Damage effect on the fracture toughness of nodular cast iron: Part I. Damage characterization and plastic flow stress mo
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THE increasing development of nodular cast iron is the result of its interesting physical properties and its economic advantages in manufacturing, including low melting temperature, high fluidity, low shrinkage, and good machinability in comparison with cast steel and other ferrous alloys.[1] These properties make nodular cast iron well suited for automotive industry components as well as for equipment such as casks for transportation of nuclear fuel elements. Fail-safe design of critically stressed components requires a thorough knowledge of the mechanical properties of a material as well as its tolerance to defects. Efforts have been devoted to better characterizing the fracture toughness of nodular cast iron as well as to better understanding the micromechanisms controlling the fracture processes. Ductile fracture, with emphasis on the limiting process for ductile cast iron plasticity, has been studied.[1–5] Therefore, the role of nodule distribution and size,[6] stress triaxiality,[7,8] and thermal treatment[9] have been extensively characterized. A problem with the measure of cast iron fracture toughness is that toughness values obtained depend strongly on the experimental procedure. Using the linear elastic fracture mechanics (LEFM) approach, an unusual dependence of the fracture toughness on specimen geometry was found, larger values being measured for larger specimens.[1] On the other hand, the upper-shelf fracture toughness of nodular cast iron, obtained from a single specimen test using an elastoplastic fracture mechanics approach, was shown to be strongly dependent on the unloading amplitude used to
monitor the crack extension.[1] This behavior was attributed to the existence of a large damaged zone ahead of the crack tip,[4,5,10,11] but no quantitative characterization of this damaged zone size has been performed yet. This insufficient understanding of nodular cast iron’s tolerance to damage is a limiting factor in the improvement of the safety of cast iron components. The present study has, therefore, been devoted to a detailed analysis of the damage processes and their consequences on the mechanical behavior of ductile cast iron. This set of articles consists of two parts. Part I is concerned with the damage mechanisms observed during uniaxial loading of nodular cast iron. Part II will be devoted to a tentative quantification of the influence of these damage mechanisms on fracture toughness, especially by experimental and theoretical study of the damage zone extension ahead of the crack tip. In this first part, an investigation of the damage processes occurring during tensile loading, as well as a modeling of the mechanical behavior taking into account the influence of these damage processes, are presented. The experiments included observations of the microstructure with a scanning electron microscope (SEM) and tensile and compression tests. These allowed monitoring of the evolution of the Young’s modulus and Poisson’s ratio during deformation. The results constitute the basis for analytical as
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