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Thermal stress is a result of a change in the temperature of an object when subjected to heat. It may cause the object to expand or contract, causing them to break.
- Thermal stress is a force that restores the body’s deformed structure to its original form.
- This type of stress depends on the thermal expansion coefficient, which varies from material to material.
- The SI Unit of thermal stress is Pascal(Pa), named after French mathematician Blaise Pascal.
- It is induced in a body when the body temperature is raised or lowered, and the body is not allowed to contract or expand freely.
- The stress is induced in electronic devices, such as computer chips, that generate heat during operation.
- Mathematically, thermal stress can be written as:
δT=Lα(Tf−Ti)
δT=LαΔT
- Where L : Length
- Ti : initial temperature
- Tf : final temperature
- ΔT=Tf-Ti : change in temperature
- α: coefficient of thermal expansion
Key Terms: Thermal Stress, Thermal Stress Formula, Stress, Applications of Thermal Stress, Working of Thermal Stress, Stress, Coefficient of Thermal Expansion, Temperature
What is Thermal Stress?
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Stress caused by a change in temperature of an object, forcing it either to expand or contract, is called thermal stress. These stresses can even lead to fracture or shattering of the object.
- The level of stress increases as the temperature change increases.
- Thermal stress is triggered by an object’s internal temperature change.
- It causes change in structural strength and stability if they are not appropriately calculated.
- This type of stress is used in the fields of mechanics and thermodynamics.
Example of Thermal StressExample: Let's take the example of a thermal conducting rod. As the temperature increases, it will expand. When the rod is kept at room temperature, the restoring force will act on it before bringing it back to its original position. Moreover, if it is kept at a cold temperature, it will start contracting. |
Thermal Stress
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Thermal Stress Formula
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Consider a thermal rod which undergoes thermal stress and expands on heating. The change in length is directly proportional to the amount of heat supplied. Let α indicates the temperature coefficient of structural expansion.
- Original length of rod is given by L0.
- Increase in length is given by dL.
- The rod was heated and the increase in temperature = ΔT
- Due to thermal stress, the length of the rod decreases from (L0 + dL) to L.
- Thermal stress formula is given as:
δT=Lα(Tf−Ti)
δT=LαΔT
- Where L : Length
- Ti : initial temperature
- Tf : final temperature
- ΔT=Tf-Ti : change in temperature
- α: coefficient of thermal expansion
Working of Thermal Stress
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The working process of thermal stress includes changes in the length of the material with respect to changes in the surrounding temperature. When bimetallic strips are placed at room temperature, the contact point remains in physical contact.
- The temperature reaches a maximum limit when it is heated at the extreme point.
- Due to heating and changes in surrounding temperature, bimetallic strips are bent.
- The changes in the length of the structure represent the changes in the temperature.
- Initially, the length of the material is given as L and changes to L−ΔL at cold temperature.
- The length changes to L+ΔL when the temperature changes to hot.
Applications of Thermal Stress
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The applications of thermal stress are as follows:
Railway Track
Thermal stress is used on railway tracks. The gap between two railway tracks is what allows the train to move. In summer, the gap expands due to the increase in temperature, while in winter, when the temperature drops, it contracts. Thermal stress helps balance these situations and keeps the train intact.
Thermostat
Another important common application of thermal stress is the thermostat. It is a device that is used to regulate the constant temperature. These devices are used in the construction of various electronic devices, such as air conditioners and iron boxes.
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Things to Remember
- Thermal stress is caused by the result of changes in external and internal temperature.
- The amount of heat that is required to cause changes in temperature is known as heat capacity.
- Stress is a force that, when applied to a body, brings about a change in its shape, ultimately causing its deformation.
- Strain measures the deformity of an object which has undergone deformation due to stress.
- It cannot exist without stress, but stress can exist even without strain.
- The CGS unit of thermal stress is dyne cm2, whereas the SI Unit is Nm2.
Sample Questions
Ques: What do you mean by thermal stress? (2 marks)
Ans: Stress which occurs as a result of temperature change in an object, is known as thermal stress. This change in temperature may be high or low. The application of thermal stress can be seen in railway tracks, bridges, industrial equipment etc. The level of stress increases, as the temperature change increases.
Ques: What is stress and it types? (2 marks)
Ans: Stress is something which measures the forces which cause deformation of an object. It can be further said that it is defined as a force applied on a unit area. The various kinds of stress are,
- Normal stress
- Longitudinal stress
- bulk/volume stress
- Shear/tangential stress
- Tensile stress
- Compressive stress
Ques: What is strain? (2 marks)
Ans: Strain basically calculates to what extent an object is deformed. It has no unit and is of three types.
- Normal or longitudinal strain,
- Shearing strain, and
- Volumetric or bulk strain
Ques: What is the difference between stress and strain? (3 marks)
Ans: There are various points of difference between stress and strain. These are,
- Stress is defined as the applied force per unit area, whereas strain measures the deformation per unit area.
- The SI unit of stress is N/m2 or N/mm2, whereas strain doesn't have any unit.
- It is possible for stress to exist without strain, but strain cannot exist without stress.
Ques: How to calculate Thermal stress? (2 marks)
Ans: Thermal stress formula is given as follows,
δT=Lα(Tf−Ti)
δT=LαΔT
- Where L : Length
- Ti : initial temperature
- Tf : final temperature
- ΔT=Tf-Ti : change in temperature
- α: coefficient of thermal expansion
Ques: What is the unit of Thermal stress? (2 marks)
Ans: Stress is symbolized as the force per unit area, which is required to deform an object. Hence, the formula of Stress happens to be [ML -1 T-2]. The CGS units of stress are dyne cm-2 and the SI unit of Thermal Stress is Nm-2.
Ques: Give some examples of thermal stress? (3 marks)
Ans: A thermostat is the most common example of this. The mechanism of the railway tracks, a piece of glass subjected to high temperature, are all common examples. The train moves because of the space between two railroad lines.
- They begin to expand in the summer due to the rising temperature, and they contract in the winter due to the falling temperature.
- In order to maintain the train's integrity, thermal stress helps maintain these circumstances in balance.
Ques: What is longitudinal stress? (2 marks)
Ans: When the shape of a body changes along its length, we call it a longitudinal stress. Suppose there is a pipe. When you apply stress on it, it acts in a perpendicular direction. Further, this stress lies parallel to the longitudinal axis of its centerline axis. This is the reason the stress acts along the length of the pipe.
Ques: What is bulk/volume stress? (3 marks)
Ans: When forces act in a perpendicular manner on a body’s surface from all directions, we call it bulk stress. Bulk stress results in a change in volume of the body.
- For example, when a scuba diver jumps into the ocean, the water exerts a uniform pressure everywhere on his body.
- The force of the water causes a slight change in the volume of the body.
- This is because, the pressure in a fluid goes on increasing, as the depth increases.
- This is the major factor behind bulk stress or volume stress.
Ques: What is a real-life example of shear stress? (4 marks)
Ans: Let's say you are chewing your food. The force is caused by your teeth, that is your lower jaw and upper jaw. It is important to notice that these forces act tangentially, or are usually sliding forces. This is called shear stress and it tends to change the shape of the object by deforming it. There is no volume change here.
Other examples of shear stress include
- When we lie down on the bed or sofa, the cushion’s surface experiences shear stress.
- While cutting a paper with a scissor both the blades apply shear stress on the paper and cut it.
- When we run on the ground, both of the legs are exerting shear force.
Ques: What is tensile stress? (2 marks)
Ans: When a force is applied on a body along the length, that is so strong that it may deform it or even break it. In this case, a stress emerges, in response to this outside attack. This stress protects the object from being broken and is called tensile stress(σ). Tensile stress occurs when you pull a rubber band.
Formula: (σ) = force / per unit area
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