Expert: John Locke - 9/28/2010

Question
I know that cellulose is a tough material, but is it true that cellulose fibers have high tensile strength almost equal to that of steel? This is not a homework question and it is just for knowledge.

Answer
Thanks for using AllExperts. Cellulose is a polysaccharide consisting of many thousands of units of glucose linked together in a repeating, overlapping pattern. Cellulose does in fact have a tensile strength in the range of some types of steel. A36 structural steel, for instance, has an ultimate strength of 400 MPa (megapascals). This means that generally A36 steel will fail when under a tensile stress of 400 MPa; it deforms irreversibly above 250 MPa, but will retain its structural integrity until it reaches the ultimate stress.

Compare this with a 2005 study of root strength (The influence of cellulose content on tensile strength in tree roots) from the journal Plant and Soil. This study compared the ultimate strength of various plant roots to their diameter; since cellulose is the component of plants that gives them their tensile strength, the cellulose content of a root might be expected to scale with its ultimate strength. Those were exactly the findings: as cellulose content increased in some of the smaller roots, the ultimate strength of those roots increased in a regular, predictable fashion. Some of the roots tested failed at stresses of 130-200 MPa, which is very close to cast iron or copper and in the range of most types of steel.

Some people find this surprising, but it just suggests that there are other properties of materials that should be considered when characterizing them. Tensile strength is one measure; compressive strength is another. Hardness is still another. Steel has high tensile and compressive strength and is hard; cellulose has high tensile strength, low compressive strength, and is soft.

Think about this thought experiment. Take a steel bar six inches long and pull on either end; it does not deform at all. Take a six-inch piece of celery and try the same thing; it takes a surprising amount of force to deform it. This is a testament to the high tensile strength of both materials.

Now imagine pressing inward on the ends of the steel bar and the ends of the celery. The results are very different for compressive strength, as they would be if you tried to cut the bar with a knife. The tensile strength of cellulose is impressive, but it is simply one characteristic of the material as a whole.