18.6:
Thermal Stress
A body experiences stress when it undergoes temperature variations; this stress is known as thermal stress, which can even permanently deform the body.
For example, a space is intentionally maintained between two joints of a rod on the railway track to prevent the deformation or bending of rods.
To calculate the thermal stress in an object, first, determine the thermal expansion or contraction, then calculate the stress.
Consider a rod of length L0.. When heat is applied, the rod will expand in length by delta 'L,' and the fractional change in the length of the rod can be evaluated.
Now, the same rod is fixed to both ends. When the heat is applied, the rod experiences strain as it cannot expand. To restore the original length of the rod, the stress can be calculated.
Recalling the definition of Young's modulus, it is the ratio of stress to strain.
Therefore, the thermal stress can be determined by combining the equations and rearranging the terms.
18.6:
Thermal Stress
If the temperature of an object is changed while it is prevented from expanding or contracting, the object is subjected to stress. The stress is compressive if the object expands in the absence of constraint and tensile if it contracts. This stress resulting from temperature change is known as thermal stress. It can be quite large and can cause damage. To avoid this stress, engineers may design components so they can expand and contract freely. For instance, on highways, gaps are deliberately left between blocks to prevent thermal stress from developing. When no gaps can be left, engineers must consider thermal stress in their designs. Thus, the reinforcing rods in concrete are made of steel because steel's coefficient of linear expansion is nearly equal to that of concrete.
Thermal stress can explain many phenomena, such as the weathering of rocks and pavements by the expansion of ice when it freezes. Railroad tracks and roadways can buckle on hot days if they lack sufficient expansion joints, and power lines sag more in the summer than in the winter and will snap in cold weather if there is insufficient slack. Cracks open and close in plaster walls as a house warms and cools. Glass cooking pans will crack if cooled rapidly or unevenly because of differential contraction and the resulting stresses.
Suggested Reading
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from https://openstax.org/details/books/university-physics-volume-2; pp 18–19.