Fractographic-examination.

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Fractographic-examination, the study of Fracture Appearance, is an important tool in failure analysis and in the research of fracture causes for bodies of all kinds of materials.

The Fractographic-examination has been applied to study the fracture of prehistoric bones but also to investigations of the sudden fracture of Liberty Ships during the second World War.

Fractographic-examination is The Study of Fractured Surfaces

This discipline, referring to different materials, is called Fractography.

Besides for failure analysis it is used also in research to study and evaluate theoretical models of crack growth behavior.

Fractographic-examination helps to determine the cause of failure by characterizing features and patterns of a fractured surface.

Different types of crack propagation leave characteristic marks on the surface, interpreted to identify failure modes like fatigue, stress corrosion, hydrogen embrittlement.

The appearance of the fracture, when correctly interpreted, provides clues related to the type of failure undergone by the part.

Together with additional information achieved by different means, it can point to the underlying reasons for the separation of the body in two or more parts.

In Fractographic-examination, the observation of the fractured surface features under suitable light, is best carried out visually or through low power enlargement.

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For the record, suitable photographic pictures should be taken, caring that telltale features be readily observable.

But first one should care that the surface to be observed by Fractographic-examination be kept intact, not damaged or obliterated by intervening corrosion or by careless manipulation like trying to match back the two separated stumps.

By so doing the wear of the surfaces against each other might cancel invaluable signs that could have been conclusive in the following investigation.

Considering the fracture of a regular round specimen subjected to tensile testing, three zone of different appearance can readily be observed.

The central part of the ruptured specimen is called fibrous zone characterized by slow crack growth.

This zone is generally followed by a radial zone, characterized by rapid crack propagation, leaving radial marks converging back toward the origin of the cracks.

Then the annular shear-lip zone occurs, near the specimen external surface, where definite plastic deformation is readily seen.

Depending on the material, hardness condition, geometry and testing temperature, the various zones may be more or less evident. The shapes and the extent of each zone depend on the local relative toughness.

The Fractographic-examination shows features related to the ductility or to the brittleness of different parts of the fracture.

Ductile modes of fracture are accompanied by plastic deformation as readily seen in the shear-lip part of a ruptured specimen.

Brittle modes show limited or nil deformation near the fractured surface.

A typical pattern of fracture is generated by fatigue, consisting in the progressive extension of cracks under the action of cyclic loads.

Surface marks consisting in generally concentric partial rings, typical of fatigue fractures, are called beach marks.

Other features connected to fatigue are called striations and ratchet marks.

The grains characteristic of metallic crystal structure are seldom visible on a fractured surface.

The exception is observable in specimens tensile tested at elevated temperature.

In that case the failure is generally intergranular so that the separation happens at the grain boundaries, clearly defining them even at low magnification.

Fractographic-examination may readily show inclusions, blow-holes and other discontinuities responsible for the weak path followed by the fracture under stress.

Differentiation among the different types of such defects may need some experience with similar patterns.

The first screening is performed visually, with photographic evidence being taken for the record.

Additional and more precise determinations to complete Fractographic-examination, can be done at higher magnification with electron microscopes.

This is best conducted by professional of this type of research with knowledge and experience in specific kinds of failures.

An Article on Asset Integrity Management was published (11) in Issue 140 of Practical Welding Letter for April 2015.
Click on PWL#140 to see it.

An Article on Example of a Failure Analysis was published (7) in Issue 147 of Practical Welding Letter for November 2015.
Click on PWL#147.

If you wish to get an idea of a Scanning Electron Microscope workings, watch the 6 Videos pointed to hereafter.

To learn and to compare the appearance of fractured surfaces as seen by Fractographic-examination one can use Atlases of Fractographs.

These are collections of pictures where the characteristic features are shown and explained, with additional information on known material and condition.

One such Atlas is Volume 12 of ASM International Handbook.

ASM Handbook Volume 12: Fractography
ASM International / 01-Mar-1987 / 517 pages

See also Fractography of Metals and Plastics (7 pages)
http://www.asminternational.org/pdf/Fract_PFA.pdf

Watch the following Video on

Scanning Electron Microscope: Pt 1 of 6

http://www.youtube.com/watch?v=c7EVTnVHN-s

The other parts can be viewed as follows:

Scanning Electron Microscope: Pt 2 of 6, at
http://www.youtube.com/watch?v=SaaVaILUObg

Scanning Electron Microscope: Pt 3 of 6, at
http://www.youtube.com/watch?v=CNIrvGRXugU

Scanning Electron Microscope: Pt 4 of 6, at
http://www.youtube.com/watch?v=NP5VckJfv04

Scanning Electron Microscope: Pt 5 of 6, at
http://www.youtube.com/watch?v=04Fvq5HWebA

Scanning Electron Microscope: Pt 6 of 6, at
http://www.youtube.com/watch?v=pu_pMYMYBlw

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