Chapter 3 Experimental Observations on the Fracture of Metals C. A. Sciammarella, L. Lamberti, and F. M. Sciammarella Abstract This paper deals with the onset of plasticity and the transition to fracture in metallic rectangular tensile specimens. The plastic instability manifests itself by the propagation of wave fronts that sweep the specimen and finally localize at a given site during the fracture process. The plastic instability is associated with changes in specimen geometry. In optical recordings, propagation and localization of structural instability manifest in the form of wide bands. The configuration of these bands is influenced by degradation of properties of a metal under analysis. Understanding the meaning of band configurations is vital for evaluating mechanical properties of a metal and its effective use. In this paper, a family of fringes called iso-derivatives is utilized to analyze the configuration and properties of the wide bands. The retrieval of the information contained in the wide bands depends on the spatial and temporal resolution of the recordings. Keywords Plasticity · Fracture mechanics · Metals · Optical methods 3.1 Introduction The process of failure in materials is a complex problem that present challenges from a fundamental point of view, full understanding of diverse mechanisms leading to the instability of dynamic equilibrium in loaded solids. In spite of theoretical and experimental developments, actual dynamics of a fracturing solid, connection between motion equations with molecular structure of the solid, presents many unknown aspects. Basic questions that theoretical efforts of atomistic based models, as well as the Continuum approach try to answer, have deep practical consequences for scientists and engineers. Which are the mechanisms of atomic configurations or of the parameters of the Continuum approach that make it possible to predict the onset of plasticity in a given material under a variety of externally applied loadings? This question is followed by another question: transition of the onset of plasticity to failure and fracture. Currently, computer science, computing hardware, and numerical methods for solving partial differential equations provide the means to solve complex theoretical models of molecular dynamics. It is possible to summarize the current state-of-the-art of the partial obtained answers. The problem of stability of crystalline arrays is formulated in terms of the stability of internal energy W expressed as a function of suitable tensorial forms of displacement gradient fieldFand a corresponding form of a stress tensor P. In the case of metals, onset of plasticity implies the motion of dislocations through crystalline array. This is the approach of dynamic time-dependent plasticity evolved in Materials Science. There is another important event, the transition from plasticity to fracture. From the engineering point of view, this transition is handled by Fracture Mechanics. The near field at the tip of a crack is modeled by an approximate solution of the theory of elasticity that assumes the existence of a singularity where stresses become infinite. The extensions of this approach including the presence of a plastic zone at the crack tip also assume the existence of a singularity at crack tip. This approach leads to the formulation of transition from plasticity to fracture by introducing C. A. Sciammarella ( ) Department of Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, Chicago, IL, USA e-mail: sciammarella@iit.edu L. Lamberti Dipartimento Meccanica, Matematica e Management, Politecnico di Bari, Bari, Italy e-mail: luciano.lamberti@poliba.it F. M. Sciammarella MXD Corporation, Chicago, IL, USA e-mail: federico.sciammarella@mxdusa.org © The Society for Experimental Mechanics, Inc. 2021 M.-T. Lin et al. (eds.), Advancement of Optical Methods & Digital Image Correlation in Experimental Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-59773-3_3 19
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