|MadSci Network: Engineering|
Hello, Ting... Both relate to the response of an object to applied tensile (i.e. pulling it apart) force, but they are not the same. Tensile strength usually means the applied force at which the material fails. Stretch is also a function of the material, and has to do with how much it elongates for a given force applied. Here's a practical example. Nylon can be made into various kinds of ropes. Dynamic climbing ropes are made so that they stretch more, and have relatively lower breaking strength, than static rescue ropes. In this case, both are made of the same material (nylon) but differ in methods of construction. "Stretch" consists of two kinds of process. One is what is called elastic, in which the stretched material recovers from the stretching and is essentially unchanged (a metal coil spring is an example) and the second is plastic deformation, which leads to permanent change in the material (does not fully recover). Plastic deformation eventually leads to failure, and happens by such means as slipping along grain boundaries, dislocations traveling through the mateiral (both in metals), molecules sliding past each other (plastics and polymers), and propagation of cracks initiated from imperfections (ceramics). Many introductory textbooks on materials science go into this in far greater detail, and the subject is a fascinating one. Videos of dislocation motion, for example, are all over the web. Here's one by Cambridge University: www.doitpoms.ac.uk/.../dislocations/dislocation_motion.php and you can enter the keywords "dislocation" "motion" and "video" into a search engine to find many more.
Try the links in the MadSci Library for more information on Engineering.