Thermal Stresses And Strain in Hindi || Thermal Stress kya hoti hai

Gear Institute Mechanical Engineering Videos

2 Oct 202013:51

Summary

TLDRThe video covers the concept of thermal stresses and thermal strain, explaining how materials expand when heated and contract when cooled. It discusses free expansion, where no stress occurs, and compares it to situations where expansion is restricted, leading to compressive or tensile stresses. The speaker uses relatable examples, like family dynamics and railway tracks, to explain the effects of thermal changes on material stress. The video concludes by explaining how these stresses can be calculated using material properties like the coefficient of thermal expansion and Young's modulus.

Takeaways

🌡️ Thermal stress and strain are caused by changes in temperature that affect a material's expansion or contraction.
📏 A material expands when the temperature rises, and contracts when the temperature falls.
💡 Free expansion of materials under temperature change does not induce stress unless their expansion is restricted.
✋ When a material's expansion is restricted, compressive stress is induced, resisting the material's attempt to expand.
🛠️ Tensile stress occurs when the contraction of a material is restricted under falling temperature, preventing it from shrinking.
🚂 Railways leave gaps between rails to prevent thermal stress during temperature changes, avoiding damage to the tracks.
📐 Thermal expansion is proportional to the change in temperature and the length of the material.
🔧 Thermal strain is defined as the change in length (delta L) divided by the original length (L).
💪 The formula for thermal stress is derived by multiplying thermal strain with Young's modulus (E).
📊 Partial restriction of expansion allows some free expansion, reducing overall thermal stress in the material.

Q & A

What happens to a material's length when its temperature increases?
-When the temperature of a material increases, the material expands, resulting in an increase in its length. This phenomenon occurs because the material's particles gain energy and move apart, causing the expansion.
What is free expansion in the context of thermal expansion?
-Free expansion occurs when a material is allowed to expand without any restriction as the temperature increases. In this case, no stress is generated in the material because it can expand freely.
What happens if the expansion of a material is restricted?
-If the expansion of a material is restricted, it leads to the generation of compressive stress. The material tries to expand outward, but the restriction prevents it, causing internal forces to develop that result in compressive stress.
How does compressive stress occur in materials due to thermal expansion?
-Compressive stress occurs when a material is prevented from expanding freely due to a rise in temperature. The restriction causes the material to develop inward forces, leading to compressive stress.
What example is used in the transcript to explain free expansion?
-The transcript uses the example of children in English families, where they are allowed to explore and grow freely without restrictions. This example is compared to the free expansion of materials, where no stress is generated.
What analogy is used to explain compressive stress in the transcript?
-The analogy used for compressive stress is how parents in some cultures restrict their children's freedom as they grow, leading to frustration. This is compared to a material being restricted from expanding, which causes compressive stress.
Why do railway lines have gaps between them?
-Railway lines have gaps to allow for the expansion of the metal due to temperature changes. If the expansion is restricted without gaps, the rails can push against each other, causing bending or breakage due to thermal stresses.
What happens when a material's temperature decreases after being restricted?
-When a material's temperature decreases, it tends to contract. However, if the material is restricted from contracting, tensile stress develops as the material is pulled outward.
How is thermal strain related to temperature change?
-Thermal strain is the change in length of a material due to a change in temperature. It depends on the material's coefficient of thermal expansion (alpha), the original length of the material, and the temperature change (ΔT).
How can thermal stress be calculated from thermal strain?
-Thermal stress can be calculated by multiplying the thermal strain by Young's modulus (E) of the material. The formula is: Thermal stress = E × Thermal strain.

Outlines

00:00

🌡️ Understanding Thermal Stresses and Strain

The first paragraph explains the concept of thermal stresses and strain, focusing on how materials expand when the temperature rises. The example discusses a material at 30°C, which expands as the temperature increases to 50°C. This expansion happens naturally as materials expand when heated. If the material cools down again, the expansion reverses, causing contraction. This natural phenomenon doesn't pose any issues unless the expansion is restricted, which would result in compressive stress. The paragraph illustrates how preventing expansion by holding the material can lead to compressive forces and how this contradicts initial learning expectations where it was assumed tensile stress would occur.

05:03

⚙️ The Impact of Restricting Growth and Stress Development

The second paragraph draws an analogy between thermal expansion and the growth of children in different cultures. In Western cultures, children are often allowed to 'freely expand' without restrictions, meaning they face no stress while growing up. Conversely, in other cultures, when restrictions are imposed, such as parents limiting their child’s freedom, this leads to 'compressive stress' akin to the physical stresses in materials. The analogy extends to railway lines, where gaps are left between the tracks to allow for thermal expansion without causing stress. If no gap is left, the tracks would bend and break under stress, similar to how restricting natural growth can lead to frustration.

10:04

📏 Calculating Thermal Stress and Strain

In the third paragraph, the speaker delves into the calculations for thermal expansion and stress. The expansion or contraction of materials is based on the temperature change (ΔT) and material length (L), where ΔL = αLΔT. The strain is defined as the change in length (ΔL) over the original length (L), and the stress is derived from the Young’s modulus (E) multiplied by the strain. This calculation helps understand how much stress is developed due to temperature variations. The speaker also explains a specific scenario where a material is restricted after some expansion, causing partial expansion and stress.

Mindmap

Keywords

💡Thermal Stress

Thermal stress refers to the stress generated in a material due to a change in temperature. As the temperature increases or decreases, materials expand or contract, leading to stresses if the material's movement is restricted. In the video, thermal stress occurs when the expansion of the material is blocked, creating compressive or tensile forces, depending on the constraints.

💡Thermal Expansion

Thermal expansion is the increase in the length or volume of a material when it is heated. As described in the video, when the temperature of a material rises from 30°C to 50°C, the material expands. This is a natural phenomenon, but problems arise when the expansion is restricted, leading to thermal stress.

💡Compressive Stress

Compressive stress occurs when a material is compressed or prevented from expanding under thermal changes. The video explains how when a material tries to expand due to heat but is restrained, compressive stress is induced. This stress pushes the material inward, preventing it from expanding freely.

💡Tensile Stress

Tensile stress refers to the stress that occurs when a material is stretched or pulled outward. In the context of the video, if a material is restrained and the temperature drops, it tries to contract. If this contraction is blocked, tensile stress is generated as the material is being pulled apart.

💡Free Expansion

Free expansion is the unrestrained expansion of a material as its temperature rises. In the video, the concept is compared to children growing up without restrictions. Just as children expand their horizons without stress when not held back, a material that expands freely without obstruction does not develop stress.

💡Delta L (ΔL)

Delta L (ΔL) represents the change in the length of a material due to thermal expansion or contraction. The video explains that ΔL is directly proportional to the change in temperature (ΔT) and the original length of the material. The greater the temperature change or the longer the material, the larger the expansion or contraction.

💡Alpha (α)

Alpha (α) is the coefficient of thermal expansion, which quantifies how much a material expands per degree of temperature change. In the video, it is a constant used in the formula to calculate the change in length (ΔL) due to a change in temperature. Different materials have different values of α, affecting how much they expand or contract.

💡Young's Modulus (E)

Young's Modulus (E) measures a material's stiffness or elasticity. It is the ratio of stress to strain in a material under tension or compression. In the video, Young's Modulus is used in the formula to calculate thermal stress, illustrating how the stiffness of the material influences its response to thermal changes.

💡Railway Track Expansion

The example of railway track expansion in the video illustrates the practical application of thermal expansion. Gaps are left between railway tracks to account for thermal expansion during hot weather. Without these gaps, the tracks would push against each other, creating stress and potentially causing the tracks to bend or break.

💡Delta T (ΔT)

Delta T (ΔT) refers to the change in temperature, which directly affects thermal expansion. In the video, ΔT is used to calculate the change in length (ΔL) and the resulting thermal stress. A larger ΔT results in more significant expansion or contraction of a material, influencing the overall stress experienced.

Highlights

Thermal stress occurs when temperature changes cause expansion or contraction in a material.

Materials expand when the temperature rises and contract when the temperature decreases.

When free expansion is allowed, no stress develops in the material.

Restricting a material's expansion or contraction leads to the development of compressive or tensile stress.

The concept of free expansion, where a material can expand without any restriction, results in no stress.

If a material is restricted from expanding, compressive stress develops within the material.

When temperature decreases and contraction is prevented, tensile stress develops in the material.

The example of railway tracks shows how gaps are left to avoid stress due to thermal expansion.

In real-world applications, thermal expansion must be accounted for to avoid material failure.

The change in length due to temperature change is directly proportional to the original length and the temperature change.

The formula for thermal strain is the change in length divided by the original length.

Young’s modulus (E) helps calculate thermal stress by using the formula stress = E * strain.

Thermal stress can be positive (tensile) or negative (compressive), depending on whether expansion or contraction is prevented.

In some situations, partial expansion is allowed, leading to reduced but not eliminated stress.

If a material is allowed partial expansion, the final thermal stress can be calculated by subtracting the allowed expansion from the total expansion.