Effect of different preparation methods on mechanical behaviors of carbon fiber-reinforced PEEK-Titanium hybrid laminates
Introduction
Fiber metal laminates (FMLs), composed of alternating stacked layers of metallic sheets and fiber-reinforced polymer-matrix composites, are increasingly used in aircraft structures [1,2]. Compared to conventional monolithic metals, FMLs have several attractive mechanical properties such as high specific strength and stiffness, excellent fatigue behavior and fire resistance [[3], [4], [5]]. Since the introduction of Aramid-reinforced aluminum laminates (ARALL) by Delft University of Technology at 1970s, FMLs have undergone several renewals [6,7]. The Glass-reinforced aluminum laminates (GLARE), which is currently commercialized applied in the skin panel of A380 [8], has a modulus and service temperature limit that constraints its development on high-performance aircraft, while titanium alloys have superior high temperature resistance compared to aluminum alloys. Therefore, the application of titanium alloy instead of aluminum alloy could further enhance the specific strength and stiffness of the laminates. Meanwhile, combined with the high temperature resistant resin, such as polyetheretherketone (PEEK), the service temperature range of traditional FMLs could be greatly expanded, which makes Graphite/Carbon fiber reinforced titanium laminates (TiGr) an important choice for next-generation supersonic aircrafts and aerospace vehicles [9,10].
Multi-interface is one of the structural characteristics of FMLs, which will inevitably cause the performance of material to abruptly change at the interface, therefore, the inter-laminar performance largely restricts the stiffness, strength and fatigue properties of FMLs. To improve the bonding strength of the interface between metal and composites, several pre-treatment types including surface treatment of metal [11,12], surface modification of composites [13,14] and design of adhesives [15,16] have been attempted by most researchers. Mechanical roughening, chemical corrosion, anodizing and silanization are four primary methods for surface treatment of metal, which are usually processed by the combination while the single method has little effect on reinforcing the strength [17]. Among the above methods, silanization treatment on metal surfaces by silane coupling agents (SCA) has become a hot research topic in recent years [[18], [19], [20], [21]]. The morphology and characteristics of silane films are the determinant factors affecting the bonding behaviors of resins and metals. The most popular theory to explain the mechanism is the chemical bonding theory, it suggests that SCA usually contains functional groups that react with inorganic and organic materials, which act as a bridge by chemical bonding to connect two materials with different properties. For the connection with PEEK, the organophilic functional group of SCA could form a chemical bonding with the functional group of PEEK [22,23]. In addition, as a complement to the chemical bonding theory, the interpenetrating polymer network (IPN) theory suggests that the polymer could diffuse into the condensed silane interphase to form a tangled network [24]. For instance, Hon [18] evaluated the effects of five types of SCAs on adhesive performance of Ti sheet and dental composite resin, the acceptable similar results were obtained and the storage conditions had a significant effect on the adhesive strength. Moreover, Kang [19] investigated the interfacial bonding strength of Ti plate and shape memory polymer nanocomposite. It was found that the surface treatment of Ti plates with SCA improved the adhesive behavior.
The mechanical performance of fiber metal laminates at different temperatures is a significant issue in aviation industry [9]. Previous generations of hybrid laminates (ARALL, GLARE, CARE) can hardly apply in high temperature environments since metal layers and resins cannot withstand high temperatures, or the overall stiffness is relatively poor [[25], [26], [27]]. TiGr hybrid laminates make up for the gap in such application of FMLs, and some results from previous investigations have been acquired and applied. Cortés and Cantwell [28] carried out tension-tension fatigue tests on notched TiGr laminates, which have shown that the fatigue lives were 50 times greater than that of traditional notched titanium alloy with the same dimensions. The authors [29] also studied the effect of fiber orientation on the tensile performances of TiGr, which resulted in a decrease of the strength with increasing the angle as expected. Yu et al. [13] prepared the FMLs by anodizing Ti sheets and grafting multi-walled carbon nanotube onto carbon fibers, which have shown a significant increase of inter-laminar fracture toughness compared with untreated cases. Du et al. [30] investigated the open-hole tensile properties of TiGr laminates by using PEEK, and predicted the open-hole failure behaviors by employed three different damage models considering metal, composite and adhesive layers separately. It was noted that the predicted results consisted well with experiments.
Currently, the combination methods of mechanical roughening, chemical corrosion or anodizing to improve the interlayer behaviors of TiGr laminates have been well investigated [31,32], but few studies have been focused on the surface treatment of titanium through SCA. This paper aims to investigate the mechanical behaviors of TiGr laminates prepared by different surface treatment conditions using SCA. In the present work, in order to enhance the bonding properties of metal and PEEK, the titanium layers of Ti/Cf-PEEK laminates were subjected to sandblasting roughening and coupling agent silanization. The curing process was explored by setting different concentrations of silane coupling agent, curing temperatures and curing times, respectively. Moreover, 3/2 laminated structure was prepared by hot pressing method and the interface of PEEK-titanium was analyzed. Subsequently, the optimum process parameters were determined by comparing the tensile, bending and inter-laminar shearing properties of Ti/Cf-PEEK laminates treated through different preparation methods.
Section snippets
Surface pre-treatment of TA2 sheets
In this research, the pure titanium sheet of TA2 was chosen as the metal layer of the laminates which was supplied from Baoji titanium industry Co., Ltd. in the form of 0.3 mm-thick, with the chemical composition (in wt.%) of 0.3Fe, 0.25O, 0.08C, 0.03 N, 0.015H and balance Ti, the property parameters are given in Table 1. The unidirectional carbon fiber-PEEK pregreg named CETEX TC1200 (resin used PEEK, fiber used AS-4) was used as the composite layer with a thickness of 0.125 mm and fiber
Tensile behavior
In this part, the tensile curves of groups A-C are presented in Fig. 4(a–c). The tensile failure of the TiGr laminates can be roughly divided into two stages. Firstly, the load was undertaken by the metal and the fibers at the linear elastic deformation stage, there was no permanent plastic deformation occurred in the metal. Moreover, the slope of the curve changed slightly with increasing the tensile stress, which reveals that the metal has reached the yield limit. Since the elongation at
Conclusions
In this research, a surface treatment method by silane coupling agent for titanium sheet to improve the bonding performance with PEEK was proposed. The TiGr hybrid laminate was prepared by hot pressing method, the curing process and the mechanical properties of the laminates were investigated. It is found that, with the increase of SCA concentration, the mechanical properties include tensile, bending and inter-laminar shearing strengths increase first and then decrease, similar to curing
Data availability
The raw/processed data required to reproduce these findings will be made available on email request.
CRediT authorship contribution statement
Chunming Ji: Conceptualization, Methodology, Investigation, Writing - original draft. Bing Wang: Conceptualization, Validation, Resources, Writing - review & editing. Jiqiang Hu: Formal analysis, Visualization. Chang'an Zhang: Investigation, Data curation. Yuguo Sun: Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (Grant Number 11972008) and the Innovation Project of New Energy Vehicle and Intelligent Connected Vehicle.
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