Understanding Component Failures
(Part 1: Mechanisms)
When a component part fails, it is only natural to ask why and then strive to determine the root cause. Gathering all possible information about the damage event and performing a thorough failure analysis is a critical first step in the process. This type of information helps the heat treater create a set of do’s and don’ts that are invaluable in avoiding a repetition of the problem. Let’s learn more.
In simplest terms, a failure is the inability of a component part to perform its intended function (Fig. 1). In service, components experience different types of conditions/environments, damage mechanisms and applied loading, including tension, compression, bending, torsion and mixed modes (combinations). The failures that result may be categorized in a broad sense as those related to fracture, wear, corrosion and dimensional change/distortion. As heat treaters, we must also consider that residual stresses can often play an important role.
Types of Fracture: Macroscopic Scale
Applied loads (Fig. 2) may be unidirectional or multi-directional in nature and occur singularly or in combination. The result is a macroscopic stress state comprised of normal stress (perpendicular to the surface) and/or shear stress (parallel to the surface). In combination with the other application conditions, the result is one of four primary modes of fracture: dimpled rupture (also called microvoid coalescence), cleavage, decohesive rupture and fatigue. These will be discussed in more detail in Part 2.
Types of Fracture: Microscopic Scale
Virtually all engineering metals are polycrystalline. As a result, the two basic modes of deformation/fracture (under single loading) are shear and cleavage (Table 1). The shear mechanism, which occurs by sliding along specific crystallographic planes, is the basis for the macroscopic modes of elastic and plastic deformation. The cleavage mechanism occurs very suddenly via a splitting action of the planes with very little deformation involved. Both of these micro mechanisms primarily result in transgranular (through the grains) fracture.
Ductile and Brittle Fractures
Numerous factors influence whether a fracture will behave in a ductile or brittle manner (Table 2). In ductile materials, plastic deformation occurs when the shear stress exceeds the shear strength before another mode of fracture can occur, with necking typically in evidence before final fracture. Brittle fractures occur suddenly and exhibit very little, if any, deformation before final fracture.
Ductile fractures typically have the following characteristics:
• Considerable plastic or permanent deformation in the failure region
• Dull and fibrous fracture appearance
Brittle fractures typically have the following characteristics:
• Lack of plastic or permanent deformation in the region of the fracture
• Principle tensile stress is perpendicular to the surface of the brittle fracture
• Characteristic markings on the fracture surface pointing back to where the fracture originated
When examined under a scanning electron microscope, fracture surfaces seldom exhibit entirely dimpled rupture (i.e. ductile fracture) or entirely cleavage (i.e. brittle fracture), although one or the other may dominate. Other fracture modes include intergranular fractures, combination (quasi-cleavage) fractures and fatigue fractures.
Beachmarks (aka stop marks, arrest marks, clamshell marks, conchoidal marks) are a classic fracture feature (Fig. 3) found during failure analysis, and they identify fatigue failures by their presence. These manifest themselves as visible ridges and are indicative of interruptions in crack propagation. Beachmarks (as well as striations) identify the position of the tip of the fatigue crack at any given point in time. They are formed by plastic deformation at the crack tip and by differences in the time of corrosion (when present) in the propagating crack. Beachmarks expand outward from the fatigue origin and are often circular or semicircular in appearance. They will not be present if the part saw only brief interruptions in service.
Wear (Table 3 - online only) is a type of surface destruction that involves the removal of material from the surface of a component part under some form of contact produced by a form of mechanical action. Wear and corrosion are closely linked, and it is important not only to evaluate the failure but to take into consideration design and environment and have a good understanding of the service history of a component.
Corrosion is the destruction of a material or component by the actions of chemical or electrochemical reactions with the component environment. The major types of corrosion include galvanic action, uniform corrosion, crevice corrosion, stress-corrosion cracking and corrosion fatigue. The mechanisms and effects created by each of these are well documented in the literature.[ 8, 9] It is critical to understand that the effects of corrosion are present to some degree in every failure analysis, which is one of the reasons why protecting fracture surfaces is so critical to performing a proper failure analysis.
This brief introduction to the types and mechanisms involved with the failure of component parts in service is intended to invite the reader to learn more about this subject as it relates to his/her specific area of interest. One of the important takeaways is that no product failure should be treated lightly, and determination of the (single) root cause is critical to the success of any engineering design.
Part 2 will address failure-analysis methods and introduce the subject of fractography as a tool in determining root cause. IH
1. Wulpi, Donald J., Understanding How Components Fail, ASM International, 1985.
2. Lawn, B.R. and T. R. Wilshaw, Fracture of Brittle Solids, Cambridge University Press, 1975.
3. ASM Handbook, Volume 11: Failure Analysis and Prevention, R. J. Shipley and W. T. Becker (Eds.), ASM International, 2002.
4. ASM Handbook, Volume 11: Failure Analysis and Prevention, R. J. Shipley and W. T. Becker (Eds.), ASM International, 2002.
5. Mr. Craig Darragh, AgFox LLC, private correspondence and technical editing.
6. Ms. Debbie Aliya, Aliya Analytical, Inc. (www.itothen.com), private correspondence/
7. Mr. George Vander Voort, George Vander Voort Consulting (www.georgevandervoort.com), private correspondence.
8. Fontana, M. G., and N. D. Greene, Corrosion Engineering, 3rd Edition, McGraw-Hill Book Company, 1985.
9. Uhlig, H. H., Corrosion and Corrosion Control, John Wiley & Sons, 1963.