Understanding Strength

Strength not only depends on the material, but the structure of the members.
Strength not only depends on the material,
but the structure of the members.

Level: Middle School to H.S.

Time involvement: 15 – 20 minutes.

This lesson utilizes objects made of wood, aluminum, carbon fibers and composite materials. Primitive Man made practically everything out of rock, wood, animal or plant parts. Much later the Bronze Age and the Iron Age introduced new materials. With metals, tools and weapons could be made stronger and more effective. The strength of metals came with a price: weight. Sometimes weight was necessary. To drive nails a steel hammer is better than a wood hammer. A dugout canoe hollowed from a log could carry passengers and cargo but it was heavy. A birch bark canoe used lighter materials, but could be carried from river to river.

It soon became evident that improving almost anything, required careful design, engineering, craftsmanship and lighter and stronger materials. Ships could be made of steel but a steel airplane was almost impossible. The cost of aluminum, before better production methods were developed, was higher than gold! Although aluminum has about one third the weight of steel it has about half the strength. For many years airplanes made extensive use of aluminum. Today carbon fibers are replacing aluminum in many areas.

The photo above shows several aircraft components using different materials. What they have in common is minimizing materials where they carry little or no load and maximizing material where the stresses are highest. Wing ribs often incorporate weight saving holes and the concept of an aircraft fuselage (body) being essentially a hollow tube came very early in the design of airplanes.

understanding-strength-2This simple diagram illustrates an important concept in weight savings. Assume the beam is a wood board. The fibers on the top are being squeezed together. The fibers on the bottom are being stretched. What happens to the fibers in the middle? No stress! This simple fact carries an important implication. If the outer “skin” can be kept in place all the inner material can be eliminated! This is the secret that makes possible lightweight airplanes, canoes, tennis rackets, even bones in our bodies.

Discussion: Model bridge contests emphasize high strength to weight of the bridge itself. Buildings use lots of steel in their framework but the steel is not solid rods but “I” beams with thick plates on the top and bottom connected with a thinner web to keep the plates in alignment. Many variations of truss construction are used in bridges: Warren Truss, Pratt Truss, Lattice Truss, etc. The humble cardboard box of corrugated paper was patented over a hundred years ago. Nature uses this “secret” constantly. From the super light weight of their bones to the construction of feathers birds are engineering marvels. Even the eggshell provides excellent protection for the developing bird. Plants grow hollow stems to save energy for the important task of producing seeds.

The insect world “perfected” the shell idea. Biologists use the term exoskeleton for the method of creating a strong protective structure for internal organs. Dissect a grasshopper and you will find no internal bones. Camping/backpacking gear often uses carbon fiber tent poles; backpack frames are often made from aluminum tubing. Racing bicycles incorporate lightweight Kevlar. Interestingly although diamonds are easily the hardest material know due to their tight carbon crystal structure diamonds are brittle and can shatter! Material engineers use many terms to describe the characteristics of materials: toughness, flexibility, corrosion resistance, UV resistance, etc. may often be more important than just hardness. By combining materials of different characteristics a composite may offer advantages. Fiberglass boats, car parts, tool handles, etc. have superior performance when combined than merely the glass fibers and the resin binder.

In conclusion, the award for the best material design goes to Nature itself. The marine Abalone shell is 3,000 times tougher than the strength of the chalk and protein used in combination!

Notes: The Internet offers many ideas for challenging student projects requiring building towers, bridges, and structures using lightweight materials.

How to Build a Model Bridge (large pdf).