Mechanical properties of materials Flashcards
Brittle
Breaks suddenly without deforming plastically, tend to shatter
Ceramics, glass
Ductile
Can be drawn into wires without losing its strength
Copper
Strong
Can withstand high stress without deforming or breaking
Steel
Hard
Very resistant to cutting, indentation and abrasion
Diamond, cutting tools (needs to be harder than what its cutting)
Stiff
High resistance to bending and stretching
Safety helmets
How do we measure stiffness?
Young modulus
Tough
Really difficult to break, can absorb a lot of energy without breaking
Polythene
What is the limit of proportionality?
Point where a material stops obeying Hooke’s law but would still return to its original shape when the force is removed
What is the elastic limit of a material?
Point when the material starts to behave plastically (won’t return to original shape once forces are removed, its permanently stretched)
What is the yield stress of a ductile material?
When the material suddenly starts to stretch without any load (lots of plastic deformation)
What is Hooke’s law?
That the extension of a material is directly proportional to the force applied to it
What is the equation for Hooke’s law?
F=kx
k is the spring constant/stiffness of an object
What is the unit of k?
Nm-1
What do tensile forces do to a spring?
They pull on the spring at both ends so stretch it
What happens beyond the elastic limit?
The material will stretch further for a given force
What is plastic deformation?
Atoms in the material move position relative to one another and when the load is removed, the atoms dont return to their original positions
How do you calculate stress? units
stress = tension / cross-sectional area
Nm-2 or Pa
What is stress?
The force applied per unit area
What is strain?
The extension divided by the original length
How do you calculate strain? units
extension / original length
no units
What is the fracture stress?
The amount of stress required to break a material
(when the atoms are separated completely)
What is the effect of stress on a material?
Pulls the atoms in a material apart
What is the ultimate tensile strength?
The maximum stress a material can withstand before breaking
What is elastic strain energy?
The energy stored in a stretched material
What is the area under a force - extension graph?
Work done in deforming a material
How do you calculate elastic strain energy (work done in stretching a material)?
E = 1/2 Fx
E = 1/2 kx^2
Why isn’t all the energy when the forces are removed from a material that’s exceeded its elastic limit released?
Some work is done separating the atoms so this energy isn’t stored as elastic strain energy
What is true of stress and strain up to the limit of proportionality?
They are directly proportional
What is Young modulus?
A measure of stiffness, the ratio of stress to strain
E = stress/strain (Nm-2 or Pa)
When finding the Young modulus of a wire what are the ideal dimensions of the wire?
Long and thin so that it extends more for the same force which reduces the uncertainty in the measurements
What do you need to find before you can start the young modulus experiment and how do you find it?
Cross sectional area of the wire
Use a micrometer to find the diameter of the wire in three places and take an average
How can you reduce random errors when finding the Young modulus of a wire?
Use a thin marker on the wire and always look directly above the marker and ruler when measuring the extension
What is the gradient of a stress - strain graph?
Young modulus
What does the area under a stress - strain graph represent?
Elastic strain energy per unit volume
Why is copper ideal for electric wires?
Ductile, high electrical conductivity and keeps its strength when stretched
Why do we use steel beams in bridges?
Very strong so can withstand large forces (like lots of cars) without bending or breaking
Describe the structure of metals?
Crystalline structure with delocalised electrons making them good electrical and thermal conductors
What are the properties of metals?
Stiff - strong metallic bonds
Tough - Strongly bonded lattice structure
Ductile - when force is applies the ions move
Describe elastic deformation in metals
When force is applied the interatomic spacing increases uniformly
Describe plastic deformation in metals
The layers slip over each other
What happens to the amount of stress required to make ions slip when there are dislocations?
Stress needed is less
What does alloying do?
Pins down dislocations within a metal which increases the stress needed to cause slipping so it makes them harder and less ductile
Examples of ceramics
Pottery, bricks and glass
How are ceramics made?
Melting certain materials and then letting them cool
What is the arrangement of atoms in ceramics?
crystalline, polycrystalline or amorphous
What is a polycrystalline structure?
When there are many grains (regions) of crystalline structure
What is a grain?
A group of atoms which line up in the same direction
What is an amorphous arrangement?
There’s no overall pattern, the atoms are arranged randomly eg glass
What increases a ceramics chances of being amorphous?
If a molten ceramic is cooled quickly
Why are amorphous structures brittle?
There are no slip planes or mobile dislocations so they dont deform plastically before they fracture
What makes ceramics stiff?
Ceramics are either ionically or covalently bonded in a giant rigid structure so have lots of strong bonds
Why do cracks spread through brittle materials when they fracture?
The applied force acts on a small area (tip of the crack) meaning the stress is high
What is a polymer?
A molecular chain made up of monomers
What is the bonding in polymers?
Covalent
Why are polymers strong?
Covalent bonds makes them hard to seperate
Why are polymers flexible?
The polymer chains are often entangled and can be unravelled by rotating about their bonds when you pull them
What makes a more rigid polymer?
Lots of strong cross linking bonds
What was Rayleigh’s oil experiment and what did it find?
Calculating atomic size
Olive oil was realised one drop at a time into a tub of water until it just covered the entire surface, assuming it spreads as much as it could the thickness would be the size of a molecule of oil
If you know the surface area of the bath then thickness of a molecule = volume of oil dropped / surface area
Thickness of an atom is thickness of an oil molecule divided by no. atoms in an oil molecule
What methods do we now use for measuring size of atoms and atomic spacing?
X-ray crystallography
STM (Scanning tunneling microscopes)
SEM (Scanning electron microscopes) and AFM (Atomic force microscopes)
How does X-ray crystallography work?
Firing electrons at a sample and using their diffraction patterns to investigate atomic spacing and structure
How do Scanning Tunnelling Microscopes work?
They have a very fine tip which a voltage is applied to, electrons from the samples surface tunnel from the surface to the tip and cause a current to flow.
The tip is moved across the sample surface and the height of the tip is adjusted to keep the current constant so any small bumps or dips in the surface are detected.
An STM has such a fine resolution that individual atoms can be resolved and their size and spacing measured
How do Scanning Electron Microscopes and Atomic Force Microscopes work?
Measure atomic sizes by building up an atom by atom image of the surface on a computer screen. By knowing the magnification of the image on the computer screen and the size of the blobs representing each atom, atomic size can be calculated
