Stress Relaxation & Creep Flashcards
What is creep?
Viscoelastic flow - change in strain as function of time usually under constant load and temperature
What is stress relaxation?
Change in stress as a function of time usually under constant deformation (strain) and temperature
What is chain scission?
Permanent breakage of polymer chains, leading to irreversible change
What are causes of chain scission?
Oxidation and hydrolysis
What is bond interchange?
Reversible breaking and reforming of chemical bonds
What are some examples of polymers that undergo bond interchange?
Polyesters and polyether
What are some reasons for stress relaxation and creep?
- Segmental relaxations
- Viscous flow
- Molecular motion (strain) induced by stress which relates to conformational changes
What are segmental relaxations?
- localized movement of polymer chain segments
- Occur around Tg
- To a smaller extent, generate secondary relaxations
What is viscous flow?
- Slippage of chains past one another
- Fast reptation
What are some factors that effect creep and stress relaxation?
- Polymer structure
- Amorphous has better resistance
- Filler or reinforcements
- Glass filler is better
- Temperature
- Stay below Tg by at least 50 C for amorphous
- Stress level
- Environment
- Avoid swell due to moisture
What is a failure that is unique to polymers?
Stress rupture
How much does stress rupture account for all plastic part failures?
30-40%
What is stress rupture and environmental cracking greatly influenced by?
- Temperature
- Environment
- Stress
What is the Maxwell model?
- Creep experiment
- It assumes the material can undergo viscous flow and respond elastically
- Assume stress is constant and strain is additive
What is the Kelvin-Voigt model?
- Creep experiment
- it assumes the material can undergo viscous flow and respond elastically
- Assume strain is constant and stress is additive
Explain the stress relaxation experiment using a Maxwell element.
- Shows how stress diminishes over time when a material is held at a constant strain
What is the goal of the Four Element Model?
- To describe both stress relaxation and creep
- Combine the Maxwell and Kelvin-Voight models
What’s the difference between spring and dashpot models?
- Spring shows elastic behavior
- Dashpot shows viscous behavior
