Study of the fracture of ferritic ductile cast iron under different loading conditions (original) (raw)
2014, Fatigue & Fracture of Engineering Materials & Structures
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This work is a continuation of the studies presented in a recent paper by the authors, where the fracture surfaces of pearlitic ductile cast iron under different loading conditions were exhaustively analysed. In this study, fracture surfaces of ferritic ductile cast iron (or ferritic spheroidal graphite cast iron) generated under impact, bending and fatigue loading conditions were characterised and compared. The fracture surfaces were characterised qualitatively and quantitatively from the observation under a scanning electron microscope. The fracture mechanisms in each case were identified. For impact tests, as test temperature increases, the dominant fracture mechanism changes from brittle to ductile. For bending tests, a fully ductile fracture micromechanism dominates the surface. In fatigue tests, the surface shows a mix of flat facets that appear to be cleavage facets and ductile striations, but the typical fatigue striations are not easily found on the fracture surface. Methodologies for the determination of the macroscopic direction of main crack propagation in both ductile and brittle failure modes are proposed. These allow identifying main crack propagation direction with good approximation. The results are potentially useful to identify the nature of loading conditions in a fractured specimen of ferritic spheroidal graphite cast iron. The authors believe that it is necessary to extend the methodologies proposed in samples with different geometry and size, before they can be used to provide additional information to the classical fractographic analysis.
Related papers
2014
In this article, the characteristic fractographic features of ductile cast irons with different metallic matrices were analysed. Several heat treatment cycles were carried out on samples from the same cast iron melt in order to obtain ferritic ductile iron, pearlitic ductile iron and austempered ductile iron. The specimens were broken under impact and bending tests and the fracture surfaces were analysed by means SEM observation. Two methodologies to evaluate the main crack propagation based on some characteristic features of the fracture surface were proposed. The investigation involves a systematic study of the fracture surfaces of ductile iron of different matrices. The authors propose methodologies suitable for identifying the main crack propagation direction of fracture processes of unknown origin applicable to brittle and ductile fracture cases.
Effects of microstructure and crystallography on crack path and intrinsic resis.PDF
The paper focuses on the effective resistance and the near-threshold growth mechanisms in the ferritic-pearlitic and the pure pearlitic steel. The influence of microstructure on the shear-mode fatigue crack growth is divided here into two factors: the crystal lattice type and the presence of different phases. Experiments were done on ferritic-pearlitic steel and pearlitic steel using three different specimens, for which the effective mode II and mode III threshold values were measured and fracture surfaces were reconstructed in three dimensions using stereophotogrammetry in scanning electron microscope. The ferritic-pearlitic and pearlitic steels showed a much different behaviour of modes II and III cracks than that of the ARMCO iron. Both the deflection angle and the mode II threshold were much higher and comparable to the austenitic steel. Mechanism of shear-mode crack behaviour in the ARMCO iron, titanium and nickel were described by the model of emission of dislocations from the crack tip under a dominant mode II loading. In other tested materials the cracks propagated under a dominance of the local mode I. In the ferritic-pearlitic and pearlitic steels, the reason for such behaviour was the presence of the secondary-phase particles (cementite lamellas), unlike in the previously austenitic steel, where the fcc structure and the low stacking fault energy were the main factors. A criterion for mode I deflection from the mode II crack-tip loading, which uses values of the effective mode I and mode II thresholds, was in agreement with fractographical observations.
Acta Materialia, 2009
3D tomographic images of a nodular graphite cast iron obtained using a laboratory X-ray source were used to analyze the opening of a fatigue crack during in-situ mechanical loading. Direct image analysis and digital image correlation are utilized to obtain the 3D morphology and front location of the crack as well as the displacement fields in the bulk of the specimen. From DIC results, it is possible to extract the Crack Opening Displacement (COD) map in the whole sample cross-section and to compute Stress Intensity Factors (SIF) all along the crack front even for COD values that are less than the image resolution. The comparison of COD maps with local values of SIF enabled for an estimation of the opening SIF, K op , equal to 6 MPa√m.
Pearlitic ductile cast iron fatigue crack propagation: damaging micromechanisms at crack tip
Ductile cast irons (DCIs) are characterized by a wide range of mechanical properties, mainly depending on microstructural factors, as matrix microstructure (characterized by phases volume fraction, grains size and grain distribution), graphite nodules (characterized by size, shape, density and distribution) and defects presence (e.g., porosity, inclusions, etc.). Versatility and higher performances at lower cost if compared to steels with analogous performances are the main DCIs advantages. In the last years, the role played by graphite nodules was deeply investigated by means of tensile and fatigue tests, performing scanning electron microscope (SEM) observations of specimens lateral surfaces during the tests ("in situ" tests) and identifying different damaging micromechanisms. In this work, a pearlitic DCIs fatigue resistance is investigated considering both fatigue crack propagation (by means of Compact Type specimens and according to ASTM E399 standard) and overload effects, focusing the interaction between the crack and the investigated DCI microstructure (pearlitic matrix and graphite nodules). On the basis of experimental results, and considering loading conditions and damaging micromechanisms, the applicability of ASTM E399 standard on the characterization of fatigue crack propagation resistance in ferritic DCIs is critically analyzed, mainly focusing the stress intensity factor amplitude role.
Experimental Fracture Assessment Diagram by Ultimate Crack Resistance
2013
The report is devoted to application of the two-parameter fracture criterion to experimental results for the specimens with cracks. Tension specimens were cut out from the defective rolled steel plate with stratifications in the middle of its thickness. The plate thickness is 100 mm and the length of the specimens is same. In the middle of specimens there were cracks of different shapes and sizes which were measured after specimen fracture. The maximum critical (fracture) load was recorded. For each crack configuration, at a known fracture load, there were critical stress intensity factors. Crack sizes, fracture loads and character of fracture for all specimens were different. Therefore critical stress intensity factors are also different. By the definition, these factors are called the ultimate crack resistance [Morozov, 1968]. Dependence of the ultimate crack resistance on the fracture stress is called the failure assessment diagram (twoparameter fracture criterion). Experimental ...
Fractographic analysis of austempered ductile iron
Fatigue & Fracture of Engineering Materials & Structures, 2015
investigation involves a systematic study of the fracture surfaces of two grades of austempered ductile iron (ADI) broken under quasi-static, dynamic and cyclic loading conditions. The study used electron microscopy, optical microscopy and image postprocessing. The results show that the predominating fracture mechanism in ADI upon impact loading changes from quasi-cleavage to ductile (with little areas of cleavage facets) as the testing temperature increases. Noticeably, even at the lower temperatures tested, the fracture surface of ADI shows clear signs of ductile fracture mechanisms. In particular, graphite nodule cavities suffer marked plastic deformation. Fracture after bending tests at room temperature was characterized by a mix of quasi-cleavage facets, deformation of the contour of nodular cavities and microvoid coalescence. In the case of fatigue fracture at room temperature, the fracture surfaces show a flat appearance which has notorious differences with those reported for other loading conditions, but the typical fatigue striations were not found. The particular features identified on the fatigue fracture surfaces can be used to identify fatigue failures. It was also shown that the determination of the direction of main crack propagation by using the experimental methodology proposed earlier by the authors is applicable to ADI fractured by impact and quasi-static loads. The results provide information potentially useful to fractographic analyses of ADI, particularly in samples that fail in service under unknown conditions.
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