Short Answer:

Temperature has a significant effect on the strength of materials. When the temperature increases, most materials become weaker because their atomic bonds lose strength, and plastic deformation occurs more easily. At high temperatures, metals tend to soften, reducing their yield strength and ultimate tensile strength.

In simple words, as temperature rises, the ability of a material to resist stress or load decreases. On the other hand, at low temperatures, materials become harder and more brittle. Thus, temperature directly influences mechanical properties such as hardness, toughness, ductility, and elasticity, which determine how materials behave under load.

Detailed Explanation:

Effect of Temperature on Material Strength

Temperature plays a crucial role in determining the mechanical strength and behavior of engineering materials. The atomic structure of materials, the movement of dislocations, and the bonding between atoms are all affected by changes in temperature. These changes influence the ability of a material to withstand forces without failure.

At high temperatures, atoms in the material vibrate more intensely, making it easier for dislocations to move. This reduces the resistance to deformation and lowers the strength. At low temperatures, atomic motion is restricted, dislocation movement is limited, and the material becomes brittle.

Thus, temperature affects not only strength but also other mechanical properties like ductility, hardness, elasticity, and fracture toughness.

1. Effect of Increasing Temperature

When temperature increases, the following changes occur in a material:

1. Reduction in Yield Strength:
   • Yield strength is the stress at which a material begins to deform plastically.
   • With increasing temperature, atomic vibrations increase, and the material becomes more ductile.
   • This allows plastic deformation to occur more easily, thus reducing yield strength.

For example, mild steel that has a yield strength of about 250 MPa at room temperature may have much lower yield strength at 500°C.

1. Reduction in Ultimate Tensile Strength (UTS):
   • The maximum stress a material can withstand before breaking decreases with temperature.
   • As temperature rises, the cohesive forces between atoms weaken, leading to softening of the material.
2. Decrease in Hardness:
   • Hardness is the resistance to indentation or scratching.
   • At elevated temperatures, the material becomes softer because of reduced atomic bonding strength, thus decreasing hardness.
3. Increase in Ductility:
   • At higher temperatures, metals can undergo more plastic deformation before fracture.
   • This makes them more ductile and less likely to fail suddenly.
   • This property is used in hot working processes like rolling, forging, and extrusion.
4. Reduction in Modulus of Elasticity:
   • The stiffness of a material, represented by the modulus of elasticity (E), decreases with increasing temperature.
   • The material becomes less rigid and deforms more under the same load.
5. Reduction in Fatigue Strength:
   • The ability of a material to withstand cyclic loading decreases at high temperatures due to enhanced atomic movement and creep.
6. Occurrence of Creep:
   • At very high temperatures (usually above 0.4 of the melting temperature in Kelvin), materials experience creep, which is a time-dependent deformation under constant stress.
   • Creep leads to gradual elongation and eventual failure.

Example:
In turbine blades, which operate at high temperatures, the material must have high creep resistance. Nickel-based superalloys are used because they maintain strength even at high temperatures.

2. Effect of Decreasing Temperature

When temperature decreases, materials behave differently from their high-temperature behavior:

1. Increase in Strength:
   • At low temperatures, atomic motion decreases, which restricts dislocation movement.
   • As a result, yield strength and tensile strength increase.
2. Reduction in Ductility:
   • The material becomes less capable of plastic deformation and more likely to fracture suddenly.
   • This leads to brittle behavior.
3. Reduction in Toughness:
   • Toughness is the ability of a material to absorb energy before fracture.
   • At low temperatures, materials lose toughness and become prone to brittle fracture.
4. Brittle Fracture in Steels:
   • Steels and similar materials show a ductile-to-brittle transition at certain low temperatures.
   • Below this transition temperature, they fail suddenly without much deformation.

Example:
The brittle fracture of steel in ships during World War II occurred because of low-temperature embrittlement, where materials became brittle in cold climates.

3. Temperature Effect on Different Types of Materials

1. Metals:
   • Metals generally lose strength and hardness as temperature increases.
   • At very low temperatures, they become brittle.
   • Special alloys are developed to maintain strength across wide temperature ranges (e.g., Inconel, stainless steel).
2. Polymers:
   • At high temperatures, polymers soften and lose strength due to chain mobility.
   • At low temperatures, they become glassy and brittle.
3. Ceramics:
   • Ceramics maintain their strength at high temperatures but are inherently brittle at all temperatures.
   • They fail without much plastic deformation.
4. Composites:
   • Composites are designed to retain strength across varying temperatures, but their matrix (polymer or metal) limits temperature resistance.

4. Practical Implications in Engineering

1. Machine and Engine Components:
   Components like pistons, valves, and turbine blades operate at high temperatures. They must be made from materials with high-temperature strength and creep resistance.
2. Bridges and Structures:
   In bridges or steel structures exposed to cold climates, materials are selected to prevent brittle failure.
3. Aerospace Applications:
   Aircraft materials experience both high and low temperatures. Hence, alloys with stable properties over a wide temperature range are used.
4. Manufacturing Processes:
   High-temperature operations like forging and rolling are designed to utilize the reduced strength and increased ductility of metals for shaping.

5. Summary of Temperature Effects on Material Properties

Property	Effect of High Temperature	Effect of Low Temperature
Strength	Decreases	Increases
Ductility	Increases	Decreases
Toughness	Decreases	Decreases
Hardness	Decreases	Increases
Elastic Modulus	Decreases	Slightly increases
Creep	Increases	Negligible

(Note: Table for understanding only — not as a data table.)

Conclusion

Temperature greatly influences the strength and mechanical behavior of materials. At high temperatures, materials soften, lose strength, and show greater ductility, while at low temperatures, they become hard and brittle. These changes are due to variations in atomic motion and dislocation movement. Engineers must consider temperature effects while designing machines, engines, bridges, and structural components. Proper material selection ensures that components maintain adequate strength, safety, and performance over a wide range of temperatures.