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Visual Examination and Light Microscopy ASM Handbook, Volume 12: Fractography Copyright © 1987 ASM International® ASM Handbook Committee, p 91-165 All rights reserved. DOI: 10.1361/asmhba0001834 www.asminternational.org Visual Examination and Light Microscopy George F. Vander Voort, Carpenter Technology Corporation THE VISUAL EXAMINATION of fractures by light microscopy. Interesting features can be sections, to determine the origin of the failure, is deeply rooted in the history of metals pro- marked with a scribe, microhardness indents, and to separate the fractures according to the duction and usage, as discussed in the article or a felt-tip pen and then examined by SEM time sequence of failure, that is, which frac- "History of Fractography" in this Volume. and other procedures, such as energy- tures existed before the event versus which ones This important subject, referred to as macro- dispersive x-ray analysis, as required. occurred during the event. This article will fractography, or the examination of fracture The techniques and procedures for the visual assume that such work has already been accom- surfaces with the unaided human eye or at low and light microscopic examination of fracture plished and will concentrate on fracture exam- magnifications (--<50), is the cornerstone of surfaces will be described and illustrated in this ination and interpretation. Other related topics failure analysis. In addition, a number of qual- article. Results will also be compared and specific to failure analyses are discussed in ity control procedures rely on visual fracture contrasted with those produced by electron Volume 11 of the 9th Edition of Metals Hand- examinations. For failure analysis, visual in- metallographic methods, primarily SEM. Vi- book. spection is performed to gain an overall under- sual, light microscopic, and electron micro- standing of the fracture, to determine the frac- scopic methods are complementary; each has ture sequence, to locate the fracture origin or particular advantages and disadvantages. Opti- Macroscopic Examination origins, and to detect any macroscopic features mum results are obtained when the appropriate Locating the fracture origin is a primary goal relevant to fracture initiation or propagation. techniques are systematically applied. of fractography and is vital to successful failure For quality control purposes, the fracture fea- In addition to examination of the gross frac- analyses. The fracture markings formed during tures are correlated to processing variables. In ture face, it is often useful to examine second- the event are like a road map that the analyst this article, examples of visual fracture exami- ary cracks, when present. In many cases, the uses to evaluate the fracture. Fracture initiation nation will be given to illustrate the procedure secondary cracks exhibit less damage than the and propagation produce certain characteristic as it applies to failure analysis and quality gross fracture, which may often be damaged by marks on the fracture face, such as river marks, determination. rubbing, handling, postfracture events (such as radial lines, chevrons, or beach marks, that Although the light (optical) microscope can repair attempts), or fire or corrosion. These fine indicate the direction of crack growth. The be used to examine fracture surfaces, most cracks can be opened by careful sectioning and analyst traces these features backward to find fracture examinations at magnifications above breaking, or they can be examined on polished the origin or origins. The appearance of these 50x (microfractography) are conducted with cross sections. Also, it is often informative to marks on the fracture face is a function of the the scanning electron microscope, as described examine the microstructure ahead of the sec- type of loading, for example, tension, shear, in the article "Scanning Electron Microscopy" ondary cracks, or adjacent to the primary frac- bending, fatigue, or torsion; the nature of the in this Volume. However, if a scanning elec- ture, to detect cracking in constituents or mi- stress system; its magnitude and orientation; the tron microscope is not available, light micros- crovoids, either pre-existing or produced by the presence of stress concentrators; environmental copy can be applied and usually provides sat- deformation associated with crack formation. factors; and material factors. Examples of these isfactory results. In this article, details will also be presented fracture patterns are illustrated in the section Regardless of the equipment available to the concerning the characteristic macro- and micro- "Interpretation of Fractures" in this article and analyst, it is still very useful to examine the scopic features associated with different frac- in the article "Modes of Fracture" in this fracture profile on a section perpendicular to ture mechanisms. These features are detected Volume. the fracture origin. In this way, the origin of and used to characterize the nature of both After a service failure has occurred or after a the fracture can be examined to determine if crack initiation and propagation. Examples will crack has been observed in a component, cer- important microstructural abnormalities are be presented, using a variety of techniques, to tain steps must be taken to ensure that the present that either caused or contributed to illustrate the procedure for classifying frac- fracture features are not obliterated (Ref 1). In fracture initiation. It is also possible to tures. some cases, the fracture face is destroyed in the determine if the fracture path at the initiation incident; alternatively, postfracture events can site is transgranular or intergranular and to occur that drastically alter the fracture face and determine if the fracture path is specific to any Techniques material condition. Such problems can often phase or constituent present. Although such make a conclusive fracture interpretation and specimens can be examined by scanning All fracture examinations should begin with failure analysis difficult, if not impossible. In electron microscopy (SEM), light microscopy visual inspection, perhaps aided by the use of a some instances, however, satisfactory results is more efficient for such work, and certain simple hand lens (up to 10 x ). In failure anal- can be obtained (see the article "Failures of information, such as the color or polarization yses, it may be necessary to examine the entire Locomotive Axles" in Volume 11 of the 9th response of constituents, can be assessed only component or structure to find the broken Edition of Metals Handbook). 92 / Visual Examination and Light Microscopy The fractured sections must be protected compression, shear, bending, and so on), cracks for examination. The damage to second- from further damage after the incident. It is the relative stress level (high, medium, or ary cracks is usually less than that of the main often necessary to section the failed component low), and the stress orientation. fracture. It is important to remember that the or structure, sometimes at the failure site, so Examine areas selected by macroscopic ex- crack mechanism for propagation may be dif- that it can be studied more extensively in the amination at higher magnifications by light ferent from that at the initiation site. Damage laboratory. Sectioning must be carried out in microscopy, SEM, or replica transmission done to the main fracture may prohibit success- such a manner that the fracture and adjacent electron microscopy (TEM) to determine the ful microfractography, bUt the gross macro- material is not altered. If burning is used, it fracture mode, to confirm the fracture mech- scopic features may still be visible and amena- should be conducted well away from the fail- anism (observation of cleavage facets, duc- ble to interpretation. ure. Similarly, band saw cutting or abrasive tile dimples, fatigue striations, and so on), Mechanical damage to the fracture surface wheel cutting must be conducted well away and to detect features at the fracture origin may occur during crack propagation, for exam- from the fracture. It is generally necessary to Examine metallographic cross sections con- protect the fracture during such work. This taining the origin to detect any microstruc- subject is treated at length in the article "Prep- tural features that promoted or caused frac- aration and Preservation of Fracture Speci- ture initiation, and determine if crack mens" in this Volume. propagation favors any microstructural con- Macroscopic examination is the first step in stituent fracture interpretation. In most cases, the origin must be determined in order to obtain conclu- sive results. Careful macroscopic examination Visual macroscopic examination is the most should always precede any microscopic exam- efficient procedure for fracture evaluation. This ination. Macroscopic examination will gener- should be followed by stereoscopic examina- ally permit determination of the manner of tion of the fracture features using magnifica- loading, the relative level of applied stress, the tions up to about 50 ×. Before the fracture is mechanisms involved, the sequence of events, sectioned, relevant details should be recorded and the relative ductility or brittleness of the photographically (see the article "Photography material. Other details can be revealed by gross of Fractured Parts and Fracture Surfaces" in (al fracture examination--for example, the pres- this Volume). Sketches are also very useful. ence of hardened cases; apparent grain size or Dimensions should be recorded
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