Stress strain curves

Structure of Metals

• Metals consist of atoms arranged in layers.

• Atoms bonded by interatomic bonds can be thought of as connected by springs.

Types of Deformation

Elastic Deformation

• Occurs under small loads.

• Atoms stretch like springs and return to the original length once the load is removed.

• Defined as reversible deformation.

• Elastic Limit: The maximum extent to which a material can be stretched without permanent deformation.

Plastic Deformation

• Occurs when external load exceeds a certain threshold.

• Leads to the breaking of bonds between atoms.

• Characteristic of ductile metals such as copper.

• Permanent deformation; the material does not return to its original shape.

Stress-Strain Curve

• A graph illustrating how a metal responds to stress (load applied) over time.

• Key points include:

  • Limit of Proportionality:

    • Up to this point, the stress is proportional to strain (linear relationship).

    • Can calculate Young's modulus from this point.

  • Elastic Limit (Yield Point):

    • Beyond this point, materials begin to undergo plastic deformation.

    • The metal yields, resulting in permanent changes.

  • Breaking Point:

    • The point where the metal ultimately fails or snaps.

  • Yield Stress:

    • The stress at which material begins to deform plastically.

  • Breaking Stress (Ultimate Tensile Strength):

    • The maximum stress that a material can withstand before failure.

Material Behavior

Ductile Metals (e.g., Copper)

• Ability to undergo significant plastic deformation.

• Layers of atoms slide over each other before breaking occurs.

• Resistance to breaking due to toughness.

Brittle Materials (e.g., Glass)

• Exhibit minimal plastic deformation.

• Breaks instantly after yielding without significant deformation.

• Lacks the capability for atomic layers to slide.

Rubber

• Displays unique behavior where it becomes less stiff with initial stretching.

• Stiffness increases significantly due to polymer chain rearrangement.

Steel

• Intermediate behavior compared to ductile metals and brittle materials.

• Shows some plastic deformation.

• Features a specific bump in the stress-strain curve indicative of its unique properties.