Deformation and failure in nodular cast iron

doi:10.1016/j.actamat.2011.12.037

Acta Materialia

Volume 60, Issue 5, March 2012, Pages 2359-2368

https://doi.org/10.1016/j.actamat.2011.12.037 Get rights and content

Abstract

Ductile failure in nodular cast iron is explored through uniaxial tension and notched tension experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution were examined through quantitative microscopy to discern the mechanisms of failure and to quantitatively evaluate the local strain evolution. Fractographic observations were used to identify the onset and evolution of damage processes during the deformation and failure of nodular cast iron. These tests and observations reveal that void growth and coalescence occurred only within a narrow localized band, whose size is comparable with the size of the graphite nodules; no statistically significant changes in the porosity were observed outside this zone.

Introduction

The failure of many ductile materials used in structural applications is typically considered to occur by the progressive nucleation, growth and coalescence of damage. Microscopic models of such damage are usually considered at the continuum level, through the introduction of the idea of a representative volume element (RVE), where damage can be represented by internal variables. Homogenization requires that the size of the RVE be selected so as to contain a statistically uniform representation of the damage process within the RVE [1], [2]. The size of the RVE can vary from about 10⁻³ mm³ for metals and ceramics to about 10⁶ mm³ for concrete. The damage discontinuities in the RVE are considered to be “small” with respect to the size of the RVE, but large compared with the atomic spacing and even grain size [3]. Numerous studies have been performed aimed at exploring the development of damage. The class of micromechanical models that incorporate void nucleation, growth and coalescence as the damage process are typically called Rousselier or Gurson–Tvergaard–Needleman (GTN) models [4], [5], [6], and the general framework of these models is now rather well established. Tvergaard [7] and Benzerga and Leblond [8] provided comprehensive reviews of this approach to modeling of ductile failure. Calibration and validation of these models have been attempted by numerous investigators (see, for example, Brocks et al. [9], Dong et al. [10], Decamp et al. [11], Pardoen et al. [12], Steglich and Brocks [13], and Benseddiq and Imad [14]). However, this process is inherently non-unique, since many different combinations of parameters appear to capture the softening part of the material/structural response [15]. In this paper we investigate the process of deformation and evolution of failure in an initially porous material. This material was chosen with the goal of avoiding the difficulty of modeling the complex void nucleation process and examining the process of void growth. For this purpose three different materials, sintered iron, spheroidized sintered iron and nodular cast iron (NCI), were considered as suitable candidates. However, the sintered iron specimens could not sustain strains greater than 2% and while the spheroidized sintered iron specimens could be strained up to 16%, the poorly controlled heat treatment produced a highly non-homogeneous microstructure in which no meaningful correlation could be made between specimens interrupted at different stages of the deformation. Therefore, sintered and spheroidized sintered iron specimens were not considered further in this study and attention is restricted to the NCI specimens.

This paper is organized as follows. The microstructure of NCI used in the present work is described in Section 2. The results of uniaxial tension tests and notched tension tests are discussed in Sections 3 Uniaxial tension test, 4 Flat notched tension test, respectively. Particular attention is paid to the development of microstructural features corresponding to different macroscopic strain levels and triaxiality conditions. The conclusions from this study regarding the growth of voids and their coalescence are summarized in Section 5.

Section snippets

Material

Ductile cast iron or nodular cast iron (NCI) is a type of cast iron in which the graphite nodules take on a nearly spherical shape. Magnesium particles are usually added as an inoculant in the casting process to provide the nuclei from which the graphite nodules grow. The cooling time during the manufacturing process plays a very important role in determining the microstructure and the mechanical properties of the cast iron. Therefore, the microstructure of different sections of the cast

Uniaxial tension test

Uniaxial tension tests were performed on standard dumb-bell shaped specimens in order to characterize the elastic–plastic behavior of the materials. Dumb-bell specimens of 2.54 mm thickness and 6.35 mm width were cut in the longitudinal direction from a blank of 38.1 × 38.1 × 160.0 mm. Tensile tests were run on three specimens in an Instron 4482 universal testing machine at a cross-head speed of 0.254 mm min⁻¹, resulting in quasi-static loading at a strain rate of 10⁻⁴ s⁻¹. The strain and displacement

Flat notched tension test

Flat notched specimens cut longitudinally into strips of 2.54 mm thickness, 24.13 width and 152.4 mm length from the center portion of the cast iron blank were used in order to extend the strain range over which the evolution of porosity was examined (Fig. 8a). Two semicircular cut-outs of radius 4 mm were introduced along the sides in order to increase the stress triaxiality in the central portion of minimum cross-section. This modification allows us to promote and evaluate the void growth

Summary

In this paper we investigated the process of deformation and failure in initially porous materials. NCI was used as an example of a material with pre-existing porosity, thus avoiding the complexity of exploring the nucleation process. The NCI used in this study is an ASTM A536 grade 65-45-12 ductile cast iron with a ferritic matrix that contains about 5–25% pearlite. The average grain size of ferrite and the diameter of the graphite nodules were found to be about $26 \pm 9$ and $27 \pm 25$ μm,

Acknowledgement

This work was performed during the course of an investigation into ductile failure under two related research programs funded by the Office of Naval Research: MURI project N00014-01-1-A00001 and FNC project: N00014-08-1-0189. This support is gratefully acknowledged.

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Deformation and failure in nodular cast iron

Abstract

Introduction

Section snippets

Material

Uniaxial tension test

Flat notched tension test

Summary

Acknowledgement

Nucl Eng Des

Adv Appl Mech

Comput Mater Sci

Acta Mater

Int J Pres Ves Pip

Comput Mater Sci

Image Vis Comput

Int J Solids Struct

Int J Solids Struct

Statistical continuum theories

Appl Mech Rev

Engineering damage mechanics: ductile, creep, fatigue and brittle failures