Metal Fracture, fatigue and creep

Question 1

What are the two types of failure?

Answer 1

Ductile and Brittle

Question 2

What are the causes of failure?

Answer 2

Tension, compression, shear, torsion, heat & pressure

Question 3

Describe an indication of ductile fracture in metals.

Answer 3

A material that experiences a large amount of plastic deformation. In the case of metals, necking occurs in the gauge area.

Question 4

What is the name given to a ductile fracture surface?

Answer 4

Cup and cone fracture.

Question 5

Describe a Brittle fracture

Answer 5

A fracture that occurs after very little stretching/plastic deformation. Sudden fracture without any warning. Cracks propagate perpendicular to the surface.

Question 6

Where to cracks propagate from?

Answer 6

Through the weakest part of materials, this is the grains.

Question 7

What is Intergranular Fracture?

Answer 7

Cracks that propagate along grain boundaries.

Question 8

What is transgranular fracture?

Answer 8

Cracks that propagate through grains.

Question 9

What are fracture mechanics? And what is this based on?

Answer 9

It is a concept based on the fact that materials have flaws or defects. These are called stress concentrations.

Question 10

How can stress concentrations be induced?

Answer 10

Geometry, voids, inclusions and imperfections

Question 11

The sharper the crack tip…..

Answer 11

The higher the stress magnification

Question 12

The duller the tip…….

Answer 12

The lower the stress magnification.

Question 13

What are two impact tests we can use and what do they measure?

Answer 13

Charpy and Izod. they measure the impact energy.

Question 14

what is fatigue?

Answer 14

a form of failure that occurs in structures subjected to dynamic and fluctuating stresses. usually under loads well below that of tensile strengths.

Question 15

what does the S-N curve measure?

Answer 15

Stress vs the log of the number of cycles before failure.

Question 16

what is the fatigue limit?

Answer 16

the fatigue limit is where the S-N Curve becomes straight. below this line, failure will not occur. it represents the largest value of fluctuating stress that will not cause failure for essentially and infinite number of cycles.

Question 17

what is creep?

Answer 17

time dependent and permanent deformation of materials when subjected to a constant load or stress.

Question 18

what are the 3 stages of creep? this can be shown on the generic creep curve

Answer 18

primary, secondary, tertiary

Question 19

dimples surface fracture indicates a

Answer 19

ductile fracture

Question 20

the process used to create compressive residual stresses in a material so that fatigue is delayed is called

Answer 20

shot peening.

Question 21

for metals, creep deformation can only occur if the load on a material exceeds the materials yield strength.

Answer 21

false

Question 22

charpy impact testing is used to measure

Answer 22

fracture toughness.