Learning Goal:To develop an understanding of ductility (or brittleness) and how it can be determined by the behavior exhibited on a stress vs. strain plot. Materials can be either ductile or brittle depending on their stress-strain behavior. Generally, ductile materials experience large strains before failure, whereas brittle materials experience relatively little strain before failure. How ductile (or brittle) a material is provides engineers insight into what different materials may be best suited for certain applications.

To save energy, the air supply to a 2000 ft office has been shut off overnight and the room temperature has dropped to 40°f. in the morning, the thermostat is reset to 70°f and warm air at 120 f begins to flov in at 200ft'/min. the air is well mixed within the room, and an equal mass flow of air at room temperature (changing with time) is withdrawn through a return duct. the air pressure is nearly 1 atm everywhere. ignoring heat transfer with the surroundings and kinetic and potential energy effects, estimate how long it takes for the room temperature to reach 70°f. plot the room temperature as a function of time.