Module 8 Corrosion and Failure Analysis Objective Six Flashcards
fractography
the study and analysis of fractured surfaces
Macroscopic examination
no magnification or low magnification optical microscope or magnifying lenses
Microscopic examination
involves high magnification using electron microscopy such as scanning electron microscopy and transmission electron microscopy
Metallographic examination
of cross-sections involves optical microscopy (100 to 1000X)
Macroscopic features: ductile fracture
- 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
microscopic features: ductile fracture
- 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
macroscopic features: brittle fractures
- 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
Cleavage fracture
most common mechanism of brittle fracture and are typical when there are very high strain rates
Intergranular fractures
follow grain boundaries that have weakened due to embrittling reasons.
Macroscopic features: fatigue failures
- 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
Microscopic features: fatigue failures
- 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
Macroscopic features: creep failure
- 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
Microscopic features: creep failure
- voids, cavities and cracking at grain boundaries
- cavities at grain boundary triple points
Adhesive wear
caused by the microwelding of surface asperities under load. sliding action the surface of the welded junction tears and wears
abrasive wear
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
Erosion
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
Fretting:
similar to adhesive wear, but occurs at stationary joints. Minute elastic deflections or vibrations with very small amplitudes cause micro welding and tearing
Surface fatigue
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
Cavitation
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
False brinnelling:
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
composite materials failure mechanisms
dependent on the type of fibres, type of matric and interface, also dependent on the type of loading applied to the composite
longitudinal tensile loading
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
Longitudinal compression loading
micro-buckling, results in kink bands with excessive deformation or fracture
Transverse tensile loading
high stress and strain concentration in the matrix and the interface.
local failure mechanisms include matrix cracking, interface disbonding and fibre splitting
Transverse compression load
- 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
shear loading
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
Multidirectional laminates
laminates may fail due to the seperation within an individual plies in the laminate (intralaminar failure) or separation of adjacent plies
Compression loading
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
Bending loading
initial failure may start in either compression or tension faces and may be caused by insufficient structure thickness, skin thickness or skin strength
