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GENERAL FAILURE ANALYSIS

http://en.wikipedia.org/wiki/Failure_analysis#See_also

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Non-destructive methods

General failure analysis procedures

Fracture surface detection and analysis

Mechanical Testing

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NONDESTRUCTIVE TESTING

Nondestructive testing (NDT) or Nondestructive evaluation (NDE) means to perform some evaluation on a piece of material or

structure to determine on a piece of material or structure to

determine if it contains any flaw that could affect serviceability.

The evaluation process must not destroy or alter the material or structure that is being assessed.

It is used on metals, plastics, ceramics, composites, cermets, and coatings.

It is used on standard shapes of materials as they come from their manufacture (rods, billets, flats, sheets, bars, and others)

It is used to detect cracks, internal voids, surface cavities, delamination, incomplete or defective welds any type of flaw that could lead to premature failure.

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COMMONLY USED NDT

TECHNIQUES

Technique Capability Limitations

Visual inspection Macroscopic

surface flaws

Small flaws are difficult to detect,

no subsurface flaws

Microscopy

(optical/electron)

Small surface

flaws

Not applicable to large structures;

no subsurface flaws

Radiography (x-

ray gamma rays)

Subsurface flaws Smallest defect detectable is 2%

of the thickness; radiation

protection needed

Dye Penetration Surface flaws No subsurface flaws, not for

porous materials

Ultrasonics Subsurface flaws Material must be good conductor

of sound

Magnetic particle Surface and near-

surface flaws

Limited subsurface capability;

only for ferromagnetic materials

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COMMONLY USED NDT

TECHNIQUES

Technique Capability Limitations

Eddy current Surface and near-

surface flaws

Difficult to interpret in some

application; only for metals.

Acoustic

emission

Can analyze entire

structures

Difficult to interpret; expensive

equipment

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FAILURE ANALYSIS

PROCEDURES

Collecting history and information related to the failure

Visual examination: low-magnification examination of the two fracture surfaces.

Stress analysis involved on the operations for which the components or devices were designed: some

experimental work may need to be done.

Metallographic evaluation of the material used and its heat treatment: to examine the microstructure of the

material used to manufacture the wire cutters.

Detailed fractographic examination: SEM fractography to examine the fracture surface by using Scanning

Electron Microscope (SEM).

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FAILURE ANALYSIS

PROCEDURES

Chemical analysis to determine whether any corrosion failure involved.

Characterize the properties, especially, mechanical properties of the materials used, sometime, it is need a

simple experiment in which cannot destroy the samples.

Hardness test is the most common used method for such

purpose.

Simulation of failure: sometimes looking for a similar component (made of the same batch, same material, same

process etc), let it run in the real conditions as the failed

sample, however, this may very expensive.

Conclusions to be made.

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HANDLING THE FRACTURE

PIECES

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HANDLING THE FRACTURE

PIECES

Fracture surface contains a wealth of information, it is important to understand the types of damage that can obscure or obliterate

fracture features and obstruct interpretation.

There are two types of damage, i.e., chemical and mechanical damage. These damage can occur during or after the fracture

event.

If damage occurs during the fracture event, very little can usually be done to done to minimize it. However, proper handling and

care of fractures can minimize damage that can occur after the

fracture.

Education in the proper handling of specimens prior to any fractography examination is strongly recommended for anyone

dealing in fractures either in the field or in the laboratory.

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VISUAL EXAMINATION

Schematic representation of

the information conveyed by

crack branching with regard

to the location of the crack

origin

Schematic representation of the

T-junction method of

determining which fracture

surface to search to locate the

crack origin. Because B does not

cross A but meets it at about

90, B occurred later and cannot contain the crack origin.

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Intergranular cracking

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IDENTIFY THE FRACTURE

ORIGIN

Locating the origin in an impact

fracture, produced by two hammer

blows, in a notched bar of 12% Cr

steel. Fracture origin can be found in

three ways: by tracing the radial marks

in the lower portion of the fracture to

their point of convergence (the arrows

on the curved lines indicate the

direction of crack propagation); by

drawing normals to the crack-arrest

fronts labeled A and B; and by

projecting the tangents to the final

radial marks at C and D toward the

bottom. The crack came to a full stop

at B with the first hammer blow and

resumed motion at the second hammer

blow. Light fractograph. 3

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EXAMINE FRACTURE BY SEM

The principal categories of fracture features are:

Cleavage features

Quasicleavage features

Dimples from microvoid coalescence

Tear ridges

Fatigue striations

Separated-grain facets (intergranular fracture)

Mixed fracture features, include mixture of above features

Features of fractures resulting from chemical and thermal

environments.

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EXAMINE FRACTURE BY SEM

Cleavage fracture in a notched impact specimen of hot-rolled 1040 steel broken at

-196 C (-321 F), shown at three magnifications. The specimen was tilted in the scanning electron microscope at an angle of 40 to the electron beam. The cleavage planes followed by the crack show various alignments, as influenced by

the orientations of the individual grains. Grain A, at the center in fractograph (a),

shows two sets of tongues (see arrowheads in fractograph b) as the result of local

cleavage along the {112} planes of microtwins created by plastic deformation at

the tip of the main crack on {100} planes. Grain B and many other facets show

the cleavage steps of river patterns. The junctions of the steps point in the

direction of crack propagation from grain A through grain B, at an angle of about

22 to the horizontal plane. The details of these forks are clear in fractograph (c).

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EXAMINE FRACTURE BY SEM

Dimples and cleavage facets exhibited in three aspects of a Charpy impact fracture at

room temperature in a specimen of hot-rolled 1040 steel, tilted in the scanning

electron microscope at an angle of 30 to the electron beam. The machined notch of the specimen was below the region shown in (a). The overall direction of crack

propagation was upward. Although equiaxed dimples pre-dominate, certain grain

orientations near the top of (a) were unfavorable for ductile fracture by microvoid

coalescence and local cleavage occurred, as shown in detail in (b), which is a higher-

magnification view of the outlined area in (a). Fractograph (c), a higher-magnification

view of the region at the center of (a), shows a deep dimple, which initiated the local

ductile fracture immediately surrounding it. The smooth surface at A shows no river

patterns and should not be identified as a cleavage facet; it could be a grain-boundary

surface, or perhaps a region of stretching.

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EXAMINE FRACTURE BY SEM

Intergranular brittle fractures in tungsten, iridium, and a tungsten-3 wt% rhenium

alloy. (a) Sintered tungsten rod drawn to 1.5 mm (0.060 in.) diam, recrystallized for

100 h at 10-6 torr and 2600 C (4712 F), and fractured in tension. (b) Iridium sheet annealed for 50 h in purified helium at 1700 C (3092 F) and broken by bending. (c) Tungsten-3 wt% rhenium alloy that was prepared in the same manner as the

sintered tungsten rod in fractograph (a). Microvoids ("bubbles") at grain boundaries

resulted from segregation of potassium (an impurity).

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Materials Failure Analysis

Mechanical Physical Chemical

Static, dynamic,

strength of the

Materials, etc

Physical property

Thermo-condition

Fluid exchange

Chemistry, fluid

mechanics,

thermodynamics

English

Report Social Knowledge Engineering Ethics

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Tensile test

Bending test Shear test Compressive test

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Mechanical Testing

Fatigue

Creep

Friction and wear

Fracture toughness

Corrosion

etc.

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