FRACTURE TOUGHNESS EXAMINATION OF THE ALUMINIUM MATRIX COMPOSITE REINFORCED WITH CHOPPED CARBON FIBRES

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1 44/18 ARCHIWUM ODLEWNICTWA Rok 2006, Rocznik 6, Nr 18 (1/2) ARCHIVES OF FOUNDRY Year 2006, Volume 6, N o 18 (1/2) PAN Katowice PL ISSN FRACTURE TOUGHNESS EXAMINATION OF THE ALUMINIUM MATRIX COMPOSITE REINFORCED WITH CHOPPED CARBON FIBRES Z. KONOPKA 1, P. CHMIELOWIEC 2, A.ZYSKA 3, M. ŁĄGIEWKA 4, Częstochowa University of Technology, Armii Krajowej 19, Częstochowa. R. BALAWENDER 5 POLMAG, Kędzierzyn-Koźle SUMMARY The results of the stress intensity factor measurements of the A356 alloy matrix composites with chopped carbon fibres were presented in this paper. Composites containing different length and volume fractions of chopped carbon fibres were prepared by stirring and squeeze casting method. Insignificant increase of the stress intensity factor in composite containing 12.5% vol. of carbon fibres was observed in comparison with the squeezed pure A356 alloy, however the significant increase was noticed in comparison with the gravity cast A356 alloy. Key words: aluminium, composites, fracture toughness, squeeze casting 1. INTRODUCTION The squeeze casting process has a many of advantages over sand casting and gravity die casting for Al alloys. Solidification process takes place under a high applied pressure where excellent feeding of solidification shrinkage and a refined microstructure due to high cooling rates are achieved. The absence of shrinkage porosity, combined with a fine-scale microstructure results in very good mechanical properties for both conventional aluminium alloys castings and aluminium alloys matrix 1 dr hab. inż.prof.p.cz. konopka@mim.pcz.czest.pl 2 dr inż.chmiel@mim.pcz.czest.pl. 3 dr inż. zyska@mim.pcz.czest pl. 4 dr inż. cis@mim.pcz.czest.pl 5 mgr inż. 279

$composites. [1]. Stress-transfer analysis for fibre-matrix interfaces in short fibrereinforced composites shows strength and resistance fracture toughness improving of these materials [2].$

2 composites. [1]. Stress-transfer analysis for fibre-matrix interfaces in short fibrereinforced composites shows strength and resistance fracture toughness improving of these materials [2]. It was found that the end interface of a short fibre is easy to debond in the loading process. After the debonding of the end interface, the stress transfer from matrix to fibre depends on shear stress on the axial interface only [3]. 2. EXPERIMENTAL PROCEDURE 2.1. Production technology of tested composite The A356 aluminium alloy as a composite matrix and short carbon fibres, 7 µm in diameter and 3mm or 7mm long were used to prepare of composites. Two different volume fractions 7.5% and 12.5% of carbon fibres in composites were added. Aluminium alloy was overheated to 1000K and carbon fibres were incorporated into liquid metal during mechanical stirring. Melting and mixing were realised in argon atmosphere. After mixing the composite slurry was placed inside a metallic die heated to 573K. Indirect squeeze casting process was applied with a pressure of 75 MPa. 2.2 Fracture toughness testing procedure Two fracture parameters were used to describe fracture toughness of the material. The first one was stress intensity factor K Ic and the second one was J Ic integral [4]. The testing was conducted on the Single Edge Notch Bend specimens, which were cut of the cast flats (Fig.1). Each specimen was loaded by three-point bending using the MTS-810 tensile machine. During the loading, force P, crack opening displacement COD and load-line displacement f were measured with the aid of standard MTS instrumentation. Fig. 1. Proportional dimensions and tolerances for tested three-point bend specimen Rys. 1. Wymiary i tolerancje próbki do próby trójpunktowego zginania 3. RESULTS OF EXPERIMENTS Calculations of stress intensity factor K Ic were carried through the assistance of load P crack opening displacement COD curves. An example curve, which represents composite with fibres length 3 mm and volume fibres fraction 7,5 % is shown in Fig.2 280

3 The critical load P C which initialises the crack growth was found with the assistance of the graphical technique described in form [5]. P Y K = c (1) Q B W where: P C - load at initial cracking point; B - thickness; W - width; Y = f(a 0 /W). The values of stress intensity factor K Ic for all of the tested specimens were determined using equation (1). The composites during testing showed some small plastic deformation near the tip of crack as well. Fig. 2. Typical curve recorded during loading the specimens. (P C initial cracking point) Rys. 2. Typowa krzywa zarejestrowana podczas obciążenia próbki. (P c -punkt inicjujący pękanie) The plastic zone before crack tip might have an effect on K Q value. Therefore it is recommended to check if the tested specimens were loaded in presence of plane strain state. That condition is mathematically identified [6] with equation (2). B min K 2.5 Q = σ u 2 where: σ u - ultimate tensile strength, MPa ; KQ - stress intensity factor, MPa m ; B min -minimum thickness of specimen required for plane strain state condition. In Table 1 the results of fracture toughness are shown. Answer Yes means that the calculated K Q does not depend on the geometry of tested specimens and it could be used to describe fracture toughness of the composites. In a opposite case the value of K Q should not be used as fracture parameter, which classify fracture toughness in tested (2) 281

4 composites The J-integral testing was based on the ASTM E procedure [7]. The authors resigned themselves to the impossibility of finding out the initial cracking point for the tested composites following the procedure accurately. The techniques described there were modified to measure fracture toughness of the tested composites. J-integral value was evaluated following equation (3). 2 E J = (3) max. B ( W a ) 0 where: E - energy absorbed by specimen at maximum load, B-thickness; W-width; a 0 -crack length Table 1. Fracture toughness of composites measured by stress intensity factor Tabela 1. Odporność na pękanie mierzona współczynnikiem intensywności naprężenia Fiber Fibre length B min K Q Is % mm mm MPa m K Q =K Ic? 0 (matrix) YES YES NO ,4 YES NO Energy E absorbed by the specimen at maximum load was measured carried through the assistance of load P load-line displacement curves. Absorbed energy E was identified with the area under the curve shown for example in Fig.3 for composite containing 7.5% vol. of 3 mm carbon fibres. Fig. 3. Load P vs. load-line displacement f curve Rys. 3. Krzywa obciążenie-przemieszczenie 282

The measurements of the area were done using the planimeter. Each area under P vs. f curves was measured three times with relative error lower then 5%.

5 The measurements of the area were done using the planimeter. Each area under P vs. f curves was measured three times with relative error lower then 5%. The complete results of calculations J integral at the maximum load are shown in Table 2. Each value of J max inserted in the table is an average of three J max results obtained on three-point bended specimens. Example of the fracture surface (SEM) of the composite tested is shown in Fig.4. Table 2. Fracture toughness of composites measured by J integral parameter at maximum load Tabela 2. Odporność na pękanie kompozytów mierzona całką J przy maksymalnym obciążeniu Fiber Fibre length J max. % mm N/mm 0 (matrix) - 0, , , , ,59 4. CONCLUSIONS Fig. 4. Fractograph of composite (7.5% carbon fibres, 3mm long) Rys. 4. Fraktografia kompozytu (7.5% włókien węglowych, 3mm długości) The technology used for the production of composites Al-Si - carbon fibres allowed their fracture toughness to increase compared to pure Al-Si matrix. 1. The fracture toughness of Al-Si-carbon fibres composites gradually increased as a function of the weight fibres fraction. The maximum value of stress intensity factor 283

6 K Ic was 8,4 MPa m for composite with fibre friction 12.5% and length of fibre 7 mm. 2. Fracture toughness testing carried out using K Ic parameter should be taken on specimens with larger thickness, which guarantees plain strain state in tested specimens. REFERENCES [1] C.P. Hong, H.F. Shen, S.M. Lee: Prevention of macrodefects in squeeze casting of an Al-7wt pct Si alloy Metallurgical and materials Transaction B Vol.31B, 2000, p.297. [2] M.R. Ghomashchi, A.Vikhrov: Squeeze casting: an overview Journ. Of Materials Processing Technology 101, 2000, p.1. [3] H. Akbulut, M. Durman: Temperature dependent strength analysis of short fibre reinforced Al-Si metal matrix composites. Materials Science and Engineering A262, 1999, p.214. [4] ISO/DIS 12135, Metallic Materials - Unified Method of Test for the Determination of Quasistatic Fracture Toughness. [5] BS 5447, Methods of Test for Plane Strain Fracture Toughness K IC of Metallic Materials. [6] Broek D.: The Practical Use of Fracture Mechanics. Dordrecht, Kluwer Academic Publishers, [7] ASTM E Standard Test Method for J IC, A Measure of Fracture Toughness. STRESZCZENIE BADANIE ODPORNOŚCI NA PĘKANIE KOMPOZYTU NA OSNOWIE STOPU ALUMINIUM UMOCNIONEGO CIĘTYMI WŁÓKNAMI WĘGLOWYMI Wytworzono kompozyty na osnowie stopu A356 z cietymi włóknami węglowymi o średnicy 7µm i długości 3 i 7 mm oraz udziale objętościowym 7.5% i 12% objętościowo. Kompozyty wykonano metodą mieszania i prasowania w stanie ciekłostałym. Wykonano badania odporności na pękanie badanych kompozytów..zaobserwowano wzrost współczynnika intensywności naprężeń i odporności na pękanie kompozytów w porównaniu ze stopem osnowy. Recenzował: Prof. Zbigniew Stradomski 284

A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S Volume Issue 2 DOI: /amm

A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S Volume 59 2014 Issue 2 DOI: 10.2478/amm-2014-0116 M. ŁĄGIEWKA MECHANICAL AND TRIBOLOGICAL PROPERTIES OF METAL MATRIX COMPOSITES REINFORCED