ASM Metals HandBook Volume 12 - Fractography
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ASM Metals HandBook Volume 12 - Fractography

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Modes of Fracture 
Victor Kerlins, McDonnell Douglas Astronautics Company Austin Phillips, Metallurgical Consultant 
METALS FAIL in many different ways and for different reasons. Determining the cause of failure is vital in preventing a 
recurrence. One of the most important sources of information relating to the cause of failure is the fracture surface itself. 
A fracture surface is a detailed record of the failure history of the part. It contains evidence of loading history, 
environmental effects, and material quality. The principal technique used to analyze this evidence is electron 
fractography. Fundamental to the application of this technique is an understanding of how metals fracture and how the 
environment affects the fracture process. 
This article is divided into three major sections. The section "Fracture Modes" describes the basic fracture modes as well 
as some of the mechanisms involved in the fracture process. The section "Effect of Environment" discusses how the 
environment affects metal behavior and fracture appearance. The final section, "Discontinuities Leading to Fracture," 
discusses material flaws where fracture can initiate. 
The valuable technical discussions and advice provided by Richard A. Rawe, McDonnell Douglas Astronautics Company, 
during the preparation of this article is sincerely appreciated. 
Modes of Fracture 
Victor Kerlins, McDonnell Douglas Astronautics Company Austin Phillips, Metallurgical Consultant 
Fracture Modes 
Fracture in engineering alloys can occur by a transgranular (through the grains) or an intergranular (along the grain 
boundaries) fracture path. However, regardless of the fracture path, there are essentially only four principal fracture 
modes: dimple rupture, cleavage, fatigue, and decohesive rupture. Each of these modes has a characteristics fracture 
surface appearance and a mechanism or mechanisms by which the fracture propagates. 
In this section, the fracture surface characteristics and some of the mechanisms associated with the fracture modes will be 
presented and illustrated. Most of the mechanisms proposed to explain the various fracture modes are often based on 
dislocation interactions, involving complex slip and crystallographic relationships. The discussion of mechanisms in this 
section will not include detailed dislocation models or complex mathematical treatment, but will present the mechanisms 
in more general terms in order to impart a practical understanding as well as an ability to identify the basic fracture modes 
Dimple Rupture 
When overload is the principal cause of fracture, most common structural alloys fail by a process known as microvoid 
coalescence. The microvoids nucleate at regions of localized strain discontinuity, such as that associated with second-
phase particles, inclusions, grain boundaries, and dislocation pile-ups. As the strain in the material increases, the 
microvoids grow, coalesce, and eventually form a continuous fracture surface (Fig. 1). This type of fracture exhibits 
numerous cuplike depressions that are the direct result of the microvoid coalescence. The cuplike depressions are referred 
to as dimples, and the fracture mode is known as dimple rupture. 
Fig. 1 Influence of direction of maximum stress (\u3c3max) on the shapes of dimples formed by microvoid 
coalescence. (a) In tension, equiaxed dimples are formed on both fracture surfaces. (b) In shear, elongated 
dimples point in opposite directions on matching fracture surfaces. (c) In tensile tearing, elongated dimples 
point toward fracture origin on matching fracture surfaces 
The size of the dimple on a fracture surface is governed by the number and distribution of microvoids that are nucleated. 
When the nucleation sites are few and widely spaced, the microvoids grow to a large size before coalescing and the result 
is a fracture surface that contains large dimples. Small dimples are formed when numerous nucleating sites are activated 
and adjacent microvoids join (coalesce) before they have an opportunity to grow to a larger size. Extremely small dimples 
are often found in oxide dispersion strengthened materials. 
The distribution of the microvoids nucleation sites can significantly influence the fracture surface appearance. In some 
alloys, the nonuniform distribution of nucleating particles and the nucleation and growth of isolated microvoids early in 
the loading cycle produce a fracture surface that exhibits various dimple sizes (Fig. 2). When microvoids nucleate at the 
grain boundaries .(Fig. 3), intergranular dimple rupture results. 
Fig. 2 Examples of the dimple rupture mode of fracture. (a) Large and small dimples on the fracture surface of 
a martempered type 234 tool steel saw disk. The extremely small dimples at top left are nucieated by 
numerous, closely spaced particles. (D.-W. Huang, Fuxin Mining Institute, and C.R. Brooks, University of 
Tennessee). (b) Large and small sulfide