2.3 Mechanical properties of materials

Mechanical properties are the backbone of material selection in engineering. They determine how materials behave under different stresses and strains, guiding choices for specific applications. From strength and hardness to ductility and toughness, these properties shape the performance of everything from buildings to smartphones.

Understanding the relationship between a material's structure and its properties is key. Crystal structure, grain size, and microstructural features all play a role in determining mechanical behavior. This knowledge allows engineers to tailor materials for optimal performance in diverse applications, balancing factors like strength, weight, and cost-effectiveness.

Key Mechanical Properties

Strength and Hardness

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• Strength measures a material's ability to withstand applied stress without failure
  • Encompasses tensile strength, compressive strength, and yield strength
  • Varies depending on type of stress applied (tension, compression, shear)
• Hardness quantifies a material's resistance to localized plastic deformation
  • Measured through indentation tests (Brinell, Rockwell, Vickers scales)
  • Correlates with other properties like wear resistance and machinability
• Stiffness characterizes resistance to elastic deformation under applied stress
  • Defined by elastic modulus (Young's modulus for linear elastic materials)
  • Higher stiffness materials experience less elastic deformation under load

Ductility and Toughness

• Ductility represents ability to plastically deform without fracture
  • Quantified by percent elongation or reduction in area during tensile testing
  • Allows materials to be formed into various shapes (wire drawing, sheet metal forming)
• Toughness measures energy absorption before fracture
  • Calculated as area under stress-strain curve up to failure point
  • Crucial for impact-resistant applications (automotive crumple zones, protective equipment)
• Fatigue strength indicates maximum cyclic stress withstood without failure
  • Critical for components under repeated loading (aircraft wings, engine parts)
  • Determined through cyclic stress testing to generate S-N curves

Time and Temperature Dependent Properties

• Creep resistance describes ability to resist time-dependent deformation under constant stress
  • Particularly important at elevated temperatures
  • Relevant for high-temperature applications (turbine blades, furnace components)
• Stress relaxation characterizes decrease in stress over time under constant strain
  • Observed in materials under sustained loading (bolted joints, prestressed concrete)
  • Can lead to loss of preload or clamping force in fastened assemblies
• Temperature sensitivity of properties must be considered
  • Many materials soften and lose strength at high temperatures
  • Some materials become brittle at low temperatures (ductile-to-brittle transition)

Elastic vs Plastic Deformation

Elastic Deformation Mechanisms

• Elastic deformation involves reversible atomic displacements within a material
  • Follows Hooke's Law where stress is proportional to strain ($\sigma = E\epsilon$)
  • Occurs below yield strength, allowing material to return to original shape
• Atomic bonds in crystalline materials act like springs during elastic deformation
  • Stretch or compress but return to original positions upon stress removal
  • Energy stored during elastic deformation is recovered upon unloading
• In polymers, elastic deformation involves stretching of covalent bonds and rotation of chain segments
  • Results in higher elasticity compared to most metals and ceramics
  • Enables applications like rubber bands and elastic fibers

Plastic Deformation Processes

• Plastic deformation occurs when applied stress exceeds material's yield strength
  • Results in permanent shape change due to breaking and reforming of atomic bonds
  • Characterized by non-linear stress-strain relationship beyond yield point
• Dislocation movement serves as primary mechanism for plastic deformation in crystalline materials
  • Involves propagation of line defects through crystal lattice
  • Allows for large-scale deformation with relatively small energy input
• Slip systems facilitate dislocation movement in specific crystallographic planes and directions
  • Determine material's ductility and formability
  • Face-centered cubic (FCC) metals typically have more slip systems, leading to higher ductility
• Work hardening occurs during plastic deformation as dislocations interact and multiply
  • Increases material's strength but reduces ductility
  • Utilized in processes like cold working to strengthen metals

Deformation in Different Material Classes

• Metals typically exhibit both elastic and plastic deformation regions
  • Ductile metals show extensive plastic deformation before fracture (copper, aluminum)
  • Brittle metals have limited plastic deformation and fail soon after yield (cast iron)
• Ceramics primarily undergo elastic deformation with minimal plasticity
  • High strength but low toughness due to limited dislocation movement
  • Prone to brittle fracture under tensile stresses
• Polymers display viscoelastic behavior, combining elastic and viscous characteristics
  • Time-dependent deformation even under constant load
  • Deformation mechanisms include chain stretching, uncoiling, and slippage

Structure and Properties Relationship

Crystal Structure Effects

• Crystal structure significantly influences mechanical properties
  • Close-packed structures (FCC, HCP) generally exhibit higher ductility and lower strength
  • More open structures (BCC) tend to have higher strength but lower ductility
• Atomic packing factor (APF) correlates with material density and properties
  • Higher APF structures (FCC, HCP) tend to be more ductile
  • Lower APF structures (BCC) often have higher strength and hardness
• Anisotropy in single crystals leads to direction-dependent properties
  • Important in applications like semiconductor manufacturing and turbine blades
  • Polycrystalline materials average out anisotropy, resulting in more uniform properties

Microstructural Influences

• Grain size affects mechanical properties through Hall-Petch relationship
  • Smaller grains typically result in higher strength and hardness
  • Relationship: $\sigma_y = \sigma_0 + k_y d^{-1/2}$ (where d is grain diameter)
• Alloying elements and impurities can strengthen materials
  • Create solid solutions or precipitates, impeding dislocation movement
  • Increases strength at the cost of ductility (solid solution strengthening, precipitation hardening)
• Phase composition, distribution, and morphology in multiphase materials impact mechanical behavior
  • Examples include pearlite and martensite in steels, affecting strength and toughness
  • Eutectic structures often provide a balance of strength and ductility
• Defects act as obstacles to dislocation movement, influencing strength and ductility
  • Point defects (vacancies, interstitials) cause local stress fields
  • Line defects (dislocations) interact with each other, leading to work hardening
  • Planar defects (grain boundaries, stacking faults) impede dislocation motion

Material-Specific Structure-Property Relationships

• Degree of crystallinity in polymers affects mechanical properties
  • Higher crystallinity leads to increased strength and stiffness but reduced toughness
  • Semicrystalline polymers (polyethylene, nylon) balance properties of crystalline and amorphous regions
• Fiber orientation in composite materials determines directional properties
  • Unidirectional fibers provide maximum strength in fiber direction
  • Multi-directional layups offer more isotropic properties
• Porosity in ceramics and powder metallurgy parts influences strength and toughness
  • Higher porosity generally decreases strength but can enhance toughness in some cases
  • Controlled porosity used in applications like filters and biomedical implants

Material Selection for Applications

Balancing Properties for Specific Requirements

• Material selection involves balancing various mechanical properties
  • Often requires trade-offs between strength, ductility, and toughness
  • Consideration of both primary and secondary properties crucial
• Strength-to-weight ratio critical in aerospace and automotive applications
  • High-strength, low-density materials preferred (aluminum alloys, composites)
  • Enables fuel efficiency and performance improvements in transportation
• Fatigue resistance essential for components under cyclic loading
  • Applications include aircraft structures, rotating machinery
  • Materials with high endurance limits and good crack propagation resistance selected (high-strength steels, titanium alloys)

Application-Specific Material Considerations

• Wear resistance, related to hardness, critical for surface contact applications
  • Important for cutting tools, bearings, and wear plates
  • Materials like tungsten carbide, tool steels, and ceramics often used
• Fracture toughness key for materials in critical structural applications
  • Particularly important for components susceptible to brittle fracture
  • High-toughness steels and advanced composites used in pressure vessels and aircraft structures
• Creep resistance crucial for high-temperature applications
  • Necessary for gas turbine blades, nuclear reactor components
  • Superalloys, refractory metals, and certain ceramics preferred

Practical Aspects of Material Selection

• Environmental factors must be considered when selecting materials
  • Temperature extremes can alter material properties significantly
  • Corrosive media may necessitate use of corrosion-resistant alloys or coatings
  • Radiation exposure in nuclear applications requires special material considerations
• Cost-effectiveness and manufacturability important practical considerations
  • Balance performance requirements with economic constraints
  • Consider ease of processing, joining, and finishing in material selection
• Sustainability and lifecycle analysis increasingly important in material selection
  • Factors include recyclability, energy consumption in production, and environmental impact
  • Bio-based materials and recycled composites gaining importance in certain applications