insects that would eat the plant. Exposed to air, latex polymerizes into a soft, resilient material that can be heated and remolded.
Charles Goodyear discovered that when heated with a bit of sulfur, latex becomes much firmer, stronger, and abrasion resistant. He called the process vulcanization and the tire industry was born. Chemically, the sulfur crosslinks individual polymer chains making the material more rigid and resilient.
Natural rubber is still used to make premium tires, especially for trucks. But during and after WWII, demand for rubber exceeded the natural supply leading to intense research into synthetic alternatives. Today we have dozens, with all sorts of special properties. The first commercial synthetic success, still widely used, was based on polybutadiene (butane with two double bonds). The second double bond both makes the chains curl into springy coils and provides opportunities for cross-linking.
Rubber can be obtained pre-formed in a variety of shapes. Commonly available forms include O-rings, corks, sheet material, and rubber bands. Big, specially-shaped chunks show up as isolation mounts in a variety of machinery. Sheet material can be cut with scissors, and there are special tools for cutting round holes in corks or other solid forms, but otherwise, cured rubber is difficult to shape. It glues well with special cements, and can be attached to rigid substrates with superglue. Because superglue is rigid, it does not work well for rubber-to-rubber contacts that will be stretched.
Silicone rubbers are elastic polymers having a silicon-containing backbone (instead of or in addition to carbon). Note that these polymers are distinct from elemental silicon, a brittle, silvery material from which computer chips are made.
Some silicones are available in an uncured form that sets on exposure to air to a rubbery solid (e.g. "silicon-seal"). These materials have a variety of uses in robot construction. They are ideal for making gaskets in place, and for making joints between assembled components waterproof or even airtight. They can also be used to "glue" certain materials to each other that cannot normally be glued, in particular glass and metal. The bonds are not strong in an absolute sense, but they are quite secure for holding components in place against modest forces since the flexibility prevents fracture failure.
Tensile elements can be considered a distinct material class in the same sense as rigid solids and elastomers. They have strength only in tension in one dimension, and are otherwise flexible. The most familiar form is multi-strand twisted or braided cable, but solid wires are occasionally used. Chains and flexible belts are more specialized examples.
Structural cables are made from relatively few materials, the most important being stranded steel wire (also known as wire rope), and a handful of polymer fibers including polyester (Dacron), aramid (Kevlar), and UHMWPE (Spectra, Dyneema). Nylon and polypropylene are frequently used for rope, but they are too stretchy for most structual applications. Chains are usually made of steel links, and belts from a combination of tensile fibers with elastomers.
Tension elements find two primary uses in robotics. The first is simply as tensile reinforcement in rigid structures. A rectangular frame is mechanically weak and prone to failure by diagonal skewing. Reinforced by a pair of diagonal cables, the structure is strong and rigid in two dimensions, and very light. In three dimensions, there is still a racking (twisting) failure mode, but rigid 3-D structures can be made by combining such 2-D panels. The other use is in transmissions - as tendons, belts, or chains. Transmissions are a separate subject, but some of the simplest and best depend on the existance of flexible tension elements.
