Fracture Under Stress

Question 1

3 factors which will determine whether a material will fail?

Answer 1

• 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)

Question 2

What is a tensile test?

Answer 2

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

Question 3

Name of the stress at fracture?

What is the value of this for aluminium?

Answer 3

Rupture strength

274.8MPa

Question 4

Why is rupture strength less than ultimate strength?

Answer 4

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

Question 5

When will a material fracture/rupture?

Answer 5

When it is subject to a load greater than its ultimate strength

Question 6

What type if material will necking occur in?

Answer 6

Ductile

Question 7

2 types of fracture which can occur when a material is subject to a steady load?

Answer 7

Ductile fracture - when it ruptures after necking. e.g. aluminium

Brittle fracture - when it ruptures without necking, e.g. glass

Question 8

What is a ductile fracture?

Answer 8

When a material ruptures after considerable plastic deformation e.g. handles of a plastic shopping bag

Question 9

Mechanism of ductile fractures?

Answer 9

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

Question 10

3 characteristics of a ductile fracture?

What 2 mechanisms contribute to this?

Answer 10

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

Question 11

If a ductile material has been exposed to fatigue loading (repeated loading & unloading) how will it fracture?

Answer 11

It will respond like a brittle material rather than ductile

Question 12

What are brittle fractures?
What does the fracture site look like?
Materials which fracture in a brittle manner?

Answer 12

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

Question 13

How can materials which normally undergo ductile fracture then undergo brittle fracture? (2)

Answer 13

• 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)

Question 14

What are stress concentrations?

What causes them?

Answer 14

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

Question 15

What are stress trajectories?

What will they be like in uniform and irregular bars?

Answer 15

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

Question 16

Where is the weakest point of a structure?

What design feature is highly prone to stress concentrations?

Answer 16

The point of highest stress

Sharp changes in shape - when designing a structure these should be avoided

Question 17

What is the process of fracture propagation?

Answer 17

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

Question 18

What features can be added to structures to halt fracture propagation, and why is this done?

Answer 18

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

Question 19

What is notch sensitivity?

How can it be measured?

Answer 19

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

Question 20

What is impact loading?

Answer 20

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

Question 21

What can be used to test the impact load resistance of a structure?
What does it entail?

Answer 21

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

Question 22

How to measure the energy absorbed by a specimen in a Charpy impact test?

Answer 22

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)

Question 23

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?

Answer 23

As temperature increases, the material changes from brittle to more ductile behaviour

• yield strength decreases
• ultimate strength decreases
• elongation increases
• energy absorbed increases

Question 24

What is fracture due to fatigue loading?

Answer 24

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

Question 25

What is catastrophic failure?

Answer 25

Failure due to a large load

Question 26

Mechanism of fracture by fatigue loading?

Answer 26

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

Question 27

When are aircraft wings subject to fatigue loading?

Answer 27

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

Question 28

2 biological devices which must have good resistance to fatigue loading?

Answer 28

Artificial heart valves (must withstand 30-40 million cycles per annum)

Artificial joints

Question 29

Describe the appearance of a fatigue fracture?

Answer 29

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)

Question 30

Difference between fatigue fracture and fracture from steady load?

Answer 30

Fatigue - repeated application of a small load

Steady load - single large load applied over a period of time

Question 31

What is corrosion?

Answer 31

Chemical reaction between a metal and its environment

Question 32

What is wet corrosion?

Answer 32

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

Question 33

When is corrosion particularly bad?

Answer 33

Corrosion is particularly bad when two different metals are joined together

Question 34

How can corrosion lead to failure?

Answer 34

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

Question 35

How are metals for orthopaedic implants designed to resist corrosion?

Answer 35

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

Question 36

How can orthopaedic metals be corroded?

Answer 36

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.

Question 37

2 groups of metals?

Answer 37

Ferrous - containing iron

Non-ferrous - e.g. aluminium

Question 38

What are metal alloys?

Answer 38

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

Question 39

What are alloying elements?

Examples?

Answer 39

impurities deliberately added to steel to change its properties

examples include manganese, silicon, phosphorus, nickel, chromium, titanium and tungsten

Question 40

How are steels classed?

Answer 40

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

Question 41

What are stainless steels?

What

Answer 41

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

Question 42

Examples of medical uses of stainless steel?

Answer 42

Cardiac pacemakers
Limb prostheses
Orthoses
Acupuncture needles
Dental devices

Question 43

What may be used instead of steel or ferrous alloys for biomedical applications?

Answer 43

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

Question 44

3 advantages of titanium-based alloys?

Answer 44

• Lower density (4500kgm⁻³) compared to steel (8000kgm⁻³)
• Higher strength-to-weight ratio compared to aluminium
• Excellent corrosion resistance

Question 45

3 disadvantages of titanium-based alloys?

Answer 45

• 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)

Question 46

How can low Young’s Modulus of titanium-based alloys be overcome?

Answer 46

Using thicker sections, increasing stiffness, and thus taking advantage of the high strength-to-weight ratio

Question 47

What are polymers?

Answer 47

Lightweight, corrosion-resistant, electrical insulators with relatively low tensile strengths, melting points and density (1000kgm⁻³ compared to steels 8000kgm⁻³)

Question 48

What are polymers composed of?

Answer 48

Long chains of molecules (monomers)

The monomers are responsible for the characteristics properties of the polymer

Question 49

Stress-Strain behaviours of polymers?

Modulus of elasticity/rigidity and strengths of polymers?

Answer 49

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.

Question 50

2 main subtypes of polymers?

Answer 50

Plastics

Elastomers

Question 51

2 subtypes of plastics according to behaviour when heated?

Answer 51

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

Question 52

What are elastomers?

Characteristic?

Answer 52

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%

Question 53

What are ceramics?

Answer 53

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.

Question 54

Everyday examples of ceramics?

Extreme example of a ceramic and its properties?

Answer 54

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

Question 55

How are ceramics used in orthopaedics and why?

Answer 55

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

Question 56

What are composites?

Categories?

Answer 56

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

Question 57

What are particulate reinforced composites?

e.g.?

Answer 57

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

Question 58

What are fibre-reinforced composites?
What does it change about the material? (4)
e.g.?

Answer 58

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

Question 59

What are laminar-reinforced composites?

Answer 59

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