ME4255 NUS Failure Analysis

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1 GENERAL FAILURE ANALYSIS http://en.wikipedia.org/wiki/Failure_analysis#See_also

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Materials Failure

Transcript of ME4255 NUS Failure Analysis

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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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