Lecture 9 Notes (BME 296)

Lecture Overview

• Lecture 9, BME 296

Key Concepts

Molecular Causes for Material Properties

• Understand elastic and plastic behavior in materials.

  • Molecular cause of elasticity involves reversible deformation due to atomic interactions.

  • Molecular cause of plasticity involves irreversible deformations under stress.

Semicrystalline Polymers

• Definition

  • Occurs in non-branched linear structures.

  • Composed of spherulites with crystalline lamellae and amorphous regions.

• Structure

  • Crystalline lamellae radiate from a center.

  • Amorphous regions act as tie molecules connecting lamellae.

Plastic Deformation in Semicrystalline Polymers

• Stages of Deformation

  1. First Stage: Tie chains extend, and lamellae slide.

  2. Second Stage: Lamellae become re-oriented along loading axis; crystal blocks still attached via tie molecules.

  3. Final Stage: Blocks and tie molecules align with tensile force.

  • Reference: http://www.dynamicscience.com.au/tester/solutions1/chemistry/crystallinestructures.html

Necking in Polymers

• After yield point, a neck develops and polymer chains orient along the load direction.

• Increase in stress before fracture corresponds to overcoming strong primary bond interactions in aligned chains.

Importance of Amorphous Regions

• Absence of amorphous regions makes polymers brittle, limiting applications.

Factors Affecting Deformation

• Changes that inhibit chain motion increase strength but decrease ductility:

  • Crystallinity: Higher percent increases mechanical properties.

  • Molecular Weight: Greater weight strengthens polymers via physical entanglements.

  • Cross-Linking: Increases strength and brittleness through covalent linkages.

Application of Tensile Force

• Apply tensile force along the direction of polymer chain orientation for greater resistance.

• Resistance in non-aligned direction relies on weaker secondary forces.

Influence of Strain Rates

• As strain rates increase, the stress-strain curve shifts from ductile (3) to brittle (1) due to reduced chain orientation time.

Bending Properties of Polymers

• Bending tests determine stress-strain properties under beam-like load applications.

• Conditions:

  • Magnitude and type of stress vary across the sample.

  • Key parameters include sample thickness, bending moment, and moment of inertia.

• Flexural Modulus vs. Tensile Modulus

  • Flexural Strength: Not the same as modulus of elasticity; indicates stiffness against bending.

  • Tensile Modulus: Measures flexibility along strain axis, not normalized for thickness.

Time-Dependent Properties

• Properties vary under long loading times; tested via creep.

Creep in Materials

• Definition and Stages

  • Creep is plastic deformation under constant load, involving three stages:

    1. Primary Creep: Strain increases, creep rate decreases.

    2. Secondary Creep: Equilibrium established; linear relationship between strain and time.

    3. Tertiary Creep: Rapid failure due to defects in the material.

• Molecular Causes

  • Metals: Grain boundary sliding and vacancy migration are key.

  • Ceramics: More resistant to creep due to rigidity in ion and vacancy diffusion.

  • Polymers: Creep is influenced by chain movement in amorphous regions; increases with decreasing crystallinity.

Fatigue in Materials

• Fatigue fractures occur at stresses below yield strength, with crack initiation through repeated loading cycles.

• Three Stages of Fatigue Failure:

  1. Crack Initiation: Small cracks form at high stress areas.

  2. Crack Propagation: The cracks grow with cycles.

  3. Final Failure: Rapid failure after reaching critical crack size.

Key Influencing Factors

• Mechanical Properties: Type of metal, crystallinity, molecular weight.

• Pores and Porosity: Decrease elastic modulus and strength by acting as stress concentrators.

• Degradation Rate: Biodegradable materials lose strength over time, affecting performance (e.g., poly(glycolic acid)).