Anelasticity and Mechanical Properties

Anelasticity

• Anelasticity describes materials' time-dependent response to applied stress.
• Unlike elastic materials, anelastic materials exhibit a delayed recovery upon stress removal.
• This delayed response is often characterized by a time-dependent strain component.
• Anelastic behavior is often observed in crystalline materials, polymers, and composites.
• Several factors influence anelasticity, including temperature, frequency of stress application, and material microstructure.
• Anelasticity is related to internal stress relaxation mechanisms within the material.
• Measuring anelastic properties helps understand the material's response under sustained or cyclic loading.

Mechanical Properties

• Stress-Strain Relationship: describes the material's response to externally applied forces. The relationship can be linear (e.g., Hooke's law) or non-linear, depending on the material and loading conditions.
• Elasticity: materials' ability to deform and return to their original shape when the load is removed. The magnitude of strain is proportional to the applied stress for an elastic response.
• Plasticity: permanent deformation of a material under load. Stress exceeding the yield strength results in plastic behavior.
• Strength: resistance to failure under stress. Yield strength and ultimate tensile strength are common measures, representing the stress level at which plastic deformation begins and the maximum stress the material can withstand, respectively.
• Hardness: resistance to indentation or scratching. Various hardness scales, such as Mohs and Rockwell, measure this property.
• Ductility: ability to undergo significant plastic deformation before failure.
• Brittleness: tendency to fracture with little or no plastic deformation.
• Toughness: The ability to absorb energy during deformation before fracturing.
• Creep: time-dependent deformation under constant stress. Important at elevated temperatures.
• Fatigue: failure under cyclic loading, often leading to crack initiation and propagation.

Stress Relaxation

• Definition: Stress relaxation is the time-dependent decrease in stress in a material under constant strain.
• Mechanism: Stress relaxation occurs due to internal mechanisms like atomic rearrangements, dislocation movement, or viscous flow within the material.
• Factors affecting stress relaxation: Temperature, strain rate, time, material microstructure (and the presence of defects), and applied strain.
• Applications: Understanding stress relaxation is crucial for predicting material behavior in various engineering applications, especially those subject to prolonged stress or repeated loading.
• Typical behavior: Stress relaxation follows a specific pattern; initially, there's a rapid decrease, followed by a slower, more gradual decline, sometimes leveling off to an equilibrium stress.
• Significance in design: A material that undergoes significant relaxation under a specific load and condition would likely not be suitable for applications requiring stable performance over time.
• Methods for measuring stress relaxation: common methods include tensile testing, where strain is held constant, and measuring stress over time.