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What 3 constitutes a failure?
Excessive elastic deformation, Excessive plastic deformation (Yielding), Fracture
Name 3 types of Fracture
Sudden Brittle Fracture, Fatigue, Delayed Fracture (Stress Rupture)
What leads to better MACROSCOPIC design?
Better MICROSCOPIC understanding of materials
Name 3 types of material loading?
Compression, Tension, Shear
Name 3 important properties about metals that make them useful in the engineering world
High strength, Ability to be worked, Durable
How do metals get their distinctive properties?
From their microstructure and bonding
Describe the crystal structure of Metals
Atoms sit in regular positions and deform along preferred planes (Close packed)
What is Strength?
Ability to withstand stress
How do you calculate Strain?
Elongation / Original Length
What is the purpose of tensile testing?
Used to determine material properties such as yield, UTS, elongation.
Describe the process of tensile testing
The specimen piece is inserted in the rig with holding grips at each side. Rig then extends and applies tension on the piece whilst measuring force. Force is increased until material failure.
How do you calculate engineering stress?
Force / Area (1 MPa = 1 N/mm^2)
Definition of Engineering Stress
Instantaneous loading divided by the cross sectional area
Definition of Engineering Strain
Engineering strain is defined as the change in gauge length in the direction of applied stress divided by the original gauge length
How do you calculate Engineering strain
Change in length / Original Length
What is the Yield Point?
Point where stress is no longer proportional to the strain
What is Yielding
A material transitions from elastic to plastic, where permanent damage is done (Material does not return to original shape). Material bonds are broken.
What is Ultimate Tensile Strength?
UTS is the maximum stress on the curve of a Tensile Plot
What does UTS indicate?
Any further stress applied will cause the material to fail
Name 4 material properties obtained from a tensile test
Stiffness (Young's/Elastic Modulus), Yield Stress, UTS, Ductility
How do you find Yield Stress on a Tensile Plot
Stress found at 0.2% from linear relation
What is Poissons Ratio?
Ratio between transverse contraction strain to longitudinal extension strain in direction of stretching force
How do you calculate Poissons Ratio?
- Lateral Strain / Axial Strain
What is Hooke's Law?
Measurement of Stiffness, where strain and stress are proportional
How do you calculate Young's Modulus?
Stress / Strain
What is the definition for Stiffness?
<div>Measure of a load required to induce a certain deformation where as strength is a materials resilience to fracture </div>
What is the definition of Hardness?
Measure of its resistance to penetration by an indenter
Name some types of Hardness tests
Mohs, Rockwell, Vickers, Brinell
How does hardness testing work?
Indenter is pushed against the material surface at a set force. Larger indents indicate softer materials.
Relation between Yield Strength and Hardness level?
Yield Strength is approximately 1/3 of the Hardness level
What is the definition of Toughness?
Measure of material's ability to absorb energy, shows if a material will fracture in a brittle or ductile manner
What factors affect Toughness?
Geometry and Temperature
Name and describe methods of Toughness testing
Charpy and Izod. Notched sample of material is subjected to impact with break force measured.
What property is deformation measured by?
Strain
How does Strain accumulate?
Accumulates under the application of stress
In what direction does stress act?
Axial and Shear
How do you calculate Shear Stress?
Tau = F / A0
What does Hooke's Law relation look like for metals
<div>For most metals that are stressed in tension and at relatively low levels, stress and strain are proportional to each other</div>
Formula for Shear Modulus
Shear Stress / Shear Strain
What 3 factors affect Elastic Modulus in metals?
Temperature, Alloying Elements, Anisotropy/Loading Direction
How does temperature affect Elastic modulus in Metals
Increased temperature increases inter atomic spacing reduces the binding force between atoms, thus modulus decreases
How does Alloying Elements affect Elastic modulus in Metals
Large poorly fitting substitutional elements stretch bonds and hence reduces modulus. Smaller solute solvent elements contract bonds and hence increase modulus.
How does Anisotropy affect Elastic modulus in Metals
<div>Different crystallographic directions have different bond radii and different combinations of </div><div>tensile and shear components. Hence different loading direction relative to these directions </div><div>produce can have different moduli.</div>
What is Work/Strain Hardening?
Within the stress strain curve region, between yield stress and UTS, the occurrence of increasing strength with deformation
What does the Yield point signify?
It is the point where a material turns from elastic deformation to plastic deformation. Further stress will break bonds permanently, thus permanently deforming the material. Stress Stain is not longer proportional from this point
Explain the elastic behaviour of a material at the microscopic level
Response depends on type and strength of atomic bonds. Removing loads causes bonds to spring back to original shape, no permanent deformation. Loading beyond yield point causes permanent deformation. Some of the bonds are now broken in crystal structure.
What is slip?
It is how plastic deformation occurs. It is the shearing of one part of a crystal over another in crystalline solids. Slip occurs on a slip plane and the direction in which slip occurs is called slip direction.
Describe the components of a slip system
Slip system is a combination of slip plane and slip direction
What conditions does slip occur in
Slip always occurs under the action of shear stress. Slip occurs on a planes of closest atomic packing and in direction of closest atomic packing.
What are crystals
They are 3D arrangements of atoms. Most basic visualisation is unit cell, others include BCC, FCC, HCP.
How do you use Callister Vectors to describe crystallographic directions
[xyz] Connect dots of each point to describe plane
Describe the preferred slip system for a FCC structure
Most closely packed slip plane is {111}. Slip occurs along <110> directions within {111}. Total of 12 slip systems.
Explain the 4 types of describing directions using Miller notation
(Indicate Plane) {Indicate group of planes} [Indicate direction] <Indicate group of directions>
When does slip occur in metals?
<div>Slip will initiate when the resolved shear stress, which is created by the applied load, is sufficient to cause dislocation motion. </div>
What is a grain?
<div>A region of material where all the atoms lie in the same lattice structure and are arranged in the same orientation</div>
Describe the link between the grain and likely slip
<div>Each grain will contain various slip systems and these will in turn vary in orientation from grain to grain. Slip likely to occur first on the plane under greatest shear stress</div>
What is the formula for resolved shear stress?
Tau = (F sin theta)/(A / Cos theta) = Stress sin theta * cos theta
When is resolved shear stress at a maximum
It is at a maximum at + - 45 degrees
Name a condition where resolved shear stress is zero
Tension axis is normal to the slip plane or if it is parallel to the slip plane. This is as no shear stress is acting on those planes. Crystals orientated in this manner fracture rather than slip.
In what direction will slip take place
Will take place on slip plane nearest 45 degrees to the applied stress. In polycrystalline materials, slip path will be on average 45 degrees, zig zags through metal
What is the Schmid Factor, m, equal to?
Stress Cos lambda Cod phi
Describe Schmid's Law
<div>The value of τ Rss at which slip occurs in a given material with </div><div>specified dislocation density and purity is a constant, known as </div><div>the critical resolved shear stress τ C</div>
What is Tau RSS equal to?
Resolved force acting on a slip plane divided by area of the slip plane equals = (F Cos lambda)/(A / Cos phi) = Stress Cos lambda Cos phi = Stress m
How does resolved shear stress change with tensile load?
<div>As the tensile load is increased, the resolved shear stress on each system increases until eventually τC is reached on one of the slip systems</div>
What is the primary slip system?
The 1st orientation that plastically deforms
What is the yield stress of a single crystal?
The stress required to cause slip on the primary slip system
What happens to Tau C when the load is increased?
Tau C may be reached on other slip systems, causing them to start operating
What is the max value for Schmid Factor?
0.5
How is yield stress related to Tau C?
Yield Stress = Tau C / m (Schmid Factor)
How does Schmid factor relate to Yield?
High Schmid Factor = Low Yield
What do grains with high schmid factor exhibit?
Resolved shear stress exceeds the critical resolved shear stress, RSS = CRSS
How does purity affect Tau CRSS?
Higher purity reduces CRSS
How can you see slip lines on a material?
Use a microscope, darker lines present slip lines of crystals
How does slip vary from grain to grain?
Slip lines may vary in direction in between grains as each slip line is in direction of highest shear stress. This is due to different slip systems.
What is a polycrystalline material?
It is a material that is stronger than single crystal due to constraints imposed by the multi oriented grains.
Why is a polycrystalline material stronger than a single crystal, explain slip in both.
For a single grain to slip, the adjacent less favourable grain must be able to slip. This means a polycrystalline requires a higher stress in order to be deformed.
Describe slip in a perfect crystal.
<div>Plastic deformation usually occurs by atomic planes sliding over each other, under the influence of shear stresses. When a normal stress is applied, shear stresses are generated on planes inclined at an angle to the stress axis</div>
How do you calculate the theoretical maximum shear stress?
Tau max = G / 2 * pi
What are the key differences between experimental and theoretical maximum shear stress?
Theoretical assumes atoms on slip plane move together at same time (Slip Model). This is not true for experimental maximum shear stress, which is always significantly lower.
Describe the movement of atoms in perfect plastic deformation / Theoretical maximum resolved shear stress?
Plastic deformation for this is high as previously discussed due to the fact this requires perfect deformation of a complete layer of atoms, which requires a lot of energy
Describe the key points for the correct model for plastic deformation
Slip must start from localised region and then extend over the plane. This reduces the number of bonds broken/shifted at one time. This is analogous to a wave of breaks rather than one continuous break. This in turn requires less energy than theorised.
What two things does plastic deformation utilise in order to create bond breakages?
It uses crystal imperfections, the two notable dislocations being Edge & Screw
Explain in simple terms what a dislocation is
A dislocation is a crystallographic defect / irregularity within the crystal structure of a material. It explains why materials theoretical strength is lower than experimental. It is a region of localised lattice disturbance separating slipped and unslipped regions.
What 3 factors do dislocations strongly influence?
Plastic deformation, Electrical conductivity, Crystal growth<div><br></div>
Name 3 sources of dislocations in Metals
Solidifications, Cold Working, Applied Stress
What type of treatment can be used to reduce the number of dislocations?
Heat treatment<div><br></div>
How do dislocations change the internal energy of a system
**Thermodynamics - Increases the internal energy
What are edge dislocations?
They are extra planes of atoms inserted in the crystal lattice. It causes a region of distortion with a dislocation line. It allows the dislocation to move easily. The dislocation line here represents the boundary between slipped and unslipped material
What is a Burger Vector?
Slip is produced when one dislocation moves through the crystal structure. A Burgers Vector is the shear displacement associated with each dislocation. It denotes the direction and magnitude of the atomic displacement that occurs when the dislocation moves.
What is the difference between positive and negative edge dislocations?
Positive - 1/2 plane above slip plane. Negative - 1/2 plane below slip plane.
What happens with BOTH a positive and negative Burgers Vector?
-b + b = 0, they annihilate each other and create a perfect lattice
Describe the direction of motion with an edge dislocation
Edge dislocation moves perpendicular to extra plane. In turn produces slip perpendicular to the dislocation line. Moves in response to shear stress perpendicular to the dislocation line.
What is the definition and overall effect of a screw dislocation?
A screw dislocation does not have any extra planes, rather it generates a slip parallel to the dislocation line. Moves in response to shear stress parallel to the dislocation line. Crystal only distorts close to dislocation line, which moves to produce the required slip.
Describe a block diagram for a screw dislocation
There is no extra lattice. One side of the dislocation line is completely shifted to one side either LHS or RHS. There is a clear helical atomic displacement around the dislocation line. Dislocation direction is perpendicular to the direction of shear.
Describe a Burgers Circuit for a Screw Dislocation
Tracing a square around the point of dislocation on the plane perpendicular to shear, produces a box with a gap on the distorted line. This line represents the Burger vector, going from the inside to the outer edge.
Briefly describe what mixed dislocations are
Combination of edge and screw dislocations, result of slip extending over large curved areas. They are made of tiny alternating edge and screw dislocations extending throughout the slip line. Dislocation line is angled to Burgers Vector, not parallel or perpendicular.
What is the definition of a mixed dislocation in terms of Burgers Vectors.
It is a dislocation where the Burgers Vector is inclined to the dislocation line. This vector can be split into components representing edge and screw dislocations.
Explain the term dislocation loop
Dislocation must be a closed loop by definition. Majority of this loop will be mixed with small straight sections (Edge/Screw). The opposites of the loop have opposite polarity. Similarly one side will be LHS and the other RHS.
How do you calculate internal energy for Screw Dislocations?
Us = G * b^2 (G - Shear Modulus, b - Burgers Vector)
How do you calculate internal energy for Edge Dislocation?
Ue = (G * b^2) / (1-v) (G - Shear Modulus, b - Burgers Vector, v - Poissons ratio)
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