Composites: Part A 84 (2016) 36–42

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Composites: Part A

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The effect of thermal fatigue on the mechanical properties of the novel fiber metal laminates based on aluminum–lithium alloy ⇑ Huaguan Li a,c, Yubing Hu a, Cheng Liu a, Xingwei Zheng b, Hongbing Liu b, Jie Tao a,c, a College of Material Science & Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, PR China b Shanghai Aircraft Manufacturing Company Limited, Shanghai 200436, PR China c Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing 211100, PR China article info abstract

Article history: The effect of thermal fatigue on the mechanical properties of the novel fiber metal laminates (FMLs) Received 13 August 2015 based on aluminum–lithium alloy was investigated. The results indicated that no obvious delamination Received in revised form 7 January 2016 or defects were observed in the novel FMLs exposed to 1000 cycles. The samples treated with different Accepted 8 January 2016 cycles still exhibited stable and excellent interlaminar properties comparing with the as-manufactured Available online 13 January 2016 ones. Furthermore, the tensile and flexural strength of the FMLs even increased with the thermal fatigue cycles owing to the positive age hardening behavior of aluminum–lithium layer. The homogeneous and Keywords: fine precipitation of T phases dominated the strengthening effect of aluminum–lithium alloy. Besides, A. Hybrid 1 the novel FMLs after thermal fatigue treatments still possessed the similar resistance to fatigue crack A. Laminates B. Environmental degradation growth (FCG) when compared with the as-manufactured ones. The slight changes in the properties of B. Mechanical properties aluminum–lithium layers had no detrimental effect on the FCG. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction FMLs based on aluminum–lithium alloy significantly improve the damage tolerance of Glare [8]. Fiber metal laminates (FMLs) combine the outstanding fatigue Moreover, Glare in aircraft always serves under a variety of con- resistance and high strength of fiber reinforced composites with ditions involving temperature, loadings, and fluids [9,10]. For com- the ductility of metal alloys [1,2]. As the crucial member of FMLs, mercial aircrafts, the exposed temperature of fuselage skin and Glare is manufactured by alternating layers of 2024 aluminum wing panel usually ranges from 60 °C to +80 °C [11]. The temper- alloy and glass fiber reinforced epoxy, which possesses excellent ature variation for the high-speed aircraft is even greater. The ther- fatigue and impact resistance, good residual and blunt notch mal fatigue during flights is one of the most serious owing to the strength, convenient manufacture and repair [3,4]. Nowadays, differences in thermal expansion coefficients of the components Glare is extensively used in large aircraft, especially for the fuse- in Glare [12]. The effect of thermal fatigue on the mechanical prop- lage and wing. The upper fuselage skin structures manufactured erties as well as the failure behaviors is necessary to explore. Some with Glare lead to the 794 kg weight saving of Airbus A380 [5]. researches on the thermal fatigue behavior of conventional Glare For conventional Glare, aluminum 2024-T3 is used for the metal have been conducted [9,13]. The investigated temperature all ran- layers. However, in recent years, various novel aluminum alloys, ged from 60 °C to +80 °C. da Costa et al. [14] studied the effect of especially the aluminum–lithium alloys, offer low density, thermal fatigue (50 °C to +80 °C) on the interlaminar shear and improved specific strength, and high stiffness to weight ratio as tensile properties of Glare. The temperature changes exhibited no compared to conventional 2xxx and 7xxx series aluminum alloys obvious effect on the mechanical properties of Glare after a ther- [6]. The attractive performance of aluminum–lithium alloys, mal fatigue of 1000 cycles. including lower density, better strength, and higher stiffness, facil- The intention is to use this novel FML in the high speed aircraft. itates its wide applications in the aerospace industry [7]. The novel Comparing to commercial ones, the high speed aircraft is designed with a very limited service life of 200–500 flight times but sub- jected to the larger temperature difference (65 °C to +135 °C). ⇑ Corresponding author at: College of Material Science & Technology, Nanjing However, problems are raised. On the one hand, the temperature University of Aeronautics and Astronautics, Nanjing 211100, PR China. Tel.: +86 25 range of thermal fatigue for the high-speed aircraft is obviously 5211 2911. E-mail address: [email protected] (J. Tao). enlarged. It is necessary to know whether the differences in http://dx.doi.org/10.1016/j.compositesa.2016.01.004 1359-835X/Ó 2016 Elsevier Ltd. All rights reserved. H. Li et al. / Composites: Part A 84 (2016) 36–42 37 thermal expansion coefficients will cause the interfacial damage reinforce the laminates. The employed epoxy was SY-14 and performance degradation of the novel FMLs. On the other polysulfone- epoxy adhesive system, which was firstly developed hand, the novel aluminum–lithium alloy indeed has a desired rein- by AVIC Beijing Institute of Aeronautical Materials. forcement effect on the mechanical properties of FMLs at room Primarily, the adhesive sprayed in the metal layers was neces- temperature. Nonetheless, our previous research indicates that sary. The adhesive system determines the bonding strength the novel aluminum–lithium alloy is more sensitive to the temper- between the prepregs and metal layers, which significantly con- ature changes and usually possesses the faster aging response than tributes to the performance of the laminates [5]. W-101-S spray conventional aluminum alloys [15,16]. The microstructure evolu- gun with the constant spread quantities per seconds was used to tion and properties changes of the alloy in such conditions are spray the adhesive. The spread quantities were controlled by the rather necessary to be revealed, because the performance deterio- spray cycles and then further confirmed by weighing. It was neces- ration of the alloy during the aging treatment is fatal. Moreover, sary to spray the adhesive in the metal layers within 12 h after investigations on thermal fatigue behavior are also beneficial to anodizing, which was beneficial to protect the constructed rough understand the long term behavior of aluminum–lithium alloy, surface. Then, the 3/2 FMLs with the size of 1000 mm 500 mm, especially involved in the temperature which the age hardening consisting of three aluminum–lithium sheets and two glass/epoxy may be activated at. Through the relevant research, the potential plies, were prepared. The corresponding laminating configuration application limits of the aluminum–lithium alloy in FMLs, owing is shown in Fig. 2(a). The curing process was conducted in the auto- to the unique aging behavior, will be better recognized. clave (seeing Fig. 2(c)) operated under the optimized parameters Thus, thermal fatigue treatment of the novel FMLs was con- [8]. The effect of curing process on the microstructure and proper- ducted in our study. Furthermore, comprehensive evaluations of ties of aluminum–lithium layers could be ignored owing to the interlaminar properties, basic mechanical properties as well as very limited aging time. Moreover, the corresponding schematic fatigue properties were employed to achieve more reliable results of the stacking configuration is presented in Fig. 2(b). and better realize the possible failure behavior of the FMLs. Besides, the possible age hardening behavior of the aluminum– 2.2. Thermal fatigue of the novel fiber metal laminates lithium alloy was paid more attention. The thermal fatigue was conducted in the CTPS715C climate 2. Experimental chamber, as shown in Fig. 3(a). The four sets of the FMLs were trea- ted respectively for 250, 500, 750, and 1000 cycles, referring to the 2.1. Preparation of the novel fiber metal laminates designed service life of the high speed aircraft. The thermal fatigue temperature ranged from 65 °Cto ° ° The used novel aluminum–lithium alloy is newly-developed in +135 C, following with the heating/cooling rate of 15 C per min- recent years, aiming at the large commercial aircraft. It belongs to ute. The chamber temperature was maintained for 15 min at ° ° the aluminum–copper–lithium family but exhibits a high Cu/Li 65 C/+135 C to ensure the specimens completely heated/cooled. ratio of 5.29. The nominal chemical composition of the alu- A complete cycle, presented in Fig. 3(b), took approximately minum–lithium sheets (2 mm thickness, T8 temper) used in this 56 min in total. study was listed in Table 1. 0.3 mm aluminum–lithium layers were prepared by a series of 2.3. Characterization and testing of the novel fiber metal laminates manufacture processes, shown in Fig. 1 were applied to prepare the 0.3 mm aluminum–lithium layers. The used alloy was finally Firstly, the interface integrity of the FMLs after the thermal fati- heat treated to T3 temper to obtain better elongation and fracture gue was examined by the C-Scan ultrasonic testing and scanning toughness. The ductility and toughness, not the strength, were pri- electron microscopy (SEM). The ultrasonic C-Scan test was con- marily considered for the metal layers, since the fiber layers were ducted in through-transmission mode, using a focused broadband responsible for the strength of the FMLs but possessed limited fail- transducer (9.5 mm in diameter) with a center frequency of 5 MHz. ure strain. Meanwhile, the testing device consists of a 0.025 mm resolution Moreover, the surface treatment of the aluminum–lithium layers scanning bridge. For the SEM detection, the 10 mm 10 mm was conducted to achieve a better bonding with the fiber reinforced specimens were prepared by CNC milling prior to the thermal composites. The phosphoric acid anodizing (H3PO4 150 g/ml, 20 °C, fatigue treatments. 15 V, 15 min) was adopted to construct a rough surface. Secondly, the interlaminar properties were characterized by Except the prepared aluminum–lithium layers, S4-glass/epoxy floating roller and interlaminar shear methods. Interlaminar prepregs with the thickness of 0.125 mm were also used to shear test was conducted in accordance with ASTM D2344 by a

Table 1 Chemical composition of the novel aluminum–lithium alloy (wt.%).

Li Cu Mg Ag Zr Mn Zn Al 0.6–0.8 3.3–4.1 0.4–0.8 0.2–0.5 0.06–0.15 0.2–0.5 0.3–0.4 The rest

Fig. 1. Preparation process of the aluminum–lithium layers. Download English Version: https://daneshyari.com/en/article/7890940

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