CSC 297 Robot Construction: Materials

Structural Materials for Robots

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


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.

Sortable table: Properties of plastics from

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.


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.


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.

Flexible Tensile Elements

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.

Wikipedia on wire rope

Wikipedia on aramid fiber

Wikipedia on UHMWPE

Wikipedia on chain

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