Tests Flashcards
How is the hardness test carried out?
Indentor forces into surface of material until it reaches equilibrium depth; diameter of impression is measured and related to an arbitrary hardness scale; material must plastically deform and flow away from moving indentor
Which hardness test uses a steel ball indentor?
Brinell test
Which hardness test uses a conical indentor?
Rockwell
Which hardness test uses a pyramid shaped indentor?
Vickers
Which hardness test uses an elongated pyramid indentor?
Knoop
What temperature is considered elevated enough to exacerbate creep?
0.4 melting point (in K)
Why do turbine blades so commonly creep?
Operate at temps close to Tm; material of Ni-alloy is high density = great stress; centrifugal forces = great stress
How does creep occur?
Movement of vacancies and grain boundaries
What is the most creep-resistance way to form turbine blades?
Make a single crystal
What is the least creep-resistant way to form turbine blades?
Chill cast forms a polycrystal
What does directional solidification do?
Aligns columnar crystals in direction of loading, such that the grain boundaries are parallel to the centrifugal force
What is creep tested?
Specimen held in furnace and deformation recorded, providing a characteristic creep curve
What is primary creep?
Instantaneous deformation, followed by a short period of relatively fast deformation
What is secondary creep?
Smaller constant rate of deformation; is the operating range of interest because is the period of known and predictable deformation
What is tertiary creep?
Crack occur in grain boundaries; increasing rate of deformation; leads to failure
What is fatigue?
Occurs due to cyclic loading with a stress below the normal failure stress of a material; originates from surface flaw; concentration of applied stress occurs at tip of flaw; crack initiates; crack grows with each application of load; when CSA is less than the area which can support the load, the crack rapidly propagates across the section; sudden failure
What is fatigue failure characterised by?
Concentric rings (from slow propagation), then rough area (from rapid propagation)
Why is fatigue testing difficult?
Difficult to extrapolate into reality as fatigue loading is so variable; tests exhibit a lot of scatter; tests are lengthy
How can fatigue failure be reduced?
Improve surface finish; eliminate internal defects; design modifications to reduce stress raisers; surface treatments
What are surface treatments which can reduce fatigue failure?
Shot-peening or carburising; will impart compressive residual stresses to surface and reduce surface-related crack initiation
