Module 8 Corrosion and Failure Analysis Objective Six

Question 1

fractography

Answer 1

the study and analysis of fractured surfaces

Question 2

Macroscopic examination

Answer 2

no magnification or low magnification optical microscope or magnifying lenses

Question 3

Microscopic examination

Answer 3

involves high magnification using electron microscopy such as scanning electron microscopy and transmission electron microscopy

Question 4

Metallographic examination

Answer 4

of cross-sections involves optical microscopy (100 to 1000X)

Question 5

Macroscopic features: ductile fracture

Answer 5

• significant gross plasticity
• fractured surface appears dull and fibrous
• for tensile uniaxial loading: necking indicates gross plasticity, a tensile cup-and-cone fracture, max shear stress planes are at 45 deg from axial direction, after necking state of stress changes from uniaxial to triaxial with max stress at the center

Question 6

microscopic features: ductile fracture

Answer 6

• starts with a microvoid formation at the weakest areas, or areas of minute imperfections or inclusions
• microvoids coalesce until fracture occurs. the fractures have approximately half of each cavity on each side of the fracture surface

Question 7

macroscopic features: brittle fractures

Answer 7

• lack of gross plastic deformation
• fracture surface may be granular, sparkling and shiny
• fracture surface is perpendicular to the principle tensile stress
• chevron marks in flat bars or plates and in the outer hardened layer for case hardened components
• radial lines converging to the origin in circular and large section components

Question 8

Cleavage fracture

Answer 8

most common mechanism of brittle fracture and are typical when there are very high strain rates

Question 9

Intergranular fractures

Answer 9

follow grain boundaries that have weakened due to embrittling reasons.

Question 10

Macroscopic features: fatigue failures

Answer 10

• crack origin visible at low magnification
• lack of deformation
• beach marks, arrest marks or conchoidal marks are the characteristic features usually found on fatigue fracture surfaces
• ratchet marks are usually perpendicular to the surface from which the fatigue fracture originates
• the final fracture region is often fibrous

Question 11

Microscopic features: fatigue failures

Answer 11

• metallographic examination shows transgranular crack propagation
• fatigue striations are finely spaced parallel marks that increase in spacing as they progress from the origin of fatigue

Question 12

Macroscopic features: creep failure

Answer 12

• fracture is usually ductile, but alloy dependent, may show less ductility then a simple overload failure
• there will be creep fissures
• there will be oxidation and scaling

Question 13

Microscopic features: creep failure

Answer 13

• voids, cavities and cracking at grain boundaries

- cavities at grain boundary triple points

Question 14

Adhesive wear

Answer 14

caused by the microwelding of surface asperities under load. sliding action the surface of the welded junction tears and wears

Question 15

abrasive wear

Answer 15

caused when a harder surface applies a series of scratches in a softer material in the direction of rubbing due to the high relative hardness between two rubbing surfaces

Question 16

Erosion

Answer 16

occurs when liquid or solid particles in a moving fluid hit a surface at high kinetic energy. over time a significant amount of material is removed from the surface

Question 17

Fretting:

Answer 17

similar to adhesive wear, but occurs at stationary joints. Minute elastic deflections or vibrations with very small amplitudes cause micro welding and tearing

Question 18

Surface fatigue

Answer 18

wear occurs where contact stresses are high, like gears & ball bearings. High point stress cause local flexing which initiates subsurface cracks and some surface cracks are initiated

Question 19

Cavitation

Answer 19

may occur in pumps, and other aquatic uses. Pressure drops when a liquid increases in velocity, if pressure drops below vapour pressure of the liquid, it will boil and bubbles are created. after the orifice, the velocity slows and pressure increases causing bubbles to implode with a shockwave that causes pits to form

Question 20

False brinnelling:

Answer 20

damage caused by fretting that causes imprints that look similar to brinnelling, but are caused by a different mechanism, caused by wear from vibrations and light loads

Question 21

composite materials failure mechanisms

Answer 21

dependent on the type of fibres, type of matric and interface, also dependent on the type of loading applied to the composite

Question 22

longitudinal tensile loading

Answer 22

a fibre fractures when the stress in it reaches UTS, 3 failure mechanisms

• matric cracking perpendicular to loading direction in brittle matrix and strong interface condition
• separation between the fiber and matrix if the interface is weak
• conical shear cracking in the matrix for strong interface bond and ductile matrix

Question 23

Longitudinal compression loading

Answer 23

micro-buckling, results in kink bands with excessive deformation or fracture

Question 24

Transverse tensile loading

Answer 24

high stress and strain concentration in the matrix and the interface.
local failure mechanisms include matrix cracking, interface disbonding and fibre splitting

Question 25

Transverse compression load

Answer 25

• high compressive stress at the interface may result in compressive or shear failure in the matrix or fibre crushing
• high shear stress at the interface may cause disbonding or matrix shear failure, leading to overall shear failure mode

Question 26

shear loading

Answer 26

in-plane shear loading results in a high stress concentration at the fibre matrix interface which leads to shear failure within the matrix or the interface may disbond

Question 27

Multidirectional laminates

Answer 27

laminates may fail due to the seperation within an individual plies in the laminate (intralaminar failure) or separation of adjacent plies

Question 28

Compression loading

Answer 28

may lead to general buckling if the load exceeds a critical value, at which point the structure loses its stability. may lead to shear crimping of the structure

Question 29

Bending loading

Answer 29

initial failure may start in either compression or tension faces and may be caused by insufficient structure thickness, skin thickness or skin strength