In many engineering projects, it is critically important to know the strength of the material in your design. Strength is a measure of a material’s ability to withstand an applied tension, compression, or shear force. Tension forces pull on an object, while compression forces push on an object. Shear forces are two pushing forces that offset one another like the forces applied by the two blades of a pair of scissors. Ultimate strength is the point at which a material completely fails. Values for tension, compression, and shear strengths can be quite different depending upon the material.
Consider the strength of a length of string. The type of material is just one factor affecting the tensile strength of the string. The number of plies (the smaller strings that are twisted together to make the string), the tightness of the twist, the abrasion resistance of the fiber, the length of the sample, and whether the string is dry or wet all have an effect on a string’s ultimate breaking strength.
In this project, your challenge is to design and build a device for testing the ultimate strength of a length of string. Sewing thread, light fishing line, and yarn are good choices for testing because they often come rated by their ultimate tensile strength. You will use a Vernier Dual-Range Force Sensor for making strength measurements. The string samples should be easy to load and unload, such that one end of the string is tied to the hook of the sensor and the other end is tied to your device. Your device should be able to pull the string at a slow and steady rate until the string breaks. The maximum force should be displayed on the screen EV3 screen. In order to avoid damaging the Dual-Range Force Sensor you should select string that will break before 50 N of force is applied.