Mechanics unit 5 - Fracture under stress deck 1 Flashcards
What are the 3 factors which are important in determining if a material will fail ?
- The magnitude of the applied load
- The rate of speed at which the load is applied
- The number of times the load is applied
This is a bit like fatigue fractures
What additional factor can lead to failure of a material ?
Wear and corrosion of the material
What test is usually used to produce a stress-strain curve of a material ?
A tensile test
Describe how a tensile test is done and how the strain and stress can therefore be calculated
During the test a gradual or stepwise increase in tensile load is applied to the material until the material fractures.
- The change in length in the material is measured throughout the test and divided by the original length to give the strain
- The stress can be calculated using the applied tensile load and the cross-sectional area of the material
A stress-strain curve can then be plotted using this information
On a stress-strain curve what is the stress at the fracture point called ?
The fracture/ ruputure strength
Describe the phenomenon of necking
This is where a materials cross-sectional area is reduced during loading such that the material appears to reduce in strength
Why is the maximum calculated strength of a material ultimate strength of a material higher than the rupture/ fracture strength of material?
Due to necking - the specimen appears to carry less stress when it is calculated using the original cross-sectional shape rather than the now true reduced cross-sectional shape of the material
==> a material will fracture when it is subjected to a load greater than its ultimate strength
From application of a steady load what are the 2 ways in which a material can fracture and state the key difference between them?
- Ductile fracture - fracture occurs with necking
- Brittle fracture - fracture occurs without necking
What do ductile fractures occur after ?
Considerable plastic deformation, characterised by necking
e.g. the handles of a shopping bag when overloaded will quickly stretch before breaking
Describe the specific stages of a ductile fracture
- Application of a tensile load
- Formation of microscopic voids (small holes) at the centre of the bar - due to high stress causing the separation of the metal at the grain boundaries
- As the stress increases the microscopic voids grow and connect with one another creating larger cavities
- Eventually the actual metal to metal contact area within the bar is reduced so much due to the microscopic voids that fracture occurs
What also contributes to a ductile fracture along with the applied tensile load ?
Deformation due to shearing
What are the 4 characteristic features of ductile fracture ?
- Necking
- Flat granulated central portion and Shear lip - giving the fracture a cup and cone appearance
How do brittle fractures occur ?
The occur without any plastic deformation (no necking or elongation)
What are the 3 key features characterising the appearance of a brittle fracture ?
- The fracture surface is flat and perpendicular to the applied fracturing load
- It has a granular appearance
- It has a chevron pattern
What scenarios may result in a ductile material undergoing a brittle fracture ?
- If it has been exposed to fatigue loading (repeated loading and unloading)
- If the material contains a crack or notch
- Decreasing the temperature and increasing the strain rate
Do SAQ 2&3&4
What are stress concentrations within a material and what can they be shown by?
They are points at which the level of stress is greater than the average level of stress within the material
They can be indicated using stress trajectories
What are high levels of stress indicated by within a material ?
A high concentration of stress trajectories
What are changes in shape such as notches or holes that result in stress concentrations known as ?
Stress risers
What do the points of highest stress within a material represent ?
The weakest points within a structure and therefore the points which are likeliest to fail
What sort of changes in shape should be avoided when desiging an object and why?
Sharp changes in shape - as the shaper the change the higher the stress concentration
Describe what fracture propagation is
This is the development of a fracture from the tip of a crack or notch:
- The stress concentration will be highest at the tip of the crack/ notch
- A fracture will then propagate/ spread from the tip
What can be built into a material to hault fracture propagation ?
Smooth holes can be built into the structure to blunt a propagating (spreading) crack so that the stress is no longer concentrated at one sharp point
Do SAQ 5
In stuff just covered we looked at steady loading, but now we will look at impact loading, define what this is and do SAQ 6
Impact load = a sudden and intense blow
What test is used to test a structures resistance to impact loading ?
Charpy impact test
Describe how the charpy impact test allows you to calculate a structures resistance to impact loads
- A heavy pendulum is released from known height
- It then strikes and breaks the test specimen and continues up to its peak swing
- The height reached at the end of the swing is lower than its original height because energy is absorbed as the specimen is fractured
- The height difference is measured and allows the change in potential energy (EP) to be calculated
- The change in EP is equal to the energy absorbed by the specimen called the impact energy
What type of material is tougher a ductile or brittle ?
A ductile as it can absorb more energy
What is the ability of a material to absorb energy dependent on ?
Temperature - as temp increases the materials ability to absorb energy increases as it changes from a brittle to more ductile material
Do SAQ 8
