1. Deformation and structure of solids Flashcards
Two modes of failure
fracturing and permanent deformation
Define stress and give the symbol and units
Force/Area, sigma, N/m^-2
Define strain and give the symbol and units
extension/original length, epsilon, no units
Apparatus used to measure stress/strain (two names)
Universal Testing Machine, Tensometer
(Tensometer) Load cell measure what?
Force
The movement of the bottom bar in a tensometer represents what?
Change in length
What is the Lo (original length) of the dogbone samples used in a tensometer?
The length between the two nodes (not including the end cylinders)
What is the relationship of stress and strain in the elastic region?
Stress is directly proportional to strain
What is the elastic region also known as?
Recoverable strain region
What does Poisson’s ratio represent? What is the formula and symbol?
How thin a material gets in relation to the extension. Symbol: Nu = strain(x)/strain(z)
What is an isotropic material?
One that has the same properties in all directions
How do you calculate the elasticity (stiffness) of a material? What are the other names of this property?
Gradient of a stress vs strain graph, delta stress / delta strain, force required per unit extension. Other names: Young’s Modulus, Elastic Modulus
What is the yield point?
When plastic deformation starts
Plastic deformation does what to the yield point? What is this process called?
Makes it higher; the yield stress is higher so the material is stronger. Work hardening.
What is necking?
A localised reduction in area, the stress/strain graph at that point curves down
What is the highest stress on a stress/strain graph called?
Ultimate tensile stress
After what point does necking start?
After the ultimate tensile stress point
What is the area under the stress/strain graph proportional to?
The energy required to deform it
What is the special property of steel’s stress strain graph and what does it look like?
Discontinuous yield: the ‘spikes’ after the yield point. (After work hardening there’s no more of it.)
Does 0.2% proof stress increase accuracy or reliability or both?
Reliability only
When can we use a low-ish safety factor?
When the load and the environment are known with a high degree of certainty/accuracy
How do you get a true stress/strain graph from an engineering one?
Calculate the true stress using force/instantaneous area, as the area changes due to ‘necking’
Two ways of quantifying ductility
%elongation, change in length/original length. %reduction in area change in area/original area
What is the repeating unit of a crystal lattice called?
the unit cell
Body Centered Cubic - how many atoms per unit cell and what is the coordination number?
2 atoms per unit cell, coord number 8
symbol for unit cell dimension and how to calculate it
symbol: a. Use pythagoras and lengths in terms of atomic radii
formula for APF, Atomic Packing Factor
volume of atoms(in unit cell)/volume of unit cell
number of atoms per unit cell and co ordination number of the Face Centered Cubic unit cell
4 atoms per cell, coordination: 12
atoms in a unit cell and co ordination of hexagonal close packed
atoms: 6, coord: 12
coordination of a CPS, close packed structure
12 (highest)
Define interstitial sites
gaps between atoms (in a crystal lattice or other giant structure)
Define polymorphism
materials that can exist in more than one crystal form
Describe the polymorphism of iron
below 912 degrees C it is BCC, over it is FCC
what is the relationship between atomic bond strength and young’s modulus?
strong atomic bonds = high young’s modulus
slipping in close packed planes vs non close packed planes
close packed plane, easier to slip as less distance
relationship between slip and ductility
higher ductility = easier to slip
name of empty atomic space in crystal and impact on strength
vacancy, no effect
atom of different type replaced in crystal structure and effect on strength
substitution, slight increase (usually)
atoms in interstitial space are called
interstitials
another name for planar defects and state the two types
dislocations: edge dislocation and screw dislocation
explain briefly why slip is easier in dislocation movement
only a single line of bonds are broken
what brackets do you use to name: vectors, groups of crystallographically equivalent vectors, planes and crystallographically equivalent planes?
vectors: [], CE vectors: , planes: ( ), CE planes: { }
