Exam Questions Flashcards
Describe the behaviour of the metal at the atomic level from initial loading to point B. In your answer, explain why the stress at B is greater than A, and why the stress at D is less than C.
During elastic deformation, the bonds between atoms act like springs. However, in reality, interatomic forces pull the atoms out of their lattice.
At point A. The material starts to experience localised micro plasticity moving these dislocations from their Frank-Reed source. For these sources to exist, there needs to be an impurity or pre-existing network of dislocations (Frank network). The Frank-Reed source activates at angles of 45 deg where the resolved shear stress is greater than the critically resolved shear stress.
From point A to point B. Bulk deformation starts to occur. This is where multiple slip systems activate in multiple strains. The stress at B is greater than the stress at A due to more energy being required to create elastic deformation; therefore more repulsion and higher internal stress (work hardening) occurs.
The stress at point D is lower than the stress at point C due to the fact that the cross sectional area of the test piece at point D is smaller than the cross sectional area of the test piece at point C. As stress is a function of area, this causes the stress to be lower.
If one were to look at the specimen after faliure, what features might be expected on the fracture surface and why?
As seen from the engineering stress-strain graph. There is a large plastic region of the curve which indicates that the material is Ductile. As such, we will expect to see a cup and cone fracture, we would expect the surface of the failure to have a rough and fibrous surface in addition to seeing dimples and voids at a smaller scale.
Sketch an engineering stress-strain curve for a Brittle material. Describe 2 factors that determine a materials tendency to be brittle or ductile.
Crystal Structure: Affects the strength of individual cleavage planes within the material.
Grain Size: A material with a large grain size will have large regions for cleavage to occur over which induces fracture of the material.
Temperature: The hotter a material is, the more kinetic energy the atoms have which enables them to change planes more easily, making the material more ductile.
Describe a testing method that evaluates the ductile to brittle transition seen in some metals.
Impact testing can be used to evaluate the ductile to brittle transition seen in some metals.
Impact testing evaluates the toughness of a material by hitting it with a hammer and measuring the difference in potential energy that a freely swinging hammer would produce.
A brittle material will not impede the hammer and therefor a small change in kinetic energy will be observed.
Furnaces and cryo-chambers can be used to heat up or freeze the material prior to impact testing, and then impact testing can be used to evaluate the materials properties at different temperatures.
Define the primary material property that is used to establish a materials resistance to fracture or impact.
Fracture Toughness is the capacity of the material to absorb kinetic energy without fracture.
Materials with a high fracture toughness can absorb more energy before fracture than those with a low fracture toughness.
Fracture toughness can be expressed in fracture mechanics as the critical stress intensity factor at the tip of a crack that lead to fracture/irreversible crack growth or K1C.
Whilst fan blades require high fracture toughness values because of the likelihood of foreign object strikes, there is also a beneficial influence of high fracture toughness on resistance to crack growth. As such, all components that are predominantly subjected to fatigue loading would benefit from high fracture toughness. EG: discs, compressor blades, ect.
There are 2 methods of determining a materials fracture toughness. One is qualitative/comparative, and the other is quantitive and theoretical. Describe both approaches.
Qualitative - Impact testing.
Pendulum is swung at a Notched sample, the distance that the pendulum swings after impacting the sample will be affected by the amount of kinetic energy absorbed by the sample. These distances can be measured and converted into joules and these energies can be used to rank a range of materials in order of their fracture toughness. There are various specimen morphologies/hammer designs that can be used with a number of standard techniques (Charpy/Izod).
The test is useful as it can be performed at a range of temperatures and is simple. However results cannot be scaled for design purposes.
Quantitve - Implementation of fracture mechanics (monitoring the growth of a crack as increasing load) applied to the specimen. This had the advantage that fracture mechanics obeys similitude so that the data can be used for structural modelling/design purposes as well as providing quantitive comparable data. This added benefit comes a the cost of experimental complexity. A universal testing machine is required (but not always in a uniaxial configuration). This allows loads to be measured/applied.
The crack growth behaviour is predominately monitored by two methods. Crack opening displacement (where an extensometer is attached to the mouth of the crack) and direct current - potential difference monitoring (where a circuit is attached to the sample that monitors its increase in resistance as the crack propagates through the material. This method requires calibration as it has been shown to be exceptionally accurate at measuring the crack toughness values of materials.
Describe the difference in dislocation behaviour between a ductile and brittle material, and how this leads to very different appearance of ductile and brittle failures.
Macroscopic fracture toughness values are dictated by microscopic materials behaviour. In general, the ability of a material to absorb energy without fracture is linked to its ability to accommodate that energy through deformation and ultimately dislocation movement. In more ductile materials, the energy applied to the material is easily converted to deformation through ease of dislocation movement this detracts from the energy available to form new surfaces (Griffith Criterion) and so increase the amount of energy the material can absorb without fracture.
In materials where dislocation movement is more difficult, more of the imposed energy is used to create new surfaces and they fracture more readily. In brittle fracture (limited dislocation movement) materials to fracture across weak cleavage planes. Resulting in a flat or facetted fracture surface. In ductile materials, fracture predominately at the interface between ductile and brittle materials constituents and via a macroscopic shear. This results in a dimpled and uneven fracture surface.
