A change in temperature causes a change in the length of a metal rod. But if the two ends of the rod are rigidly fixed at fixed supports, expansion or contraction of the rod gets obstructed. Hence, a large force is generated along the rod. This force measured per unit area of the rod is called Thermal stress.
Advantages of Thermal Stress:
1. The iron rim of the bullock cartwheel has a diameter slightly smaller than that of the wheel. Hence, the rim can’t be fitted to the wheel at ordinary temperature. The rim, when heated expands and thus can be fitted easily around the wheel. When cooled to normal temperature, the iron rim contracts and fits tightly around the wheel.
2. Some holes are drilled through two metal plates by placing them one above the other. A river is heated and inserted through each hole. head of the rivet is then flattened by hammering. This keeps the two plates tightly fitted. On cooling, the rivet contracts lengthwise, fixing the two plates even tighter. Iron bridges are made by riveting large plates in this way. The flattened heads of the rivets can be easily seen on this type of bridge.
3. Thermostat: Thermostat is a device that is used for automatic temperature control. These make use of the property of a bimetallic strip which bends when heated. Thermostats are used in electrical ovens, refrigerators, electric iron, incubators, electric heater, etc. It is an automatic switch that turns on at a definite temperature and off at some other fixed temperature.
4. Fire Alarm: In a fire alarm, an electric bell is connected through a thermostat to the power supply. It is essentially a bimetallic strip of brass and invar, invar piece having the contact point with the supply. At ordinary temperatures, the thermostat remains disconnected from the power supply. When the strip gets heated up due to fire, it bends towards the contact point. This establishes current through the electric bell and the bell rings.
5. Temperature Measurement: The curvature of a bimetallic strip increases, and so its radius of curvature decreases with the increase in temperature. From the straight upright stages, it bends as temperature increases. This moves the center of curvature closer to the strip.
Disadvantages of Thermal Stress:
1. Railway tracks are made by connecting pieces of rails using fishplates. A small gap is maintained between two consecutive rail pieces. iron nuts and bolts connect and hold the fishplate with the rail. The bolt holes are slotted to allow free movement at the rail joints. Due to factors like sunlight or friction, the rails get heated up and expand. The gap between the two rails and the longitudinal slots for the blots, allows the rail to expand lengthwise.
2. Iron or steel girders are used in the construction of a bridge. One end of the girder is rigidly fixed with bricks and concrete. The other end is not fixed. Instead, the end is set on a roller over the support. When there is a rise or fall in temperature due to seasonal changes, girders may expand or contract without developing any thermal stress.
3. A thick-walled glass pot often cracks when hot water is poured into it. The inner surface of the thick glass, in contact with the hot water, warms up and expands. Glass is a bad conductor of heat, so the outer surface remains colder, and hence expansion is less. This unequal expansion sets up thermal stress and the glass cracks.
4. A metal scale can measure true reading only at the temperature at which it has been graduated. At any other temperature, the interval between two consecutive graduations, increases or decreases making the measurement inaccurate. Hence, the measured length has to be corrected using the value of α for the meatal of the scale in use.