More things about materialas

Fatigue

• Definition: Fatigue in materials refers to failure after repeated cyclical stress, even if the applied stress is below the breaking stress.

• Process:

  • Microscopic Cracking: Tiny cracks form within the material during cyclic loading.

  • Progression: With each stress cycle, these micro cracks grow larger until the material ultimately fails.

• Examples:

  • Used in worn-out shoes.

  • Components like pneumatic drills, which operate repeatedly under similar conditions.

Creep

• Definition: Creep is the gradual deformation of materials under constant stress over time, particularly at elevated temperatures.

• Characteristics:

  • Metals can slowly flow like a liquid under extreme heat and stress.

  • Long exposure to such conditions eventually leads to material failure.

• Applications:

  • Notable in turbine blades in jet engines, which can reach and maintain high temperatures and stress levels.

Energy Stored in Ductile Metals

• Beyond Yield Point: When ductile metals are strained past their yield point (elastic limit), not all energy is recoverable.

• Energy Dissipation:

  • The energy exerted turns into heat, indicating that work done during plastic deformation is permanently lost.

  • Breaking atomic bonds accounts for the irreversible energy loss, resulting in permanent deformation.

Rubber Band Behavior

• Elastic Deformation: Unlike ductile metals, a rubber band can return to its original length after stretching.

• Energy Loss Mechanism:

  • Stretching causes the long-chain rubber molecules to rub against each other, creating friction.

  • This energy transforms into heat, causing a loss of energy even though the deformation is not permanent.

• Hysteresis:

  • The energy loss during the loading and unloading cycle of a rubber band is termed hysteresis.

  • The curve representing this process is called a hysteresis curve, with its shape resembling a uterus.