Experimental and numerical assessment of fracture toughness of dual‐phase austempered ductile iron

Abstract

A B S T R A C TThe effects of the microstructure topology on the fracture toughness of dual-phase austempered ductile iron are studied in this paper by means of finite element modelling and experimental testing. To this end, specimens with matrix microstructures ranging from fully ferrite to fully ausferrite were studied and the preferential zones and phases for crack propagation were identified in every case. The effectiveness of the ausferrite phase as a reinforcement of the ferritic matrix via the encapsulation of the brittle and weak last-to-freeze (LTF) zones was confirmed. The toughening mechanism is consequence of the increment in the crack path longitude as it avoids the encapsulated LTF zones. Besides, the presence of small pools of allotriomorphic ferrite increase the crack propagation resistance of the ausferrite-ferrite matrices. 0 n = mode-I fracture energy G 0 s = mode-II fracture energy K IC = Fracture toughness t 0 n = maximum normal traction t 0 s = maximum shear traction ε ut = ultimate strain σ 0.2 = yield stress σ ut = ultimate tensile stress ν = Poisson ratio I N T R O D U C T I O NThe engineering community is strongly pressed to produce lighter, stronger and stiffer metallic parts. Ductile iron (DI) can be a material of choice to fabricate numerous parts, because it is suitable to produce high resistance cast parts of complex shape and relatively inexpensive. For this reason DI is used to successfully replace cast and forged steels parts in a large number of applications. 1 The mechanical properties of sound DI cast parts depends mainly on the matrix microstructure (type, amount and distribution of microconstituents), the graphite phase and the cast defects present mainly in the last-to-freeze (LTF) regions. A wide range of microstructures, and consequentlyCorrespondence: Adrián P. Cisilino. mechanical properties, can be obtained by using different heat treatments. 2 Currently, DI producers work in the continuous improvement of DI properties while seeking for new applications. In particular, the market of safety critical parts in the automotive industry is a main target where high strength and toughness are customary requirements. In this regard special thermal cycles have been developed in order to obtain mixed microstructures. Wade et al., 3 He et al. 4 and Rashidi et al. 5 have obtained mixed microstructures starting from rapid uncompleted austenitizations into the austenitic field. The austenite nucleated mainly surrounding graphite nodules is transformed into pearlite, bainite or martensite depending on the cooling rate. The final phases obtained consisted of ferrite and pearlite, bainite or martensite. The relationship between

The truss‐like discrete element method in fracture and damage mechanics

“…Mechanical properties of nodular cast iron vary in a wide range of values, mostly controlled by the effect of the nodule size and distribution, together with the process of microcrack nucleation, growth and coalescence (Berdin et al, 2001;Bonora and Ruggiero, 2005;Basso et al, 2009). It is presented in this example the application of the DEM to model the micromechanics of failure of a nodular cast iron.…”

Determination of effective elastic properties of ferritic ductile cast iron by computational homogenization, micrographs and microindentation tests

“…In general, higher nodular counts and nodularities promote better mechanical properties (Burditt, 1992;http://www.ductile.org, 0000). In what respects to the material-behavior constitutive data, the usual assumption is to assimilate the matrix as homogeneous (Bonora and Ruggiero, 2005;Ghosh and Moorthy, 1995;Carazo et al, 2014;Hollister and Kikuchi, 1994;Hashin, 1983;Ortiz et al, 2001a;Basso et al, 2009;Kosteski et al, 2011;Ortiz et al, 2001b). However, the experimental evidence shows that the matrix presents a high degree of heterogeneity (see Fig.…”

“…Finite, boundary and discrete element methods have been used to study the DI effective elastic response (Bonora and Ruggiero, 2005;Carazo et al, 2014) fatigue crack propagation (Ortiz et al, 2001a) and fracture (Bonora and Ruggiero, 2005;Basso et al, 2009;Kosteski et al, 2011). The hypotheses used in the geometrical description of the microstructure play a key role point when dealing with computational models.…”

“…The hypotheses used in the geometrical description of the microstructure play a key role point when dealing with computational models. There are three approaches to account for the size and spatial distribution of the nodules: they are assumed to be of one size and periodically located (Bonora and Ruggiero, 2005), they are artificially generated by means of computer algorithms (Ortiz et al, 2001a;Basso et al, 2009;Kosteski et al, 2011;Ortiz et al, 2001b) or they are directly extracted from actual micrographs (Carazo et al, 2014). Analysis based on periodic microstructures are suitable to elaborate qualitative descriptions of the material behavior, but as it has been shown by Kosteski et al (2011), they may conduct to erroneous results, especially when non-linear phenomena are involved.…”

High Silicon Ductile Iron: Possible Uses in the Production of Parts with ^|^ldquo;Dual Phase ADI^|^rdquo; Microstructure

“…This improvement is explained by the novelty of this DI microstructure, which is composed of different amounts of free (or allotriomorphic) ferrite and ausferrite (ADI regular structure). [1][2][3][4][5][6][7][8][9][10][11] Special thermal cycles have been developed by different researchers in order to obtain Dual Phase ADI. One of them has been advanced and used by Kilicli et al 7,8) and Basso et al [4][5][6] It consists in subjecting fully ferritic DI parts to an incomplete austenitization stage, at different temperatures within the intercritical interval of the Fe-C-Si diagram (see Fig.…”