4.3 Fatigue wear
7 min read•august 21, 2024
Fatigue wear is a critical concern in friction and wear engineering. It involves progressive damage accumulation in materials subjected to , leading to eventual failure. Understanding fatigue wear mechanisms helps engineers design more durable and reliable systems.
Fatigue wear occurs in three stages: , propagation, and material removal. Various factors influence this process, including material properties, surface conditions, loading conditions, and environmental factors. Engineers must consider these elements when designing wear-resistant systems for different applications.
Definition of fatigue wear
- Progressive damage accumulation in materials subjected to cyclic loading or stress
- Occurs below the of the material, leading to eventual failure
- Crucial concept in friction and wear engineering due to its impact on component lifespan and safety
Mechanisms of fatigue wear
- Involves three main stages crack initiation, propagation, and material removal
- Occurs at stress concentrations or material defects, gradually weakening the component
- Understanding these mechanisms helps engineers design more durable and reliable systems
Crack initiation
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- Begins at points (surface irregularities, inclusions, or grain boundaries)
- Involves formation of persistent slip bands due to cyclic plastic deformation
- Microstructural changes lead to the nucleation of microcracks
Crack propagation
- Microcracks grow and coalesce into larger cracks under continued cyclic loading
- Follows Paris' Law, relating crack growth rate to stress intensity factor range
- Influenced by material properties, loading conditions, and environmental factors
Material removal
- Final stage where cracks reach critical size, causing material separation
- Results in the formation of wear particles and surface degradation
- Can lead to catastrophic failure if not detected and addressed in time
Factors influencing fatigue wear
- Multiple variables affect the fatigue wear process and component lifespan
- Understanding these factors is crucial for designing wear-resistant systems
- Engineers must consider the interplay of these factors in friction and wear applications
Material properties
- Yield strength affects resistance to plastic deformation and crack initiation
- Fracture toughness influences resistance
- Microstructure (grain size, phase composition) impacts fatigue behavior
Surface conditions
- Surface roughness affects stress concentration and crack initiation sites
- Residual stresses from manufacturing processes influence fatigue life
- Surface treatments (shot peening, nitriding) can improve fatigue resistance
Loading conditions
- Stress amplitude and mean stress affect fatigue life (Goodman and Soderberg relations)
- Frequency of loading impacts heat generation and environmental interactions
- Multiaxial loading conditions complicate fatigue behavior prediction
Environmental factors
- Corrosive environments accelerate crack initiation and propagation
- Temperature fluctuations induce thermal stresses and material property changes
- Humidity affects crack growth rates in certain materials (stress corrosion cracking)
Types of fatigue wear
- Different types of fatigue wear occur depending on loading conditions and material properties
- Understanding these types helps engineers identify and address specific wear mechanisms
- Each type requires unique prevention and mitigation strategies in friction and wear applications
Surface fatigue
- Occurs due to repeated contact stresses on surfaces (rolling contact fatigue)
- Results in pitting, spalling, and surface-initiated cracks
- Common in gears, bearings, and railway tracks
Subsurface fatigue
- Initiates below the surface due to Hertzian contact stresses
- Leads to the formation of subsurface cracks that propagate to the surface
- Prevalent in rolling element bearings and heavily loaded contact surfaces
Thermal fatigue
- Caused by cyclic temperature changes inducing thermal stresses
- Results in surface cracking and material degradation
- Common in engine components, molds, and high-temperature applications
Fatigue wear in different materials
- Material selection plays a crucial role in managing fatigue wear in engineering applications
- Understanding the fatigue behavior of various materials guides design decisions
- Engineers must consider the unique properties and limitations of each material class
Metals vs polymers
- exhibit crystalline structure, allowing for slip and dislocation movement
- Fatigue in metals often follows distinct stages (crack initiation, propagation, and failure)
- Strain hardening and softening phenomena affect metal fatigue behavior
- Polymers display viscoelastic behavior, leading to different fatigue mechanisms
- Creep and stress relaxation play significant roles in polymer fatigue
- Temperature sensitivity is more pronounced in polymers compared to metals
Ceramics vs composites
- Ceramics typically have high stiffness and low fracture toughness
- Fatigue in ceramics is often dominated by slow crack growth mechanisms
- Environmental factors (humidity, temperature) significantly affect ceramic fatigue life
- exhibit anisotropic properties and complex failure modes
- Fatigue in composites involves matrix cracking, fiber-matrix debonding, and delamination
- Fiber orientation and layup sequence influence composite fatigue behavior
Fatigue wear testing methods
- Fatigue testing is essential for characterizing material behavior and predicting component lifespan
- Various testing methods simulate different loading conditions encountered in real-world applications
- Engineers use these tests to generate data for design, material selection, and quality control
Rotating bending tests
- Applies alternating bending stresses to a rotating specimen
- Generates fully reversed stress cycles (R = -1)
- Commonly used for generating S-N curves and determining endurance limits
Axial loading tests
- Subjects specimens to cyclic tension-compression or tension-tension loading
- Allows for testing at different stress ratios (R values)
- Used for generating strain-life curves and studying mean stress effects
Torsional fatigue tests
- Applies cyclic torsional loads to simulate shear stress conditions
- Important for components subjected to torsional loading (shafts, springs)
- Can be combined with axial loading for multiaxial fatigue testing
Fatigue wear analysis techniques
- Analysis techniques are crucial for understanding fatigue wear mechanisms and failure modes
- These methods help engineers diagnose problems, improve designs, and develop preventive measures
- Combining multiple analysis techniques provides a comprehensive understanding of fatigue phenomena
Microscopy
- Optical microscopy reveals surface features and large-scale crack patterns
- Scanning electron microscopy (SEM) provides high-resolution images of fracture surfaces
- Transmission electron microscopy (TEM) allows observation of dislocation structures and microstructural changes
Non-destructive testing
- Ultrasonic testing detects subsurface cracks and discontinuities
- Eddy current testing identifies surface and near-surface defects in conductive materials
- Acoustic emission monitoring detects crack growth during fatigue testing
Fracture surface analysis
- Examines fracture surface features to determine failure modes and crack propagation patterns
- Identifies beach marks and striations to reconstruct fatigue crack growth history
- Determines crack initiation sites and contributing factors (inclusions, defects, stress concentrations)
Prevention and mitigation strategies
- Implementing effective strategies to prevent and mitigate fatigue wear is crucial in engineering design
- These approaches aim to extend component lifespan, improve reliability, and enhance safety
- Engineers must consider the specific application requirements when selecting prevention methods
Material selection
- Choose materials with high fatigue strength and fracture toughness
- Consider fatigue crack growth resistance and environmental compatibility
- Utilize high-strength alloys or composite materials for critical applications
Surface treatments
- Apply shot peening to induce compressive residual stresses and improve fatigue life
- Utilize nitriding or carburizing to enhance surface hardness and wear resistance
- Implement coatings (PVD, CVD) to reduce friction and protect against environmental factors
Design optimization
- Minimize stress concentrations through proper geometry design (fillets, radii)
- Implement fail-safe and damage-tolerant design principles
- Utilize finite element analysis (FEA) to identify and address high-stress regions
Fatigue wear in engineering applications
- Fatigue wear impacts various industries and engineering disciplines
- Understanding application-specific challenges helps engineers develop targeted solutions
- Lessons learned from different sectors can be applied to improve overall fatigue wear management
Automotive components
- Suspension systems experience cyclic loading from road irregularities
- Engine components (crankshafts, connecting rods) undergo high-frequency fatigue cycles
- Wheel bearings and CV joints are subject to rolling contact fatigue
Aerospace structures
- Aircraft fuselage and wings experience cyclic pressurization and depressurization
- Turbine blades in jet engines undergo high-frequency vibrations and thermal cycling
- Landing gear components face high-impact loads during takeoff and landing
Industrial machinery
- Gears and bearings in power transmission systems experience rolling contact fatigue
- Pressure vessels and pipelines undergo cyclic pressurization and thermal stresses
- Robotic arms and manufacturing equipment face repetitive loading cycles
Modeling and prediction of fatigue wear
- Accurate modeling and prediction of fatigue wear are essential for reliable component design
- These approaches help engineers estimate component lifespan and optimize maintenance schedules
- Combining different modeling techniques provides a comprehensive understanding of fatigue behavior
Stress-life approach
- Based on S-N curves relating stress amplitude to cycles to failure
- Utilizes or endurance limit for infinite life design
- Incorporates factors for surface finish, size effect, and loading type (Marin factors)
Strain-life approach
- Considers both elastic and plastic strain components in fatigue life prediction
- Uses Coffin-Manson relationship and cyclic stress-strain curves
- Accounts for local plasticity and notch effects more accurately than stress-life approach
Fracture mechanics approach
- Applies linear elastic fracture mechanics (LEFM) principles to fatigue crack growth
- Utilizes Paris' Law to predict crack growth rate based on stress intensity factor range
- Incorporates threshold and critical stress intensity factors for crack initiation and final failure
Fatigue wear standards and specifications
- Standards and specifications provide guidelines for fatigue testing, analysis, and design
- These documents ensure consistency and reliability in fatigue-related engineering practices
- Engineers must stay updated on relevant standards to maintain compliance and best practices
ASTM standards
- ASTM E466 Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials
- ASTM E606 Standard Test Method for Strain-Controlled Fatigue Testing
- ASTM E647 Standard Test Method for Measurement of Fatigue Crack Growth Rates
ISO standards
- ISO 1143 Metallic materials — Rotating bar bending fatigue testing
- ISO 12106 Metallic materials — Fatigue testing — Axial-strain-controlled method
- ISO 12107 Metallic materials — Fatigue testing — Statistical planning and analysis of data
Industry-specific guidelines
- Aerospace MMPDS (Metallic Materials Properties Development and Standardization) handbook
- Automotive SAE J1099 Technical Report on Low Cycle Fatigue Properties Ferrous and Non-Ferrous Materials
- Nuclear ASME Boiler and Pressure Vessel Code Section III Rules for Construction of Nuclear Facility Components
Key Terms to Review (21)
Composites: Composites are materials made from two or more constituent materials with significantly different physical or chemical properties. When combined, these materials produce a composite that has enhanced properties compared to the individual components, making them particularly valuable in applications requiring high strength-to-weight ratios and improved fatigue resistance.
Crack initiation: Crack initiation is the initial phase in the development of cracks within a material, often leading to failure or fracture. This process typically occurs due to various stressors such as fatigue, environmental factors, or material defects. Understanding crack initiation is crucial for predicting the lifespan of materials and components, especially under cyclic loading conditions that contribute to fatigue wear.
Crack Propagation: Crack propagation refers to the growth and advancement of a crack within a material, often influenced by various stresses and environmental factors. It plays a critical role in the failure of materials under cyclic loading, where repeated stress can lead to fatigue wear, causing small cracks to expand over time until catastrophic failure occurs. Understanding how cracks propagate is essential for predicting material durability and designing components that can withstand cyclic stress without failing.
Cyclic Loading: Cyclic loading refers to the application of repeated loads on a material or component over time, causing variations in stress and strain. This process is critical to understanding how materials behave under conditions that mimic real-world usage, where loads may not be constant. The effect of cyclic loading can lead to fatigue wear, which ultimately contributes to the failure of materials through the initiation and propagation of cracks.
Design optimization: Design optimization is the process of adjusting and improving a design to achieve the best performance or efficiency while meeting specific constraints and requirements. This concept involves analyzing various design parameters, considering trade-offs, and utilizing mathematical and computational tools to enhance functionality and reduce costs or risks.
Fatigue crack growth test: The fatigue crack growth test is an experimental method used to evaluate how cracks propagate in materials under cyclic loading conditions. This test simulates real-world service environments where materials experience repeated stresses, allowing researchers to measure the rate at which cracks grow and ultimately lead to material failure, making it essential in understanding fatigue wear.
Fatigue limit: The fatigue limit, also known as the endurance limit, is the maximum stress level a material can withstand for an infinite number of loading cycles without experiencing failure. This concept is crucial in understanding how materials behave under repeated loading conditions, particularly when elastic and plastic deformations occur, as well as in the context of fatigue wear which leads to the eventual failure of materials.
High-cycle fatigue: High-cycle fatigue refers to the phenomenon where materials experience failure after a large number of loading cycles, typically greater than 10,000 cycles, at relatively low stress levels. This type of fatigue is crucial in understanding the durability of materials and components that undergo repetitive loading, making it essential for designing parts in various engineering applications.
Low-cycle fatigue: Low-cycle fatigue refers to the failure of materials due to cyclic loading at relatively low numbers of cycles, typically less than 10,000. This type of fatigue occurs when a material is subjected to high levels of stress that lead to significant plastic deformation, making it distinct from high-cycle fatigue, where the stresses are lower and the number of cycles is higher. Understanding low-cycle fatigue is crucial in assessing the durability and longevity of components subjected to fluctuating loads, especially in critical engineering applications.
Metals: Metals are a class of materials characterized by their high electrical and thermal conductivity, malleability, ductility, and metallic luster. They play a crucial role in various engineering applications, especially concerning friction and wear, due to their unique properties that influence adhesion, deformation, and wear mechanisms.
Paris Law: Paris Law describes the relationship between the stress intensity factor and the rate of fatigue crack growth in materials, stating that the rate of crack growth is proportional to the stress intensity factor range. This law is vital for understanding how materials behave under cyclic loading conditions and helps in predicting fatigue life, especially in engineering applications where structural integrity is crucial.
Repeated stress: Repeated stress refers to the application of cyclic loads or forces on a material, leading to the potential for fatigue over time. This kind of stress is crucial in understanding how materials behave under conditions where they are subjected to loading and unloading repeatedly, which is common in mechanical systems and components. The impact of repeated stress can lead to microstructural changes in materials, contributing to failure mechanisms such as fatigue wear.
Rotating bending test: The rotating bending test is a method used to evaluate the fatigue properties of materials by subjecting a specimen to bending stresses while it rotates. This test simulates real-world conditions where components experience cyclic loads and helps determine how materials behave under repeated stress, which is crucial for understanding fatigue wear.
S-n curve: The s-n curve, also known as the stress-number curve or Wöhler curve, represents the relationship between the cyclic stress amplitude (s) applied to a material and the number of cycles to failure (n). This graph is essential in understanding fatigue wear, as it illustrates how materials behave under repeated loading conditions and helps predict their lifespan.
Stress Concentration: Stress concentration refers to the localization of stress in a material at points of geometric discontinuity or changes in material properties. This phenomenon can significantly affect the fatigue life of a material, as areas of high stress can lead to the initiation of cracks and subsequent fatigue wear under cyclic loading conditions.
Subsurface Fatigue: Subsurface fatigue refers to the progressive failure that occurs beneath the surface of a material due to cyclic loading or repeated stress. This phenomenon is particularly relevant in engineering, where components experience fluctuating loads during operation, leading to the formation of cracks or defects below the surface that can ultimately result in catastrophic failure. Understanding subsurface fatigue is crucial for predicting material performance and longevity in various applications, especially those subjected to fatigue wear.
Surface Fatigue: Surface fatigue refers to the progressive and localized structural damage that occurs when a material experiences repeated or cyclic loading, leading to the initiation and growth of cracks at or near the surface. This phenomenon is particularly significant in applications involving components like bearings and gears, where constant motion and contact stresses contribute to wear over time.
Surface Hardening: Surface hardening is a heat treatment process used to increase the hardness and wear resistance of the surface of a material, particularly metals, while maintaining a tougher and more ductile interior. This technique is crucial for enhancing the performance of materials in applications where wear and fatigue are significant concerns, allowing them to withstand greater loads and extend their service life.
Tensile Strength: Tensile strength is the maximum amount of tensile (stretching) stress that a material can withstand before breaking. This property is crucial for understanding how materials, particularly in engineering applications, will behave under tension, which is relevant for polymers and composites used in structural applications as well as for analyzing fatigue wear in materials that undergo repeated loading cycles.
Thermal fatigue: Thermal fatigue refers to the progressive and localized structural damage that occurs in materials due to cyclic thermal loading. This phenomenon is particularly relevant when materials experience repeated temperature fluctuations, causing them to expand and contract, which can lead to the formation of cracks and eventual failure. Understanding thermal fatigue is crucial when considering the wear and longevity of materials in environments where temperature changes are frequent.
Yield Strength: Yield strength is the amount of stress a material can withstand before it begins to deform plastically. This means that once the yield strength is exceeded, the material will not return to its original shape when the load is removed. Understanding yield strength is crucial because it relates directly to how materials behave under different loads, impacting processes like friction, deformation, and wear.