9.4 Impact resistance and fatigue behavior
3 min read•july 23, 2024
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
- generally have higher impact resistance compared to 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 and failure under impact (epoxy resins, phenolic resins, unsaturated polyesters)
- Factors influencing impact behavior include molecular structure, degree of crystallinity, presence of or reinforcements, and 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
- 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 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:
- Elevated temperatures can accelerate fatigue damage while low temperatures may cause embrittlement
- Chemical exposure can degrade the polymer structure and reduce fatigue resistance
- Moisture absorption can lead to plasticization and reduced fatigue life, particularly in hydrophilic polymers
Testing methods for polymer resistance
- 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
- 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
Key Terms to Review (18)
Brittle vs. ductile fracture studies: Brittle vs. ductile fracture studies examine the two distinct ways materials fail when subjected to stress. Brittle fracture occurs suddenly and without significant deformation, often leading to a sharp break, while ductile fracture involves more extensive deformation before failure, resulting in a more gradual and fibrous break. Understanding these types of fractures is crucial for assessing impact resistance and fatigue behavior in materials, as they provide insights into how materials will perform under various loading conditions.
Charpy impact test: The Charpy impact test is a standardized method used to measure the impact resistance of materials, particularly metals and polymers, by determining the energy absorbed during fracture. This test involves striking a notched specimen with a swinging pendulum, and the energy lost in breaking the specimen provides insight into its toughness and resistance to brittle failure. Understanding the results of this test helps predict how materials will behave under sudden impacts or stress, which is crucial for ensuring safety and reliability in various applications.
Crack propagation: Crack propagation refers to the process by which a crack in a material grows and advances under stress or external forces. This phenomenon is critical in understanding how materials, especially polymers, fail when subjected to mechanical loads or environmental conditions. It involves the study of how cracks initiate, grow, and eventually lead to material failure, which is essential for evaluating the fracture toughness and durability of polymeric materials.
Crazing: Crazing refers to the formation of fine cracks or a network of cracks on the surface of a material, typically a polymer, due to stress or environmental factors. This phenomenon is important because it can significantly affect the impact resistance and fatigue behavior of polymers, leading to potential failures under mechanical loads or prolonged exposure to certain conditions.
Ductility: Ductility is the ability of a material to deform under tensile stress, which means it can be stretched into a wire or other shapes without breaking. This property is crucial for materials that will undergo bending, stretching, or twisting during use. Ductile materials can absorb energy during deformation, which plays a significant role in their impact resistance and fatigue behavior.
Elastic Modulus: Elastic modulus is a measure of a material's ability to deform elastically (i.e., non-permanently) when a stress is applied. It quantifies the relationship between stress (force per unit area) and strain (deformation) within the elastic limit of the material, indicating how much it will stretch or compress under load. The higher the elastic modulus, the stiffer the material, which directly affects its performance under different mechanical conditions.
Endurance Limit: The endurance limit refers to the maximum stress level a material can withstand for an infinite number of loading cycles without failing due to fatigue. This concept is crucial in understanding how materials behave under repeated loading, particularly in applications where impact resistance and fatigue behavior are critical for durability and longevity.
Fatigue life: Fatigue life refers to the number of cycles of stress or strain a material can endure before failure occurs due to repeated loading and unloading. This concept is crucial in understanding how materials, including polymers and composites, behave under cyclic loading conditions, which can lead to microscopic damage and ultimately failure over time. It’s essential for evaluating the durability and longevity of materials used in various applications, particularly in impact resistance and composite structures.
Fillers: Fillers are materials added to polymers to enhance their properties, reduce production costs, and improve processing characteristics. They can modify the mechanical, thermal, and aesthetic properties of the polymer matrix, allowing for tailored applications in various industries. The selection of fillers influences factors like fracture toughness, impact resistance, and the final performance of polymer products.
Izod Impact Test: The Izod impact test is a standardized method used to measure the impact resistance of materials, particularly polymers, by determining the energy absorbed by a specimen when subjected to a sudden force. This test is crucial for understanding how materials will behave under stress and can help predict their performance in real-world applications. By quantifying impact resistance, the Izod test provides valuable insights into the material's durability and susceptibility to fractures, especially in dynamic loading situations.
Loading rate: Loading rate refers to the speed at which a force is applied to a material, often expressed in terms of force per unit time. This concept is crucial in understanding how materials, especially polymers, respond to sudden loads, which directly influences their impact resistance and fatigue behavior. A higher loading rate can lead to different deformation mechanisms, changing the way a material absorbs energy during impact or how it fails under repeated loading conditions.
Plasticizers: Plasticizers are substances added to polymers to increase their flexibility, workability, and durability. By reducing the intermolecular forces between polymer chains, plasticizers allow materials to become more pliable and easier to process, which can significantly enhance the performance characteristics of various polymer applications.
Polymer blends and composites research: Polymer blends and composites research focuses on the study and development of materials made from mixing different polymers or incorporating fillers to enhance the properties of the base polymer. This area of research seeks to improve various characteristics, such as mechanical strength, impact resistance, and fatigue behavior, which are essential for many industrial applications.
Temperature: Temperature is a measure of the average kinetic energy of the particles in a substance, influencing its physical properties and behavior. In polymers, temperature plays a crucial role in determining how these materials respond to stress, their solubility, their processing conditions, and their stability over time.
Thermoplastics: Thermoplastics are a type of polymer that become soft and moldable upon heating and solidify upon cooling. This unique property allows them to be reshaped and recycled, making them incredibly versatile in various applications across different industries.
Thermosets: Thermosets are a class of polymers that undergo irreversible curing when heated or treated with a hardener, resulting in a rigid and infusible structure. Once set, thermosets cannot be remolded or reshaped, which distinguishes them from thermoplastics. Their three-dimensional crosslinked structure gives them exceptional thermal stability, mechanical strength, and chemical resistance.
Toughness: Toughness is a material's ability to absorb energy and deform plastically without breaking. It reflects a combination of strength and ductility, indicating how well a material can withstand impact and fatigue without failing. Understanding toughness is critical for evaluating how materials behave under dynamic conditions and their long-term durability.
Yield Strength: Yield strength is the amount of stress a material can withstand before it begins to deform permanently. It is a critical point in the stress-strain curve, where the material transitions from elastic deformation, where it will return to its original shape, to plastic deformation, where permanent changes occur. Understanding yield strength helps in designing materials and structures that can endure mechanical loads without failing or permanently deforming.