Vittorio Di Cocco

Ductile cast irons: microstructure influence on fatigue crack propagation resistance

Microstructure influence on fatigue crack propagation resistance in five different ductile cast irons (DCI) was investigated. Four ferrite/pearlite volume fractions were considered, performing fatigue crack propagation tests according to ASTM E647 standard (R equals to 0.1, 0.5 and 0.75, respectively). Results were compared with an austempered DCI. Damaging micromechanisms were investigated according to the following procedures: - “traditional” Scanning Electron Microscope (SEM) fracture surfaces analysis; - SEM fracture surface analysis with 3D quantitative analysis; - SEM longitudinal crack profile analysis - Light Optical Microscope (LOM) transversal crack profile analysis;

Graphite nodules features identifications and damaging micromechanims in ductile irons

Ductile irons mechanical properties are strongly influenced by the metal matrix microstructure and on the graphite elements morphology. Depending on the chemical composition, the manufacturing process and the heat treatments, these graphite elements can be characterized by different shape, size and distribution. These geometrical features are usually evaluated by the experts visual inspection, and some commercial softwares are also available to assist this activity. In this work, an automatic procedure based on an image segmentation technique is applied: this procedure is validated not only considering spheroidal graphite elements, but also considering other morphologies (e.g. lamellae).

Fatigue crack tip damaging micromechanisms in a ferritic-pearlitic ductile cast iron

Due to the peculiar graphite elements shape, obtained by means of a chemical composition control (mainly small addition of elements like Mg, Ca or Ce), Ductile Cast Irons (DCIs) are able to offer the good castability of gray irons with the high mechanical properties of irons (first of all, toughness). This interesting properties combination can be improved both by means of the chemical composition control and by means of different heat treatments(e.g. annealing, normalizing, quenching, austempering etc). In this work, fatigue crack tip damaging micromechanisms in a ferritic-pearlitic DCI were investigated by means of scanning electron microscope observations performed on a lateral surface of Compact Type (CT) specimens during the fatigue crack propagation test (step by step procedure), performed according to the “load shedding procedure”. On the basis of the experimental results, different fatigue damaging micromechanisms were identified, both in the graphite nodules and in the ferritic – pearlitic matrix.

Degenerated graphite nodules influence on fatigue crack paths in a ferritic ductile cast iron

Focusing on ferritic-pearlitic DCIs, these alloys are characterized by a microstructure that ranges from a fully ferritic to a completely pearlitic matrix, and they are widely used for many applications (e.g. wheels, gears, crankshafts in cars, exhaust manifolds, valves, flywheels, boxes bearings, hubs, shafts, valves, flanges, pipelines ...). Considering the graphite elements, their morphology can be considered as degenerated when its nodularity is too low and this can be due to different causes (e.g., a partially failed nodularization process or a wrong inoculant). In this work, a ferritic DCI with degenerated nodules was obtained by means of an annealing treatment and the fatigue crack propagation resistance was investigated by means of fatigue crack propagation tests performed according to ASTM E647, focusing on the influence of degenerated graphite nodules on the fatigue crack paths. This analysis was performed both analysing the crack path profile by means of a scanning electron microscope (SEM) and by means of a SEM fracture surfaces analysis.

Stress triaxiality influence on damaging micromechanisms in a pearlitic ductile cast iron

In the last decades, damaging micromechanisms in ductile cast irons (DCIs) have been widely investigated, considering both the matrix microstructure and the loading conditions influence. Considering the graphite nodules, they were initially considered as voids embedded and growing in a ductile metal matrix (especially considering ferritic ductile cast irons). Recent experimental results allowed to identify a more complex role played by the graphite nodules, depending on the matrix microstructure. In this work, damaging micromechanisms in a pearlitic DCI were investigated by means of tensile tests performed on notched specimen, mainly focusing the role played by graphite elements and considering the stress triaxiality influence.

A simple model to calculate the microstructure evolution in a NiTi SMA

Shape memory alloys (SMAs) are a wide class of materials characterized by the property to recover the initial shape. This property is due to ability of alloys to change the microstructure from a “parent” microstructure (usually called “Austenite”) to a “product” microstructure (usually called “Martensite”). Considering the tensile resistance, SMAs stress strain curves are characterized by a sort of plateau were the transformations from Austenite to Martensite (in loading condition) and from Martensite to Austenite (in unloading condition) take place. In this work a simple model to predict the microstructure modification has been proposed and verified with an equiatomic NiTi alloy characterized by a pseudo-elastic behavior.

Mechanical Behaviour and Phase Transition Mechanisms of a Shape Memory Alloy by Means of a Novel Analytical Model

Abstract The aim of the present paper is to examine both the fatigue behaviour and the phase transition mechanisms of an equiatomic pseudo-elastic NiTi Shape Memory Alloy through cyclic tests (up to 100 loading cycles). More precisely, miniaturised dog-bone specimens are tested by using a customised testing machine and the contents of both austenite and martensite phase are experimentally measured by means of X-Ray diffraction (XRD) analyses. On the basis of such experimental results in terms of martensite content, an analytical model is here formulated to correlate the stress-strain relationship to the phase transition mechanisms. Finally, a validation of the present model by means of experimental data pertaining the stress-strain relationship is performed.

Special Issue on ‘Modern Imaging Techniques in Fracture and Damage Analyses’: Selected papers from the 21st European Conference of Fracture (ECF 21), held in Catania, Sicily, Italy, on 20–24 June 2016

This Special Issue of Engineering Fracture Mechanics contains selected papers invited on the basis of presentations made at the 21st European Conference on Fracture (ECF 21), held in Catania, Sicily, Italy, 20–24 June 2016. The Conference was very successful being accompanied by beautiful views, ambience and weather. This conference was officially recognised by the European Structural Integrity Society (ESIS). It was organised by Francesco Iacoviello (IGF President), Giuseppe Andrea Ferro, Donato Firrao and Luca Susmel as Chairmen. Support for the conference organisation was also provided by the MTS, GOM, Instron, Anton Paar, w+b ag, Rumul, Italsigma and was run by the Italian Group of Fracture. The conference provided an interactive forum with about 650 delegates from 44 countries. There were over 660 paper presentations most of which are described in a special issue of Procedia Structural Integrity. The conference covered a very diverse range of topics. One theme that emerged as a relatively new and exciting topic for ECFs was the use of imaging/full-field techniques and for this reason we decided to invite attendees to contribute to a collection of papers that cover this topic as a special issue. The 14 papers selected for this special issue demonstrate the wide range of full-field/imaging methods now being exploited to better understand the behaviour of fracture. Methods such as high resolution X-ray diffraction, thermoelasticity/thermography [pp. 1–12, pp. 13–25, pp. 53–65], 2D [pp. 26–38, pp. 39–52, pp. 53–65, pp. 79–83, pp. 94–108, pp. 109–124] and 3D [pp. 125–146] digital image correlation (DIC), confocal laser scanning microscopy [pp. 147–158] and X-ray microtomography [pp. 159–169, pp. 170–179] and laminography [pp. 180–189] are now being used to map the total or elastic strain fields, stress fields or to image crack propagation, damage mechanisms, crack opening displacements or to identify and quantify key crack-tip shielding mechanisms. In many cases these full field measurements are being combined with numerical modelling of the crack-tip stress field [pp. 39–52, pp. 170–179, pp. 66–78] to turn such full field image data into key fracture mechanics parameters. Of course thermoelastic stress mapping is a well-established technique – here it is used to look at the effect of repair and reinforcement strategies on the stress distribution around fatigue cracks [pp. 1–12]. The period of last 10 years has seen an explosion in the use of DIC to provide total crack-tip strain fields at the surface of materials with software and camera developments radically improving the strain mapping accuracy. Even more recently 3D imaging methods have been combined to provide 3D images of displacement or total strain fields. Indeed, one of the strength of majority of the full-field imaging methods covered in this special issue is that they can be applied to a very wide range of materials from metals and alloys (e.g. [pp. 94–108, pp. 170–179]), to elastomers [pp. 79–93], to fiber reinforced composites [pp. 39–52], to marble [pp. 109–124] and bone [pp. 125–146]. Further, at the conference we heard how the spatial resolution of X-ray diffraction is sufficient to image the elastic strain field local to cracks for direct correlation with the plastic strain recorded by DIC. X-ray tomography and laminography are able to provide complementary information about the defects that give rise to failure or the morphology, or opening of the primary or nucleation of secondary microcracks during crack propagation. In this respect, the paper [pp. 180–189] is noteworthy in using high resolution synchrotron radiation X-ray laminography to study cracks generated by rolling contact fatigue because the crack opening is typically small and the cracks hard to detect. By acquiring images non-destructively at various stages of loading these techniques allow the measurement of crack closure and crack opening and the local crack driving force compared to that nominally applied.

Fatigue crack micromechanisms in a Cu-Zn-Al shape memory alloy with pseudo-elastic behavior

Shape memory property characterizes the behavior of many Ti based and Cu based alloys (SMAs). In Cu-Zn-Al SMAs, the original shape recovering is due to a bcc phase that is stable at high temperature. After an appropriate cooling process, this phase (?-phase or austenitic phase) transforms reversibly into a B2 structure (transition phase) and, after a further cooling process or a plastic deformation, it transforms into a DO3 phase (martensitic phase). In ?-Cu-Zn-Al SMAs, the martensitic transformation due to plastic deformation is not stable at room temperature: a high temperature “austenitization” process followed by a high speed cooling process allow to obtain a martensitic phase with a higher stability. In this work, a Cu-Zn-Al SMA in “as cast” conditions has been microstructurally and metallographically characterized by means of X-Ray diffraction and Light Optical Microscope (LOM) observations. Fatigue crack propagation resistance and damaging micromechanisms have been investigated corresponding to three different load ratios (R=0.10, 0.50 and 0.75).

Fatigue loading of a ferritic ductile cast iron: Damaging characterization

Ductile cast irons offer and interesting combination of overall mechanical properties and technological peculiarities, allowing to obtain a high castability (peculiar of cast irons) with good tensile strength and toughness values (peculiar of steels). This result is due to their chemical composition that allows to obtain graphite elements in nodular shape directly from the melt. Ductile cast iron mechanical properties are strongly influenced both by the matrix microstructure and by the graphite nodules. The role of these elements cannot be merely ascribed to a simple matrix-graphite “debonding” damaging mechanism, but, according to previously published results, this role is more complex. In this work, customized image processing procedures were optimized to analyze the evolution of the damaging micromechanisms in a fatigue loaded ferritic ductile cast iron, focusing the graphite elements.