Tests

Question 1

How is the hardness test carried out?

Answer 1

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

Question 2

Which hardness test uses a steel ball indentor?

Answer 2

Brinell test

Question 3

Which hardness test uses a conical indentor?

Answer 3

Rockwell

Question 4

Which hardness test uses a pyramid shaped indentor?

Answer 4

Vickers

Question 5

Which hardness test uses an elongated pyramid indentor?

Answer 5

Knoop

Question 6

What temperature is considered elevated enough to exacerbate creep?

Answer 6

0.4 melting point (in K)

Question 7

Why do turbine blades so commonly creep?

Answer 7

Operate at temps close to Tm; material of Ni-alloy is high density = great stress; centrifugal forces = great stress

Question 8

How does creep occur?

Answer 8

Movement of vacancies and grain boundaries

Question 9

What is the most creep-resistance way to form turbine blades?

Answer 9

Make a single crystal

Question 10

What is the least creep-resistant way to form turbine blades?

Answer 10

Chill cast forms a polycrystal

Question 11

What does directional solidification do?

Answer 11

Aligns columnar crystals in direction of loading, such that the grain boundaries are parallel to the centrifugal force

Question 12

What is creep tested?

Answer 12

Specimen held in furnace and deformation recorded, providing a characteristic creep curve

Question 13

What is primary creep?

Answer 13

Instantaneous deformation, followed by a short period of relatively fast deformation

Question 14

What is secondary creep?

Answer 14

Smaller constant rate of deformation; is the operating range of interest because is the period of known and predictable deformation

Question 15

What is tertiary creep?

Answer 15

Crack occur in grain boundaries; increasing rate of deformation; leads to failure

Question 16

What is fatigue?

Answer 16

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

Question 17

What is fatigue failure characterised by?

Answer 17

Concentric rings (from slow propagation), then rough area (from rapid propagation)

Question 18

Why is fatigue testing difficult?

Answer 18

Difficult to extrapolate into reality as fatigue loading is so variable; tests exhibit a lot of scatter; tests are lengthy

Question 19

How can fatigue failure be reduced?

Answer 19

Improve surface finish; eliminate internal defects; design modifications to reduce stress raisers; surface treatments

Question 20

What are surface treatments which can reduce fatigue failure?

Answer 20

Shot-peening or carburising; will impart compressive residual stresses to surface and reduce surface-related crack initiation