Mechanical testing Flashcards
What type of test is used for brittle materials and why?
3 point bend test clamping would damage the specimen reducing the measured specimens
Define stiffness
How much a material deflects under a given load
Define strength
a measure of how much force is needed to permanently deform or break a material
Define toughness
the ability of a material to resist fracture or to withstand impact
Define hardness
the ability of a material to resist local plastic deformation
Define true stress
It is defined as the force divided by the instantaneous cross sectional area true strain differs due to the change in cross sectional area of the specimen during the test
True strain equation
equation
True stress and true strain graph
Magneto- striction
materials strained by magnetic fields
Piezo-electric materials(uses)
materials which respond to electric fields Uses: piezoelectric actuators fuel injectors tyre pressure sensors engine knock sensors keyless door entry
Thermal strain
Thermal strain can cause stress in a constrained object
Isotropic mechanical properties
properties the same in the each direction regardless of the load applied
Number of properties of composites can be understood using the rule of mixtures. The density is given by…
equation
Electrical conductivity of a fibre reinforced composite along the fibres
equation
Thermal conductivity
equation
Modulus of elasticity along the fibres (Rule of mixtures)
equation the equation generally overestimates the yield strength as the matrix will not be fully extended when the fibres fail
Modulus of elasticity perpendicular to the fibres
equation
Define cohesive energy
It is defined as the energy per atom when solid bonds together
As the cohesive energy increases, the bond strength between the atoms increases
Cohesive energy is a quadratic around the minimum
Stress caused by an atom when a force is applied
equation
Types of point defects
Interstitial point defects
an extra atom is wedged into the crystal causing structural expansion
Substitutional point defects
a different atom i aadded into the crystal replacing an original atom
Self-interstitial point defects
an atom from the crystal jumps from its original position to elsewhere-remains close to a vacancy
Calculating number of vacancies
equation
Why are vacancies present?
they are present due to the increase in entropy of the system there more different ways in which the energy is distributed therefore the free energy of the system decreases
Hume-Rothery rules
Alloying elements will only form solid solutions if:
the atomic radii of the atoms differ by no more than 15% the atoms have
- similar electronegativity
- similar crystal structures
Number of self interstitial defects
equation
Difference between the interstitial and vacancy formation energy
Interstitial formation energies are generally 2-5x bigger than the vacancy formation energy
FCC interstitial sites
4 octahedral interstitial sites per fcc unit cell 8 tetrahedral interstitial sites per fcc unit cell
BCC interstitial sites
6 octahedral interstitial sites per bcc unit cell 12 tetrahedral interstitial sites per bcc unit cell
Types of dislocations
edge screw dislocations are characterised by the Burgers vector
Edge dislocation
additional half plane of atoms
Screw dislocation
when a perfect crystal is cut and sheared
Define a slip
process by which a dislocation moves
Define a slip plane
plane defined by the dislocation line and Burgers vector the plane on which the dislocation moves
Peierls-Nabarro stress
the stress required to move the dislocation
the stress which resists slip
Dislocation density
the length of the dislocation line contained in a volume of material
A dislocation will always want to be as short as possible.
The line tension associated with a unit length of the dislocation line is given by…
equation
Number of obstacles touching per unit length of the dislocation line
equation
Pinning force
equation
Different ways of strengthening a material
solution hardening precipitation hardening work hardening grain boundaries
Total stress required to move a dislocation
Solution hardening
isolated substantial atoms in a lattice the solute atoms pin the dislocation line increasing the force that needs to overcome to shear the crystal
Solid solution strengthening
equation
Precipitation hardening
occurs when strong particles are dispersed in the path of dislocations particles much larger obstacles than solute atoms
Precipitation strengthening
Work hardening
it occurs when a dislocation line is stuck between dislocation lines in different directions dislocation lines intersect with the plance along which the slip occurs there is a jump in the dislocation line when it passes through
Work strengthening
Averaging spacing between dislocations
equation
Pining force of each dislocation that is cut through
equation
Grain boundaries
occurs when two crystalline regions meet at different orientation
Grain boundary strengthening
How is yield strength measured?
it is measured when tension is applied to a specimen and measurements are taken when plastic deformation starts this depends on the inclination of the slip planes relative to the applied load
How does increased yield strength affect ductility?
increasing the yield strength of a metal results in the reduction of the ductility therefore the maximum elongation is reduced
Define proof stress
It is a stress needed to cause a small amount of plastic deformation it corresponds to 0.1-0.2% of plastic strain used
What happens when the temperature of a polymer is less than 0.75 glass transition temperature?
it will be brittle
Work hardening stress
equation
What happens when the temperature of a polymer is greater than 0.75 glass transition temperature?
the polymer chains realign under tension this process is known as drawing the drawn object is stronger and stiffer than before due to the alignment, by a factor of about 8
Define crazing
it occurs when a polymer is at too low of a temperature small cracks form over where the polymer stretches this leads to light scattering and reduced transparency
Additives in polymers
they are used to improve strength and ease of processing
Plasticisers in polymers
they are added to lower glass transition temperature and they consist of low molecular weight polymers
Reinforcements in polymers
they are added to polymers to improve their strength and are usually some form of filament
Define delomation
It is the separation of layers and it is difficult to detect
During a bend test, how is the flexural strength calculated?
equation
Identify the different methods for hardness testing
vickers method rockwell method
Benefits of hardness testing
a smaller sample is used than in tensile testing the specimen requires no prior preparation it is a non-destructive test
Define hardness
It is defined as force over an area
How is vickers hardness calculated
equation It is very useful as it correlates well with wear resistance
Impact tests
they measure the response of a material to a very high rate of deformation impact specimens are notched as to control where and how they break
How do impact tests work?
they work by releasing a hammer from a known height the hammer breaks the specimen and the final height of the hammer is measured
Impact energy
it is measured using the conservation of energy
Define fracture toughness
It is a measure off the ability of a material containing a flaw to withstand an applied load
How is fracture toughness measured?
a specimen is prepared with a flaw of known size and geometry and then a load is applied
Stress around the crack tip is higher than in the rest of the specimen
How is the local stress calculated?
equation
Stress near the tip
Mode 1 stress intensity factor
if the stress intensity factor is above a critical value, the crack propagates
Fracture toughness calculation
equation
How are microvoids formed?
they are usually formed when ductile metals are under stress they form at grain boundaries or at boundaries with inclusions under stress, the formation of cracks at flaws leads to the failure of brittle metals the crack propagates along the lattice planes leading to characteristic fracture surfaces(often flat with separated cracks visible)
How do ceramics and glasses fail?
Brittle fracture ceramics fracture along their lattice planes leading to smooth flat fracture surfaces glasses are amorphous and therefore a conchoidal fracture structure is observed
Probability of failure is governed by Weibull statistics
equation
Polymers fail through either ductile or brittle mechanisms What happens below and above the glass transition temperature
Below thermoplastics fail through brittle fracture due to the amorphous structure, sane as thermoset polymers above thermoplastic polymers fail through ductile mechanism due to the sliding of the polymer chains deformation can be observed before failure
Define fatigue
It is a reduction in strength or failure of a material due to repeated application of stresses
3 stages of fatigue
small cracks occur near the surface close to scratches, pitting, grain boundaries and inclusions localised stress near the crack pit is higher than in the rest of the material. the crack propagates at each loading cycle crack eventually propagates through the whole material and it fractures
Fatigue tests
a load is applied to the end of a bar and then rotated
Endurance limit
It exists for most materials below which fatigue will not occur regardless of the number of cycles
no endurance limit = no safe stress below which lack of failure can be guaranteed graph
Define fatigue strength
it is the maximum stress for which fatigue will not occur for a given number of cycles
Define fatigue life
It is the lifetime of a component for a given stress
Mean stress a component experiences
equation
Stress amplitude a component experiences
equation
Goodman relation
equation
Define creep
The process at which material at a high temperature can fail even when the load is below the yield stress
Which factors contribute to creep?
diffusion dislocation movement grain boundary movement
Define stress rupture
when a material breaks after creep
Creep performance of materials
It is measured by applying a load to a material in a furnace and monitoring its deflection over a long period of time
Brittle stress rupture
voids can form at the intersections of grain boundaries and diffuse along the boundaries
Ductile stress rupture
necking and many cracks don’t grow to fracture
Strain rate of the material
equation
