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UHMWPE-Elium® laminates
Composites Part B 181 (2020) 107578
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On the metal thermoplastic composite interface of Ti alloy/ UHMWPE-Elium® laminates Logesh Shanmugam a, M.E. Kazemi a, Zaiqing Rao a, Lei Yang b, Jinglei Yang a, * a b
Department of Mechanical and Aerospace Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China College of Civil and Transportation Engineering, Shenzhen University, Shenzhen, 518060, China
A R T I C L E I N F O
A B S T R A C T
Keywords: UHMWPE fibers Surface treatment Thermoplastic FML MTCI
Thermoplastic fiber metal laminate (T-FML) is a new hybrid composite material, which is a combination of sandwiched metal and complete thermoplastic fiber reinforced polymer (FRP). Due to its superior properties contributed from the unique combination of metal and FRP’s, it has been applied in various advanced fields, like aerospace, and automotive. However, poor adhesion between inhomogeneous material surfaces of fiber, metal, and matrix in T-FML makes the whole system weaker. In this work, the Ti6Al4V (titanium alloy) and ultrahigh molecular weight polyethylene fiber (UHMWPE) reinforced thermoplastic (Elium®) polymeric composite were combined together to form a T-FML. Fiber surface functionalization by PDA (polydopamine) coating with MWCNT (Multiwalled carbon nanotubes) has been adopted to enhance the bonding between the fiber and matrix. Ti6Al4V metal surface treatment by anodization with postprocessing of etching and annealing process has been adopted to enhance the interfacial bonding between metal thermoplastic composite interface (MTCI). The double cantilever beam test was utilized to evaluate the G1C (Mode I interlaminar fracture toughness at MTCI) for the T-FML sample with fiber surface functionalization and metal surface treatment. The result shows, after metal surface treatment, the average G1C can be immediately increased from 0.25 kJ/m2 (pristine titanium alloy with pristine fiber) to 1.57 kJ/m2 for surface-treated titanium alloy with pristine fiber. The PDA only coating for UHMWPE fiber enhanced the G1C from 1.57 kJ/m2 to 1.84 kJ/m2. PDA fiber surface functionalization with MWCNT coating enhanced the G1C further to 2.54 kJ/m2.
1. Introduction Fibre-reinforced polymer composites (FRPs) have a high strength to weight ratio, lightweight, and has extensive application in aerospace, marine, and automotive to name few [1]. However, fiber metal lami nates (FMLs) are hybrid composite materials, which is a combined sandwich of FRPs and thin metal, which are bonded together by either physical or chemical bonding. The fiber used can be carbon fiber, glass fiber, Kevlar fiber, UHMWPE fiber, etc., and metal can be either aluminum, titanium, or magnesium. Comparing with the traditional composite material, FMLs have superior properties, including light weight, excellent corrosion, flaming resistance, high tensile strength and modulus, high impact damage and fatigue resistance [2,3]. These ad vantages are mainly contributed by their unique FRP-metal geometrical structure. Exceptional fatigue resistance in FML is due to the fiber bridging of fatigue cracks, thanks to the residual stress system between metal layers and the composite lamina [4]. At present, the FMLs are
utilized in a wide application, especially in the cutting-edge area, like aerospace, and automotive. For one of the most used civil aircraft, Airbus A380, the aircraft’s fuselage is made up of GLARE (Glass Lami nate Aluminum Reinforced Epoxy) FML [5]. The Prepregs acts as a moisture barrier between metal (aluminium layer) and the FRP com posite layer (Glass laminate) in GLARE were the main reason for supe rior corrosion resistance [6]. Metal being ductile in nature can absorb a large amount of energy in the elastic region up to the yielding and establish large strain rate before the failure of the event of impact. Composite materials being brittle thermosetting (in some cases) in na ture may absorb the energy only in elastic region before undergoing different modes of failure in the composite laminate. FML make use of the advantage of both metal and the FRP composite system and am plifies its impact damage resistance [7]. Among the FMLs, the T-FML (Thermoplastic-FML) recently becomes a hot topic in advanced mate rials due to its high energy absorbing capability where either fiber or matrix or both made of thermoplastic material in the FRP system. Due to
* Corresponding author. E-mail address: [email protected] (J. Yang). https://doi.org/10.1016/j.compositesb.2019.107578 Received 1 August 2019; Received in revised form 18 October 2019; Accepted 31 October 2019 Available online 6 November 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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the advanced development in the industries like aerospace, automotive and marine sector, it demands such material with capable of with standing high impact and with reasonable strength as well. In such conditions, T-FML can play a pivotal role. To fabricate such material system, Ultra-high-molecular-weight polyethylene (UHMWPE) fiber, which has high impact resistance, and better chemical, moisture, and water resistance [8] can be combined with Elium® resin (thermoplastic polymer, liquid at room temperature) to form a complete thermoplastic FRP part in T-FML. Elium® resin has high fracture toughness compared to conventional thermosetting matrix. The Elium® resin is first of its kind, which can be infused at room temperature similar to conventional thermosetting matrix [9,10]. Thermoplastic Elium® resin is an infusible resin at room temperature which is newly developed has been used by few researcher to determine the Mode I interlaminar fracture toughness with brittle carbon fibers [11–13]. But in this research, combining liquid thermoplastic matrix (Elium®) with thermoplastic UHMWPE fibers in FML is first of its kind to process at room temperature. The Titanium alloy can be used as a thin metal due to its low density along with excellent mechanical properties [14]. Delamination is a severe problem in the hybrid polymeric composite and occurs due to the high inter laminar stress developed at the fiber and matrix interface during Mode I (opening mode), Mode II (Shear mode) and Mode II (torsion) type of loading [15]. To overcome this complication, hybrid T-FML composite interfaces must have high interfacial bonding quality between fiber and matrix, and at the metal thermoplastic composite interface (MTCI). At present, the major complication in T-FML (in the present work) is the adhesion between each constituent, (i) the adhesion between the UHMWPE fiber with the Elium® resin (ii) the adhesions between the titanium metal and the Elium® resin. To enhance the bonding quality, the surface treatment on both the fiber surface and the metal surface can be an ideal solution. UHMWPE fiber surface can be surface functional ized by adding the polar functional group and increasing the surface free energy by several different methods which are currently available [16]. The UHMWPE fibers can be functionalized by plasma treatment, the shear strength of UHMWPE fiber after plasma treatment with epoxy matrix has been improved 3 times when compared with the pristine sample, the enhancement comes from inducing the reactive species on the fiber surface and modifying the surface morphology [17,18]; Corona discharge, similar to plasma treatment is another method where high electrical energy discharged on the fiber surface would enhance the shear strength with epoxy matrix about 11 times than the pristine fiber epoxy composite system [19,20]; Chromic acid treatment, utilizes strong corrosive acid to treat the fiber surface to modify the surface which creates strong mechanical interlocking when combined with the epoxy resin system [21]. Sol-gel treatment, a process of coating the hybrid organic (polymers) and inorganic (silicon alkoxides) material onto the substrate is another method to improve the bonding quality between the fiber and matrix [22]. However, the above-mentioned methods have issues, such as toxic contaminations, high cost, etc., The UHMWPE fiber surface can be functionalized by immersing the fiber in simple dopamine solution to form PDA on the fiber surface. The process involves simple self-oxidative polymerization. In our previous work, along with dopamine solution, when 0.03 wt% of MWCNT nano-fillers is added, the bonding strength can be improved about 42.5% compared to that of pristine-UHMWPE/Elium® composite system [23]. Since the process is simple and does not require expensive equipment for the functionalization process, this protocol has been adopted in this research work. Titanium surface can be modified by many different methods, and the process is well matured to enhance the interfacial bonding in the metal thermoplastic composite interface. The different surface treat ment is commonly followed are shock peening [24], and sandblasting [25–27] comes under the mechanical surface treatment of titanium alloy surface. Anodization [28,29], micro-arc oxidation (MAO) [30,31], and etching [32,33] are categorized under chemical surface treatment. Other surface treatment process such as addition of interfacial layer -
sol/gel methods [34,35], coupling agent [36,37], and plasma spray [38] are some of the surface treatment processes which are followed to enhance the bonding quality at MTCI. In our previous work [39], surface treatment for titanium alloy is carried out by both mechanical and chemical treatment following the procedure from sandblasting for 20sec, anodization at 40 � C for 15min, NaOH etching for 24hrs and annealing for 5 hrs at 600 � C leads to the formation of hierarchical macro to nanopores on the Ti alloy surface. After the formation of nanopores on the titanium alloy surface, the bonding strength and the wettability of the surface with polymer significantly enhanced. The Mode I double cantilever beam (DCB) failure mechanism in hybrid T-FML is completely different from a homogenous DCB com posite [40]. Each of the ingredients in T-FML may fail: the metal layer may fail by cracking, fibers may break, or matrix may crack. However, the strong or weak interface at MTCI plays a vital role in determining the failure mechanism in T-FML. Based on the interface at fiber/matrix, and MTCI, the failure can be determined in DCB Mode I loading. The two most delaminations may occur at the interface (i) the interface between metal and polymer composite, (ii) the interface between the polymer matrix and the fiber. The special case is, “In case of delamination one of the interfaces fail or the matrix fails in a cohesive manner”. This article reports the Mode I interlaminar fracture toughness of TFML at MTCI between the surface-treated titanium alloy and the pris tine, PDA, and PDA þ CNT surface-treated UHMWPE fiber with infusible thermoplastic Elium® resin. The interlaminar fracture toughness of different surface treatment is evaluated by a double cantilever beam (DCB) test. This article also reports the failure mechanism, delamination resistance in the composite system after the surface treatment of the metal and the fiber. In this work, the coating of the fiber is characterized by Raman spectroscopy; the SEM (scanning electron microscopy) before and after the fracture is shown. Load-displacement, resistance curve for the T-FML hybrid composite system before and after surface treatment is also described to its corresponding crack growth from DCB test. 2. Experimental 2.1. Materials 1.5 mm thickness of grade 5 titanium alloy (Ti6Al4V) with element contribution of 90% Ti (titanium), 6% Al (aluminium), and 4% V (va nadium) plates were used as a metal part in the T-FML. Plain-woven QuantaFlex™ (PEP) ultrahigh molecular weight polyethylene (UHMWPE) fiber with an aerial density of 172 gsm have used as fiber reinforcement in this work. Dopamine-HCl and tris(hydroxymethyl) aminomethane (Tris) were ordered from Sigma-aldrich, China. COOHfunctionalized MWCNT (multi-walled carbon nanotubes) with outer diameter 8–15 nm and length 0.5–2 μm was purchased from time nano, China. Elium® 188, a liquid methyl methacrylate thermoplastic resin has the ability of infusion with the low viscosity at room temperature, were supplied from Arkema, China. All chemicals were of analytical reagent grade and used without further purification. 2.2. Surface treatment of metal The 2000 grid abrasive paper was used to ground the Ti alloy fol lowed by washing in distilled water and ethanol in a bath sonicator and dried for further treatment. After the polishing by ground paper, the Ti samples were further sandblasted using alumina powder with a particle diameter of 5–20 μm for 20 s, and ultrasonicated and dried. Electrolyte solution for the anodization process was a combination of sodium hy droxide solution (7.5 M), ethylene diamine tetra acetic acid (EDTA0.1 M) as impurity-ion complexing agent and Na-tartrate (0.2 M) was used as the Ti-complexing agent. The temperature of the electrolyte solution was kept constant at 40 � C throughout the anodization process on a hotplate set up with a constant voltage of 15V and cathode as stainless steel for 15min. NaOH solution (1 M) was prepared as an 2
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from equilibrium; the same forces are acting on each substrate. Clearly, for each substrate to have the same deflection requires both top and bottom adherend must have the same flexural rigidity, as shown in Equation (1).
etching solution, and the anodized samples were immersed in the so lution for 24 h at 60 � C. After the process, the sample is washed and dried and were maintained at a temperature of 600 � C for 5hrs in the hot furnace to complete the surface treatment process. Fig. 1 shows the schematic and SEM image of the Ti alloy surface treatment.
(EI)
2.3. Surface treatment of UHMWPE fabrics
top, Ti metal
¼ (EI) bottom, UHMWPE
(1)
composite 2
E is Young’s modulus of the material (N/mm ), and I is Moment of Inertia of the substrate (mm4). The Young’s modulus of UHMWPE composites were studied previously in our work [10]. Based on this condition, the thickness of the metal is chosen as 1.5 mm and composite thickness of 4.5 mm. The T-FML samples were fabricated by vacuum-assisted resin infusion (VARI) process. Elium resin were pre pared with the addition of 2 wt% benzoyl peroxide polymerization as initiator. The surface-treated Ti6Al4V metal and modified/pristine UHMWPE fabrics were arranged as per Fig. 3. For all T-FML samples, 1.5 mm thick Ti alloy plate at the bottom, and 12 fabric pieces (for PDA and PDA þ CNT samples, 2 modified fabrics, and 10 pristine fabrics were utilized) with the Teflon sheets in the middle (between MTCI) to create a 40 mm pre-crack in the DCB Mode I interlaminar fracture toughness test samples. Table 1 shows the fabric and metal combinations tested in this work.
Fresh dopamine solution (4 g/L) was prepared by adding the dopamine-HCL powder into DI water, then the pH value of the solution was modified to 8.5 by adding the Tris base. The UHMWPE fabric were immersed in the as-prepared solution and then left it for 24 h. After the PDA coating, the treated fabric were rinsed in DI water gently without squeezing it to remove the unreacted dopamine chemical residue from the fabric surface. Finally, treated fabrics were dried in the vacuum oven before the composite fabrication. Similarly, for the dopamine with CNT embedded coating, dopamine solution were prepared to the similar protocol above, but additionally, 0.03 wt% of COOH-MWCNT were added to the as-prepared solution. CNTs were well dispersed in the dopamine solution by using probe sonicator. Now, the pristine UHMWPE fabrics were immersed in the solution for 24hrs, to obtain a uniform coating of PDA þ CNT. Finally, the treated fabrics were washed and dried in the vacuum oven before the composite fabrication. The procedure followed similarly to our previous work [23]. Fig. 2a and b depict the coating of PDA and PDA þ CNT on the fiber surface.
2.5. Sample characterization and mechanical test The fiber/fabric surface after the surface treatment were observed using SEM (scanning electron microscopy – JEOL-6390) images oper ated at 20 kV. Similarly, the fracture surface after the DCB test was evaluated by SEM. Ti6Al4V surface morphology after the surface treatment was also characterized by SEM operated at 20 kV. Energydispersive X-ray spectroscopy (EDS) used to find the chemical compo sition on the surface to analyze the oxide coating on Ti alloy by SEM. 514.5 nm laser source from Raman spectrometer inVia (Reinhaw) was acquired on the surface treated fabric to confirm the presence of PDA on the fabric surface. The surface roughness of Ti6Al4V after the surface treatment was recorded using optical profilometry (Bruker NPFLEX) with a measured area of 480 μm � 640 μm at a different location. Finally, the average and standard deviation of the sample is reported. The contact angle between the Elium® and the Ti6Al4V surface treated samples were measured using contact angle measuring equipment, Biolin Theta. Mode I interlaminar fracture toughness (G1C) of T-FML at metal thermoplastic composite interface (MTCI) samples were cut into the required dimension for double cantilever beam (DCB) testing after the sample fabrication. Each sample were labelled as per the sample code mentioned in Table 1. In order to detect the crack front of each sample during the DCB test, one side of the DCB samples were painted by brittle lacquer. Modified beam theory, as mentioned in ASTM D5528, is used to evaluate the Mode I interlaminar fracture toughness of T-FML. During the DCB loading, each sample were loaded at the loading rate of 1 mm/ min. Equation (2) is used to calculate the G1C of T-FML samples,
2.4. Sample preparation of DCB test DCB specimens with dissimilar substrates (Ti metal and UHMWPE composite) are inherently mixed-mode loading specimens due to nonsymmetric flexural rigidity. To establish a Mode I specimen, the thick ness of two adherends are chosen to achieve symmetric bending during loading [41]. During DCB loading, the cantilevered portion of each substrate must have the same load-line displacement. Consequently, each portion contributes equally to the work done during the test. Since,
GIC ¼
3Pδ 2Bða þ jΔjÞ
(2)
where P: a load of crack growth (N), δ: corresponding displacement (mm), a: corresponding crack growth (mm), B: width of the specimen – P/δ. From the DCB test, load(mm), Δ ¼ slope of C1/3 vs a, and C– displacement plot is observed, and the resistance curve is plotted with GIC on X-axis and corresponding crack growth on Y-axis. 3. Results & discussion 3.1. Surface morphology and surface characterization of UHMWPE fabric and Ti6Al4V after surface treatment
Fig. 1. Schematic of metal surface treatment (a) anodization, (b) etching, (c) annealing.
Fig. 1 presents the SEM images of treated Ti alloys based on 3
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Fig. 2. UHMWPE fiber surface treatment (a) PDA, (b) PDA þ CNT.
Fig. 3. Sample Preparation (a) Schematic of T-FML fabrication, (b) VARI fabrication setup, (c) Schematic of T-FML with modified fabric layers for DCB testing.
anodization with post-process treatment. For the sample after anodiza tion of Ti alloy, the surface has many microscopic and macroscopic bumps showing a macroscopic structure (Fig .1a). The morphology of the surface has greatly influenced by NaOH etching. The surface many nano-structured bumps appeared based on the thermally activated chemical reaction in the corrosive NaOH solution (Fig. 1b). After the process of annealing treatment, the Ti alloy surface were observed with uniform hexagonal nanopores at high magnification (Fig. 1c). From the observation of SEM, a hierarchical macro-scale to nanoscale was developed on Ti-alloy. The result from TFBT test shows that on adding 0.03 wt% of CNT can improve the bonding strength with Elium® resin about 42.5% compared to that of the pristine composite system [23]. However, Raman spectroscopy can also be used to confirm the presence of PDA on the fiber surface. The detailed characterization of Raman spectroscopy on UHMWPE fiber surface PDA and PDA þ CNT coating
has been explained in our previous work [23]. Fig. 4a shows the surface roughness of Ti alloy after anodization surface treatment process, which is very important in improving the bonding strength between metal and polymer composite. The other surface-treated Ti alloy shows lower surface roughness relative to the Ti alloy after the annealing process depicting that only after the sequential process of anodization, etching, and annealing the sample can make the surface with higher roughness. On the other hand, after the annealing process, the wettability between the surface-treated metal and the ma trix has improved significantly relative to the other surface-treated process (Fig. 4b). This confirms the sequential process of anodization, etching, and annealing can create nanopores on the surface, improving the surface roughness and wettability of the resin, making the surface more oleophilic. The EDS result from Table 2 shows more oxygen con tent on the treated surface, and the percentage of titanium decreases 4
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tested to estimate the interlaminar fracture toughness at MTCI by DCB testing method. Load-displacement plot, which is shown in Fig. 5a were derived from DCB experiments, and the results show the significance of surface treatment. For all the samples, the load increases linearly until the peak load and there after the load drops gradually as the crack propagates between the metal and thermoplastic composite interface. For sample, Ti-Pris þ UHMWPE-Pris in Fig. 5a, the critical or peak load required to initiate the crack between the metal and thermoplastic composite interface is lower when compared to other samples mentioned in Table 1. The result obtained for the sample Ti-Pris þ UHMWPE-Pris can be compared with the sample Ti-Ann þ UHMWPEPris, where the difference between these two samples is only the surface treatment of Ti alloy. The peak load required to initiate the crack at MTCI is higher relatively for the sample after Ti surface treatment. This shows that nanopores, which is formed on the Ti alloy after the annealing process can significantly improve the bonding between the metal and thermoplastic composite interface. The other case to discuss here is the significance of surface treatment on the UHMWPE fabric in TFML. The load required to initiate the crack in T-FML after the surface treatment follows, Ti-Ann þ UHMWPE-PDA þ CNT > Ti-Ann þ UHMWPE-PDA > UHMWPE-Pristine. This confirms that PDA and CNT play a vital role in improving the bonding at MTCI in T-FML. Characteristic stick-slip behaviour observed for all T-FML which may arise due to the abnormality in local material properties, such as fiber or resin-rich region, non-alignment fibers, voids, fractured behaviour of fiber bridging, and moreover to the adhesion level between Ti alloy and UHMWPE composite system. But, for the sample Ti-Ann þ UHMWPEPDA þ CNT, the stick-slip behaviour is different from others; this may be due to the strong adhesion level between metal and polymer composite. This stick-slip behaviour can be defined as “no crack-growth even as the load increases” [42]. This phenomenon can improve the critical strain energy release rate at MTCI. From Fig. 5a, the load-displacement for Ti-Ann þ UHMWPE-PDA þ CNT, the load abruptly dropped at several points, immediately after the critical load corresponding to unstable and intensive crack propagation mechanism. The unstable crack propagation is due to the plastic fracture due to the shear yielding between the surface-treated fiber and the Elium® resin. The shear yielding may arise
Table 1 Prepared T-FML composite systems for Mode I interlaminar fracture toughness test. Sample code
Description
Dimension
1
Ti-Pris þ UHMWPE-Pris
Length ¼ 150 mm, width ¼ 25 mm, Thickness ¼ 6 mm, Film insert length (a’) ¼ 40 mm
2
Ti-Ann þ UHMWPE-Pris
3
Ti-Ann þ UHMWPEPDA
4
Ti-Ann þ UHMWPEPDA þ CNT
T-FML fabricated from Pristine Ti6Al4V and Pristine UHMWPE fabric reinforced neat ELIUM matrix T-FML fabricated from surface treated (annealing) Ti6Al4V, and Pristine UHMWPE fabric reinforced neat ELIUM matrix T-FML fabricated from surface treated (annealing) Ti6Al4V, and PDA treated UHMWPE fabric reinforced neat ELIUM matrix T-FML fabricated from surface treated (annealing) Ti6Al4V, and PDA embedded with 0.03 wt% CNT treated UHMWPE fabric reinforced neat ELIUM matrix
after sequential anodization with post-process treatment. This confirms that the oxide layer formed on the surface after annealing process sup ports the wettability and improve the bonding strength with the Elium® matrix. 3.2. Double cantilever beam test T-FML samples which were fabricated by combining pristine and surface treated (Annealing) Ti6Al4V with pristine and surface treated (PDA and PDA þ CNT) UHMWPE with thermoplastic Elium® were
Fig. 4. Surface treated Ti6Al4V (a) Surface roughness, (b) Contact between Ti alloy and matrix. Table 2 EDS result of surface-treated Ti6Al4V. Ti-Pristine Ti-Anodization Ti-Etching Ti-Annealing
C (at. %)
Al (at. %)
Si (at. %)
Ti (at. %)
V (at. %)
Au (at. %)
O (at. %)
Ti/O (at%)
3.74 3.26 3.16 3.18
7.45 6.86 5.49 5.42
0.43 2.52 0.08 0.15
77.97 66.86 65.06 59.46
2.90 2.70 2.45 2.02
2.38 2.30 2.32 2.10
5.13 15.49 21.43 27.67
15.20 4.32 3.04 2.15
5
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Fig. 5. Mode I opening mode result of T-FML fabricated from surface-treated Ti6Al4V with pristine, PDA, and PDA þ CNT surface-treated UHMWPE fiber. (a) Loaddisplacement, (b) Interlaminar fracture toughness, (c) Resistance curve (R-curve).
due to the strong interface, where 42.5% improved bonding has been achieved at the fiber and matrix interface when the fiber is treated with PDAþ0.03 wt% CNT [23]. In summary, the load required to initiate the crack at the MTCI is defined by the surface treatment of both metal and fabric surface. The result obtained from the DCB experiment in Fig. 5a shows that surface treatment of annealing Ti-alloy and PDA þ CNT on fiber surface shows the maximum load is required to initiate the crack at the MTCI. The determination of interlaminar fracture toughness by DCB is tested only on the symmetrical and homogenous composite with either Mode I or Mode II loading condition. But, for the case of T-FML, the fracture resistance of the hybrid composite may likely to affect due to the dissimilar materials with different mechanical and physical prop erties. Also, for T-FML sample, the initial delamination is between the UHMWPE composite and Ti alloy, where the interlaminar fracture toughness is characterized by asymmetry DCB composite system. This makes the T-FML hybrid composite system as a mixed-mode at the crack tip [43–45]. More specifically, in our T-FML, the bending stiffness dif ference between Ti alloy and UHMWPE composite is different, which resulted in Mode II testing in the fracture mechanism, even if the loading condition is Mode I. However, Mollon [46] and sebaey [47] reported that the contribution of Mode II asymmetric crack propagation is negligible when compared with Mode I. Fig. 5b shows the average Mode I interlaminar fracture toughness (G1c) at MTCI with standard deviation for 4 samples tested in each configuration. After metal surface treat ment, the G1C can be immediately increased from 0.25 kJ/m2 (pristine titanium alloy with pristine fiber) to 1.57 kJ/m2 (528%) for surface-treated titanium alloy with pristine fiber. The PDA only coating for UHMWPE fiber enhanced the G1C from 1.57 kJ/m2 to 1.84 kJ/m2
(636%, compared with Ti-Pris þ UHMWPE-Pris). PDA fiber surface functionalization with MWCNT coating enhanced the G1C further to 2.54 kJ/m2 (916%, compared with Ti-Pris þ UHMWPE-Pris). To understand the crack growth resistance offered to the specimen on applying the DCB Mode I load, R-curve or resistance curve can be used. Fig. 5c shows the resistance curve (Mode I interlaminar fracture toughness at MTCI vs corresponding crack growth) for all four different sample configurations in this work. Among all four samples, Ti-Ann þ UHMWPE-PDA þ CNT shows higher G1c at both initiation and propa gation compared to other sample. This depicts that surface treatment on both metal and fiber has a significant influence in enhancing the crack growth resistance offered in Mode I loading. In general, the thermo plastic polymer (Elium®) has higher fracture toughness (G1c ¼ 0.5 kJ/ m2) when compared to the brittle thermoset polymer (epoxy) fracture toughness (G1c ¼ 0.2 kJ/m2). The enhanced crack growth resistance in T-FML is offered by two different reasons (i) strong fiber and matrix bonding, and strong bonding at MTCI, (ii) influence by thermoplastic MMA Elium® resin. If the fiber and matrix have strong adhesion at the interface, the thermoplastic Elium® resin highly deforms at the interface upon the loading [23]. Only after the PDA þ CNT surface treatment on the fiber, significant stick-slip behaviour is observed. During stick-slip behaviour, there is no crack growth as the load increases. At this stage, crack blunt occurs at the fiber and matrix due to the strong interface. This crack blunt makes the Elium® resin to highly deform or initiate plastic rupture at the cracktip. This deformation, in turn, increases the load with no further crack growth during the DCB loading. This proves crack blunt is the domi nating mechanism in improving the interlaminar fracture toughness [48]. For the sample Ti-Ann þ UHMWPE-Pris, the failure may be due to 6
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corresponding fractured SEM image. For Ti-Pris þ UHMWPE-Pristine sample, the failure is between pristine Ti alloy and the pristine UHMWPE polymer composite system. The poor adhesion between metal and composite plays a significant role in attributing to the poor per formance of T-FML. Fig. 7a2 shows the poor matrix bonding on the nonsurface (pristine) treated Ti alloy. Adhesive failure of polymer com posite system from the pristine Ti-alloy can be confirmed from the smooth delamination in Fig. 7a3. For all T-FML sample after the surface treatment of Ti-alloy (Fig. 7b1, c1 and d1), the DCB failure is similar to Fig. 6. So, the fracture toughness is determined by the level of adhesion between fiber and matrix. In Fig. 7b2, the debonding of the fiber is very smooth, depicts poor adhe sion of the fiber to the matrix in Ti-Ann þ UHMWPE-Pristine. Also, the fiber surface were very smooth on the fracture composite adherend confirming the poor level of adhesion between fiber and matrix (Fig. 7b3). After the PDA surface treatment on the fibers, the level of adhesion between the fiber and matrix is enhanced, and this can be evidently seen from Fig. 7c2 and 7c3. The micro-fibrils, which is part of UHMWPE fiber, were observed on both the matrix surface and the fiber surface, confirms the enhanced level of adhesion between the fiber and matrix in the sample Ti-Ann þ UHMWPE-PDA. Interestingly, after the surface treatment of the fiber with CNT embedded in PDA coating has a rougher fracture on a metal surface with highly deformed matrix attributing towards crack-blunt mechanism (Fig. 7d2). The highly deformed matrix yields a plastic rupture due to the shear yielding at the fibre-matrix interface [50]. Strong fibre-matrix adhesion and high ductile nature of thermoplastic Elium® contributed towards the shear yielding at the interface. Fig. 7d3 shows the deformed matrix attached on the fiber surface with enormous of microfibril for the sample Ti-Ann þ UHMWPE-PDA þ CNT. This confirms PDA þ CNT coating on the fiber surface can create a crack-blunt at the crack tip and favouring towards strong fiber-matrix adhesion and the ductility of the matrix at the interface.
the debonding at the interface instead of plastic rupture. This failure may not contribute to the crack-blunt at the crack tip due to the poor interface. Although the failure occurs at fiber and matrix interface, it should be well noted that the Elim matrix from UHMWPE composite system has strong adhesion with Ti alloy. The poor bonding at MTCI leads to the poor performance of T-FML, even if the adhesion between fiber and matrix is stronger. Influence of strong MTCI (annealing surface treatment of Ti alloy), fiber/matrix interface (PDA þ CNT surface treatment of UHMWPE fiber), the influence of thermoplastic Elium® resin (plastic fracture), and crack-blunt mechanism plays a vital role in enhancing the Mode I interlaminar fracture toughness. Enhancing the interlaminar fracture toughness by crack blunt mechanism is similar to the stitching effect in laminated composites [49]. 3.3. Fracture surface morphology The two different failure modes were observed after the DCB fracture in T-FML, (i) For the sample Ti-Pris þ UHMWPE-Pris, the failure is in between the pristine-metal and the thermoplastic matrix due to the poor bonding of UHMWPE polymer composite with non-surface treated Ti metal alloy. (ii) For the T-FML sample fabricated after the surface treatment of Ti alloy (Ti-Ann þ UHMWPE-Pristine, Ti-Ann þ UHMWPEPDA, and Ti-Ann þ UHMWPE-PDA þ CNT) has a similar failure to Fig. 6. This failure is due to the strong adhesion of the Elium matrix from UHMWPE polymer composite with surface-treated Ti metal alloy. Fig. 6 shows the failure of T-FML in the DCB experiments for the sample fabricated after the surface treatment of Ti alloy. Due to the strong adhesion of polymer composite with surface-treated metal, the adhesion level of fiber and matrix bonding determines the failure mode in T-FML. In our case, the adhesion level between the fiber and matrix is deter mined by the surface treatment of UHMWPE fiber functionalized with PDA, and PDA þ CNT. Fig. 6 shows the macro photograph of T-FML sample after DCB fracture. The Elium® matrix strongly adheres to the surface-treated Ti alloy (Fig. 6a1), and the UHMWPE composite fiber debonded from the matrix in Fig. 6a2. The SEM image in Fig. 6c and d shows the Elium® adhere to the metal surface and debonded fibers at MTCI, respectively. Scanning electron microscopic (SEM) images were taken on both the metal surface and debonded polymer composite surface after the DCB test understand the failure mechanism at MTCI. Fig 0.7 shows the schematic and fractured SEM images from both the metal and UHMWPE composite surface. Fig. 7a1 shows the schematic representation on the failure mode between the metal and fiber composite and the
4. Conclusions Determination of Mode I interlaminar fracture toughness at Metal thermoplastic composite interface is successfully carried out for the thermoplastic fiber metal laminate fabricated from surface-treated Ti6Al4V and PDA þ CNT surface-treated UHMWPE/Elium® composite system. The test result from the DCB test at the loading rate of 1 mm/min can be drawn as follows.
Fig. 6. Failure criteria observed in T-FML (Surface treated Ti6Al4V with UHMWPE composite) after the DCB fracture (a) Original sample image of the DCB fracture surface, (b) Schematic of the DCB fracture observed, (c) SEM of the Surface treated Ti6Al4V after DCB fracture, (d) SEM of the UHMWPE fiber after DCB fracture. 7
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Fig. 7. Schematic failure mechanisms and SEM fracture in T-FML at metal thermoplastic composite interface (a1,a2,a3) Ti-Pris þ UHMWPE-Pris, (b1,b2,b3) Ti-Ann þ UHMWPE-Pris, (c1,c2,c3) Ti-Ann þ UHMWPE-PDA, (d1,d2,d3) Ti-Ann þ UHMWPE-PDA þ CNT.
1. Successful surface treatment on the Ti alloy creates a nano-pores on the surface improves the metal and polymer interface. SEM, wetta bility test, and surface roughness confirm the enhanced result after the sequential annealing process. SEM and Raman spectroscopy confirms the successful coating of PDA and CNT embedded on the UHMWPE surface. 2. The improved interlaminar fracture toughness in the T-FML samples after the surface treatment of both fiber and metal were because of three important characteristics (i) high fracture toughness of infus ible room temperature processing thermoplastic Elium® resin (ii) strong Ti alloy metal and Elium®composite interfacial adhesion (iii) strong UHMWPE fiber and Elium® resin interfacial adhesion. 3. Enhanced average interlaminar fracture toughness about 916% was achieved after annealing surface treatment on Ti alloy and PDAþ0.03 wt%CNT surface treatment on the fiber/fabric surface. Highly deformed plastic rupture observed at the crack-tip influenced by strong fiber/matrix adhesion is the primary reason for the enhancement. 4. High deformation of the matrix attributes to the crack-blunt at the interface near the crack tip was the primary reason to observe stickslip behaviour in the sample after PDA þ CNT fiber surface treat ment. Crack-blunt mechanism is dominance in enhancing the Metal thermoplastic composite interface due to the high level of adhesion between fiber and high ductile nature of thermoplastic Elium® matrix.
Joint Research Scheme of Hong Kong (Grant#: N_HKUST 631/18), Nanhai-HKUST Program (Grant #: FSNH-18FYTRI01), Guangdong Sci ence and Technology Department (Project#: 2017A050506005, 2018B050502001), and Science, Technology & Innovation Commission of Guangzhou (Project #: 201907010028). The authors would like to acknowledge Dr Dong Brian and Dr Jinchun Zhu of Arkema, Changshu Research and Development Center, China, for providing Elium® resin. References [1] Kazemi M, Kouchakzadeh M, Shakouri M. Stability analysis of generally laminated conical shells with variable thickness under axial compression. Mech Adv Mater Struct 2018:1–14. € Çoban O. A review: fibre metal laminates, [2] Sinmazçelik T, Avcu E, Bora MO, background, bonding types and applied test methods. Mater Des 2011;32(7): 3671–85. [3] Ali A, Pan L, Duan L, Zheng Z, Sapkota B. Characterization of seawater hygrothermal conditioning effects on the properties of titanium-based fiber-metal laminates for marine applications. Compos Struct 2016;158:199–207. [4] Vlot A. Impact properties of fibre metal laminates. Compos Eng 1993;3(10): 911–27. [5] Wu GaJ-MY. The mechanical behavior of GLAREL laminates for aircraft structures. J Occup Med 2005;57(1):72–9. [6] Vlot A. Impact loading on fibre metal laminates. Int J Impact Eng 1996;18(3): 291–307. [7] Chai GB, Manikandan P. Low velocity impact response of fibre-metal laminates–A review. Compos Struct 2014;107:363–81. [8] Bhatnagar DL A. Military and law enforcement applications of lightweight ballistic materials. Lightweight Ballistic Composites 2006:364–97. [9] Shanmugam L, Kazemi M, Rao Z, Lu D, Wang X, Wang B, et al. Enhanced Mode I fracture toughness of UHMWPE fabric/thermoplastic laminates with combined surface treatments of polydopamine and functionalized carbon nanotubes. Compos B Eng 2019:107450. [10] Kazemi M, Shanmugam L, Lu D, Wang X, Wang B, Yang J. Mechanical properties and failure modes of hybrid fiber reinforced polymer composites with a novel liquid thermoplastic resin. Elium®. Comp Part A: Appl Sci Manuf 2019;125: 105523.
Acknowledgment The authors are grateful to the support from The Hong Kong Uni versity of Science and Technology (Grant #: R9365), the NSFC/RGC 8
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
On the metal thermoplastic composite interface of Ti alloy/ UHMWPE-Elium® laminates Logesh Shanmugam a, M.E. Kazemi a, Zaiqing Rao a, Lei Yang b, Jinglei Yang a, * a b
Department of Mechanical and Aerospace Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China College of Civil and Transportation Engineering, Shenzhen University, Shenzhen, 518060, China
A R T I C L E I N F O
A B S T R A C T
Keywords: UHMWPE fibers Surface treatment Thermoplastic FML MTCI
Thermoplastic fiber metal laminate (T-FML) is a new hybrid composite material, which is a combination of sandwiched metal and complete thermoplastic fiber reinforced polymer (FRP). Due to its superior properties contributed from the unique combination of metal and FRP’s, it has been applied in various advanced fields, like aerospace, and automotive. However, poor adhesion between inhomogeneous material surfaces of fiber, metal, and matrix in T-FML makes the whole system weaker. In this work, the Ti6Al4V (titanium alloy) and ultrahigh molecular weight polyethylene fiber (UHMWPE) reinforced thermoplastic (Elium®) polymeric composite were combined together to form a T-FML. Fiber surface functionalization by PDA (polydopamine) coating with MWCNT (Multiwalled carbon nanotubes) has been adopted to enhance the bonding between the fiber and matrix. Ti6Al4V metal surface treatment by anodization with postprocessing of etching and annealing process has been adopted to enhance the interfacial bonding between metal thermoplastic composite interface (MTCI). The double cantilever beam test was utilized to evaluate the G1C (Mode I interlaminar fracture toughness at MTCI) for the T-FML sample with fiber surface functionalization and metal surface treatment. The result shows, after metal surface treatment, the average G1C can be immediately increased from 0.25 kJ/m2 (pristine titanium alloy with pristine fiber) to 1.57 kJ/m2 for surface-treated titanium alloy with pristine fiber. The PDA only coating for UHMWPE fiber enhanced the G1C from 1.57 kJ/m2 to 1.84 kJ/m2. PDA fiber surface functionalization with MWCNT coating enhanced the G1C further to 2.54 kJ/m2.
1. Introduction Fibre-reinforced polymer composites (FRPs) have a high strength to weight ratio, lightweight, and has extensive application in aerospace, marine, and automotive to name few [1]. However, fiber metal lami nates (FMLs) are hybrid composite materials, which is a combined sandwich of FRPs and thin metal, which are bonded together by either physical or chemical bonding. The fiber used can be carbon fiber, glass fiber, Kevlar fiber, UHMWPE fiber, etc., and metal can be either aluminum, titanium, or magnesium. Comparing with the traditional composite material, FMLs have superior properties, including light weight, excellent corrosion, flaming resistance, high tensile strength and modulus, high impact damage and fatigue resistance [2,3]. These ad vantages are mainly contributed by their unique FRP-metal geometrical structure. Exceptional fatigue resistance in FML is due to the fiber bridging of fatigue cracks, thanks to the residual stress system between metal layers and the composite lamina [4]. At present, the FMLs are
utilized in a wide application, especially in the cutting-edge area, like aerospace, and automotive. For one of the most used civil aircraft, Airbus A380, the aircraft’s fuselage is made up of GLARE (Glass Lami nate Aluminum Reinforced Epoxy) FML [5]. The Prepregs acts as a moisture barrier between metal (aluminium layer) and the FRP com posite layer (Glass laminate) in GLARE were the main reason for supe rior corrosion resistance [6]. Metal being ductile in nature can absorb a large amount of energy in the elastic region up to the yielding and establish large strain rate before the failure of the event of impact. Composite materials being brittle thermosetting (in some cases) in na ture may absorb the energy only in elastic region before undergoing different modes of failure in the composite laminate. FML make use of the advantage of both metal and the FRP composite system and am plifies its impact damage resistance [7]. Among the FMLs, the T-FML (Thermoplastic-FML) recently becomes a hot topic in advanced mate rials due to its high energy absorbing capability where either fiber or matrix or both made of thermoplastic material in the FRP system. Due to
* Corresponding author. E-mail address: [email protected] (J. Yang). https://doi.org/10.1016/j.compositesb.2019.107578 Received 1 August 2019; Received in revised form 18 October 2019; Accepted 31 October 2019 Available online 6 November 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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the advanced development in the industries like aerospace, automotive and marine sector, it demands such material with capable of with standing high impact and with reasonable strength as well. In such conditions, T-FML can play a pivotal role. To fabricate such material system, Ultra-high-molecular-weight polyethylene (UHMWPE) fiber, which has high impact resistance, and better chemical, moisture, and water resistance [8] can be combined with Elium® resin (thermoplastic polymer, liquid at room temperature) to form a complete thermoplastic FRP part in T-FML. Elium® resin has high fracture toughness compared to conventional thermosetting matrix. The Elium® resin is first of its kind, which can be infused at room temperature similar to conventional thermosetting matrix [9,10]. Thermoplastic Elium® resin is an infusible resin at room temperature which is newly developed has been used by few researcher to determine the Mode I interlaminar fracture toughness with brittle carbon fibers [11–13]. But in this research, combining liquid thermoplastic matrix (Elium®) with thermoplastic UHMWPE fibers in FML is first of its kind to process at room temperature. The Titanium alloy can be used as a thin metal due to its low density along with excellent mechanical properties [14]. Delamination is a severe problem in the hybrid polymeric composite and occurs due to the high inter laminar stress developed at the fiber and matrix interface during Mode I (opening mode), Mode II (Shear mode) and Mode II (torsion) type of loading [15]. To overcome this complication, hybrid T-FML composite interfaces must have high interfacial bonding quality between fiber and matrix, and at the metal thermoplastic composite interface (MTCI). At present, the major complication in T-FML (in the present work) is the adhesion between each constituent, (i) the adhesion between the UHMWPE fiber with the Elium® resin (ii) the adhesions between the titanium metal and the Elium® resin. To enhance the bonding quality, the surface treatment on both the fiber surface and the metal surface can be an ideal solution. UHMWPE fiber surface can be surface functional ized by adding the polar functional group and increasing the surface free energy by several different methods which are currently available [16]. The UHMWPE fibers can be functionalized by plasma treatment, the shear strength of UHMWPE fiber after plasma treatment with epoxy matrix has been improved 3 times when compared with the pristine sample, the enhancement comes from inducing the reactive species on the fiber surface and modifying the surface morphology [17,18]; Corona discharge, similar to plasma treatment is another method where high electrical energy discharged on the fiber surface would enhance the shear strength with epoxy matrix about 11 times than the pristine fiber epoxy composite system [19,20]; Chromic acid treatment, utilizes strong corrosive acid to treat the fiber surface to modify the surface which creates strong mechanical interlocking when combined with the epoxy resin system [21]. Sol-gel treatment, a process of coating the hybrid organic (polymers) and inorganic (silicon alkoxides) material onto the substrate is another method to improve the bonding quality between the fiber and matrix [22]. However, the above-mentioned methods have issues, such as toxic contaminations, high cost, etc., The UHMWPE fiber surface can be functionalized by immersing the fiber in simple dopamine solution to form PDA on the fiber surface. The process involves simple self-oxidative polymerization. In our previous work, along with dopamine solution, when 0.03 wt% of MWCNT nano-fillers is added, the bonding strength can be improved about 42.5% compared to that of pristine-UHMWPE/Elium® composite system [23]. Since the process is simple and does not require expensive equipment for the functionalization process, this protocol has been adopted in this research work. Titanium surface can be modified by many different methods, and the process is well matured to enhance the interfacial bonding in the metal thermoplastic composite interface. The different surface treat ment is commonly followed are shock peening [24], and sandblasting [25–27] comes under the mechanical surface treatment of titanium alloy surface. Anodization [28,29], micro-arc oxidation (MAO) [30,31], and etching [32,33] are categorized under chemical surface treatment. Other surface treatment process such as addition of interfacial layer -
sol/gel methods [34,35], coupling agent [36,37], and plasma spray [38] are some of the surface treatment processes which are followed to enhance the bonding quality at MTCI. In our previous work [39], surface treatment for titanium alloy is carried out by both mechanical and chemical treatment following the procedure from sandblasting for 20sec, anodization at 40 � C for 15min, NaOH etching for 24hrs and annealing for 5 hrs at 600 � C leads to the formation of hierarchical macro to nanopores on the Ti alloy surface. After the formation of nanopores on the titanium alloy surface, the bonding strength and the wettability of the surface with polymer significantly enhanced. The Mode I double cantilever beam (DCB) failure mechanism in hybrid T-FML is completely different from a homogenous DCB com posite [40]. Each of the ingredients in T-FML may fail: the metal layer may fail by cracking, fibers may break, or matrix may crack. However, the strong or weak interface at MTCI plays a vital role in determining the failure mechanism in T-FML. Based on the interface at fiber/matrix, and MTCI, the failure can be determined in DCB Mode I loading. The two most delaminations may occur at the interface (i) the interface between metal and polymer composite, (ii) the interface between the polymer matrix and the fiber. The special case is, “In case of delamination one of the interfaces fail or the matrix fails in a cohesive manner”. This article reports the Mode I interlaminar fracture toughness of TFML at MTCI between the surface-treated titanium alloy and the pris tine, PDA, and PDA þ CNT surface-treated UHMWPE fiber with infusible thermoplastic Elium® resin. The interlaminar fracture toughness of different surface treatment is evaluated by a double cantilever beam (DCB) test. This article also reports the failure mechanism, delamination resistance in the composite system after the surface treatment of the metal and the fiber. In this work, the coating of the fiber is characterized by Raman spectroscopy; the SEM (scanning electron microscopy) before and after the fracture is shown. Load-displacement, resistance curve for the T-FML hybrid composite system before and after surface treatment is also described to its corresponding crack growth from DCB test. 2. Experimental 2.1. Materials 1.5 mm thickness of grade 5 titanium alloy (Ti6Al4V) with element contribution of 90% Ti (titanium), 6% Al (aluminium), and 4% V (va nadium) plates were used as a metal part in the T-FML. Plain-woven QuantaFlex™ (PEP) ultrahigh molecular weight polyethylene (UHMWPE) fiber with an aerial density of 172 gsm have used as fiber reinforcement in this work. Dopamine-HCl and tris(hydroxymethyl) aminomethane (Tris) were ordered from Sigma-aldrich, China. COOHfunctionalized MWCNT (multi-walled carbon nanotubes) with outer diameter 8–15 nm and length 0.5–2 μm was purchased from time nano, China. Elium® 188, a liquid methyl methacrylate thermoplastic resin has the ability of infusion with the low viscosity at room temperature, were supplied from Arkema, China. All chemicals were of analytical reagent grade and used without further purification. 2.2. Surface treatment of metal The 2000 grid abrasive paper was used to ground the Ti alloy fol lowed by washing in distilled water and ethanol in a bath sonicator and dried for further treatment. After the polishing by ground paper, the Ti samples were further sandblasted using alumina powder with a particle diameter of 5–20 μm for 20 s, and ultrasonicated and dried. Electrolyte solution for the anodization process was a combination of sodium hy droxide solution (7.5 M), ethylene diamine tetra acetic acid (EDTA0.1 M) as impurity-ion complexing agent and Na-tartrate (0.2 M) was used as the Ti-complexing agent. The temperature of the electrolyte solution was kept constant at 40 � C throughout the anodization process on a hotplate set up with a constant voltage of 15V and cathode as stainless steel for 15min. NaOH solution (1 M) was prepared as an 2
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from equilibrium; the same forces are acting on each substrate. Clearly, for each substrate to have the same deflection requires both top and bottom adherend must have the same flexural rigidity, as shown in Equation (1).
etching solution, and the anodized samples were immersed in the so lution for 24 h at 60 � C. After the process, the sample is washed and dried and were maintained at a temperature of 600 � C for 5hrs in the hot furnace to complete the surface treatment process. Fig. 1 shows the schematic and SEM image of the Ti alloy surface treatment.
(EI)
2.3. Surface treatment of UHMWPE fabrics
top, Ti metal
¼ (EI) bottom, UHMWPE
(1)
composite 2
E is Young’s modulus of the material (N/mm ), and I is Moment of Inertia of the substrate (mm4). The Young’s modulus of UHMWPE composites were studied previously in our work [10]. Based on this condition, the thickness of the metal is chosen as 1.5 mm and composite thickness of 4.5 mm. The T-FML samples were fabricated by vacuum-assisted resin infusion (VARI) process. Elium resin were pre pared with the addition of 2 wt% benzoyl peroxide polymerization as initiator. The surface-treated Ti6Al4V metal and modified/pristine UHMWPE fabrics were arranged as per Fig. 3. For all T-FML samples, 1.5 mm thick Ti alloy plate at the bottom, and 12 fabric pieces (for PDA and PDA þ CNT samples, 2 modified fabrics, and 10 pristine fabrics were utilized) with the Teflon sheets in the middle (between MTCI) to create a 40 mm pre-crack in the DCB Mode I interlaminar fracture toughness test samples. Table 1 shows the fabric and metal combinations tested in this work.
Fresh dopamine solution (4 g/L) was prepared by adding the dopamine-HCL powder into DI water, then the pH value of the solution was modified to 8.5 by adding the Tris base. The UHMWPE fabric were immersed in the as-prepared solution and then left it for 24 h. After the PDA coating, the treated fabric were rinsed in DI water gently without squeezing it to remove the unreacted dopamine chemical residue from the fabric surface. Finally, treated fabrics were dried in the vacuum oven before the composite fabrication. Similarly, for the dopamine with CNT embedded coating, dopamine solution were prepared to the similar protocol above, but additionally, 0.03 wt% of COOH-MWCNT were added to the as-prepared solution. CNTs were well dispersed in the dopamine solution by using probe sonicator. Now, the pristine UHMWPE fabrics were immersed in the solution for 24hrs, to obtain a uniform coating of PDA þ CNT. Finally, the treated fabrics were washed and dried in the vacuum oven before the composite fabrication. The procedure followed similarly to our previous work [23]. Fig. 2a and b depict the coating of PDA and PDA þ CNT on the fiber surface.
2.5. Sample characterization and mechanical test The fiber/fabric surface after the surface treatment were observed using SEM (scanning electron microscopy – JEOL-6390) images oper ated at 20 kV. Similarly, the fracture surface after the DCB test was evaluated by SEM. Ti6Al4V surface morphology after the surface treatment was also characterized by SEM operated at 20 kV. Energydispersive X-ray spectroscopy (EDS) used to find the chemical compo sition on the surface to analyze the oxide coating on Ti alloy by SEM. 514.5 nm laser source from Raman spectrometer inVia (Reinhaw) was acquired on the surface treated fabric to confirm the presence of PDA on the fabric surface. The surface roughness of Ti6Al4V after the surface treatment was recorded using optical profilometry (Bruker NPFLEX) with a measured area of 480 μm � 640 μm at a different location. Finally, the average and standard deviation of the sample is reported. The contact angle between the Elium® and the Ti6Al4V surface treated samples were measured using contact angle measuring equipment, Biolin Theta. Mode I interlaminar fracture toughness (G1C) of T-FML at metal thermoplastic composite interface (MTCI) samples were cut into the required dimension for double cantilever beam (DCB) testing after the sample fabrication. Each sample were labelled as per the sample code mentioned in Table 1. In order to detect the crack front of each sample during the DCB test, one side of the DCB samples were painted by brittle lacquer. Modified beam theory, as mentioned in ASTM D5528, is used to evaluate the Mode I interlaminar fracture toughness of T-FML. During the DCB loading, each sample were loaded at the loading rate of 1 mm/ min. Equation (2) is used to calculate the G1C of T-FML samples,
2.4. Sample preparation of DCB test DCB specimens with dissimilar substrates (Ti metal and UHMWPE composite) are inherently mixed-mode loading specimens due to nonsymmetric flexural rigidity. To establish a Mode I specimen, the thick ness of two adherends are chosen to achieve symmetric bending during loading [41]. During DCB loading, the cantilevered portion of each substrate must have the same load-line displacement. Consequently, each portion contributes equally to the work done during the test. Since,
GIC ¼
3Pδ 2Bða þ jΔjÞ
(2)
where P: a load of crack growth (N), δ: corresponding displacement (mm), a: corresponding crack growth (mm), B: width of the specimen – P/δ. From the DCB test, load(mm), Δ ¼ slope of C1/3 vs a, and C– displacement plot is observed, and the resistance curve is plotted with GIC on X-axis and corresponding crack growth on Y-axis. 3. Results & discussion 3.1. Surface morphology and surface characterization of UHMWPE fabric and Ti6Al4V after surface treatment
Fig. 1. Schematic of metal surface treatment (a) anodization, (b) etching, (c) annealing.
Fig. 1 presents the SEM images of treated Ti alloys based on 3
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Fig. 2. UHMWPE fiber surface treatment (a) PDA, (b) PDA þ CNT.
Fig. 3. Sample Preparation (a) Schematic of T-FML fabrication, (b) VARI fabrication setup, (c) Schematic of T-FML with modified fabric layers for DCB testing.
anodization with post-process treatment. For the sample after anodiza tion of Ti alloy, the surface has many microscopic and macroscopic bumps showing a macroscopic structure (Fig .1a). The morphology of the surface has greatly influenced by NaOH etching. The surface many nano-structured bumps appeared based on the thermally activated chemical reaction in the corrosive NaOH solution (Fig. 1b). After the process of annealing treatment, the Ti alloy surface were observed with uniform hexagonal nanopores at high magnification (Fig. 1c). From the observation of SEM, a hierarchical macro-scale to nanoscale was developed on Ti-alloy. The result from TFBT test shows that on adding 0.03 wt% of CNT can improve the bonding strength with Elium® resin about 42.5% compared to that of the pristine composite system [23]. However, Raman spectroscopy can also be used to confirm the presence of PDA on the fiber surface. The detailed characterization of Raman spectroscopy on UHMWPE fiber surface PDA and PDA þ CNT coating
has been explained in our previous work [23]. Fig. 4a shows the surface roughness of Ti alloy after anodization surface treatment process, which is very important in improving the bonding strength between metal and polymer composite. The other surface-treated Ti alloy shows lower surface roughness relative to the Ti alloy after the annealing process depicting that only after the sequential process of anodization, etching, and annealing the sample can make the surface with higher roughness. On the other hand, after the annealing process, the wettability between the surface-treated metal and the ma trix has improved significantly relative to the other surface-treated process (Fig. 4b). This confirms the sequential process of anodization, etching, and annealing can create nanopores on the surface, improving the surface roughness and wettability of the resin, making the surface more oleophilic. The EDS result from Table 2 shows more oxygen con tent on the treated surface, and the percentage of titanium decreases 4
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tested to estimate the interlaminar fracture toughness at MTCI by DCB testing method. Load-displacement plot, which is shown in Fig. 5a were derived from DCB experiments, and the results show the significance of surface treatment. For all the samples, the load increases linearly until the peak load and there after the load drops gradually as the crack propagates between the metal and thermoplastic composite interface. For sample, Ti-Pris þ UHMWPE-Pris in Fig. 5a, the critical or peak load required to initiate the crack between the metal and thermoplastic composite interface is lower when compared to other samples mentioned in Table 1. The result obtained for the sample Ti-Pris þ UHMWPE-Pris can be compared with the sample Ti-Ann þ UHMWPEPris, where the difference between these two samples is only the surface treatment of Ti alloy. The peak load required to initiate the crack at MTCI is higher relatively for the sample after Ti surface treatment. This shows that nanopores, which is formed on the Ti alloy after the annealing process can significantly improve the bonding between the metal and thermoplastic composite interface. The other case to discuss here is the significance of surface treatment on the UHMWPE fabric in TFML. The load required to initiate the crack in T-FML after the surface treatment follows, Ti-Ann þ UHMWPE-PDA þ CNT > Ti-Ann þ UHMWPE-PDA > UHMWPE-Pristine. This confirms that PDA and CNT play a vital role in improving the bonding at MTCI in T-FML. Characteristic stick-slip behaviour observed for all T-FML which may arise due to the abnormality in local material properties, such as fiber or resin-rich region, non-alignment fibers, voids, fractured behaviour of fiber bridging, and moreover to the adhesion level between Ti alloy and UHMWPE composite system. But, for the sample Ti-Ann þ UHMWPEPDA þ CNT, the stick-slip behaviour is different from others; this may be due to the strong adhesion level between metal and polymer composite. This stick-slip behaviour can be defined as “no crack-growth even as the load increases” [42]. This phenomenon can improve the critical strain energy release rate at MTCI. From Fig. 5a, the load-displacement for Ti-Ann þ UHMWPE-PDA þ CNT, the load abruptly dropped at several points, immediately after the critical load corresponding to unstable and intensive crack propagation mechanism. The unstable crack propagation is due to the plastic fracture due to the shear yielding between the surface-treated fiber and the Elium® resin. The shear yielding may arise
Table 1 Prepared T-FML composite systems for Mode I interlaminar fracture toughness test. Sample code
Description
Dimension
1
Ti-Pris þ UHMWPE-Pris
Length ¼ 150 mm, width ¼ 25 mm, Thickness ¼ 6 mm, Film insert length (a’) ¼ 40 mm
2
Ti-Ann þ UHMWPE-Pris
3
Ti-Ann þ UHMWPEPDA
4
Ti-Ann þ UHMWPEPDA þ CNT
T-FML fabricated from Pristine Ti6Al4V and Pristine UHMWPE fabric reinforced neat ELIUM matrix T-FML fabricated from surface treated (annealing) Ti6Al4V, and Pristine UHMWPE fabric reinforced neat ELIUM matrix T-FML fabricated from surface treated (annealing) Ti6Al4V, and PDA treated UHMWPE fabric reinforced neat ELIUM matrix T-FML fabricated from surface treated (annealing) Ti6Al4V, and PDA embedded with 0.03 wt% CNT treated UHMWPE fabric reinforced neat ELIUM matrix
after sequential anodization with post-process treatment. This confirms that the oxide layer formed on the surface after annealing process sup ports the wettability and improve the bonding strength with the Elium® matrix. 3.2. Double cantilever beam test T-FML samples which were fabricated by combining pristine and surface treated (Annealing) Ti6Al4V with pristine and surface treated (PDA and PDA þ CNT) UHMWPE with thermoplastic Elium® were
Fig. 4. Surface treated Ti6Al4V (a) Surface roughness, (b) Contact between Ti alloy and matrix. Table 2 EDS result of surface-treated Ti6Al4V. Ti-Pristine Ti-Anodization Ti-Etching Ti-Annealing
C (at. %)
Al (at. %)
Si (at. %)
Ti (at. %)
V (at. %)
Au (at. %)
O (at. %)
Ti/O (at%)
3.74 3.26 3.16 3.18
7.45 6.86 5.49 5.42
0.43 2.52 0.08 0.15
77.97 66.86 65.06 59.46
2.90 2.70 2.45 2.02
2.38 2.30 2.32 2.10
5.13 15.49 21.43 27.67
15.20 4.32 3.04 2.15
5
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Fig. 5. Mode I opening mode result of T-FML fabricated from surface-treated Ti6Al4V with pristine, PDA, and PDA þ CNT surface-treated UHMWPE fiber. (a) Loaddisplacement, (b) Interlaminar fracture toughness, (c) Resistance curve (R-curve).
due to the strong interface, where 42.5% improved bonding has been achieved at the fiber and matrix interface when the fiber is treated with PDAþ0.03 wt% CNT [23]. In summary, the load required to initiate the crack at the MTCI is defined by the surface treatment of both metal and fabric surface. The result obtained from the DCB experiment in Fig. 5a shows that surface treatment of annealing Ti-alloy and PDA þ CNT on fiber surface shows the maximum load is required to initiate the crack at the MTCI. The determination of interlaminar fracture toughness by DCB is tested only on the symmetrical and homogenous composite with either Mode I or Mode II loading condition. But, for the case of T-FML, the fracture resistance of the hybrid composite may likely to affect due to the dissimilar materials with different mechanical and physical prop erties. Also, for T-FML sample, the initial delamination is between the UHMWPE composite and Ti alloy, where the interlaminar fracture toughness is characterized by asymmetry DCB composite system. This makes the T-FML hybrid composite system as a mixed-mode at the crack tip [43–45]. More specifically, in our T-FML, the bending stiffness dif ference between Ti alloy and UHMWPE composite is different, which resulted in Mode II testing in the fracture mechanism, even if the loading condition is Mode I. However, Mollon [46] and sebaey [47] reported that the contribution of Mode II asymmetric crack propagation is negligible when compared with Mode I. Fig. 5b shows the average Mode I interlaminar fracture toughness (G1c) at MTCI with standard deviation for 4 samples tested in each configuration. After metal surface treat ment, the G1C can be immediately increased from 0.25 kJ/m2 (pristine titanium alloy with pristine fiber) to 1.57 kJ/m2 (528%) for surface-treated titanium alloy with pristine fiber. The PDA only coating for UHMWPE fiber enhanced the G1C from 1.57 kJ/m2 to 1.84 kJ/m2
(636%, compared with Ti-Pris þ UHMWPE-Pris). PDA fiber surface functionalization with MWCNT coating enhanced the G1C further to 2.54 kJ/m2 (916%, compared with Ti-Pris þ UHMWPE-Pris). To understand the crack growth resistance offered to the specimen on applying the DCB Mode I load, R-curve or resistance curve can be used. Fig. 5c shows the resistance curve (Mode I interlaminar fracture toughness at MTCI vs corresponding crack growth) for all four different sample configurations in this work. Among all four samples, Ti-Ann þ UHMWPE-PDA þ CNT shows higher G1c at both initiation and propa gation compared to other sample. This depicts that surface treatment on both metal and fiber has a significant influence in enhancing the crack growth resistance offered in Mode I loading. In general, the thermo plastic polymer (Elium®) has higher fracture toughness (G1c ¼ 0.5 kJ/ m2) when compared to the brittle thermoset polymer (epoxy) fracture toughness (G1c ¼ 0.2 kJ/m2). The enhanced crack growth resistance in T-FML is offered by two different reasons (i) strong fiber and matrix bonding, and strong bonding at MTCI, (ii) influence by thermoplastic MMA Elium® resin. If the fiber and matrix have strong adhesion at the interface, the thermoplastic Elium® resin highly deforms at the interface upon the loading [23]. Only after the PDA þ CNT surface treatment on the fiber, significant stick-slip behaviour is observed. During stick-slip behaviour, there is no crack growth as the load increases. At this stage, crack blunt occurs at the fiber and matrix due to the strong interface. This crack blunt makes the Elium® resin to highly deform or initiate plastic rupture at the cracktip. This deformation, in turn, increases the load with no further crack growth during the DCB loading. This proves crack blunt is the domi nating mechanism in improving the interlaminar fracture toughness [48]. For the sample Ti-Ann þ UHMWPE-Pris, the failure may be due to 6
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corresponding fractured SEM image. For Ti-Pris þ UHMWPE-Pristine sample, the failure is between pristine Ti alloy and the pristine UHMWPE polymer composite system. The poor adhesion between metal and composite plays a significant role in attributing to the poor per formance of T-FML. Fig. 7a2 shows the poor matrix bonding on the nonsurface (pristine) treated Ti alloy. Adhesive failure of polymer com posite system from the pristine Ti-alloy can be confirmed from the smooth delamination in Fig. 7a3. For all T-FML sample after the surface treatment of Ti-alloy (Fig. 7b1, c1 and d1), the DCB failure is similar to Fig. 6. So, the fracture toughness is determined by the level of adhesion between fiber and matrix. In Fig. 7b2, the debonding of the fiber is very smooth, depicts poor adhe sion of the fiber to the matrix in Ti-Ann þ UHMWPE-Pristine. Also, the fiber surface were very smooth on the fracture composite adherend confirming the poor level of adhesion between fiber and matrix (Fig. 7b3). After the PDA surface treatment on the fibers, the level of adhesion between the fiber and matrix is enhanced, and this can be evidently seen from Fig. 7c2 and 7c3. The micro-fibrils, which is part of UHMWPE fiber, were observed on both the matrix surface and the fiber surface, confirms the enhanced level of adhesion between the fiber and matrix in the sample Ti-Ann þ UHMWPE-PDA. Interestingly, after the surface treatment of the fiber with CNT embedded in PDA coating has a rougher fracture on a metal surface with highly deformed matrix attributing towards crack-blunt mechanism (Fig. 7d2). The highly deformed matrix yields a plastic rupture due to the shear yielding at the fibre-matrix interface [50]. Strong fibre-matrix adhesion and high ductile nature of thermoplastic Elium® contributed towards the shear yielding at the interface. Fig. 7d3 shows the deformed matrix attached on the fiber surface with enormous of microfibril for the sample Ti-Ann þ UHMWPE-PDA þ CNT. This confirms PDA þ CNT coating on the fiber surface can create a crack-blunt at the crack tip and favouring towards strong fiber-matrix adhesion and the ductility of the matrix at the interface.
the debonding at the interface instead of plastic rupture. This failure may not contribute to the crack-blunt at the crack tip due to the poor interface. Although the failure occurs at fiber and matrix interface, it should be well noted that the Elim matrix from UHMWPE composite system has strong adhesion with Ti alloy. The poor bonding at MTCI leads to the poor performance of T-FML, even if the adhesion between fiber and matrix is stronger. Influence of strong MTCI (annealing surface treatment of Ti alloy), fiber/matrix interface (PDA þ CNT surface treatment of UHMWPE fiber), the influence of thermoplastic Elium® resin (plastic fracture), and crack-blunt mechanism plays a vital role in enhancing the Mode I interlaminar fracture toughness. Enhancing the interlaminar fracture toughness by crack blunt mechanism is similar to the stitching effect in laminated composites [49]. 3.3. Fracture surface morphology The two different failure modes were observed after the DCB fracture in T-FML, (i) For the sample Ti-Pris þ UHMWPE-Pris, the failure is in between the pristine-metal and the thermoplastic matrix due to the poor bonding of UHMWPE polymer composite with non-surface treated Ti metal alloy. (ii) For the T-FML sample fabricated after the surface treatment of Ti alloy (Ti-Ann þ UHMWPE-Pristine, Ti-Ann þ UHMWPEPDA, and Ti-Ann þ UHMWPE-PDA þ CNT) has a similar failure to Fig. 6. This failure is due to the strong adhesion of the Elium matrix from UHMWPE polymer composite with surface-treated Ti metal alloy. Fig. 6 shows the failure of T-FML in the DCB experiments for the sample fabricated after the surface treatment of Ti alloy. Due to the strong adhesion of polymer composite with surface-treated metal, the adhesion level of fiber and matrix bonding determines the failure mode in T-FML. In our case, the adhesion level between the fiber and matrix is deter mined by the surface treatment of UHMWPE fiber functionalized with PDA, and PDA þ CNT. Fig. 6 shows the macro photograph of T-FML sample after DCB fracture. The Elium® matrix strongly adheres to the surface-treated Ti alloy (Fig. 6a1), and the UHMWPE composite fiber debonded from the matrix in Fig. 6a2. The SEM image in Fig. 6c and d shows the Elium® adhere to the metal surface and debonded fibers at MTCI, respectively. Scanning electron microscopic (SEM) images were taken on both the metal surface and debonded polymer composite surface after the DCB test understand the failure mechanism at MTCI. Fig 0.7 shows the schematic and fractured SEM images from both the metal and UHMWPE composite surface. Fig. 7a1 shows the schematic representation on the failure mode between the metal and fiber composite and the
4. Conclusions Determination of Mode I interlaminar fracture toughness at Metal thermoplastic composite interface is successfully carried out for the thermoplastic fiber metal laminate fabricated from surface-treated Ti6Al4V and PDA þ CNT surface-treated UHMWPE/Elium® composite system. The test result from the DCB test at the loading rate of 1 mm/min can be drawn as follows.
Fig. 6. Failure criteria observed in T-FML (Surface treated Ti6Al4V with UHMWPE composite) after the DCB fracture (a) Original sample image of the DCB fracture surface, (b) Schematic of the DCB fracture observed, (c) SEM of the Surface treated Ti6Al4V after DCB fracture, (d) SEM of the UHMWPE fiber after DCB fracture. 7
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Fig. 7. Schematic failure mechanisms and SEM fracture in T-FML at metal thermoplastic composite interface (a1,a2,a3) Ti-Pris þ UHMWPE-Pris, (b1,b2,b3) Ti-Ann þ UHMWPE-Pris, (c1,c2,c3) Ti-Ann þ UHMWPE-PDA, (d1,d2,d3) Ti-Ann þ UHMWPE-PDA þ CNT.
1. Successful surface treatment on the Ti alloy creates a nano-pores on the surface improves the metal and polymer interface. SEM, wetta bility test, and surface roughness confirm the enhanced result after the sequential annealing process. SEM and Raman spectroscopy confirms the successful coating of PDA and CNT embedded on the UHMWPE surface. 2. The improved interlaminar fracture toughness in the T-FML samples after the surface treatment of both fiber and metal were because of three important characteristics (i) high fracture toughness of infus ible room temperature processing thermoplastic Elium® resin (ii) strong Ti alloy metal and Elium®composite interfacial adhesion (iii) strong UHMWPE fiber and Elium® resin interfacial adhesion. 3. Enhanced average interlaminar fracture toughness about 916% was achieved after annealing surface treatment on Ti alloy and PDAþ0.03 wt%CNT surface treatment on the fiber/fabric surface. Highly deformed plastic rupture observed at the crack-tip influenced by strong fiber/matrix adhesion is the primary reason for the enhancement. 4. High deformation of the matrix attributes to the crack-blunt at the interface near the crack tip was the primary reason to observe stickslip behaviour in the sample after PDA þ CNT fiber surface treat ment. Crack-blunt mechanism is dominance in enhancing the Metal thermoplastic composite interface due to the high level of adhesion between fiber and high ductile nature of thermoplastic Elium® matrix.
Joint Research Scheme of Hong Kong (Grant#: N_HKUST 631/18), Nanhai-HKUST Program (Grant #: FSNH-18FYTRI01), Guangdong Sci ence and Technology Department (Project#: 2017A050506005, 2018B050502001), and Science, Technology & Innovation Commission of Guangzhou (Project #: 201907010028). The authors would like to acknowledge Dr Dong Brian and Dr Jinchun Zhu of Arkema, Changshu Research and Development Center, China, for providing Elium® resin. References [1] Kazemi M, Kouchakzadeh M, Shakouri M. Stability analysis of generally laminated conical shells with variable thickness under axial compression. Mech Adv Mater Struct 2018:1–14. € Çoban O. A review: fibre metal laminates, [2] Sinmazçelik T, Avcu E, Bora MO, background, bonding types and applied test methods. Mater Des 2011;32(7): 3671–85. [3] Ali A, Pan L, Duan L, Zheng Z, Sapkota B. Characterization of seawater hygrothermal conditioning effects on the properties of titanium-based fiber-metal laminates for marine applications. Compos Struct 2016;158:199–207. [4] Vlot A. Impact properties of fibre metal laminates. Compos Eng 1993;3(10): 911–27. [5] Wu GaJ-MY. The mechanical behavior of GLAREL laminates for aircraft structures. J Occup Med 2005;57(1):72–9. [6] Vlot A. Impact loading on fibre metal laminates. Int J Impact Eng 1996;18(3): 291–307. [7] Chai GB, Manikandan P. Low velocity impact response of fibre-metal laminates–A review. Compos Struct 2014;107:363–81. [8] Bhatnagar DL A. Military and law enforcement applications of lightweight ballistic materials. Lightweight Ballistic Composites 2006:364–97. [9] Shanmugam L, Kazemi M, Rao Z, Lu D, Wang X, Wang B, et al. Enhanced Mode I fracture toughness of UHMWPE fabric/thermoplastic laminates with combined surface treatments of polydopamine and functionalized carbon nanotubes. Compos B Eng 2019:107450. [10] Kazemi M, Shanmugam L, Lu D, Wang X, Wang B, Yang J. Mechanical properties and failure modes of hybrid fiber reinforced polymer composites with a novel liquid thermoplastic resin. Elium®. Comp Part A: Appl Sci Manuf 2019;125: 105523.
Acknowledgment The authors are grateful to the support from The Hong Kong Uni versity of Science and Technology (Grant #: R9365), the NSFC/RGC 8
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