Issue 26

Anno VII Numero 26 Ottobre 2013

Rivista Internazionale Ufficiale del Gruppo Italiano Frattura Fondata nel 2007

Editor-in-chief: Francesco Iacoviello Associate Editors: Alfredo Navarro Robles

ISSN 1971-8993

Luca Susmel John Yates

Editorial Advisory Board:

Harm Askes Alberto Carpinteri Andrea Carpinteri

Donato Firrao M. Neil James Gary Marquis

Robert O. Ritchie Darrell F. Socie Cetin Morris Sonsino Ramesh Talreja David Taylor

Frattura ed integrità strutturale The International Journal of the Italian Group of Fracture

www.gruppofrattura.it

Frattura ed Integrità Strutturale, 26 (2013); Rivista Ufficiale del Gruppo Italiano Frattura

Table of Contents

A. Boschetto, L. Bottini, F. Campana, L. Consorti, D. Pilone Investigation via morphological analysis of aluminium foams produced by replication casting …………... 1 A. De Santis, D. Iacoviello, V. Di Cocco, F. Iacoviello Graphite nodules features identifications and damaging micromechanims in ductile irons ……………… 12 A . Namdar, I. B. Zakaria, A. Bt Hazeli, S. J. Azimi, A. Syukor Bin Abd. Razak, G. S. Gopalakrishna An experimental study on flexural strength enhancement of concrete by means of small steel fibers …........ 22 S. Agnetti Strength on cut edge and ground edge glass beams with the failure analysis method …………...……… 31 S. Bulatovic, V. Aleksic, L. Milovic Failure of steam line causes determined by NDT testing in power and heating plants ………………... 41 A. Tridello, D.S. Paolino, G. Chiandussi, M. Rossetto Comparison between dog-bone and Gaussian specimens for size effect evaluation in gigacycle fatigue …..… 49 A. De Iorio, M. Grasso, F. Penta, G.P. Pucillo About the certification of railway rails …………………….…………………………………… 57 M. Grasso, F. Penta, P. Pinto, G.P. Pucillo A four-parameters model for fatigue crack growth data analysis ….………………………………... 69 E. Salvati, P. Livieri, R. Tovo Fattori di intensificazione delle tensioni per cricche ad angolo triangolari in corrispondenza di intersezione di fori sollecitate a modo I …………………..………………………………………………... 80 R. Citarella, M. Lepore, J. Fellinger, V. Bykov, F. Schauer Coupled FEM-DBEM method to assess crack growth in magnet system of Wendelstein 7-X ………… 92 L. Allegrucci, F. De Paolis, A. Coletta, M. Bernabei Rottura di un attacco del servocomando del piatto oscillante di un elicottero: failure analysis e lessons learned ………………………………………………………………………..…………… 104 S. Foletti, A. Lo Conte, S. Salgarollo, F. Bassi, A. Riva High temperature initiation and propagation of cracks in 12%Cr-steel turbine disks ………………… 123 T. Ingrassia, M. Mucera, V. Nigrelli Comportamento di un fucile subacqueo con testata innovativa …………………………………… 132 G. Fargione, D. Tringale, E. Guglielmino, G. Risitano Fatigue characterization of mechanical components in service ……………………………………… 143

I

Frattura ed Integrità Strutturale, 26 (2013); ISSN 1971-9883

Editor-in-Chief Francesco Iacoviello

(Università di Cassino e del Lazio Meridionale, Italy)

Associate Editors Alfredo Navarro Robles

(Escuela Superior de Ingenieros, Universidad de Sevilla, Spain)

Luca Susmel John Yates

(University of Sheffield, UK) (University of Manchester, UK)

Advisory Editorial Board Harm Askes

(University of Sheffield, Italy) (Politecnico di Torino, Italy) (Università di Parma, Italy) (Politecnico di Torino, Italy) (University of Plymouth, UK)

Alberto Carpinteri Andrea Carpinteri

Donato Firrao M. Neil James Gary Marquis

(Helsinki University of Technology, Finland)

Robert O. Ritchie Darrell F. Socie Cetin Morris Sonsino

(University of California, USA)

(University of Illinois at Urbana-Champaign, USA)

(Fraunhofer LBF, Germany) (Texas A&M University, USA) (University of Dublin, Ireland)

Ramesh Talreja David Taylor

Journal Review Board Stefano Beretta

(Politecnico di Milano, Italy)

Nicola Bonora Lajos Borbás Francesca Cosmi

(Università di Cassino e del Lazio Meridionale, Italy) (Budapest University Technology and Economics, Hungary)

(Università di Trieste, Italy) (EADS, Munich, Germany)

Claudio Dalle Donne Vittorio Di Cocco Josef Eberhardsteiner Giuseppe Ferro Tommaso Ghidini Mario Guagliano Carmine Maletta Liviu Marsavina

(Università di Cassino e del Lazio Meridionale, Italy)

(IMWS, Wien, Austria)

(Politecnico di Torino, Italy)

(European Space Agency - ESA-ESRIN) (Politecnico di Milano, Italy) (Università della Calabria, Italy) (University of Timisoara, Romania) (University of Porto, Portugal)

Lucas Filipe Martins da Silva

Marco Paggi

(Politecnico di Torino, Italy) (Università di Parma, Italy)

Alessandro Pirondi

Ivatury S. Raju

(NASA Langley Research Center, USA)

Giacomo Risitano Roberto Roberti

(Università di Messina, Italy) (Università di Brescia, Italy) (Università di Bologna, Italy) (Università di Parma, Italy)

Marco Savoia

Andrea Spagnoli

II

Frattura ed Integrità Strutturale, 26 (2013); Rivista Ufficiale del Gruppo Italiano Frattura

Descrizione e scopi Frattura ed Integrità Strutturale è la rivista ufficiale del Gruppo Italiano Frattura . E’ una rivista open-access pubblicata on-line con periodicità trimestrale (luglio, ottobre, gennaio, aprile). Frattura ed Integrità Strutturale riguarda l’ampio settore dell’integrità strutturale, basato sulla meccanica della fatica e della frattura, per la valutazione dell’affidabilità e dell’efficacia di componenti strutturali. Scopo della rivista è la promozione di lavori e ricerche sui fenomeni di frattura, nonché lo sviluppo di nuovi materiali e di nuovi standard per la valutazione dell’integrità strutturale. La rivista ha un carattere interdisciplinare e accetta contributi da ingegneri, metallurgisti, scienziati dei materiali, fisici, chimici e matematici. Contributi Frattura ed Integrità Strutturale si prefigge la rapida disseminazione di contributi originali di natura analitica, numerica e/o sperimentale riguardanti la meccanica della frattura e l’integrità strutturale. Si accettano lavori di ricerca che contribuiscano a migliorare la conoscenza del comportamento a frattura di materiali convenzionali ed innovativi. Note tecniche, lettere brevi e recensioni possono essere anche accettati in base alla loro qualità. L’ Editorial Advisory Board sollecita anche la pubblicazione di numeri speciali contenenti articoli estesi presentati in occasione di conferenze e simposia tematici. Istruzioni per l’invio dei manoscritti I manoscritti devono essere scritti in formato word senza necessità di utilizzare un particolare stile e devono essere inviati all'indirizzo iacoviello@unicas.it. Il lavoro proposto può essere in lingua Italiana (con riassunto in inglese di almeno 1000 parole e didascalie bilingue) o Inglese. La conferma della ricezione avverrà entro 48 ore. Il processo di referaggio e pubblicazione on-line si concluderà entro tre mesi dal primo invio. Journal description and aims Frattura ed Integrità Strutturale (Fracture and Structural Integrity) is the official Journal of the Italian Group of Fracture. It is an open-access Journal published on-line every three months (July, October, January, April). Frattura ed Integrità Strutturale encompasses the broad topic of structural integrity, which is based on the mechanics of fatigue and fracture, and is concerned with the reliability and effectiveness of structural components. The aim of the Journal is to promote works and researches on fracture phenomena, as well as the development of new materials and new standards for structural integrity assessment. The Journal is interdisciplinary and accepts contributions from engineers, metallurgists, materials scientists, physicists, chemists, and mathematicians. Contributions Frattura ed Integrità Strutturale is a medium for rapid dissemination of original analytical, numerical and experimental contributions on fracture mechanics and structural integrity. Research works which provide improved understanding of the fracture behaviour of conventional and innovative engineering material systems are welcome. Technical notes, letters and review papers may also be accepted depending on their quality. Special issues containing full-length papers presented during selected conferences or symposia are also solicited by the Editorial Board. Manuscript submission Manuscripts have to be written using a standard word file without any specific format and submitted via e-mail to iacoviello@unicas.it. The paper may be written in English or Italian (with an English 1000 words abstract). A confirmation of reception will be sent within 48 hours. The review and the on-line publication process will be concluded within three months from the date of submission.

Publisher Gruppo Italiano Frattura (IGF) http://www.gruppofrattura.it ISSN 1971-8993 Reg. Trib. di Cassino n. 729/07, 30/07/2007

III

Frattura ed Integrità Strutturale, 26 (2013); ISSN 1971-9883

Videos!!

C

ara Lettrice, caro Lettore, utilizziamo queste poche righe per darti qualche novità riguardanti la vita della rivista IGF, ormai arrivata al suo ventiseiesimo numero. Anzitutto l’acronimo che abbiamo deciso di adottare per la rivista IGF nell’ultima assemblea dei Soci IGF: F&IS . Da oggi questa sarà la denominazione abbreviata di Frattura ed Integrità Strutturale ! Come avrai già notato, abbiamo pubblicato alcuni “numeri dedicati”. Non vogliamo chiamarli “special issue”. Si tratta piuttosto di numeri pubblicati secondo la cadenza normale che, oltre a contenere articoli proposti e revisionati secondo la normale procedura, contengono anche una serie di articoli dedicati ad un argomento oppure ad un evento particolare. Per questi numeri i due Board e gli Editor sono coadiuvati da uno o più Guest Editor. Si tratta, ad esempio, del numero 24 (aprile 2013), dedicato alla Russian Fracture Mechanics School , oppure del numero 25 (luglio 2013), dedicato al Workshop organizzato a Malaga dal titolo Characterization of Crack Tip Stress Field . Quindi, se vuoi proporre una special issue dedicata ad un argomento particolare oppure ad un evento di interesse per la comunità IGF, contattaci!! Altro evento importante è l’arrivo di un nuovo Associate Editor: Alfredo Navarro Robles della Escuela Superior de Ingenieros, Universidad de Sevilla, Spagna! Siamo certi che con il suo contributo faremo crescere ulteriormente la qualità e la diffusione della rivista IGF. Infine, e forse questa è la novità più importante, desideriamo comunicarvi che a partire da questo numero sarà possibile inserire dei video all’interno degli articoli e, ovviamente, commentare questi video all’interno del testo dell’articolo. Si tratta di una modalità assolutamente innovativa di divulgazione. A causa della molteplicità di sistemi operativi e di browser, e delle diverse modalità di fruizione dei lavori (desktop, laptop e mobile) inseriremo i video come link multimediali che potranno funzionare in maniera differente in funzione del device, del sistema operativo e del browser. In corrispondenza delle figure che potranno attivarsi come video troverete queste due icone: La prima icona (sicuramente funzionate su sistemi Windows desktop), permette di fruire il video in maniera assolutamente interessante: selezionandola si apre infatti il video all’interno di una finestra mobile, che consente di scorrere il testo dell’articolo e visionare contemporaneamente il video (l’icona è collegata ad un file flv). La seconda icona è più utile in altri sistemi operativi (ad esempio Apple-ipad) e permette di fruire i video in una nuova finestra (l’icona è collegata ad un file mp4). E’ da sottolineare che la dimensione del file pdf resta immutata, in quanto i file sono solo collegati ai differenti file. Si tratta evidentemente di una modalità di pubblicazione innovativa che nel prossimo futuro sicuramente vedrà delle evoluzioni. Speriamo che questa novità riscuota il vostro apprezzamento, a presto Francesco Iacoviello Direttore F&IS

IV

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.26.01

Investigation via morphological analysis of aluminium foams produced by replication casting

A. Boschetto, L. Bottini, F. Campana, L. Consorti Dip.di Ingegneria Meccanica e Aerospaziale, Sapienza Università di Roma, Roma (Italy) alberto.boschetto@uniroma1.it, luana.bottini@uniroma1.it, francesca.campana@uniroma1.it, lconsorti@gmail.com D. Pilone Dip. Ingegneria Chimica Materiali Ambiente, Sapienza Università di Roma, Roma (Italy) daniela.pilone@uniroma1.it A BSTRACT . Foams and porous materials with cellular structure have many interesting combinations of physical and mechanical properties coupled with low specific weight. By means of replication casting it is possible to manufacture foams from molten metal without direct foaming. A soluble salt is used as space holder, which is removed by leaching in water. This can be done successfully if the content of space holding fillers is so high that all the granules are interconnected. One of the main advantages of using the replication casting is a close control of pore sizes which is given by the distribution of particle sizes of the filler material. This contrasts with the pore size distribution of the materials foamed by other processes where a wider statistical distribution of pores is found. On the other hand, the maximum porosities that can be achieved using space holders are limited to values below 60%, whereas the other methods allow for porosities up to 98%. Temperature of the mould and infiltration pressure are critical process parameters: a typical problem encountered is the premature solidification of the melt, especially due to the high heat capacity of the salt. In this work foam properties such as cell shape, distribution and anisotropy and defect presence are investigated by using digital image processing technique. For this purpose replicated AlSi7Mg0.3 alloy foams are produced by infiltrating preforms of NaCl particles, varying the metal infiltration pressure and the mould preheating temperature. An original procedure based on image analysis has been set up to determine size, morphology and distribution of cells. The paper demonstrates that this methodology, coupled with microstructural analysis, is a useful tool for investigating the effects of process parameters on foam properties. K EYWORDS . Aluminium foams; Foam morphology; Image analysis; Replication casting; Watershed method.

I NTRODUCTION

pattern[1]. M

etal foams are a class of materials with interesting physical, mechanical, thermal, electrical, structural and acoustic properties. The current understanding of production methods ranges from ancient to novel processing techniques, many of which still under development. The processes can be divided into several classes: melt gas injection, gas-releasing particle decomposition in the melt or in semi-solids, casting using a polymer or wax precursor as template, metal decomposition on cellular preforms, entrapped gas expansion, metal infiltration through a leachable

1

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.25.01

This last technique is characterised by the following steps: a bed of particles of a leachable material is infiltrated by liquid metal under pressure and allowed to cool [2], afterwards leaching of the particles gives a cellular metallic structure of great uniformity. Although salt grains offer the advantage that they can be sintered to enhance the connectivity of the salt and change the structure of the pattern, the obtainable density depends on the packing efficiency of the granules, which is related to their size distribution. This process was pioneered by Polonsky et al. [3] that produced aluminium alloy foams. Only in the recent 15 years the production of high-purity foams by replication has been revived [4]. Adair et al. [5] studied the crystal shape and the growth of salt crystals to obtain several structures. In Gaillard et al. [6] the authors set up a procedure to fabricate NaCl powders by controlling their shape. Foams with spherical cells have been produced by Jiang et al. [7]. Goodall et al. [8, 9] varied the foam relative density by densifying the NaCl preform before infiltration. The results showed that cold pressing rather than sintering yields superior Young modulus. Despois et al. [10] investigated the effect of infiltration pressure on mechanical properties and permeability of foams. They found that as the pressure is increased, small finger-like protrusions appear lowering the foam permeability to fluid flow. On the other hand mechanical properties such as Young ’ s modulus and yield stress increase with increasing relative density. Kadar et al. [11] studied the compression behaviour of foams manufactured by replication casting of an eutectic Al – Si alloy. The acoustic emission has been employed to study that behaviour. Quadrini et al. [12] observed different microstructures along the height of the samples: the grain size decreases from the top, near the cooled piston, to the bottom of the foam. This behaviour can be related to the cooling rate and pressure during solidification [13]. It is noteworthy that the grain size distribution is everywhere fine if compared with the bulk sample: this is due not only to the holding pressure but also to the salt precursor. Thus good mechanical properties are expected and a further increase could be obtained after heat treatment. Porosity of the foam has been measured by Boschetto et al. [14] by means of digital image processing. The morphological analysis of the foam showed sharpened voids, especially in comparison with the ones obtained by compact powder processes. This is justified by the salt pattern that determines the formation of angular and faceted cells. At present metal foams have a very wide range of applications due to their properties [15]. Examples are: lightweight structures due to excellent stiffness-to-weight ratio [16]; heat exchangers and refrigerators due to their high specific surface area [17]; energy absorbers for the ability to absorb energy at constant pressure [18, 19]; acoustic absorbers for their sound absorbing capacity [19]; sandwich cores due to low density and fracture strength; flame arrester panels for their conductivity; filters for gas and liquid filtration; catalyst carriers for their good conductivity and high specific surface area; shock wave dissipation devices due to dumping capacity; air batteries. Considering that each application requires specific foam properties, knowing the relationship between manufacturing process parameters and foam properties is of paramount importance to tailor properties for a given application. For that reason in this work an original procedure based on image analysis has been set up to determine size, morphology and distribution of cells. This methodology, coupled with microstructural analysis, is a useful tool for investigating the effects of process parameters on foam properties. In order to demonstrate the capabilities of the proposed method, in this paper the results related to twelve AlSi7Mg0.3 alloy specimens produced by using different process parameters are discussed. First the experimental set up to produce the foams is described together with the proposed specimen analysis method, then results and discussion are reported and finally the conclusions are drawn. Specimen fabrication he alloy used for producing foams is AlSi7Mg0.3 (A356.0). This alloy is commonly used in pressure and gravity casting for fabricating electric motors, automotive and structural components. The pattern was made of commercial NaCl that allows to set up a quite cheap industrial process. The mould was composed by a cylindrical core (Fig.1), a positioning base and an upper conveyor. The resulting specimen is 28 mm in diameter and 50 mm in height. A hollow oven allowed to keep constant the temperature of the model by a thermostatic controller. A hydraulic cylinder was employed to provide the pressure for the infiltration and solidification stage. The assembled apparatus is reported in Fig. 2. Pressure, piston speed, mould and crucible temperatures were acquired by an A/D input device system and controlled through Labview. The specimen manufacturing process includes several steps. First a sample (40 g) was taken from an aluminium ingot. The alloy was put in a graphite crucible heated in a muffle at 700 ° C. The salt (9 g) was sieved by means of a 3 mm mesh sieve in order to eliminate all the smaller particles, placed in a second crucible and heated at 700 ° C. The hollow oven was used T E XPERIMENTAL

2

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.26.01

to keep constant the mould temperature. Then the salt was poured into the mould and subsequently the aluminium alloy was cast. The piston was pushed at a speed of 32 mm/s to close the mould and provide the predefined pressure to infiltrate the pattern. The pressure was maintained for 30 s. After solidification and cooling the specimen was extracted by opening the mould. Then the salt was dissolved in water. By means of this process 12 specimens were produced changing mould temperature and injection pressure on two levels: (500, 550) ° C and (20,30) bar.

Figure 1 : Mould used for foam production.

(c)

(a)

(b)

Figure 2 : Experimental apparatus: (a) hollow oven, (b) mould, (c) piston.

Specimen analysis The analysis was performed on longitudinal sections of every specimen after cutting by a Minitom Struers cut-off machine (125 mm in diameter and 0,5 mm thick wheel, with bakelite bonded SiC abrasive). Images of these sections were acquired in order to perform image analysis of voids. To this end the surfaces were painted using a black dye in order to obtain good contrast. The specimens were ground with a series of SiC papers and afterwards polished with 1 μ m alumina. Then they were acquired by an Image Sensor Type CCD with 5.04 Megapixel resolution and f\2.8 Carl Zeiss optic lens. The image of each specimen was calibrated to avoid aberrations and positioning errors [19]. It was enhanced by means of contrast equalisation and bright correction. A median filter was applied in order to avoid random error and impulse noise. A binarisation separated features (cell cavities) from background. The segmentation was performed by using an automatic watershed method [14] developed to highlight each cavity and each cell contributing to cavity formation. In fact some process problems can produce larger cavities by cell intersection. The automatic watershed method allows to isolate these cells by using seeds positioned at the intersections of the segments that define the binary image skeleton (Fig. 3c and Fig. 3d). Fig. 3e shows the results of the seed definition overlapped to the original specimen image. From these seeds a segmentation of the cells that contribute to each cavity is determined (Fig. 3f).

3

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.25.01

(a)

(a)

(c)

(b)

(d)

(e)

(f)

Figure 3 : Automatic watershed method isolates cell contributing to cavity formation. (a-f) is the sequence of image processing from image binarisation (a) to segmentation (f) . Shape indicators considered in this study are circularity, rectangularity and elongation. Circularity is a ratio of the perimeter of a circle with the same area as the particle divided by the perimeter of the actual particle image. Rectangularity is measured by the ratio of the area of the region to the area of its minimum bounding box. Elongation is calculated as 1- width/height where width is the smallest axis of the best-fit ellipse of data and height is the largest one.

R ESULTS AND D ISCUSSION

T

ab. 1 shows the global characteristics of the specimens in terms of weight, density and height according to process parameters. It can be observed that the mean density value is 1.045 ± 0.110 g/cm 3 . A close observation of the specimens reveals that four of them are characterised by a discontinuous external wall. This is related to the mould preheating temperature that affects cooling rates and that can determine a premature solidification of the foam external layer. This assumption is confirmed by the fact that four of the six specimens obtained by preheating the mould at 500 ° C have this defect (Fig. 4).

weight [g]

height [mm]

density [g/cm 3 ]

Specimen

T [°C]

P [bar]

#1 #2 #3 #4 #5 #6 #7 #8 #9

500 500 500 500 500 500 550 550 550 550 550 550

20 20 20 30 30 30 20 20 20 30 30 30

30.2 32.9 32.4 28.8 30.7 24.9 33.5 31.8 34.1 34.7 33.2 27.1

52.84 49.21 56.18 49.97 45.26 44.44 48.80 52.00 47.24 47.45 44.92 45.25

0.928 1.086 0.937 0.936 1.102 0.910 1.115 0.993 1.172 1.188 1.200 0.973

#10 #11 #12

Table 1 : Global characteristics of the specimens.

4

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.26.01

#1 #3 #9 #12 Figure 4 : Macrographs showing the external appearance of specimens characterised by a discontinuous external layer.

Fig. 5 shows foam density as a function of temperature and pressure. Foam density decreases by decreasing mould temperature. In fact a mould temperature decrease raises cooling rates with consequent quick solidification of the alloy occurring probably before the mould is completely filled. This phenomenon explains also the reason why specimens #1, #3, #9, #12 have a discontinuous external layer. As far as the pressure is concerned Fig. 5 shows that it does not affect specimen density at the lower considered mould temperature while at higher temperature its increase seems to minimally raise density. This can be explained considering that at 550°C solidification is slightly delayed and then an higher applied pressure may enhance metal infiltration through the salt pattern. The Analysis of Variance (ANOVA) with a 95% confidence interval, confirmed that there are no significant evidences for pressure single effect as well as pressure- temperature interaction.

Figure 5 : Foam density according to temperature and pressure variations on two levels. The error bars correspond to the standard deviation. In order to find the relationship between manufacturing process parameters and foam morphology, image analysis has been used to study specimen longitudinal sections. Void distribution and averaged density as well as void circularity, elongation and rectangularity have been analysed and their mean values have been reported in Tab. 2. Specimen #12 has been evaluated as an outlier sample since its external wall is strongly discontinuous and moreover its section is affected by anomalous cavities, probably due to inefficient infiltration through the salt pattern (Fig. 6). This is confirmed by standardised residual (the ratio of residual to the estimate of its standard deviation) of this specimen because its value, 2.11, is greater than 2, thus it must be classified as an unusual observation [20]. In figure 6 it is evident

5

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.25.01

how the voids in specimen #12 are unevenly distributed and of greater size than desired, and in the specimen centre part they also are merging together to create a single great cavity.

averaged void density

Specimen

circularity

elongation

rectangularity

#1 #2 #3 #4 #5 #6 #7 #8 #9

0.676 0.581 0.666 0.661 0.589 0.667 0.645 0.563 0.515 0.568 0.535 0.738 0.617 0.068

0.612 0.600 0.602 0.603 0.584 0.608 0.585 0.599 0.577 0.560 0.570 0.591 0.017 -

0.437 0.434 0.415 0.443 0.423 0.451 0.414 0.443 0.416 0.411 0.442 0.430 0.014 -

0.483 0.459 0.489 0.462 0.481 0.488 0.479 0.469 0.480 0.475 0.474

#10 #11 #12

-

mean

0.476

std. dev. 0.010 Table 2 : Morphological indicators of the investigated sections (mean values).

Figure 6 : Section of specimen #12.

Concerning the morphological indicators (circularity, elongation and rectangularity) standard deviations are less than 4%. The correspondent ANOVA, made with a 95% confidence interval on circularity, reveals a significant single effects of temperature and a less remarkable effect of the pressure, while no significant effects on the other indicators are recognised through the ANOVA calculations on elongation and rectangularity values. Standard deviation of the averaged void density shows the highest value (about 11%). Two factors can affect this variation: the local distribution of void density along the investigated section and the variation of the process parameters. Fig. 7 shows the averaged void density according to temperature and pressure variations. The results are consistent with the foam density graphs reported in Fig. 5. In fact when the temperature level increases the averaged void density decreases due to a higher foam density. Pressure effects on void density seem to be slightly different from the results achieved in Fig. 5 probably due to a major role played by the local distribution of voids on the sections. To better asses this matter in Fig. 8 the normalised occurrence histograms of the local distribution of foam density (FD=1-void density ) are shown. The density maps have been computed by considering the grey level image’s array of pixel values, i.e. a matrix of real values scaled to the range 0 to 1.

6

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.26.01

Each point of the maps is obtained calculating the mean of a moving fragment of this matrix corresponding to a square 4 mm in length, in order to not consider single void variation.

Figure 7 : Averaged void density according to pressure and temperature variation on two levels. The error bars correspond to the standard deviation. Fig.8 shows that the specimens, in which the salt particles are well replicated and the cavities are spread regularly, have symmetric frequency histograms (#2, #5, #8, #9, #10, #11 ). Conversely specimens #1, #3, #4, #6, #7, #12 present large localized cavities and their frequency histograms are asymmetric. From the analysis of the macrographs it can be observed that specimens obtained with a 550 ° C preheated mould exhibit more regular cavities distribution; among them, the ones infiltrated at 30 bar are the most uniform. This confirms the assumption that higher pressure coupled with higher temperature optimises metal infiltration. The interaction between temperature and pressure to improve the infiltration is also investigated by analysing void circularity. As shown in Fig.9a the void circularity at the lower mould temperature does not depend upon the applied pressure (as suggested by standard deviations); at the higher preheating temperature as the pressure increases the circularity decreases. Moreover at low and high pressure levels the temperature increase determines the reduction of the circularity. It can be noticed that for the set parameters 550 ° C and 30 bar the lowest circularity value (0.565) is achieved. This behaviour can be explained considering that in these conditions the liquid metal infiltrates more effectively the salt bed and therefore the salt particles are well replicated and cavities tend to become less rounded (generally speaking circularity of 0.886 is related to square shape, while 0.709 is the circularity of the rectangular shape with an aspect ratio of 1:4). As already said introducing table 2 the ANOVA analysis confirms that temperature has a significant effect on the circularity with a confidence interval of 95%, while there is not evidence for pressure effect. As far as elongation (Fig.9b) and rectangularity (Fig.9c) values are concerned they appear to be insensitive to changes of process parameters. Voids seem slightly more elongated only when metallic foam is produced by applying 30 bar pressure and by preheating the mould at 500 ° C. This is probably due to the fact that, in these operative conditions, there is a considerable chill from the mould face. The consequence is that the alloy is characterised by a poor fluidity and it is pushed by the piston through the salt bed producing more elongated cavities. As far as the rectangularity is concerned, it can be noticed that the specimens produced with the set parameters 550 ° C and 30 bar have a standard deviation markedly smaller than the other sets. The small sensitivity of these indicators to process parameters can be probably due to the shape of the salt pattern adopted during the manufacturing process. A further aspect that needs to be analysed is the effect of process parameters on the alloy microstructure. The microstructure of the alloy is typical of a hypoeutectic Al-Si alloy, with primary α -Al dendrites and eutectic Si particles distributed around the Al dendrites to form a cell pattern periodically repeated across the surface.

7

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.25.01

Figure 8 : Specimens macrographs and normalised occurrence histograms of FD.

8

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.26.01

Figure 9 : Circularity (a) , Elongation (b) and Rectangularity (c) according to temperature and pressure variations on two levels. The error bars corresponds to the standard deviation.

(a)

(b)

Figure 10 : Optical micrographs showing the microstructure of specimen #1 (a) and of an area close to a void in specimen #2 (b) . Microstructural analysis highlighted that the microstructure of alloys cast in different operative conditions are very similar to each other. The specimens showing a finer structure (Fig. 10) are only the ones produced by preheating the mould at 550 ° C and by applying a pressure of 30 bar. In those conditions the melt infiltrates efficiently the salt pattern and there is a considerable chill from the salt surface that locally increases cooling rates. By analyzing the foam microstructure it is also apparent that the most heterogeneous areas are the ones close to the voids where solidification conditions are far away from equilibrium. Those areas are richer of eutectic Si. As far as the eutectic Si particles are concerned the used manufacturing process, which is characterized by very high cooling rates, determines the formation of rounded and short lamellae that improve the mechanical properties of the alloy. A close examination of silicon and intermetallic phases, carried out on deep etched specimens, highlighted that both of them may have platelet and rod-like morphology (Fig. 11).

Figure 11 : SEM micrographs showing the morphology of a silicon (a) and of an iron-rich intermetallic particle (b) .

9

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.25.01

C ONCLUSIONS

I

n this work replicated AlSi7Mg0.3 alloy foams are produced by infiltrating preforms of NaCl particles, varying the metal infiltration pressure and the mould temperature. An original procedure based on image analysis has been set up to determine size, morphology and distribution of cells. The results highlighted that when the mould is preheated at 500 ° C there is an excessive chill and the liquid metal does not develop a smooth skin through intimate contact with the mould surface. When the mould is preheated at 550 ° C the void density decreases by increasing the applied pressure. Moreover an increased infiltration pressure enhances metal infiltration through the salt pattern. In these conditions the void density distribution is more regular and there is a more effective replication of salt particles with a consequent reduced void circularity. This confirms the assumption that higher infiltration pressure coupled with higher mould preheating temperature optimises metal infiltration. The proposed procedure used to analyse the foam sections appears an effective way to analyse the effect of process parameters on foam morphology. [1] Ashby, M.F., Evans, A.G., Fleck, N.A., Gibson, L.J., Hutchinson, J.W., Wadley, H.N.G., Metal Foams: A Design Guide, Edited by Butterworth-Heinemann, USA (2000). [2] Degischer, H.P., Kriszt B., Handbook of Cellular Metals: Production, Processing, Applications, Edited by H.P. Degischer, and B. Kriszt, Austria (2003). [3] Polonsky, L., Lipson, S., Markus, H., Light weight cellular metal, Mod. Cast., 39 (1961) 57-71. [4] Al-Jibbouri, S., Ulrich, J., The influence of impurities on crystallization kinetics of sodium chloride, Cryst. Res. Technol., 36 (2001) 1365-1375. [5] Adair, J.H., Suvaci, E., Morphological control of particles, Curr. Opin. Colloid In., 5 (2000 ) 160-167. [6] Gaillard, C., Despois, J.F., Mortensen, A., Processing of NaCI powders of controlled size and shape for the microstructural tailoring of aluminium foams, Mat. Sci. Eng. A-Struct., 374 (2004) 250-262. [7] Jiang, B., Zhao, N.Q., Shi, C.S., Li, J.J., Processing of open cell aluminum foams with tailored porous morphology, Scripta Mater., 53 (2005) 781-783. [8] Goodall, R., Marmottant, A., Salvo, L., Mortensen, A., Spherical pore replicated microcellular aluminium: Processing and influence on properties, Mat. Sci. Eng. A-Struct., 465 (2007) 124-135. [9] Goodall, R., Despois, J.F., Marmottant , A., Salvo, L., Mortensen, A., The effect of preform processing on replicated aluminium foam structure and mechanical properties, Scripta Mater., 54 (2006) 2069-2073. [10] Despois, J.F., Marmottant, A., Salvo, L., Mortensen, A., Influence of the infiltration pressure on the structure and properties of replicated aluminium foams, Mat. Sci. Eng. A-Struct., 462 (2007) 68-75. [11] Kádár, C., Chmelĺk, F., Cieslar, M., Lendvai , J., Acoustic emission of salt-replicated foams during compression, Scripta Mater., 59 (2008) 987-990. [12] Quadrini, F., Boschetto, A., Rovatti, L., Santo, L., Replication casting of open-cell AlSi7Mg0.3 foams, Mater. Lett., 65 (2011) 2558-2561. [13] Boschetto, A., Costanza, G., Quadrini, F., Tata, M. E., Cooling rate inference in aluminum alloy squeeze casting, Mater. Lett., 61 (2007) 2969-2972. [14] Boschetto, A., Campana, F., Giordano, V., Pilone, D., Morphological characterisation of cellular materials by image analysis, In: P. Di Giamberardino, D. Iacoviello, M. João (Eds), Computational Modelling of Objects Represented in Images III: Fundamentals, Methods and Applications, Italy (2012) 391-396. [15] Banhart, J., Manufacture, characterisation and application of cellular metals and metal foams, Prog. Mater. Sci. 46 (2001) 559-632. [16] Baumeister, J., Banhart, J., Weber, M., Aluminium foams for transport industry , Mater. Design, 18 (1997) 217-220. [17] Vaidya, U.K., Pillay, S., Bartus, S., Ulven, C.A., Growc, D.T., Mathewc, B., Impact and post-impact vibration response of protective metal foam composite sandwich plates, Mat. Sci. Eng. A-Struct., 428 (2006) 59-66. [18] Campana, F., Pilone, D., Effect of wall microstructure and morphometric parameters on the crush behaviour of Al alloy foams, Mat. Sci. Eng. A-Struct., 479 (2008) 58-64. [19] Russ, J.C., The Image Processing Handbook, 5th ed., CRC Press Taylor & Francis Group, USA (2006). R EFERENCES

10

A. Boschetto et alii, Frattura ed Integrità Strutturale, 26 (2013) 1-11; DOI: 10.3221/IGF-ESIS.26.01

[20] Montgomery, D.C., Runger, G.C., Applied Statistics and Probability for Engineers, 3rd ed., John Wiley &Sons Inc., New Jersey (2003).

V IDEO REFERENCES

[V1] http://www.gruppofrattura.it/video/FIS26/Pilone/V1.mp4

11

A. De Santis et alii, Frattura ed Integrità Strutturale, 26 (2013) 12-21; DOI: 10.3221/IGF-ESIS.26.02

Graphite nodules features identifications and damaging micromechanims in ductile irons

Alberto De Santis, Daniela Iacoviello Sapienza Università di Roma, Dipartimento di Ingegneria Informatica, Automatica e Gestionale Antonio Ruberti, Italy Vittorio Di Cocco, Francesco Iacoviello Università di Cassino e del Lazio Meridionale, DICeM, via G. Di Biasio 43, 03043 Cassino (FR), Italy iacoviello@unicas.it A BSTRACT . 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). K EYWORDS . Ductile irons; Damaging micromechanisms; Image segmentation; Level sets. amaging micromechanisms evolution in ductile cast irons (DCIs) is strongly influenced by the matrix microstructure and by the graphite nodules morphology. Considering recent experimental results [1-6], the role played by the graphite nodules is not merely connected to a matrix-graphite debonding mechanism, followed by voids nucleation and growth as described in [7-10]. Graphite nodules damaging micromechanisms can be classified as follows: - Graphite – matrix debonding (Fig.1 – Video 1); - “Onion-like” mechanism (Fig. 2 – Video 2); - Crack initiation and propagation in the “nodule core” (Fig. 3 – Video 3). Fig. 1 – 3 (and Videos 1 – 3) are obtained according to the procedure described in [1-6] and refers to a fully pearlitic DCI. In order to obtain the videos, a “step by step” testing procedure allowed “in situ” scanning electron microscope (SEM) observations and a commercially available morphing software was used (at least, 10 step for each video). The pearlitic matrix – graphite nodule debonding (Fig. 1 - V1) is characterized by a debonding nucleation that is obtained for loading conditions that corresponds to the elastic stage: debonding nucleation is evident in the nodule “pole” and becomes more and more evident with the increase of the macroscopic deformation. The “onion –like” mechanism is probably connected to a mechanical properties gradient in the graphite nodule, probably due to the graphite nodule nucleation and growth mechanisms. This gradient has been observed in ferritic DCI by means of nano indentation tests [11]. The “onion-like” mechanism nucleates already corresponding to a stress value of about 600 MPa (elastic stage): some cracks nucleate on the higher-left side of the nodule (Fig. 2 - V2). The increase of the macroscopic deformation implies a propagation of the first cracks and the nucleation of new cracks, allowing to define a D I NTRODUCTION

12

A. De Santis et alii, Frattura ed Integrità Strutturale, 26 (2013) 12-21; DOI: 10.3221/IGF-ESIS.26.02

both sort of “nodule core” (obtained during the solidification from the melt) and a “graphite shield” (obtained by means of Carbon solid diffusion through the austenite shield during the DCI cooling) that is connected to the pearlitic matrix.

Figure 1 : Pearlitic DCI: graphite – matrix debonding [V1] .

Figure 2 : “Onion-like” mechanism [V2] . The last mechanism (Fig. 3 - V3) is the crack nucleation and growth corresponding to the nodule center, probably the nodule nucleation site. Always corresponding to a loading value that is lower than DCI yield strenght, a crack nucleates in the nodule center: the macroscopic deformation increases, implying both a crack propagation inside the graphite element (with a crack path that is orthogonal to the loading direction) and an increase of the crack opening. Corresponding to the higher deformation level, both the “onion-like” and the matrix – graphite debonding mechanisms are also active.

Figure 3 : Crack initiation and propagation in the “nodule core” [V3] .

13

A. De Santis et alii, Frattura ed Integrità Strutturale, 26 (2013) 12-21; DOI: 10.3221/IGF-ESIS.26.02

The importance of the different damaging micromechanisms is influenced by the matrix microstructure, but it is worth to note that, considering ferritic-pearlitic ductile irons, matrix – graphite nodule debonding is not the most important damaging micromechanism. Focusing the role played by graphite elements in a completely different cast iron (a fully pearlitic flake cast iron), the flaky graphite tends to open in the middle and a void appears inside, considering both graphite elements orientated perpendicular and parallel to the loading direction [12]. The load increase implies an increase of the cast iron internal damage, with a graphite elements - pearlitic matrix debonding. This mechanism is more evident with the graphite elements oriented perpendicular to the loading direction. Furthermore, a microplasticity at graphite tips is observed in the pearlitic matrix (Fig. 4).

Figure 4 : Damaging micromechanims in a fully pearlitic flake cast iron [12].

Considering the results in Fig.-1-4 and in Videos 1-3, it is evident the strong influence of the graphite nodules morphology on the damaging micromechanisms: the visual qualitative approach followed in the EN ISO standard [13] does not seem to be sufficient to fully characterize the graphite elements in a cast iron and therefore a quantitative approach seems to be necessary. Images obtained by means of LOM, despite a good visual appearance, are represented by a quite irregular signal due to various kind of degradations stemming from the acquisition process: additive noise, albedos due to dust and specimen oxidation, artifacts coming from scratches occurring during the specimen preparation. High performance image analysis procedure to distinguish the nodules from the background can be obtained within the framework of image segmentation: the original image is partitioned into disjoint domains where the signal has homogeneous characteristics, and passing from one domain to another these characteristics vary significantly [14]. For the cast iron (and ductile iron) classification it suffices to choose the class of piecewise constant functions to approximate the gray level distribution of the LOM images, so that in the segmented image the nodules are sharply enhanced over a uniform background. Then any standard software can quantify the graphite elements morphological parameters of interest; in particular, the Image Processing Toolbox of MatLab© provided a good performance. The segmentation problem can be solved by various techniques [15-16] with different pros and cons. The method of active contours [17] was preferred since can deal with the complex topology of the graphite elements without compromising the numerical complexity: as a consequence the cast iron metallographies can be reliably segmented and evaluated in real time. In this work, different graphite elements morphologies have been considered, ranging from lamellae to nodules, and image segmentation by the active contours method has been optimized in order to perform a quantitative analysis and characterization. A complete automatization of this approach allows to perform statistical analysis of many morphological parameters (e.g., graphite elements density, distribution, shape), allowing to fully characterize the investigated cast iron. espite a visual inspection can distinguish the objects from the background on the available real data, the signal is quite irregular and does not feature a clear cut between the background and the different objects of interest in the picture. In Fig. 5-8, some examples of specimens are presented; as it can be noted, in each image there are some scratches and dust and, obviously, graphite elements like exploded graphite and flakes. It is important also to distinguish among the different kind of objects. D I MAGE SEGMENTATION BY THE ACTIVE CONTOURS METHOD

14

A. De Santis et alii, Frattura ed Integrità Strutturale, 26 (2013) 12-21; DOI: 10.3221/IGF-ESIS.26.02

Figure 5 : Flakes and irregular spheroidal graphite

Figure 6 : Flakes and compacted (vermicular) graphite.

Figure 7 : Flakes and slightly irregular spheroidal graphite.

Figure 8 : Flakes (higher magnification than Fig. 5-6)

The problem is solved once real data are segmented. The segmentation is the partition of the image in regions homogeneous with respect to some properties, for example the gray level, the shape, the colour, the texture and so on. For the images considered in this paper, a segmentation with respect to the gray level will adequately suit the purpose, obtaining the various objects and the background clearly separated as sub regions of the image domain with constant gray level. The procedure that will be briefly recalled in the following was first proposed in [17] and applied in [18]. Let  R 2 be a compact subset representing the image domain; the image signal is modeled as a continuous function   : 0, 1 g  . The image segmentation we will refer to is given by

N

N

N 

,   i C     



g

i c I

,

(1)



s

i

i



i

1





i

i

1

1

where i  . The segmentation problem can be stated as follows: Given   : 0, 1 g  , find s i I  is the characteristic function of set

1 ,..., N   and 1 c

g , that means finding a finite partition

c such that s

g represents g according to

,..., N

some criterion. The active contour method considered in this work has been suitably adapted to the problem of ductile iron obtaining a very efficient algorithm. Without loss of generality, from now on we will refer to an image with just one object P on the background; we denote by 1  the subset of  points corresponding to the object. Therefore 1 C   defines the object contour, while 2 1 \     denotes the region outside the object. The following energy functional   1 2 , , F c c C is assumed:





2

2

  

( ) C A   

 

1 

( , ) g x y c dxdy 



2 

( , ) g x y c dxdy 

F c c C

1 2 ( , , )

1 ( )



(2)

1

2





1

2

15