Materials scientists often have to perform tests to characterize the physical properties of different materials. Some tests are difficult to carry out at home, but there are a few properties that we can classify with a simple force spring, for example calculating the tensile strength of a material with a tensile test.
The tensile strength of a material is its resistance to breaking under tension, and it can be easily determined by pulling on the material until its breaking point. This breaking point, or in other words the maximum amount of tension that can be applied to the material by stretching or pulling until it breaks, is the ultimate tensile strength of the material.
Tensile tests use stress vs strain plots to display their results. Stress (the amount of tension per unit of cross-sectional area) and strain (the change in length divided by the original length) are words that material scientists often use to describe the physical properties of materials.
Here we perform the test on silk, but experiment with different materials and see how tensile strength differs between them!
- Force spring
- Length of silk (or any other material you want to test)
- Take a length of silk and measure its initial length and its thickness using a ruler. (Tip: a single strand of silk may be too thin to measure its thickness. Try twisting a few strands together to make one, thicker strand that can then be measured.)
- Tightly tie one end of the silk in a knot around the hook of the force spring.
- Secure the top of the force spring to something stable, like a table or door handle.
- Slowly begin to pull on the silk, keeping an eye on the scale reading.
- Record the value of the scale reading right before the silk breaks. (If your fiber is breaking too early or too late for you to record, you may have to twist the fiber so that it has a higher or lower cross sectional area.)
- Calculate the cross sectional area (CSA) by squaring the thickness of the silk and multiplying by pi.
CSA = π × (thickness of your strand of silk)²
- The ultimate tensile strength of your silk is equal to the value you measured just before the silk broke, divided by the CSA that you calculated in step 6.
Tensile strength = (scale reading) ÷ (CSA)
Analysis & Reflection
- How does the tensile strength of your silk compare to other biomaterials or synthetic materials? Try calculating the tensile strength of other materials around your home (some ideas: cotton, spider silk, hair)!
- Could different degumming conditions affect the tensile strength of your silk?