Transactions on Engineering Sciences vol 13, 1996 WIT Press, ISSN

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1 Fracture toughness of unidirectional fiber reinforced titanium alloy and titanium intermetallic matrix composites K. Hirano/K. Etoh/M. Kikuchi' "Mechanical Engineering Labororatory, Agency of Industrial Science and Technology, MITI, Namiki 1-2, Tsukuba-shi, Ibarakiken 305, Japan * Graduate Student, Science University of Tokyo 'Science University of Tokyo Abstract Fracture toughness testing are firstly conducted on continuous fiber unidirectionally reinforced both titanium alloy and titanium aluminide intermetallic matrix composites. There is a remarkable initial notch length dependence of fracture toughness experimentally determined at an unstable fracture. It is resulted from reinforcement fiber bridging during the stable crack growth behavior. Nextly, the crack tip stress intensity factor are calculated based on fiber pressure model, and successfully predicted the initial notch length dependence of fracture toughness. 1 Introduction Various advanced structural materials have recently been researched and developed in the field of aeronautics, aerospace and power generator technologies. Metal matrix composites (MMCs) are some of these advanced structural composites and are generally considered to be attractive as structural materials because of their superiority to polymer matrix composites (PMCs) in high temperature applications. It is therefore the most important and urgent to characterize damage tolerant behavior for these advanced MMCs for a wide practical use in primary structural applications. We have already done fracture mechanics of discontinuous and continuous fibers reinforced MMCs in order to establish high performance materials design concepts for superior strength, toughness and durability (Hirano, et al. [1-7]). Mechanical and fracture mechanics test methods were also researched and developed in these research projects, and the newly developed techniques were transferred to the standardization activities of Japanese Industrial Standards (JIS) for MMCs (Hirano[8]).

2 410 Localized Damage This paper presents fracture toughness testing of continuous fiber unidirectionally reinforced titanium alloy and titanium aluminide intermetallic matrix composites. Stable crack growth behavior results from reinforcement fiber bridging. The initial notch length dependence of fracture toughness KC in the reinforcement fiber (L-) orientation is successfully predicted by calculating crack tip stress intensity factor on the basis of fiber pressure model (e.g. Ghosn et al. [9]). 2 Materials and experimental procedure 2.1 Materials The materials chosen for this investigation are unidirectional six-plies SiCcvE/Ti- 15V-3Al-3Cr-3Sn (Ti-15-3), five-plies SiCcvD/Ti-15Mo-5Zr-3Al (Ti-15-5) and four-plies SiCcvD/TigAl+Nb. The reinforcing phase (approximately 35 volumepercent) was continuous 140 mm diameter SiC fibers, commercially designated as SCS-6. The matrix materials were p type titanium alloys and titanium aluminide intermetallic. These composites were fabricated by hot isostatically pressing (HIP-ing) alternate layers of fibers and thin foils of titanium alloy and titanium aluminide intermetallic. The average room temperature mechanical properties of the composite are also listed in Table 1. It is highly anisotropic properties, whose tensile strength and elastic modulus value is much greater parallel to the fiber direction (L) than in the transverse direction (T). Table 1. Mechanical properties. Materials Ori. E (GPa) Gurs(MPa) f(%) SiCcvo/ Ti-15-3 (6 plies/vf=35%) L T SiCcvo/Ti-15-5 L (5 plies/vf=37%) SiCcvD / Tis Al+Nb L (4 plies/vf=35%) The specimens used are single edge notched (SEN) specimen with the tensile axis oriented along and parallel to the reinforcement fiber direction (L- and T- orientation) as shown in Fig. 1. Relative notch length a/w (W=10 mm) ranged from 0.15 to 0.6 with approximately 0.1, 0.2 and 0.4 mm width was electrodischarged in each specimen. In a case of SiCcvD/TisAl, some specimen have a wide width specimen of 20 mm. All specimens were not fatigue precracked, because the reinforcement fibers bridging is occurred. 2.2 Fracture toughness testing The fracture toughness tests were conducted in a room temperature environment at a constant cross-head speed of 0.5 mm/min. using a MTS closed-loop electrohydraulic materials testing system. The initiation of fracture load Ppop is determined at the onset of the pop-in behavior in the load versus COD and/or strain

3 Localized Damage 411 Clip-on gauge (Gauge length 5mm) Strain gauge (Gauge length 1mm) 10 Figure 1: Schematic illustrations of experimental procedure. curves measured by strain guage at initial notch tip. Fracture toughness KC and Kpop are determined at the maximum and pop-in loads (see Fig. 1). After the tests, fractographic examinations were performed on the cross section and fracture surface of specimen using a scanning electron microscope. 3 Fracture toughness 3.1 Relative notch length dependence Fracture toughness KC are shown in Fig. 2 as a function of relative initial notch length a/w. Fracture toughness, especially in the L-orientation, have a tendency of increase with increasing the initial notch length. There is a great difference in fracture toughness between L- and T-orientations. As compared with fracture toughness for matrix material [10], there is a remarkable improvement in the L- orientation while fracture toughness is degraded in the T-orientation. It is also found that SiCcvr/Ti-15-3 is fairly inferior to SiCcvo/Ti-15-5 in the both L- and T-orientations. 3.2 Notch tip radius dependence Dependence of fracture toughness on notch tip radius p are shown in Fig. 3 with a/w of 0.3 in the L-orientation. In a case of Ti-alloys matrix composites, KC are almost constant regardless of notch tip radius within the limits of this experiment. On the other hand, there is a remarkable dependence of fracture toughness on notch tip radius in a case of SiCcvD/TigAl, and KC increases with increasing

4 412 Localized Damage v p=o.05mm p=0.1mm A p=0.2mm (0 9=100 L-Ori. 50 T-Ori. v a/w 0.8 Figure 2: Dependence of fracture toughness on relative notch length a/w for SiCcvD/Ti ^100 Q_ -# P Figure 3: Dependence of fracture toughness on notch tip radius p.

5 Localized Damage 413 notch tip radius. This is considered to be due to difference of interfacial properties between SiCcvr/Ti-alloys and SiCcvr/TigAl+Nb. 3.3 Fractographic examinations Fractographic examinations of the specimen unloaded just after first pop-in behavior observed in the Load versus COD curve show that there exits the matrix cracking initiated from the initial notch tip, debonding at interface and the reinforcement fiber bridging. The fiber pull-out are also observed at final fracture surface. Fracture mechanism is summarized in Figure 4 along with the Load versus COD curve measured during fracture toughness test. Fiber B ridging ** Lj sj 1 "> J n COD, 8 Figure 4: Schematics of fracture mechanism. 4 Simulation of fracture toughness 4.1 Modeling and analysis of stress intensity factor The effect of fibers bridging on the crack faces can be modeled by applying a closure pressure in the bridged region as shown in Fig. 5. For the fiber pressure model [9], a closure pressure c(x) is given by the following expression. c(x) = a W 6W-ao{0.5 x (1)

6 414 Localized Damage The homogenized composite effective stress intensity factor Ktip(a) for a partially bridged SEN specimen can be expressed by a linear summputation of the unbridged region (0<x<a0) and bridged region (ag<x<a) and is given by the following equation. H(a,x) dx (2) where, H(a,x) = 1 + m/a-x)/a + (3) and where m^ and ir^ are given as a function of relative crack length a/w. m, = l7.1884(a/wf (a/wf,_ 9 R (4) mg = (a/w) (a/w) Figure 5: Analytical model for fiber bridging. 4.2 Comparisons with experimental data Computer simulations were done on the basis of Equation (2). It is supposed here that unstable fracture occurs at maximum load Pmax when the bridged region is reached at a critical value Aac(=a-ao), and K%pp(a) and Ktip(a) are equal to Kc(Pmax» a) and KIC of matrix materials, respectively. In this computer simulation, Aa^ is also assumed to be constant (0.9 mm for W=10 mm and 3.0 mm for W=20 mm) regardless of both the relative notch length and matrix materials. Comparisons of predicted curve with experimental data are shown for SiCcvD/Ti-15-3 and SiCcvo/TigAl+Nb in Figs 6(a) and (b). There is a good relationship between the predicted curve and experimental data. It is concluded here that the relative initial notch length dependence of fracture toughness is successfully explained on the basis of fiber pressure model. 5 Conclusions (l)there is a remarkable initial notch length dependence of fracture toughness KC experimentally determined at an unstable fracture. It is resulted from reinforcement fiber bridging during the stable crack growth behavior. (2)The fractographic examinations clearly show that there also exists matrix cracking, debonding at interface and fiber bridging. (3)The crack tip stress intensity factor are calculated on the basis of fiber pressure model, and successfully simulated the initial notch length dependence of fracture toughness for continuous fiber unidirectionally reinforced Ti-alloys and titanium aluminide intermetallic matrix composites.

7 Localized Damage # Q_ A p=0.05mm p=0.1mm p=0.2mm Predicted curve a/w 0.8 (a) SiCcvE/Ti V W=10mm W=20mm Predicted curve Q_ a/w 0.6 (b) Si Figure 6: Comparisons of predicted curve with experimental data

8 416 Localized Damage Acknowledgments This research was supported by Industrial Science and Technology Frontier Program of Agency of Industrial Science and Technology, MITL The authors also specially thank Mr. T. Fujiwara (MHI Ltd) for providing the TigAl intermetalmc matrix composite. References 1. Hirano, K., "Current R&D Trends of Advanced Metallic and Inorganic Materials and Its Technical Problems", Trans. JSME (A), 1992, , , (in Japanese). 2. Hirano, K., "R&D Trends of Advanced Metal Matrix Composites and Fracture Mechanics Characterization" ISIJ International, 1992, 32-12, Hirano, K., "High Performance Materials for Severe Environments in the Field of Aerospace and Power Generator Technologies in Japan "(Invited Lecture), Metal Matrix Composites, Proc. 9th Int. Conf. on Composite Materials, Vol. 1, 1993, pp Hirano, K., "Fatigue of Metal Matrix Composites", /. Soc. Mat. Sci., Japan, 1994, , , (in Japanese). 5. Hirano, K., "Fatigue Crack Growth Characteristics of Metal Matrix Composites "(Invited Paper), Mechanical Behavior ofmaterials-vi, Proc. of the 6th Int. Conf. on Mechanical Behavior of Materials, Pergamon Press, Vol. 3, 1991, pp Hirano, K., M. Kashiwagi and M. Kikuchi, "Fatigue Failure Mechanism of Continuous Fiber Unidirectionally Reinforced Intermatallic Compound Matrix Composite", Metal Matrix composites, Proc. 9th Int. Conf. on Composite Materials, Woodheed Publishing Limited, Vol. 1, 1993, pp Hirano, K., M. Kashiwagi and M. Kikuchi, Trans. JSME,(A.), 1993, , , (in Japanese). 8. Hirano, K., "Current Status of Standardization Activities on Mechanical Testing for Metal Matrix Composites in Japan "(Invited Paper), Composites Testing and Standardisation, Proc. European Conf. on Composites Testing and Standardisation, Woodheed Publishing Limited, 1994, pp Ghosn, J. Louis, Kantzos, P. and Telesman, J., "Modeling of Crack Bridging in a Unidirectional Metal Matrix Composite", International Journal of Fracture, 1992,54, Hirano, K., K. Etoh and M. Kikuchi, Preprint of 2nd Materials and Processing Conf. (M&P'94), , 1994, pp.48-49, (in Japanese).