We continue our discussion of component failures by talking about how to analyze what went wrong and some of the laboratory tools we use. If our analysis is accurate, one of the fracture mechanisms discussed in Part 1 will be consistent with the cause. If the mechanism is not properly understood or the analysis incomplete or inadequate, all contributory factors and, therefore, the root cause will not be properly identified, making corrective action ineffective. Let’s learn more.
The Role of Fractography
Fractography is the term used to describe the study of fracture surfaces. Fractographic methods are routinely used to help determine the root cause of a failure. One of the goals of fractography is to establish and examine the origin of cracks in an attempt to reveal the cause of crack initiation. Initial fractographic examination is commonly carried out on a macro scale using stereo microscopy and oblique lighting techniques (e.g., low angle, often from various sides) to identify the extent of cracking, possible modes and likely origins. Optical or scanning electron microscopy is then used to pinpoint the nature of the failure and the causes of crack initiation and growth (if the loading pattern is known).
Common features that may cause crack initiation are inclusions; voids or holes in the material; contaminants; and stress concentrators such as dents, bruises, coarse surface finishes, sharp corners, insufficient radii or other abrupt changes in cross section. Lines on fracture surfaces often show crack direction. Some modes of crack growth leave characteristic marks on the surface that identify the mode of crack growth and the origin on a macro scale (e.g., beachmarks or striations on fatigue cracks). Also revealing are areas that exhibit subcritical cracks (i.e. cracks that have not grown to completion). They can indicate that the material was faulty when loaded or, alternatively, that the sample was overloaded at the time of failure. Other clues such as cusps, which typically form where brittle cracks meet, may be present.
Fracture in most engineered materials occurs either by intergranular (along the grain boundaries) or transgranular (through the grains) fracture paths. As mentioned in Part 1, the type of loading experienced produces different types of stresses in the material, which result in different modes of fracture. These can be classified as dimpled rupture, cleavage, decohesive rupture and fatigue.
Dimpled rupture (aka microvoid coalescence, or MVC) is essentially transgranular and ductile in nature. This type of fracture exhibits cuplike depressions (Fig. 1) commonly referred to as dimples. Dimple rupture is the mode of fracture when overload is the principal cause of failure in a ductile manner and typically proceeds in three stages: nucleation, growth and coalescence of microvoids. The dimple shape is dictated by the stress state of the material and can be caused by particle cracking or interfacial failure between precipitate particles and the matrix. It usually occurs under single load or tearing.
When a material is put under uniaxial tensile loading, equiaxed dimples appear that have complete rims. Under tear loading, the dimples are elongated, the rims of the dimples are not complete and the dimples are in the same direction as the loading when viewing the mating fracture surfaces. Shear loading has the same features as tear loading except the dimples are in opposite directions on the mating surfaces. Microvoids grow during plastic flow of the matrix, and microvoids coalesce when adjacent microvoids link together or the material between microvoids experiences necking.
Cleavage is also essentially transgranular and brittle in nature being a low-energy fracture propagating along well-defined crystallographic planes. It is normally flat with shallow features. Cleavage occurs in brittle materials at low temperature and generally results from high stress along multiple axes with a high rate of deformation. Characteristics of cleavage are cleavage steps (i.e. a cleavage facet joining two parallel cleavage fractures); feather markings (i.e. very fine, fan-like markings on a cleavage fracture); chevron patterns; herringbone structures; tongues and microtwins; Wallner lines (i.e. distinct V-shape created by pattern intersection of two groups of parallel cleavage steps, primarily found in brittle materials); and quasi-cleavage (i.e. cleavage with micro-dimples).
Fatigue failures (c.f. Part 1) are essentially transgranular brittle fractures and the result of repetitive or (high or low) cyclic loading. The observed characteristic striations (clamshell marks, beachmarks, etc.) show the propagation of the crack front.
Decohesive rupture is almost exclusively intergranular in nature and typically occurs by one of several phenomena in weaker areas such as those between the individual grains.
Tools of the Trade
The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals from the surface of a specimen. These signals reveal information such as external morphology (texture), chemical composition, crystalline structure and orientation. In most applications, data are collected over a selected area of the surface of the sample and a two-dimensional image is generated that displays spatial variations in these properties. Areas ranging from approximately 1 cm to 5 microns in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20X to approximately 30,000X and spatial resolution of 50-100 nm). The SEM is also capable of performing analyses of selected point locations on the sample. This approach is especially useful in qualitative or semi-quantitative analysis, helping to determine chemical compositions (using EDS), crystalline structure and crystal orientations (using EBSD).
The transmission electron microscope (TEM) operates on the same basic principles as the light microscope but uses electrons as a “light source” because their lower wavelength makes it possible to obtain a resolution 1,000 times better than with an optical light microscope. The TEM is generally used to study ultrathin sections (50-60 nanometers), allowing the user to observe and analyze internal microstructures. Stained areas of the sample absorb or scatter the beam, producing dark spots; unstained areas appear light.
Scanning transmission electron microscopy (STEM) combines the principles of TEM and SEM and is increasing in popularity for failure analysis. Like the TEM, a STEM requires very thin samples and looks primarily at a focused beam of electrons. One of its principal advantages over TEM is in enabling the use of other of signals that cannot be spatially correlated in TEM, including secondary electrons, scattered-beam electrons, characteristic X-rays and electron energy loss. The STEM looks like a TEM but produces images one spot at a time, as in the SEM, rather than all at one time as in the TEM. Like the SEM, the STEM technique scans a very finely focused beam of electrons across the sample, which is correlated with beam position to build a virtual image in which the gray level at the corresponding location in the image represents the signal level at any location in the sample. Its primary advantage over conventional SEM imaging is the improvement in spatial resolution.
What the Heat Treater Needs To Know
The important aspect of failure analysis for the heat treater to understand is that tools and investigative practices exist to help us pin-point where and how a component failed. If it happens that heat treatment is the root cause, the information obtained by failure analysis allows us to make informed decisions regarding changes to recipes, processes or equipment as part of the corrective action. IH
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5. Nishida, Shin-Ichi, Failure Analysis in Engineering Applications. Butterworth Heinemann Ltd, Jordan Hill, 1992.
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8. Mr. Craig Darragh, AgFox LLC, private correspondence and technical editing.
9. Ms. Debbie Aliya, Aliya Analytical, Inc. (www.itothen.com), private correspondence.
10. Mr. Alan Stone, Aston Metallurgical Services Co., Inc. (www.astonmet.com), private correspondence.