Podcast
Questions and Answers
Which material is described as having the lowest stiffness?
Which material is described as having the lowest stiffness?
Polymers generally have high elastic modulus (E) values.
Polymers generally have high elastic modulus (E) values.
False
What is the range of Poisson's ratio (ν) for most metals?
What is the range of Poisson's ratio (ν) for most metals?
0.25 to 0.45
Polymers with many cross-links that cannot melt after solidification are called __________.
Polymers with many cross-links that cannot melt after solidification are called __________.
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Which of the following best describes Poisson's ratio (ν)?
Which of the following best describes Poisson's ratio (ν)?
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Match the following materials with their characteristics:
Match the following materials with their characteristics:
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Natural cork has a Poisson's ratio approximately equal to 0.
Natural cork has a Poisson's ratio approximately equal to 0.
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Why do composites have a large range of elastic modulus (E) values?
Why do composites have a large range of elastic modulus (E) values?
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What type of motion does dislocation climb represent?
What type of motion does dislocation climb represent?
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Dislocation climb can occur at low temperatures without additional energy.
Dislocation climb can occur at low temperatures without additional energy.
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What is the effect of dislocation width on frictional stress?
What is the effect of dislocation width on frictional stress?
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Dislocation climb involves the ______ motion of atoms.
Dislocation climb involves the ______ motion of atoms.
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What factor influences the width of a dislocation?
What factor influences the width of a dislocation?
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Match the following materials with their corresponding characteristics:
Match the following materials with their corresponding characteristics:
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Slip in close-packed directions occurs where the atomic slip distance is at a maximum.
Slip in close-packed directions occurs where the atomic slip distance is at a maximum.
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Which of the following materials are classified as thermoplastics?
Which of the following materials are classified as thermoplastics?
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Why are ceramics typically considered brittle?
Why are ceramics typically considered brittle?
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Dislocation glide is the only mechanism for the motion of edge dislocations.
Dislocation glide is the only mechanism for the motion of edge dislocations.
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What is the midpoint called in the range of thermoplastic behavior?
What is the midpoint called in the range of thermoplastic behavior?
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The _____ vector defines the dislocation slip direction.
The _____ vector defines the dislocation slip direction.
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Match the following dislocation types with their descriptions:
Match the following dislocation types with their descriptions:
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Which of the following correctly describes the edge dislocation Burger's vector relation?
Which of the following correctly describes the edge dislocation Burger's vector relation?
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Dislocation climb involves conservative motion.
Dislocation climb involves conservative motion.
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What is a common application of thermosetting materials?
What is a common application of thermosetting materials?
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What happens to the critical resolved shear stress (CRSS) of fcc metals with temperature changes compared to bcc metals?
What happens to the critical resolved shear stress (CRSS) of fcc metals with temperature changes compared to bcc metals?
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Alloying generally decreases the critical resolved shear stress (CRSS) of metals.
Alloying generally decreases the critical resolved shear stress (CRSS) of metals.
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Which material class is considered the strongest among ceramics, metals, and polymers?
Which material class is considered the strongest among ceramics, metals, and polymers?
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Ultra-pure metals are typically quite ______.
Ultra-pure metals are typically quite ______.
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What is a common characteristic of strong materials in relation to ductility?
What is a common characteristic of strong materials in relation to ductility?
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Match the following strengthening mechanisms with their descriptions:
Match the following strengthening mechanisms with their descriptions:
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The Hall-Petch equation indicates that a smaller grain size results in a higher driving force for dislocation movement.
The Hall-Petch equation indicates that a smaller grain size results in a higher driving force for dislocation movement.
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What is the general trend of yield strength when comparing high-strength engineering metals to ultra-pure metals?
What is the general trend of yield strength when comparing high-strength engineering metals to ultra-pure metals?
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Which boundary type allows dislocation penetration?
Which boundary type allows dislocation penetration?
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Dislocation bowing occurs when a dislocation encounters an ordered interphase boundary.
Dislocation bowing occurs when a dislocation encounters an ordered interphase boundary.
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What happens to the precipitation process at temperatures approaching the solvus temperature?
What happens to the precipitation process at temperatures approaching the solvus temperature?
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In the creep curves, the phase where the dislocation density increases is called __________.
In the creep curves, the phase where the dislocation density increases is called __________.
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At what stage of creep does continuous increase in creep rate occur?
At what stage of creep does continuous increase in creep rate occur?
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Match the types of interphase boundaries with their characteristics:
Match the types of interphase boundaries with their characteristics:
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Recovery effects during steady-state creep require immobilization of vacancies.
Recovery effects during steady-state creep require immobilization of vacancies.
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What determines the hardening effect in particle-reinforced materials?
What determines the hardening effect in particle-reinforced materials?
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Which type of creep is primarily accomplished through diffusional mass transport?
Which type of creep is primarily accomplished through diffusional mass transport?
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Coble creep operates independently of grain size.
Coble creep operates independently of grain size.
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What is a primary characteristic of Nabarro-Herring (NH) creep?
What is a primary characteristic of Nabarro-Herring (NH) creep?
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Coble creep is important in __________ grained materials.
Coble creep is important in __________ grained materials.
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Match the following creep types with their characteristics:
Match the following creep types with their characteristics:
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What does Coble creep depend on more strongly compared to Nabarro-Herring creep?
What does Coble creep depend on more strongly compared to Nabarro-Herring creep?
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Superalloys are primarily based on iron, nickel, or cobalt.
Superalloys are primarily based on iron, nickel, or cobalt.
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What type of environments do turbine applications require materials to be resistant to?
What type of environments do turbine applications require materials to be resistant to?
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Study Notes
True Stress and Strain
- Engineering stress and strain are not always "real" materials behaviour.
- True stress (σT) = F/Ai , where F is the force and Ai is the instantaneous cross-sectional area
- True strain (εT) = ln(A0/Ai) = ln(1+εE), where A0 is the original cross-sectional area, Ai is the instantaneous cross-sectional area, and εE is the engineering strain
- For elastic deformation, true stress is roughly equal to engineering stress, and true strain is equal to engineering strain
- For plastic deformation, true stress is approximately equal to engineering stress, and true strain is equal to engineering strain
The Range of E
- Different materials exhibit vastly different Young's moduli (E).
- Diamond has a very high E, indicating it is very stiff.
- Graphite has a lower E than diamond.
- Graphene has a very high E, comparable to diamond.
- Ice has a lower E compared to MgO.
- Lead has the lowest E among metals, meaning it's the least stiff.
Poisson's Ratio (v)
- Poisson's ratio (ν) is an elastic constant describing the proportionality between normal strain in one axis and the resulting normal strain in an orthogonal direction.
- Primarily, it relates extension to contraction.
- For incompressible materials, ν = 0.5.
- For most metals, ν is between 0.25 and 0.45
- For ceramics and glass, ν is between 0.1 and 0.3
- Natural cork has a Poisson's ratio near 0: meaning it is very easy to compress
Generalized Hooke's Law
- The total strain in one direction is equal to the sum of the strains generated by the various stresses in that direction.
- Equations for strains ε11, ε22, ε33 are presented relating stresses and strains
Compliance and Stiffness Constants
- Equations that define compliance (S) and stiffness (C) constants are provided
- α, β, and γ are the direction cosines of [hkl] direction
Polymers
- Thermoplastics can melt and re-melt
- Thermosets cannot melt after solidifying
- Differences lie in cross-linking
Thermoplastic Behavior
- Glassy, amorphous, partially crystalline, and leathery states of polymers
- Glass transition temperature (Tg)
Edge/Screw Dislocations
- Edge, screw displacements are mechanisms for the movement of edge dislocations.
- Images are provided of edge dislocation motion and screw dislocation motion
The Burgers Vector (b) and Burgers Circuit
- Right-hand/finish-start (RH/FS) convention for defining the Burgers vector
- Definition of b for edge and screw dislocations
Climb of Edge Dislocations
- Dislocation glide and climb are conservative and nonconservative motions, respectively
- Climb requires addition or removal of atoms/vacancies
- Climb is only important at elevated temperatures
Cross Slip in a Face-Centered Cubic Metal
- Sequence of events in cross slip are shown and described
Dislocation Core and Lattice Frictional Stress
- Frictional stress is low when a is large
Slip Systems in FCC and BCC
- Slip systems and directions in FCC and BCC are indicated, graphically
Slip Systems in Crystalline Materials
- Summarizes the number of possible slip systems in various materials, including FCC, BCC, and HCP structures
The Width of Dislocation Core
- Width of dislocation core affects Peierls-Nabarro stress and atomic displacements required for motion
- Temperature affects dislocation width
Why Ceramics are Brittle?
- Dislocation cores in ceramics are very small, leading to extremely high frictional stresses exceeding fracture strength, making dislocation motion difficult
- This causes stress concentration at crack tips, leading to brittle fracture
Dislocation Glide vs. Twinning
- Dislocation glide and twinning are competitive mechanisms for plastic deformation.
- FCC metals tend to exhibit slip over twinning at higher temperatures and strain rates
- BCC metals can exhibit twinning more readily due to yield strength dependence on temperature
Dislocation Multiplication - Frank-Read Source
- Explanation and diagrams of the Frank-Read source mechanism for dislocation multiplication
Plastic Flow in Single Crystals
- Schmid factor and Taylor factor are explained and pictured
Stress-Strain Behavior of FCC and BCC Metals
- Stages I,II,III of the stress-strain curves depending on temperature and strain rate
- Cross slip occurs readily in materials with high SFE
Temperature and Strain Rate Dependence of CRSS
- Temperature and strain rate dependence of critical resolved shear stress (CRSS) at various temperature regions, both thermally dependent (τ*) and athermal (τa) components
Creep Activation Energies vs. Self-Diffusion Activation Energies
- Correlation between high-temperature creep activation energies and self-diffusion activation energies
- Diffusional flow is associated with creep deformation
Nabarro-Herring Creep
- Description of the Nabarro-Herring (NH) creep mechanism
- NH creep is diffusion-controlled, important at high temperatures but less significant in metals than ceramics
Coble Creep
- Coble creep is a grain-boundary diffusion-controlled process
- High temperature creep is more sensitive to grain size than Nabarro-Herring creep
Creep Mechanisms Involving Dislocation and Diffusional Flow
- Description and equations for solute drag creep and climb-glide creep
Independent Creep Processes
- NH and Coble creep mechanisms operate independently (parallel or series)
- Coble creep dominates in fine-grained materials while NH creep is prevalent in larger grains
Diffusional Flow and Dislocation Creep
- Description of different creep mechanisms and their dominance based on stress and temperature
Summary
- Summary of creep, hardening mechanisms and controlling factors
Basic Principles of Reinforcement
- Overview of stress for the phases are the same
- Equations for stress in composite materials, and phase volume
Lamellar Arrangements of Two Phases
- Lamellar arrangements in composites utilizing phases, and forces applied to the phases
Tensile Test of a Fiber Composite
- Description of the force effect in a fiber composite
Stress-Strain Curves of Composites
- Discussion of stress-strain curves and stages in composites (Stages 1,2,3)
Composite Tensile Strength vs. Secondary Tensile Strength
- Relations between composite tensile strength to secondary tensile strength in composites
Case Studies - Superalloys
- Overview of superalloys, their importance, and composition, typically in Ni-based materials.
Strain-Rate Sensitivity
- Strain-rate sensitivity is important in materials behaviour
- High values of m (strain-rate sensitivity) indicates resistance to neck development in tension -Strain rate sensitivities increase with temperature
Crystalline, Polycrystalline, and Amorphous Structures
- Illustrations of the three basic crystal structures
- Crystal structures vary widely and this impacts their behaviour
Hardness vs. Strength
- Overview of the relationship between hardness and strength in materials
Oliver-Pharr Method
- Description of the Oliver-Pharr method for measuring the reduced elastic modulus
Multiaxial Loading Conditions
- Multiaxial loading and its implications for material response
Tresca Yield Criterion
- Overview of the Tresca yield criterion for biaxial loading conditions
The von Mises Yield Criterion
- Overview of the von Mises yield criterion for multiaxial stress states, and comparison to Tresca
(Note that some documents listed concepts not included in the notes if they are not discussed or relevant to the topics covered in the other sections.)
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Description
Test your knowledge on the properties and characteristics of different materials in this quiz focused on elasticity, stiffness, and dislocations. Discover how polymers, metals, and composites compare in terms of elastic modulus and Poisson's ratio. Challenge yourself with matching materials to their respective properties and understanding dislocation motion.
