Understanding the Interdependencies Between Composition, Microstructure, and Continuum Variables and Their Influence on
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fracture toughness (K) of a material describes how the material will respond when subjected to a load with a critical flaw size. This property, while important for the design engineer, is an often ill-studied problem, costly to measure, and is exceptionally difficult to predict. While progress has been made to predict the yield strength and fatigue properties of engineering
P.C. COLLINS is with the Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011 and also with the Center for Advanced Non-Ferrous Structural Alloys, an NSF I/ UCRC, Ames, IA 50011. Contact e-mail: [email protected] S. KODURI and V. DIXIT are with Intel Corporation, Hillsboro, OR 97124. H.L. FRASER is with the Department of Materials Science and Engineering, The Center for the Accelerated Maturation of Materials, The Ohio State University, Columbus, OH 43210. Manuscript submitted December 6, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys given a specific composition and microstructure (including titanium based alloys[1–6]), and while fundamental relationships between composition and slip mechanisms have been determined,[7–11] the prediction of fracture toughness based upon composition and microstructure is far less mature. In other words, there is a ‘‘knowledge gap’’ or ‘‘predictability gap’’ when it comes to fracture toughness. This is especially true for ductile materials, such as a/b-processed Ti-6Al-4V. The origin of this ‘‘predictability gap’’ may be attributed to the complex manner by which fracture occurs (i.e., by which cracks propagate). The area immediately in front of a crack tip is subjected to local stresses which may exceed the tensile strength of the material (rys ¼ fðcomposition, microstructureÞ). The specimen geometry and dimensions, initial flaw size, state of stress at the crack tip, and the response of the material to the concentrated stress and crack propagation all contribute to the fracture toughness. The degree to which these relationships are known varies significantly. For example, there is a reasonable level of
maturity with regard to the understanding of the relationships between applied stress, basic sample geometries, and initial crack shapes/sizes and the resulting fracture toughness. Conversely, there is generally a dearth of legacy knowledge regarding the influence of the individual continuum material parameters (e.g., yield strength and the state of stress at the crack tip, rz) on the measured toughness. This knowledge gap includes (generally) any understanding of how the microstructure responds to the presence of a crack under load, including crack tip opening and growth and the development of damage (e.g., secondary microcracks) in the immediate vicinity of the primary crack. This knowledge asymmetry is shown schematically in Figure 1 with the well-established connection between geometry and crack size and the material property shown with a solid line. In a similar fashion, the various interconnected material variables are shown with dotted lines to indicate the lack of u
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