Fracture Under Stress Flashcards
3 factors which will determine whether a material will fail?
- Magnitude of applied load
- Rate of speed at which the load is applied
- Number of times that the load is applied
(in addition, wear and corrosion can lead to failure)
What is a tensile test?
A specimen is subject to tensile load using tensile testing equipment, and the load is usually increased gradually or in steps until fracture
Throughout the test the length of the specimen in measured to calculate the strain.
Using the calculated values of stress and strain, a stress-strain curve can be plotted
Name of the stress at fracture?
What is the value of this for aluminium?
Rupture strength
274.8MPa
Why is rupture strength less than ultimate strength?
Yield strength is where the material begins to fail and necking begins to occur.
Necking is when the cross section of begins to decrease, therefore the specimen appears to carry less stress
When will a material fracture/rupture?
When it is subject to a load greater than its ultimate strength
What type if material will necking occur in?
Ductile
2 types of fracture which can occur when a material is subject to a steady load?
Ductile fracture - when it ruptures after necking. e.g. aluminium
Brittle fracture - when it ruptures without necking, e.g. glass
What is a ductile fracture?
When a material ruptures after considerable plastic deformation e.g. handles of a plastic shopping bag
Mechanism of ductile fractures?
As the tensile load increases, microscopic voids begin to form at the centre of the bar, caused by separation of the metal at grain boundaries, or interfaces between metal grains and inclusions.
As local stress increases, the microscopic voids grow and connect, producing larger cavities
Eventually the metal-to-metal contact area within the bar is reduced so that it is unable to support the applied load and complete fracture occurs
3 characteristics of a ductile fracture?
What 2 mechanisms contribute to this?
Necking
Flat granulated central portion
Shear lip (which gives the fracture a cup & cone surface)
The tensile load, and shear deformation at a maximum angle of 45ᵒ contribute
If a ductile material has been exposed to fatigue loading (repeated loading & unloading) how will it fracture?
It will respond like a brittle material rather than ductile
What are brittle fractures?
What does the fracture site look like?
Materials which fracture in a brittle manner?
Fractures which occur suddenly without any appreciable plastic deformation (i.e. no necking or elongation)
The fracture site is flat, perpendicular to the load, and has a granular appearance.
It may also have a chevron pattern, caused by separate crack fronts fanning out from the origin of the crack
Occurs in glass, ceramics, concrete and high-strength metals
How can materials which normally undergo ductile fracture then undergo brittle fracture? (2)
- if the material has a notch or crack, the concentration of stress at its tip will result in rapid brittle fracture
- Decreasing temp and increasing strain rate by rapidly loading the material (e.g. impact loading)
An example of this is chips - if frozen can be snapped (brittle), if room temp/hot can be bent (ductile) - if exposed to fatigue loading (repeated loading & unloading)
What are stress concentrations?
What causes them?
Points at which the level of stress is greater than the average stress of the material
Stress concentrations are caused by any sudden changes in shape, such as notches or holes
What are stress trajectories?
What will they be like in uniform and irregular bars?
Lines in a diagram to show the position of stress concentrations. They may be thought of as the paths along which the internal forces and stresses act within a material
In a uniform bar, they will be a series of straight lines. In an irregular bar, they will be deflected to accommodate the shape, helping to indicate the points of stress concentration
Where is the weakest point of a structure?
What design feature is highly prone to stress concentrations?
The point of highest stress
Sharp changes in shape - when designing a structure these should be avoided
What is the process of fracture propagation?
Stress concentrations are highest at the tip of cracks and notches, and makes a fracture much more likely to develop at this point.
If it does so, it will spread (propagate) from the tip of the defect - which is fracture propagation
What features can be added to structures to halt fracture propagation, and why is this done?
Smooth holes - if the fracture spreads into them, the stress will no longer be concentrated at one sharp point and will reduce the likelihood of fracture propagation. However, it means the structure will be weaker due to loss of material.
This is done because all materials will contain microscopic defects, such as scratches, pores and cracks, which can develop into fractures
What is notch sensitivity?
How can it be measured?
The phenomenon of stress being concentrated at the ti[ of a crack or notch
It can be evaluated by comparing the energy absorbed by notched and unnotched specimens during a notch sensitivity test
What is impact loading?
A sudden intense blow
e.g. an aeroplane touching down creates an impact load on the landing gear much greater than the static weight of the plane. When parked, the constant static weight of the plane on the landing gear is a steady load
What can be used to test the impact load resistance of a structure?
What does it entail?
Charpy impact test
A heavy pendulum is released from a known height and strikes the specimen at the bottom of its trajectory, and if it breaks the specimen will continue to the peak of its swing
How to measure the energy absorbed by a specimen in a Charpy impact test?
Potential energy is dependent on height, therefore measuring the difference in height between the start and peak of the pendulum’s swing allows the potential energy lost to be calculated.
This will also give the work done, since PE=W (both in J so they are equivalent)
How does temperature change the behaviour of a material?
What does this mean in terms of yield strength, ultimate strength, elongation and energy absorbed in impact loading?
As temperature increases, the material changes from brittle to more ductile behaviour
- yield strength decreases
- ultimate strength decreases
- elongation increases
- energy absorbed increases
What is fracture due to fatigue loading?
Fracture caused by repeat loading of a load much lower than that required to break the object by impact loading
Both the number of loadings and the magnitude of loadings is important
What is catastrophic failure?
Failure due to a large load
Mechanism of fracture by fatigue loading?
A small load will not cause catastrophic failure but may cause the structure to fracture. The number of cycles required to cause fatigue failure will vary from a few to a few million depending on the amplitude of the applied load and properties of the material
When are aircraft wings subject to fatigue loading?
During take off and landing - this is why aeroplane lifespans are based on the number of take off and landings, rather than the flight hours clocked
2 biological devices which must have good resistance to fatigue loading?
Artificial heart valves (must withstand 30-40 million cycles per annum)
Artificial joints
Describe the appearance of a fatigue fracture?
There are 2 distinct regions:
The relatively smooth region has concentric markings, called conchoidal marks, which indicate the various positions at which the crack has stopped as it propagates intermittently through the component. The source of the load is usually in the middle of these markings.
The second region is either granular or fibrous in appearance:
- granular is produced by rapid brittle fracture that occurs when the remaining cross section can no longer support the applied load
- Fibrous is produced if the material undergoes ductile fracture
(in some materials the whole surface may have a granular appearance indicating only a very small fatigue fracture was necessary to produce failure)
Difference between fatigue fracture and fracture from steady load?
Fatigue - repeated application of a small load
Steady load - single large load applied over a period of time
What is corrosion?
Chemical reaction between a metal and its environment
What is wet corrosion?
Wet corrosion is in the presence of water, and is an electrochemical process involving an anode and cathode joined electrically by an aqueous electrolyte
Oxidation occurs at the anode and reduction occurs at the cathode
The metal gradually dissolves away into the solution, weakening the structure
When is corrosion particularly bad?
Corrosion is particularly bad when two different metals are joined together
How can corrosion lead to failure?
Imperfections on the surfaces of metals are prone to attack by corrosion - and once started, the corrosive process accelerates due to differences in oxygen concentrations. The imperfections quickly develops into a crevice as metal ions migrate away from the imperfections.
The crevice gives rise to stress concentrations in a loaded structure, which ultimately leads to fracture. Thus, corrosion can severely limit the fatigue life and ultimate strength of a material
How are metals for orthopaedic implants designed to resist corrosion?
There is an inert film, usually an oxide, covering the surface of the metal - this is called the passivation layer, and such metals are called passive metals
How can orthopaedic metals be corroded?
The junction of two components is prone to corrosion due to the slight relative movements, which results in a small crevice between them and potentially damaging the passivation layer - this is ripe for chemical attack.
2 groups of metals?
Ferrous - containing iron
Non-ferrous - e.g. aluminium
What are metal alloys?
Materials formed by adding various elements to a basic metal in order to improve the mechanical and corrosive properties.
e.g. steels are alloys of iron and carbon
Pure metals are rarely used to construct engineering structures
What are alloying elements?
Examples?
impurities deliberately added to steel to change its properties
examples include manganese, silicon, phosphorus, nickel, chromium, titanium and tungsten
How are steels classed?
Based on their carbon content by weight:
- wrought iron: <0.03% C as well as quantities of slag from the iron making process
- iron: <0.05% C
- carbon steels: 0.2-0.6% C
- cast iron: 2-4.5% C
What are stainless steels?
What
Steel alloys that contain 12%-18% chromium by weight (nickel and manganese also usually added)
The chromium gives corrosion-resistant properties and forms the protective oxide passivation layer that can only be broken down under extreme conditions
Examples of medical uses of stainless steel?
Cardiac pacemakers Limb prostheses Orthoses Acupuncture needles Dental devices
What may be used instead of steel or ferrous alloys for biomedical applications?
Non-ferrous metals or alloys, in particular titanium and titanium-based alloys are commonly used for heart valve components, joint replacement endoprostheses and fracture fixation plates
3 advantages of titanium-based alloys?
- Lower density (4500kgm⁻³) compared to steel (8000kgm⁻³)
- Higher strength-to-weight ratio compared to aluminium
- Excellent corrosion resistance
3 disadvantages of titanium-based alloys?
- high material cost compared to steel and aluminium
- high fabrication cost compared with aluminium and steel (difficult to machine)
- low Young’s Modulus (110GPa) compared with stainless steel (200GPa)
How can low Young’s Modulus of titanium-based alloys be overcome?
Using thicker sections, increasing stiffness, and thus taking advantage of the high strength-to-weight ratio
What are polymers?
Lightweight, corrosion-resistant, electrical insulators with relatively low tensile strengths, melting points and density (1000kgm⁻³ compared to steels 8000kgm⁻³)
What are polymers composed of?
Long chains of molecules (monomers)
The monomers are responsible for the characteristics properties of the polymer
Stress-Strain behaviours of polymers?
Modulus of elasticity/rigidity and strengths of polymers?
The stress-strain behaviour of polymers is generally non-linear and time-dependent - they exhibit both elastic and plastic behaviour.
Generally, the moduli and strengths of polymers are low in comparison with other materials. however, the actual properties of one polymer is very much dependent on the degree of crystallisation.
2 main subtypes of polymers?
Plastics
Elastomers
2 subtypes of plastics according to behaviour when heated?
Thermoplastics - display plastic behaviour at high temperatures. Their structure is quite stable when heated so they can be formed at high temperatures, cooled, then reheated/reformed without altering their behaviour. e.g. PMMA used as bone cement.
Thermosets - cannot be reformed, because a by-product is released during their formation at high temperatures, meaning they harden and set. This restricts their recyclability
What are elastomers?
Characteristic?
AKA rubbers - can deform enormous amounts without changing shape permanently. An ordinary rubber band may stretch to 7x its original length (700% strain) where as steel can only be considered perfectly elastic for strains of 0.1%
What are ceramics?
Hard, brittle crystalline materials with a high melting point, low electrical and thermal conductivity, good thermal stability and high compressive strength. They also do not creep.
Everyday examples of ceramics?
Extreme example of a ceramic and its properties?
Brick and glass
Diamond - very hard (resistant to localised plastic deformation) , and has the highest known Young’s modulus (1200GNm⁻¹). The crystal structure allows it to cleave along certain planes, meaning it is not particularly tough
How are ceramics used in orthopaedics and why?
For heads of hip prostheses
Their hardness means they are wear resistant and have low friction. They have low density, high compressive strength and good chemical stability.
However, low toughness means they cannot be used for the stem of a hip prosthesis as they would easily snap
What are composites?
Categories?
When 2 or more materials are joined to give a combination of properties that cannot be obtained from the original materials
3 categories:
- Particulate
- Fibre
- Laminar
What are particulate reinforced composites?
e.g.?
When particles of a hard, brittle material are dispersed within a softer, more ductile material
e.g. concrete is a mixture of gravel and cement
What are fibre-reinforced composites?
What does it change about the material? (4)
e.g.?
When fibres of a strong, stiff, brittle material are within a softer, more ductile material.
This improves strength fatigue resistance, stiffness and strength-to-weight ratio
Fibre-glass contains glass fibres within a polymer; concrete columns are reinforced with steel rods; wood is a natural example
What are laminar-reinforced composites?
These take several different forms.
A very thin coating may cover a material to improve corrosion resistance
Thicker layers may be laminated together to improve strength e.g. in plywood which has alternating layers of wood veneer with the grain directions running at right angles in successive layers
