9.4 Impact resistance and fatigue behavior

Impact resistance and fatigue behavior are crucial properties for polymers in various applications. These characteristics determine how materials withstand sudden forces and repeated stress, affecting product safety and durability.

Understanding impact resistance helps engineers select the right polymers for specific uses. Fatigue behavior is equally important, as it influences a material's long-term performance under cyclic loading. Both properties are essential for designing reliable polymer products.

Impact Resistance

Impact resistance in polymer applications

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• Ability of a material to withstand sudden, high-speed loading or impact without fracturing or failing measured by the amount of energy absorbed by the material during impact
• Crucial for ensuring the safety, durability, and reliability of products in applications where polymeric materials may be subjected to impact forces (automotive parts, protective gear, packaging materials, consumer products)

Impact behavior of polymer types

• Thermoplastics generally have higher impact resistance compared to thermosets due to their ductile behavior which allows for plastic deformation and energy absorption during impact (polycarbonate (PC), acrylonitrile butadiene styrene (ABS), high-density polyethylene (HDPE))
• Thermosets typically have lower impact resistance due to their highly crosslinked structure leading to brittle behavior with rapid crack propagation and failure under impact (epoxy resins, phenolic resins, unsaturated polyesters)
• Factors influencing impact behavior include molecular structure, degree of crystallinity, presence of plasticizers or reinforcements, and temperature where thermoplastics may exhibit brittle behavior at low temperatures while thermosets can maintain their properties over a wider temperature range

Fatigue Behavior

Fatigue in polymer performance

• Progressive damage and failure of a material subjected to repeated, cyclic loading occurring at stress levels below the material's yield strength
• Relevant to polymer performance as many polymeric components experience cyclic loading during their service life (vibration dampening parts, flexible tubing, medical implants) and fatigue failure can lead to premature product failure and potential safety hazards

Factors affecting polymer fatigue

• Stress amplitude: higher stress amplitudes lead to shorter fatigue life and stress concentration points (notches, defects) can accelerate fatigue failure
• Frequency: higher frequencies generally result in shorter fatigue life with more pronounced effects in viscoelastic polymers due to their time-dependent behavior
• Environmental conditions:
  1. Elevated temperatures can accelerate fatigue damage while low temperatures may cause embrittlement
  2. Chemical exposure can degrade the polymer structure and reduce fatigue resistance
  3. Moisture absorption can lead to plasticization and reduced fatigue life, particularly in hydrophilic polymers

Testing methods for polymer resistance

• Charpy impact test measures the impact energy absorbed by a notched specimen subjected to a pendulum impact, commonly used for thermoplastics and provides a measure of notch sensitivity
• Izod impact test is similar to the Charpy test but the specimen is held vertically and impacted by a swinging pendulum, useful for comparing the impact resistance of different materials or formulations
• Fatigue testing involves cyclic loading tests (tension-tension, tension-compression, flexural fatigue) to generate stress-life (S-N) curves characterizing fatigue behavior and determining fatigue limit, with fractography and microscopic analysis providing insights into fatigue failure mechanisms