9.3 Fracture and toughness of polymers

Polymer fracture comes in three flavors: brittle, ductile, and intermediate. Each type depends on factors like temperature and crosslinking. Understanding these modes helps engineers design safer, more durable plastic products.

Fracture toughness is key in preventing cracks from spreading in polymers. It's influenced by molecular weight, crystallinity, and temperature. Testing methods like J-integral and essential work of fracture help measure a material's resistance to breaking.

Modes of polymer fracture

• Brittle fracture involves sudden, rapid crack propagation with little plastic deformation occurs below the glass transition temperature ($T_g$) or in highly crosslinked polymers (thermosets) fracture surface appears smooth and glossy
• Ductile fracture involves extensive plastic deformation before failure occurs above the glass transition temperature ($T_g$) or in lightly crosslinked polymers (elastomers) fracture surface appears rough and fibrous
• Intermediate fracture exhibits a combination of brittle and ductile fracture characteristics occurs near the glass transition temperature ($T_g$) or in moderately crosslinked polymers (semi-crystalline polymers)

Fracture toughness in polymers

• Fracture toughness measures a material's resistance to crack propagation quantifies the energy required to create new surfaces during fracture represented by the critical stress intensity factor ($K_{IC}$) or critical strain energy release rate ($G_{IC}$)
• Fracture toughness is crucial in determining a material's ability to withstand stress concentrations and prevent catastrophic failure important in load-bearing applications (automotive parts, aerospace components, medical devices) influences the design and material selection process for various products

Factors influencing fracture behavior

• Molecular weight higher molecular weight increases entanglement density and improves fracture toughness longer polymer chains can more effectively dissipate energy during crack propagation
• Crystallinity increasing crystallinity can enhance fracture toughness up to a certain point crystalline regions act as physical crosslinks and can impede crack growth excessive crystallinity may lead to brittleness due to reduced chain mobility
• Temperature fracture behavior is temperature-dependent, especially near the glass transition temperature ($T_g$) below $T_g$, polymers are brittle and exhibit low fracture toughness above $T_g$, polymers become more ductile and display improved fracture resistance
• Other factors influencing fracture behavior include:
  1. Polymer chain architecture (linear, branched, crosslinked)
  2. Presence of plasticizers or fillers
  3. Processing conditions and residual stresses

Testing methods for fracture resistance

• J-integral method measures the energy required for crack initiation and propagation applicable to both linear elastic and elastic-plastic materials determined by calculating the J-integral around the crack tip provides a measure of the material's resistance to fracture ($J_{IC}$)
• Essential work of fracture (EWF) method separates the total work of fracture into essential (fracture process) and non-essential (plastic deformation) components suitable for thin, ductile polymer films (packaging materials) determined by plotting specific work of fracture versus ligament length provides a measure of the material's inherent fracture toughness ($w_e$)
• Other testing methods for fracture resistance include:
  1. Charpy and Izod impact tests
  2. Compact tension (CT) and single-edge notched bend (SENB) tests
  3. Double cantilever beam (DCB) and end-notched flexure (ENF) tests for adhesive joints