Compacted and Spheroidal Graphite Irons : Experimental Evaluation of Poisson ’ s Ratio

Spheroidal Graphite cast Iron (SGI), also known as ductile iron, represents a perfect material for a very large range of modern applications. It is successfully used wherever optimal strength, stiffness and even low costs are required: high values of tensile strength and Young’s modulus are between its most appreciable propеrties. At the same time, nowadays, a different cast iron, known as Compacted Graphite Iron (CGI) or vermicular graphite iron, is taking the first steps in replacing SGI respect to some specific applications. It depends on a better castability, machinability and thermal resistance. CGI is an ideal cast iron in the case of components under simultaneous mechanical and thermal loadings, such as cylinder blocks and heads. Unfortunately, while SGI benefits of a wide scientific literature, CGI is a relatively unknown material. Moreover, dut to its particular microstructure, the production of CGI presents additional difficulties and it is not easy to obtain stable properties in the CGI alloy. This paper illustrates a way for the experimental evaluation of the Poisson’s ratio by tensile specimens, comparing this propriety in the case of SGI and CGI.


INTRODUCTION
Although Compacted Graphite Iron (CGI) was first observed in 1940s, an unstable foundry production precluded its use for high volume production until advanced process control technologies became available [1][2][3]. Nowadays CGI is used for complex shape applications such as cylinder heads and engine blocks. As the demand for higher torque, lower weight and, consequently, lower emissions continues to grow, engine designers are forced to seek stronger materials with higher heat transfer efficiency. This is particularly true in the diesel sector where resolution of the conflicting performance objectives requires increased cylinder bore pressures. At these operating levels, the strength, stiffness and fatigue properties of traditional cast iron or aluminium alloys may not be sufficient to satisfy the design requirements maintaining low production costs. Referring to this, more detailed studies on mechanical and physical properties of CGI as a function of the content and morphology of its microstructural constituents have been carried out [4]. Due to its properties, several automotive manufacturers have therefore evaluated CGI for their petrol and diesel cylinder head applications [5][6][7].
Despite that, the mechanical behaviour of CGI is not yet as adequately known as those of spheroidal grey iron (SGI) and that fact leaves empty space for further CGI studies.
Cast irons are differentiated by the shape of the graphite particles. As shown in Figure 1, the graphite particles in CGI are randomly oriented and elongated, but they are shorter, thicker with respect to other common families of cast irons. Adding, these particles begin to present rounded edges, up to real spheroidal nodules, distinctive for SGI. In contrast to grey and ductile iron (SGI), the entangled compacted graphite 'clusters' interlock themselves into the iron matrix to provide a strong adhesion between the iron and the graphite [8]. The rounded edges of the CGI particles suppress the crack initiation that would otherwise occur at the sharp flake SGI white grey CGI edges, while the complex shape of the clusters and the good iron/graphite adhesion impairs crack propagation. Together, these factors account for the increase in strength relative to grey iron and the improved thermal conductivity relative to ductile iron.
An example of difference in mechanical properties between the cast irons, including CGI, are summarized in Table 1. This comparative table, in line with [9], represents a simplification of information coming from several investigations and represents a valid base for further analysis. It is noteworthy that the final properties for these cast materials strongly depend on the chemical-physical peculiarities of the alloys (as amount of perlite, grade of nodularity, etc.) [10][11][12][13][14].
Referring to the CGI, beyond other traditional studies [as [15][16][17], two interesting investigations were realized by some of the present authors using similar process conditions and test methodologies. In particular, in [18]  On the other hand, in [19] several tribological aspects of CGI and SGI were investigated via fracture toughness permitting to relate several proprieties of fracture mechanics to microstructures. During these investigations, standard test methods were used for the determination of fracture toughness (KIC) and plasticelastic energy (JIC) required to grow a crack in these alloys. It was realized under predominantly linearelastic, plane-strain conditions using fatigue precracked specimens. Experimental values of KIC and JIC can be used to the design of structures to ensure that a cast does not fail by brittle or ductile fracture. Moreover, in [20,21] the practical advantages in using CGI, instead SGI are detailed.
The present paper completes the previous analysis proposing an experimental estimation of Poisson's ratio.

Poisson's ratio measurement
Poisson's ratio is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. Tensile deformation is considered positive and compressive deformation is considered negative.
The definition of Poisson's ratio contains a negative sign, so that normal materials have a positive ratio.
Specifically referring to the scheme in Fig. 2, a cube with sides of length L of an isotropic linearly elastic material subject to tension along the x axis, with a Poisson's ratio of 0.5. The green cube is unstrained, the red is expanded in the x direction by ΔL due to the tension, and contracted in y and z directions by ΔL'.

Figure 2. Example of Poisson's ratio
The Poisson's ratio is defined as: where: dε trans is transverse strain (negative for axial tension (stretching), positive for axial compression) dε axial is axial strain (positive for axial tension, negative for axial compression).
Poisson's ratio was evaluated by experimental tensile tests in accordance with a the EN 10002-1, using specific "dog-bone" tensile specimens (Fig. 3).

Specimens design and manufacturing
Tensile tests were carried out on the uniaxial loading tensile testing machine INSTRON® 1343 (in Fig. 4), according to the EN 10002-1 standard where a procedure for the tensile testing of metallic materials at ambient temperature is described [22].
Referring to the tensile specimens, the standardized geometry is reported in Fig. 3A. This shape was machined according to the EN 1563 standard (Fig. 3B). The EN 1563 is the European standard for spheroidal B graphite cast iron. It outlines legal and regulatory requirements as well as industry grades for materialsproviding ways to classify the quality and hardness of spheroidal cast irons. Its specifications are based on the mechanical properties of machine tested materials [23]. Six specimens were shaped using a CNC tool machine, three in CGI (labelled as I, II, III) and three in SGI (labelled as IV, V, VI).

Specimens preparation
Before testing, a strain gauge was mounted on each specimen following a delicate procedure aiming at assuring the perfect match between gauge and surface.
This action started by removing grease and oil on the specimen surface using a solvent (alcohol) and cotton wool. Then, the surface was abraded by silicon-carbide paper (320 grit first) to sand away uneven surface, paint, and rust. It also permitted to smooth the gaging area. Particular attention was reserved to prevent over abrading. Finally, a proper neutralizer provided the right pH level at the specimen surface for better bonding with adhesive.
The strain gauge was bonded on the surface of the specimen applying a film of adhesive; the surface was abraded by sandpaper and finally cleaning by alcohol. Strain gauges were completed by wires connections and specimens ready for cables mounting (Fig. 5).

Figure 5. Specimen with strain gauges and wires connection, ready for cables mounting
After the installation on specimens, strain gauges were electrically tested. In particular, the measuring of electrical resistance in condition of open-circuit and close-circuit voltages correctly providing the values of, nearly, respectively, 0 Ω and 120 Ω.

RESULTS
Before other considerations, it is noteworthy to report the chemical compositions of castings (Table 2). With the aim at better understanding these values, it is noteworty to recall that specimens were extracted from CGI and SGI green sand castings. Before the pouring, the melt (with a sulphur content lower than 0.01% wt.) was inoculated by adding ferrosilicon alloys and modified with Fe-Si-Mg master alloys. In the production of CGI castings also Ti was added. In all cases, the pouring temperature was 1400°C.  Tables 3 and 4. These data are also compared in Fig. 7 in terms of stress-strain (σ-ε) curves, and in Fig. 8 in terms of axial -transversal strains (ε axial -ε transversal ) distribution.
In particular, graphs show a linear trend. It means that there is a direct proportionality between stress and strain confirming that the experiments remained inside the elastic region for both materials. Furthermore, these trends demonstrated a low variability in experimental data, confirming the possibility to use this procedure for the direct measure of Poisson's Ratio in CGI and SGI.  Poisson's Ratio was calculated as individual values (by data experimentally measured for each specimen), as average, medium and standard deviations. These values are reported in Table 4.
Average values for Poisson's ratio can be considered: ν = 0.226 for Compacted Graphite Iron (CGI) ν = 0.240 for Spheroidal Graphite Iron (SGI) These results were in line with the indications of a Poisson's Ratio between 0.21 and 0.26 for cast iron. In two cases regarding CGI, the experimental results were apparently lower (0.16) or higher (0,28) than expected. But after a deeper analysis it was established that all mechanical properties were in the expected range. In particular, experimental data from tests performed over different lots of production, as detailed in [18], permitted to estimate a large variability in material properties in the case of CGI (related to intrinsic factors of processing). This variability is in line with the range of Poisson ratio. Moreover, also the chemical composition and microstructure as reported [19], in appeared in line with the theory. Thus, these extreme values were considered as a statistical divergence.

CONCLUSIONS
Poisson's Ratio for Compacted Graphite Iron (CGI) and Spheroidal Graphite Iron (SGI) were measured using tensile tests. The procedure for a direct testing of this mechanical propriety was described in details, including several remarks and suggestions. For both alloys it was possible to verify that Poisson's Ratio is in line with the overall expectations. At the same time, for both of them, it was possible to acquire, also in a comparative way, accurate experimental information as stress-strain curves and intensity of necking effects.
In general, it is important to note that, although SGI is quite largely investigated material in consideration of its wide use for industrial applications and market SGI CGI SGI CGI products, very few studies refers to CGI. A better knowledge on CGI mechanical proprieties, especially regarding proprieties as Poisson's Rate, frequently consider of secondary importance, conversely represents a fundamental step toward CGI formal classification between the other cast irons. Several organizations for standardization are moving in that direction with the aim at promoting its larger utilization in consideration of specific technology advantage offered by CGI respect to other cast irons.

ACKNOWLEDGEMENT
This investigation was realized thanks to the support of CRIF -Center of Research for Industrial Foundries inside the IPA AdriaHub collaborative project [24]. In particular, all specimens in SGI and CGI were realized inside the SCM sand casting foundry in Rimini, Italy.