- Email: [email protected]
metal laminate
Composites Part B 174 (2019) 107043
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Quasi-static and fatigue bending behavior of a continuous fiber-reinforced thermoplastic/metal laminate Camilo Zopp a, *, Axel Dittes b, Daisy Nestler a, Ingolf Scharf b, Lothar Kroll a, Thomas Lampke b a b
Chemnitz University of Technology, Lightweight Structures and Polymer Technology Group, Reichenhainer Strasse 31/33, 09126, Chemnitz, Germany Chemnitz University of Technology, Materials and Surface Engineering Group, Erfenschlager Strasse 73, 09125, Chemnitz, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid laminates Fiber metal laminates Fiber-reinforced thermoplastics Static properties Fatigue Bending
Thermoplastic hybrid laminates are a novel class of material compounds that offer the possibility for large-scale production routes. The laminates consist of an assembly of fiber-reinforced semi-finished layers and metallic sheets in an alternating arrangement. Through the targeted combination of the individual sub-components, a high performing lightweight material with tailor-made properties is achieved. The paper focuses on an advanced thermoplastic hybrid laminate of the CAPAAL type, which is made up of continuous carbon fiber-reinforced polyamide sheets, aluminum top sheets and with continuous glass fiberreinforced polyamide interlayers in between. The mechanical performance under quasi-static and 3-point bending fatigue, the corresponding damage as well as failure mechanisms are investigated. Under quasi-static bending, a comparatively high stiffness (52 GPa) and strength (645 MPa) in comparison to the commercially established thermosetting hybrid laminates were achieved while even at high strains no interfacial delamination was observed. The bending fatigue tests show that pure elastic loading of the laminate is below the fatigue limit and the laminate withstands one million load cycles without any measurable damages in the individual sub-components. Higher loads are leading to crack propagation inside the aluminum top sheet, whereas only the highest loads applied induced additional damage occurrence inside the thermoplastic matrix. For the first time, investigations on the mechanical performance of a CAPAAL type hybrid laminate under fatigue bending loads are presented. Hereby, in combination with the fast manufacturability of the laminate as a semi-finished product and its subsequent high formability, the suitability for e.g. automotive applications is demonstrated.
1. Introduction Due to their high lightweight construction potentials, hybrid lami nates based on continuous fiber-reinforced thermosetting plastics and aluminum as well as titanium alloys are commercially well established. Especially for aircraft applications, GLARE (GLAss fiber-REinforced epoxy/aluminum laminate), ARALL (ARamid fiber-reinforced epoxy/ ALuminum Laminate), CARALL (CARbon fiber-reinforced epoxy/ ALuminum Laminate) as well as TiGr (Titanium/Graphit Laminate) count among the best known hybrid laminates [1–4]. The beneficial approach is to combine the advantages of each single component and thereby to compensate individual weaknesses. For instance, fiber-reinforced plastics (FRP) show high failure stresses and enable excellent long-term fatigue behavior in consequence of the fiber bridging mechanism [2,5,6]. On the other side, metal layers are placed
at the outside of the hybrid laminate to protect the inner FRP layers e. g. from moisture absorption, UV radiation and additionally enable an improved energy absorption under impact loading [7,8]. Thermoplastic hybrid laminates are in general composed of contin uous FRP tapes and metallic interlayers and top sheets in an alternating arrangement. The thermoplastic matrix is typically made up of PA 6 (e. g. CAPAAL – CArbon fiber-reinforced PolyAmide/ALuminum laminate), TPU (e. g. CATPUAL– CArbon fiber-reinforced Thermoplastic Poly Urethane/ALuminum laminate), PA 6.6, Polypropylene (PP) or Poly etheretherketone (PEEK) [4,9–16]. Hybrid laminates based on thermoplastic FRP instead of thermoset ting FRP offer significant benefits and new fields of application can be obtained [9,10,17]. For example, the impact loading behavior and the recyclability are improved. Accounting to economic aspects, the op portunity for continuous, large-scale production routes and especially
* Corresponding author. E-mail address: [email protected] (C. Zopp). https://doi.org/10.1016/j.compositesb.2019.107043 Received 29 January 2019; Received in revised form 17 May 2019; Accepted 12 June 2019 Available online 12 June 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
C. Zopp et al.
Composites Part B 174 (2019) 107043
the possibility of subsequent forming and deep drawing processes after consolidation is given [18,19]. In particular, rapid production is feasible through the one-shot thermoforming process, whereby consolidation and forming takes place simultaneously [9,10,17]. The use of a ther moplastic matrix also allows tighter forming radii [20]. Forming tests were carried out, for example, on self-reinforced PP/Al composite compounds and on conventional CAPAAL [21,22]. Particularly with the hybrid laminate CAPAAL, no delamination perpendicular to the fiber direction under variation of the bending radii was observed. The fatigue properties of thermosetting hybrid laminates under tensile load and the crucial mechanisms of crack initiation, propagation and failure are well known due to investigations as part of the qualifi cation of GLARE and ARALL laminates [23]. However, only a few types of thermoplastic hybrid laminates are known and their mechanical performance, especially the fatigue behavior, was rarely investigated yet. The fundamental axial fatigue behavior of thermoplastic hybrid laminates was characterized e.g. for glass fiber and self-fiber-reinforced PP (GF-PP, SR-PP) hybrid laminates by Cort� es and Cantwell as well as Reyes and Kang [12,24]. The fatigue behavior of hybrid laminates under bending loads is not conclusively considered yet, only investigations on the thermosetting hybrid laminate ARALL are reported [25]. Referring to the state of art, investigations on the fatigue perfor mance of carbon fiber (CF) as well as mixed CF/GF-reinforced thermo plastic hybrid laminates have not been realized yet. However, these laminates are of particular interest for lightweight construction due to their high specific strength and stiffness of such laminates. This moti vates the focus on a novel CAPAAL type hybrid laminate of this contri bution. Aiming on suitability for large-scale production, a rapid manufacturing process for semi-finished products with excellent form ability for final shaping operations is presented. Thus, new industrial application potentials in the railway and the automotive sector can be raised. For the latter, generic demonstrators for the roof bow [17] and the wishbone [26] have been realized based on advanced CAPAAL. In relation to the load case of the roof bow, the paper presents in vestigations on the quasi-static and the bending fatigue properties. In comparison to the original CAPAAL hybrid laminate, an advanced lay-up is utilized in order to overcome the disadvantage of strong re sidual stresses as a result of the highly differing coefficients of thermal expansion (CTE) of aluminum and CFRP as well as to increase the reproducibility [27].
prevented and reproducibility is increased [10,17,20,31]. Additionally, the GFRP interlayers act as an insulator in between the electrochemi cally noble CF and the ignoble aluminum and whence contact corrosion is prevented [9,10]. Furthermore, an improved mechanical performance of the laminate is achieved compared to the utilization of unreinforced thermoplastic interlayers [32,33]. The individual unidirectional tapes of polyamide 6 were aligned with their fiber direction and compliant with the direction of rolling of the aluminum sheets in order to achieve high bending performance. No additional polymer interlayers or adhesion promoters were used. The pre-treatment of the aluminum sheets and the production of the selfdeveloped tapes is described in detail in section 2.2. 2.2. Preparation of sub-components The metallic top sheets of the hybrid laminate, each with a thickness of 0.5 mm, consist of EN AW 6082-T4. This alloy is distinguished by high ductility, good weldability, cold formability, good resistance to corro sion and secured availability. Furthermore, the alloy has a low spring back after forming and a good fatigue strength. For this reason, the aluminum alloy is also used in the automotive and railway industry. In order to achieve high adhesion towards the polyamide 6 based FRP, a suitable pretreatment of the aluminum is essential. In Refs. [31, 34], different approaches, e.g. mechanical blasting, anodic oxidation, brushing and etching were compared regarding the achievable joint strength. Blasting enables sufficient adhesion by mechanical inter locking, is economically cheap and offers additional advantages such as the removal of possible surface contamination and integrability into large-scale production and was hence applied as pre-treatment for the aluminum sheets [9,27,31]. Blasting was conducted at the bottom and the top side to avoid warpage owing to residual stresses. The surfaces were subsequently cleaned with compressed air and rinsed in iso propanol. In Ref. [20], the influence of the blasting abrasive and the process parameters on the roughness and the achievable adhesion is investigated. Considering these results, a special fused aluminum oxide (Co. Cerablast) with a size distribution between 125 μm and 180 μm was used. The blasting was performed under an angle of 45� using a pressure of 5 bar [20,27]. The roughness parameters Ra and Rz were determined before and after blasting by the profile method using a MarTalk device (Co. Mahr). The achieved Ra (Table 1) are comparable to other in vestigations illustrating the suitability for high achievable joint strengths [14]. The satisfying uniformity of the roughness feature dis tribution accounts from three-dimensional images (Fig. 2). For this, stripes light microscopy using a confocal-microscope Duo Vario (Co. Confovis) was conducted. The FRP tapes were produced by a self-developed fiber-foil-tape unit (FFTU). The exact process is described in Refs. [10,17,35]. Each ther moplastic tape was manufactured by combining two PA 6 foils (Cast-PA 6, Co. mf-Folien) accounting to 50 μm thickness each plus either an intermediary 50 K HT-carbon fiber roving (Panex 35, Co. Zoltek) or an E-glass fiber roving (TuFRov 4588, Co. PPG). The fiber sizings were applied by the supplier and are chemically unknown. The combination with polyamide 6 is particularly recommended. The average thickness of the pre-consolidated tapes is about 0.25 mm. The fiber volume content is about 54% in CFRP tapes and 49% in GFRP tapes. Quantification was performed by the thermogravimetric analysis according to DIN EN ISO 11358 [36]. The density was deter mined according to DIN EN ISO 1183 [37] using three specimens of the
2. Materials and manufacturing 2.1. Lay-up of the hybrid laminate A CAPAAL type hybrid laminate was manufactured using a 2/1 arrangement of the sub-components (Fig. 1). The 2/1 lay-up was chosen, since it generally offers greater formability than e.g. a 3/2 arrangement [28–30]. The laminate is composed of two aluminum top sheets (EN AW 6082-T4) and a FRP core unit, which consists of six unidirectional fiber-reinforced polyamide 6 based, pre-consolidated tapes (AL 6082/1 � GFRP/4 � CFRP/1 � GFRP/AL 6082). In this context, the continuous GFRP sheets are integrated as interlayers in between the aluminum- and the continuous CFRP layer. Thus, delamination is
Table 1 Surface roughness prior and after corundum blasting. Alloy EN AW 6082-T4
Fig. 1. Schematic 2/1 lay-up of the produced CAPAAL type hybrid laminate. 2
Prior blasting
After blasting
Rz [μm]
Ra [μm]
Rz [μm]
Ra [μm]
2.64 � 0.08
0.45 � 0.03
25.73 � 1.87
3.18 � 0.09
C. Zopp et al.
Composites Part B 174 (2019) 107043
time (20 min) is used for a polyetheretherketone based hybrid laminate in Ref. [39], whereby the heating and cooling times are unknown. 3. Experimental procedure 3.1. Residual stress X-ray diffractions (XRD) measurements were performed at the outwardly oriented surfaces of the aluminum top sheets in order to investigate the influence of the pre-treatment (corundum blasting, see section 2.2) and the effect of the consolidation process (see section 2.3). For this purpose, two specimens were studied under the following conditions: (a) as-delivered, (b) just-blasted, (c) blasted and annealed by applying the consolidation process without using the FRP tapes and (d) after consolidation into the hybrid laminate. The measurement areas were located at the zone of the maximum strain of the corresponding 3point bending tests (see sections 3.2 and 3.3). Three measurements were chosen in that area oriented perpendicular the final fiber direction. For the experimental studies a diffractometer D8 Discover (Co. Bruker AXS) equipped with a Co anode (40 kV, 40 mA, point focus, 1 mm collimator) was used. The aluminum phase (311) lattice planes was evaluated by applying the sin2 (ψ) method. The corresponding Young’s modulus and Poisson’s ratio were E {311} ¼ 69.3 GPa and η {311} ¼ 0.35, respectively.
Fig. 2. Surface topography of the aluminum sheet after corundum blasting.
dimensions 20 mm � 20 mm � 1 mm. Other properties of the thermo plastic tapes are presented in Ref. [35]. 2.3. Consolidation
3.2. Quasi-static 3-point bending test
The consolidation of the hybrid laminates was carried through a hot pressing process at a P300 P/M (Co. Collin). Further GFRP and CFRP composites were manufactured, consisting of 11 pre-consolidated tapes each and mixed CFRP/GFRP composites, which were produced from six tapes (1 � GFRP/4 � CFRP/1 � GFRP). Overall, unidirectional arrange ments of the tapes were applied for all the specimens. A mold tool with an effective press area of 260 mm � 260 mm was used to produce the composites. The utilized temperature-pressure-time regime (Fig. 3) is based on prior investigations including three funda mental steps [10]. First, heating of the tool and melting of the thermo plastic matrix take place. During the consolidation phase, the fibers are fully impregnated with the matrix while applying additional pressure. The last step is used to cool the hybrid laminate under defined and time-controlled increasing of the pressure. In comparison to Ref. [10], the adapted process, in particular the stepwise pressure increase during the consolidation and cooling steps is performed to obtain an improved impregnation and an increased reproducibility. The overall cycle time including heating, consolidation and cooling accounted to 29 min, wherein the consolidation phase endures 7 min. Compared hereto, the overall cycle time of 26.5 min utilized for the production of the PA 6 based CFRP/steel laminate is lower [38]. Nevertheless, in this study, fully consolidated CFRP tapes were used and whence a lower consolidation time (3.5 min) was needed. In contrast, the heating (70 min) and the consolidation (30 min) times for the pro duction of the thermoplastic SR-PP/Al laminate are higher and the cooling time is not provided [13]. Furthermore, a higher consolidation
Quasi-static 3-point bending tests were carried out on the manufac tured FRPs (GFRP, CFRP, mixed CFRP/GFRP) and the hybrid laminate in accordance to DIN EN ISO 14125 [40] using a Z 5.0 universal testing machine equipped with a bending test apparatus (Co. Zwick). Five specimens were tested respectively. The specimens were cut in a rect angular shape (100 mm � 15 mm) with water jet cutting and tested as soon as possible after finishing the manufacturing process to avoid sig nificant influences of the moisture uptake on the mechanical properties. An extra conditioning step was not performed. The long side of each specimen was chosen in accordance to the fiber direction. For testing, the support distance was 80 mm. The radii of the cylindrical bearings were 2 mm and the radii of the bending die was fixed with 5 mm. A cross-head displacement rate of 5 mm/min was used and the tempera ture was 23 � C. The deflection of the bending die was measured via an inductive position transducer, which was attached to the bottom of the specimen. The tests were either stopped if the force signal dropped below 80% of the maximum value or if the applied deflection reached 22 mm. As described in section 3.3, the 3-point fatigue bending tests were performed with an adapted geometry (140 mm � 40 mm). In order to guarantee comparability between the fatigue and quasi-static bending tests, additional quasi-static bending tests using this geometry and a support distance of 130 mm were performed. For this, the experimental set-up of the fatigue tests (see section 3.3) was used. The cross-head displacement rate was set to 5 mm/min. 3.3. 3-point bending fatigue test The bending fatigue tests were performed using a MTS Landmark (Co. MTS Systems) and a 3-point bending test device (Co. Grasse Zur). The radii of the cylindrical bearings were chosen with 5 mm and the experiments were carried out at a temperature of 23 � C. The hybrid laminate specimens were investigated without any initial notches or precracks and testing was conducted as soon as possible after consolidation in order to keep the influence of moisture absorption under control. In addition, the specimens were stored in sealed polyethylene plastic bags before testing. A sine-shaped deflection-controlled alternating excitation with a load ratio of R ¼ 1 was applied. This implies, in contrast to
Fig. 3. Pressure-temperature-time regime of the manufacturing process. 3
C. Zopp et al.
Composites Part B 174 (2019) 107043
unsymmetrical load ratios (0 � R < þ1), that possible non-symmetrical mechanical behavior caused by the material inhomogeneity of the hybrid laminates can be identified. Since no appropriated test device for the small standard geometry (DIN EN ISO 14125 [40]) was commer cially available, it was necessary to use an extended specimen geometry (140 mm � 40 mm) and to set the support distance to 130 mm. In this context, the experimental setup allows the monitoring of the tempera ture of the specimens during the bending fatigue tests with an infrared camera. The applied peak deflections were chosen, such that pure elastic as well as plastic material behavior in the laminates were examined. The applied displacement corresponds to the signal of the MTS testing ma chine. The excitation frequencies were chosen in dependence of the peak deflection in order to gain an equal average strain rate for each parameter set, cf. Table 2. The corresponding force signal was recorded to monitor the progressing laminate damage. Due to the high inertia of the bending device and the high load frequencies, a load cycle with a tenth of the actual load frequency was performed every 5000 normal load cycles and the peak forces in the reversal points were recorded and compared in-situ. The laminates were considered as damaged and the testing was automatically stopped, as soon as one out of the two current peak force values fell below 80% of the comparative initial value of the very first load cycle. The number of applied load cycles was limited to 1,000,000.
determined at different manufacturing steps and the results are given in Table 3. The orientation of the RS is given parallel and perpendicular to the rolling direction (RD), which is identical to the fiber direction (FD|| RD). Negative values indicate compressive stresses and vice versa. For each condition, the measured maximum and minimum values are given. Whereas the as-delivered aluminum sheets are almost free of residual stresses, the blasting process introduces strong compression stresses in the near-surface areas. For blasted aluminum sheets that were subjected to the consolidation process (see section 2.3) without adding the FRP tapes, almost no residual stresses were measured in the aluminum, which is probably a result of recovery effects (e. g. annihilation of dis locations) and partial recrystallization enforced through the applied elevated temperatures. In this context it is not predictable, if the residual stresses resulting from blasting have an influence on the total residual stress distribution of the hybrid laminate. It is unclear whether the compression stresses were reduced to zero before or after the form fit in between the aluminum sheets and the FRPs is formed during the consolidation. For the CAPAAL type hybrid laminate, medium tensile stresses (54 MPa up to 59 MPa) in alignment with the RD were observed at the aluminum top sheets. Perpendicular the RD, almost zero stresses or small compressive stresses were found. Residual tensile stresses in the top sheets were also reported for thermosetting hybrid laminates, such as GLARE and ARALL [42–44]. For example in CARALL hybrid lami nates, high tensile stresses of about 100 MPa were determined [44]. Since XRD measurements can be only performed at the outer surface of the aluminum, the residual stresses at the aluminum/GFRP interface are unknown. Indeed, the CFRP core unit is the origin of the residual stresses and it is thereby to expect that even higher tensile stresses are present in the aluminum next to the GFRP.
4. Results and discussion 4.1. Manufactured hybrid laminate The manufactured CAPAAL type hybrid laminates do not show macroscopic defects, pre-damage or delamination. The laminates have an average thickness of about 2.09 mm, which implies a metal volume content of about 48%. However, a theoretical thickness of 2.5 mm and thus a lower metal volume content (40%) are expected, when taking the thicknesses of the individual pre-materials into account. The differences are caused by the post-consolidation of the tapes, whereby a volume decrease is induced by further impregnation of the thermoplastic tapes. In principle, a complete overall impregnation is achieved by the applied consolidation process. As can be seen in the cross-sectional images (Fig. 4), a full interlock of the thermoplastic matrix and the inner aluminum surface, without any intermediary pores, was reached. Generally, the Aluminum/GFRP interface is characterized by a pure thermoplastic sub-area of a thickness between 4 μm and 30 μm, which is partly greater compared to GLARE with about 10 μm [41]. This ensures a high interfacial load transfer. Furthermore, sub-areas of uniformly distributed fibers as well as fiber agglomerations and pure thermoplastic zones are present, which result from the high viscosity of the thermoplastic PA 6 melt avoiding a homogenous fiber distribution even at high pressures. Additionally, the interfacial zones of the former FRP tapes are of pure polymeric kind accounting to a vertical extent in between 10 μm and 50 μm. The waviness of the FRP layers is a result of the FFTU tape production, whereby fibers are locally stacked during the movement through the gap of the calenders rolls. The applied pressure is not sufficient for the full impregnation of the tapes, nevertheless the wavy shape manifests during the consolidation into the hybrid laminates. Generally, strong residual stresses can be expected in consolidated aluminum/FRP hybrid laminates due to the highly differing CTE. For this reason, the residual stresses (RS) of the aluminum top sheets were
4.2. Quasi-static 3-point bending test The stress-strain-charts of the CFRP, the GFRP, the mixed CFRP/ GFRP composites and the unidirectional CAPAAL type hybrid laminate are given in Fig. 5. For the laminate and each composite five specimens were tested. Since the stress-strain charts show a good reproducibility only one representative chart is shown in the figure. The bending modulus and the maximum bending strengths were calculated each under the assumption of pure elastic deformation according to DIN EN ISO 14125 [40]. The bending moduli were determined using an evalu ation zone in between a strain of 0.05% and 0.25% for the FRPs and a strain of 0.05% and 0.15% for the hybrid laminate. The mechanical properties and the specific values are listed in Table 4. The bending modulus of the produced CFRP (85 GPa) is higher compared to the GFRP (36 GPa). Thus, the core unit of the hybrid laminate, which consists of a mixed CFRP/GFRP composite, shows an intermediate bending modulus (64 GPa). The bending modulus of the CAPAAL type laminate was calculated with means of the classical laminate theory [39,45]. For this, the experimental results of the GFRP and the CFRP as well as a Young’s modulus of 70 GPa of the aluminum alloy [46] were taken into account. In comparison, the calculated value (65 GPa) of the hybrid laminate is about 20% higher than the experi mentally determined bending modulus (52 GPa). The difference is caused by the manufacturing process (inhomogeneous fiber and matrix distributions, estimation of a uniform tape thickness, pressing temper ature). It is to assume, that interlaminar shear stresses cause plastic deformation in the pure thermoplastic interfacial regions (AL/GFRP, CFRP/GFRP, CFRP/CFRP, cf. section 4.1) during bending, which is not considered in the calculation. The bending modulus of the investigated hybrid laminate with GFRP interlayers exceeds the value of the classical CAPAAL laminate (43 GPa) without GFRP interlayers [9]. This results from the higher stiffness of the GFRP interlayers compared to the pure thermoplastic interlayers with a thickness of 100 μm, which are used in the conventional CAPAAL laminate. The specific bending modulus of the presented CAPAAL type hybrid laminate is 23.63 GPa � cm3/g and thus
Table 2 Set up parameters of the strain controlled alternating 3-point bending fatigue testing. Deflection amplitude [mm] Load frequency [Hz]
�0.5
�1
�1.25
�1.5
�2
15
7.5
6
5
3.75
4
C. Zopp et al.
Composites Part B 174 (2019) 107043
Fig. 4. Cross-sectional images of the hybrid laminate.
lower than the specific modulus of the thermosetting hybrid laminate CFRP/Al5052 (29.82 GPa � cm3/g, 3/2 lay-up, bidirectional) [47]. Maximum bending strengths of 956 MPa for the CFRP, 860 MPa for the GFRP and 828 MPa for the mixed CFRP/GFRP were achieved. The maximum bending strength of the CAPAAL type laminate can only serve as an estimate due to the plastic deformation induced in the aluminum during the bending tests. A lowered maximum strength of 645 MPa is achieved compared to the FRP composites. This is caused by the high aluminum content, which shows a lower strength itself. Nevertheless, the value is higher compared to the classical CAPAAL (608 MPa) [9]. The achieved bending strength is also higher in comparison to the steel based thermoplastic hybrid laminate CF-PA 6/HC220Y (529 MPa, 2/1 lay-up, unidirectional) [38]. Nevertheless, the bending strength is slightly lower than that of the thermosetting laminate CFRP/Al2024 (700 MPa, 2/1 lay-up, unidirectional) [48], which can be related to the higher strength of the Al2024 alloy, compared to the Al6082 alloy. The specific bending strength of the CAPAAL type hybrid laminate (295.83 MPa � cm3/g) is higher in comparison to thermosetting lami nate CFRP/Al5052 (222.7 MPa � cm3/g, 3/2 lay-up, bidirectional) [47]. The specific bending strength is almost equal to the thermoplastic laminate long-GF-PA 6.6/Al2024 (302.21 MPa � cm3/g, 2/1 lay-up) [14]. For the GFRP a bending strain at Fmax of about 3.7% was calculated, whereas a strain of only 1.2% applies for the CFRP, which agrees with the sustainable maximum strain of the carbon fibers. The CFRP shows an abrupt failure, whereas the GFRP is characterized by progressing failure in the individual sub-layers above strains of about 2.8%. This behavior corresponds to the sustainable maximum strain of the applied glass fi bers. For the hybrid laminate, the bending strain of 4.3% at the peak bending deflection, without damage in the fiber core, shows the good formability. The significantly higher strain tolerance of the glass fibers and the aluminum layers improves the damage tolerance as well as energy absorption capacity, compared to the CFRP. The bending tested CAPAAL type laminates show no damage and no delamination of the sub-components (Fig. 6). In contrast to in vestigations on laminates with long GF-PA 6.6/2024 or CF-PA 6/ HC220Y [14,38] no cracks were found in the aluminum. In this context,
Table 3 Residual stresses (RS) at the aluminum top sheets in dependence of the manufacturing process. Residual stress Parallel RD [MPa] Perpendicular RD [MPa]
Asdelivered 4.0 � 3 10.0 � 4 1.0 � 5 3.0 � 4
Justblasted 104 � 2 133 � 3 103 � 2 130 � 3
Blasted þ annealed
Consolidated
12 � 1 7�1 0�1 4�1
54 � 2 59 � 1 26 � 2 3�1
Fig. 5. Stress-strain charts for quasi-static 3-point bending of the produced CFRP, GFRP, the mixed CFRP/GFRP composite and the CAPAAL type hybrid laminate.
Table 4 Key properties of the CFRP, CFRP, mixed CFRP/GFRP and CAPAAL type hybrid laminate. Material Density [g/cm3] Thickness [mm] Bending modulus [GPa] Bending strength [MPa] Strain at Fmax [%] Specific bending modulus [GPa � cm3/g] Specific bending strength [MPa � cm3/g]
CFRP
GFRP
CFRP/GFRP
CAPAAL type
1.51 1.95 84.66 � 5.34 955.76 � 40.94 1.17 � 0.08 56.07 632.95
1.87 1.91 36.35 � 0.74 859.94 � 11.48 3.69 � 0.11 19.44 459.86
1.63 1.10 63.77 � 0.95 827.78 � 4.45 2.06 � 0.12 39.12 507.84
2.18 2.09 51.51 � 1.53 644.92 � 16.10 4.28 � 0.32 23.63 295.83
5
C. Zopp et al.
Composites Part B 174 (2019) 107043
Fig. 6. CAPAAL samples after quasi-static bending.
a high bending angle (β) of about 30� was measured.
tests were only stopped below one million load cycles (80 %-criterion default) for deflections of 2 mm. At all other deflections, specimens were tested for one million load cycles. Although the laminates tested with 1.25 mm and 1.5 mm run out one million load cycles, severe damages, as the failure of at least one aluminum sheet, were found after testing. For this reason, the recorded data (peak force values/number of load cycles) were evaluated applying an adapted 90% criterion, which is more meaningful and corresponds to complete cracking of at least one of the aluminum sheets. Following this, the number of sustained load cycles in dependence on the applied bending deflection is depicted in Fig. 8. The specimens tested at deflections of 0.5 mm and 1 mm sustained one million load cycles and none of the typical damage-patterns of hybrid laminates (fiber-matrix, matrix-matrix and metal-matrix delam ination as well as crack propagation inside the metal layer or fiber cracking [3,33]), were observed at cross-sectional images. A peak force of 65 N was measured for the fatigue test at a deflection of 1 mm, which corresponds to 16% of the maximum force (402 N) of the quasi-static bending test (cf. Fig. 7). In contrast, a force of only 47 N was applied
4.3. 3-point bending fatigue test Since an adapted specimen geometry was utilized for the fatigue tests, additional quasi-static 3-point bending tests were conducted using this specimen geometry. The force-deflection-charts (Fig. 7) are depicted for the range including the maximum force (402 N). At high deflections, the specimens slide through the supports without showing failure. Preliminary experimental fatigue tests were carried out monitoring the force-deflection hysteresis for different peak deflections. For de flections of 0.7 mm and above, a significant opening of the hysteresis was observed, which indicates a distinct energy consumption of the hybrid laminate due to plastic deformation of the aluminum. Hence this value is above the elastic limit. For all tested parameters (cf. Table 2), the in-situ observation of the specimens with an infrared camera showed that no significant increase in temperature occurred. For the manufactured hybrid laminates, the 3-point bending fatigue
Fig. 7. Force-deflection chart for the quasi-static 3-point bending test of the fatigue specimen geometry (dimensions: 140 mm � 40 mm, support distance: 130 mm). 6
C. Zopp et al.
Composites Part B 174 (2019) 107043
Fig. 8. Number of sustained load cycles in dependence on the applied bending deflection.
for 1 mm deflection in the quasi-static bending test. The difference is partly explained by the initial hardening of the aluminum that was observed in the fatigue tests. A distinct decrease in the peak force value below 90% of the initial value was observed during fatigue tests at higher deflections due to cracking of at least one of the aluminum sheets. Thus, the stiffness as
well as the measured peak force in the reversal points of the laminate are consequently lowered during fatigue bending. Although the tested laminates were symmetrically deflected (R ¼ 1), only one aluminum sheet was cracked at a deflection of 1.25 mm (Fig. 9a), which is reasoned by slight warpage of the consolidated laminate due to the unequally distributed residual stresses. Comparatively high numbers of load cycles
Fig. 9. a–f: Cross-sectional images of bending fatigue tested hybrid laminates at deflections of 1.25 mm, 1.5 mm and 2 mm. 7
C. Zopp et al.
Composites Part B 174 (2019) 107043
are sustained at a deflection of 1.25 mm and hence the fatigue limit is close to deflections of 1.25 mm. For laminates tested at a deflection of 1.5 mm, both aluminum sheets were completely cracked (Fig. 9b) and a few glass fibers located in front of the aluminum crack tip were found to be cracked (Fig. 9c). However, no delamination between aluminum and GFRP was observed. For deflections of 2 mm, the same damage-patterns (Fig. 9d and e) are observed as for deflections of 1.5 mm. In addition, delamination inside GFRP is found (Fig. 9f), which explains the stronger decrease in the measured peak force values of the laminate during fatigue testing. The applied force (127 N) is equal to 32% of the maximum force of the quasi-static bending test. The presented results show clearly that the prominent damage mechanism during fatigue bending of the CAPAAL type hybrid laminate is the cracking of the aluminum sheets. This is supported by the residual tensile stress (54 MPa up to 59 MPa) at the outside of the aluminum component, which is superimposed with the corresponding tensile stress, induced by bending. Even higher tensile stresses are to expect in the aluminum close to the GFRP interface (cf. section 4.1). It cannot be stated, whether the cracks initiate at the GFRP/aluminum interface or at the aluminum surface. The crack initiation is in both cases promoted by the residual tensile stresses and the fatigue strength of the hybrid laminate is consequently reduced [43,45,49]. Further, the roughening of the aluminum surface likely supports the crack initiation. To reverse the tensile stress in the aluminum into compressive stress different measures such as post-stretching are needed [42,43].
the CAPAAL type hybrid laminate, it is required to reverse the residual stresses in the aluminum, e.g. through subsequent stretching of the consolidated laminate. Further investigations on optimal processing steps for thermoplastic laminates are to be conducted. In this context, suitable measures may comprise the selection of an aluminum alloy of higher strength as well as using another interfacial design concept for the metal/FRP interface (e.g. coupling agents) to avoid roughening of the aluminum surface and to further increase the interfacial load transfer capability. The beneficial influence of using the GFRP in terlayers motivates further studies on even more finely graded lay-up concepts in order to decrease the pronounced mechanical property jumps in between the different sub-components. Acknowledgements This work was performed within the Federal Cluster of Excellence EXC 1075 “MERGE Technologies for Multifunctional Lightweight Structures” and supported by the German Research Foundation (DFG). Financial support is gratefully acknowledged. References € Çoban O. A review: fibre metal laminates, [1] Sinmazçelik T, Avcu E, Bora MO, background, bonding types and applied test methods. Mater and Des 2011;32: 3671–85. [2] Lalibert�e JF, Poon C, Stranznicky PV, Fahr A. Applications of fibre-metal-laminates. Polym Compos 2000;21:558–67. [3] Wu G, Yang J-M. The mechanical behaviour of GLARE laminates for aircraft structures. JOM 2005;57:72–9. [4] Guill�en JF, Cantwell WJ. The influence of cooling rate on the fracture properties of a glass reinforced/nylon fiber-metal laminate. Polym Compos 2002;23(5):839–51. [5] Alderliesten R. Fatigue and fracture of fibre metal laminates. Solid mechanics and its applications, vol 236. Springer International Publishing; 2017. [6] Wu HF, Wu LL. MIL-HDBK-5 design allowables for fibre/metal laminates: ARALL 2 and ARALL 3. J Mater Sci Lett 1994;13:582–5. [7] Vlot A, Gunnink JW. Fibre metal laminates. An introduction. Dordrecht: Springer ScienceþBusiness Media; 2001. [8] Botelho EC, Silva RA, Pardini LC, Rezende MC. A review on the development and properties of continuous fiber/epoxy/aluminum hybrid composites for aircraft structures. Mater Res 2006;9(3):247–56. [9] Zopp C, Nestler D, Tr€ oltzsch J, Trautmann M, Nendel S, Wagner G, et al. CATPUAL – an innovative and high-performance hybrid laminate with carbon fibrereinforced thermoplastic polyurethane. In: Hermann AS, editor. 21st symposium on composites. Trans Tech Publications; 2017. p. 294–301. [10] Zopp C, Nestler D, Buschner N, Mende C, Mauersberger S, Tr€ oltzsch J, et al. Influence of the cooling behaviour on mechanical properties of carbon fibrereinforced thermoplastic/metal laminates. Technol Lightweight Struct 2017;1(2): 32–42. [11] Cortes P, Cantwell WJ. The tensile and fatigue properties of carbon fiber-reinforced PEEK–titanium fiber-metal laminates. J Reinf Plast Compos 2004;23:1615–23. [12] Reyes G, Kang H. Mechanical behavior of lightweight thermoplastic fiber–metal laminates. J Mater Process Technol 2007;186:284–90. [13] Lee B-E, Park E-T, Kim J, et al. Analytical evaluation on uniaxial tensile deformation behavior of fiber metal laminate based on SRPP and its experimental confirmation. Composites, Part B 2014;67:154–9. [14] Kulkarni RR, Chawla KK, Vaidya UK, et al. Characterization of long fiber thermoplastic/metal laminates. J Mater Sci 2008;43:4391–8. [15] Compston P, Cantwell WJ, Jones C, et al. Impact perforation resistance and fracture mechanisms of a thermoplastic based fiber-metal laminate. J Mater Sci Lett 2001;20:597–9. [16] Reyes G, Cantwell WJ. The mechanical properties of fibre-metal laminates based on glass fibre reinforced polypropylene. Compos Sci Technol 2000;60:1085–94. [17] Nestler D, Trautmann M, Zopp C, Tr€ oltzsch J, Osiecki T, Nendel S, et al. Continuous film stacking and thermoforming process for hybrid CFRP/Aluminum laminates. Procedia CIRP 2017;66:107–12. [18] Rajabi A, Kadkhodayan M. An investigation into the deep drawing of fiber-metal laminates based on glass fiber reinforced polypropylene. Int J Eng IJE Trans C Asp 2014;27(3):349–58. [19] Rajabi A, Kadkhodayan M, Ghanei S. An investigation into the flexural and drawing behaviors of GFRP based fiber-metal laminate. Mech Adv Mater Struct 2017;25:805–12. [20] Wielage B, Nestler D, Jung H, et al. CAPAAL and CAPET – new materials of highstrength, high-stiff hybrid laminates. In: Fathi M, editor. Integrated Systems, design and technology 2010. Berlin: Springer; 2011. p. 23–36. [21] Mosse L, Compston P, Cantwell WJ, et al. The effect of process temperature on the formability of polypropylene based fibre–metal laminates. Composites, Part A 2005;36:1158–66.
5. Conclusion and outlook The paper investigates the quasi-static bending and the fatigue bending behavior of an advanced CAPAAL type thermoplastic hybrid laminate. The applied manufacturing approach is suitable for large-scale production routes, wherein the consolidation process yields complete impregnation without intermediary pores and for this an overall cycle time of 29 min was necessary. In comparison to the lay-up of the classical CAPAAL, the presented hybrid laminate contains GFRP interlayers in between the aluminum top sheets and the CFRP core unit. Is was shown by the performed quasistatic 3-point bending tests (DIN EN ISO 14125), that an increased bending modulus (52 GPa) and a higher bending strength (645 MPa) are achieved in comparison to the classical CAPAAL (43 GPa, 608 MPa). Additionally, the laminates were deflected to a maximum bending strain of 4.3% and no failure like interlaminar delamination and crack initia tion in the matrix, fibers or the aluminum was found. Together with the high bending angle (β ¼ 30� ) this implies a good formability for subse quent shaping operations. Future investigations should include the de pendency of the mechanical properties on different load directions, since a unidirectional fiber arrangement is used for the presented hybrid laminate. The 3-point fatigue bending tests were performed using alternating excitation (R ¼ 1), an adapted specimen geometry (130 mm support distance, 40 mm width). Different peak deflections of up to 2 mm were applied. It was found that, a deflection of 1 mm, which corresponds to slight plastic deformation in the hybrid laminate, is sustained for one million load cycles without damage being initiated. However, the load applied for 1 mm deflection accounted to only 65 N, which is rather low compared to the peak force (402 N) of the corresponding quasi-static bending test. Higher deflections cause damage during fatigue testing and the cracking of the aluminum top sheets was identified to be the predominant damage mechanism. Nevertheless, interlaminar delami nation was not observed. For the highest tested deflection, glass fibers were cracked and intralaminar delamination in the GFRP was found at the cross-sectional images. The cracking of the aluminum at comparably low loads is due to the residual tensile stresses (54 MPa–59 MPa) in the aluminum top sheets of the consolidated laminate. In order to improve the quasi-static and fatigue bending properties of 8
C. Zopp et al.
Composites Part B 174 (2019) 107043 [36] DIN EN ISO 11358: plastics - thermogravimetry (TG) of polymers - Part 1: general principles (ISO 11358-1:2014). [37] DIN EN ISO 1183: plastics - Methods for determining the density of non-cellular plastics - Part 1: immersion method, liquid pyknometer method and titration method (ISO 1183-1:2012). [38] Osiecki T, Gerstenberger C, Hackert A, et al. Metal/composite hybrids for lightweight applications. Mach Dyn Res 2015;39(4):117–23. [39] Cort� es P, Cantwell WJ. The prediction of tensile failure in titanium-based thermoplastic fibre–metal laminates. Compos Sci Technol 2006;66:2306–16. [40] DIN EN ISO 14125: fibre-reinforced plastic composites - determination of flexural properties (ISO 14125:1998 þ Cor.1:2001 þ Amd.1:2011). [41] Cook J, Donnellan ME. Tensile and interlaminar properties of GLARE laminates. Air Vehicle and Crew Systems Technology Department; 1991. [42] Krishnakumar S. Fiber metal laminates – the synthesis of metals and composites. Mater Manuf Process 1994;9(2):295–354. [43] Marissen R. Fatigue crack growth in ARALL. A hybrid aluminium-aramid composite material. Dissertation; 1988. [44] Xue J, Wang W-X, Takao Y, et al. Reduction of thermal residual stress in carbon fiber aluminum laminates using a thermal expansion clamp. Composites Part A 2011;42:986–92. [45] Khan SU, Alderliesten RC, Benedictus R. Post-stretching induced stress redistribution in fibre metal laminates. Compos Sci Technol 2009;69:396–405. [46] Aluminium Giesserei Hannover GmbH: Features of EN AW-6082. Product datasheet. [47] Dhaliwal GS, Newaz GM. Experimental and numerical investigation of flexural behavior of carbon fiber reinforced aluminum laminates. J Reinf Plast Compos 2016;35(12):945–56. [48] Ostapiuk M, Bienia�s J, Surowska B. Analysis of the bending and failure of fiber metal laminates based on glass and carbon fibers. Sci Eng Compos Mater 2018;25 (6):1095–106. [49] Chlupov� a A, Koz� ak V. In: Fatigue crack growth and delamination in fiber metal laminate (GLARE) during loading with positive mean stress. 18th international conference. Engineering Mechanics; 2012. p. 531–6.
[22] Kr€ ausel V, Graf A, Nestler D, et al. In: Forming of new thermoplastic based fibre metal laminates. 3rd Global Conference on Materials Science and Engineering (CMSE); 2014. p. 40–5. [23] Gunnink JW, Vlot A, De Vries J, et al. Glare technology development 1997-2000. Appl Compos Mater 2002;9:201–19. [24] Cort� es P, Cantwell WJ. The fracture properties of a fibre–metal laminate based on magnesium alloy. Composites Part B 2006;37:163–70. [25] Cook J, Donnellan M. Flexural fatigue behavior of ARALL laminates. Report No. NADC-90073-60. 1990. [26] Drebenstedt C, Zopp C, Hackert A, Kroll L. Wishbone made of hybrid aluminum foam sandwich. Lightweight Des 2018;11(2):36–40. [27] Nestler D, Jung H, Arnold S, et al. Thermoplastic hybrid laminates with varying metal component. Mat.-wiss. u. Werkstoff-techvol 45. Weinheim: WILEY-VCH; 2014. p. 531–6. 6. [28] Edwardson SP, French P, Dearden G, et al. Laser forming of fibre metal laminates. Lasers Eng 2005;15:233–55. [29] Sinke J. Manufacturing of GLARE parts and structures. Appl Compos Mater 2003; 10:293–305. [30] Mennecart T, Gies S, Khalifa NB, et al. Analysis of the influence of fibers on the formability of metal blanks in manufacturing processes for fiber metal laminates. J Mater Process Manuf Sci 2019;3(2). [31] Nestler D, Trautmann M, Nendel S, et al. Innovative hybrid laminates of aluminium alloy foils and fibre-reinforced thermoplastic layers. Mater Werkst 2016;47: 1121–31. [32] Vlot A, Gunnink JW. Fibre metal laminates. An introduction. Springer-Scineceþ Business Media, B.V; 2001. [33] Abdullah MR, Prawoto Y, Cantwell WJ. Interfacial fracture of the fibre-metal laminates based on fibre reinforced thermoplastics. Mater Des 2015;66:446–52. [34] Nestler D, Jung H, Trautmann M, Wagner G. Functionalized thermoplastic-based hybrid laminates. Lightweight Des 2015;4:20–5. [35] Stenbeck W, Schultze D, Kroll L, et al. Ready for large-scale production. Thermoplastic polyurethane composite sheet with continuous carbon fiber reinforcement. In: Kunststoffe international 08/2016. München: Hanser; 2016. p. 101–3.
9
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Quasi-static and fatigue bending behavior of a continuous fiber-reinforced thermoplastic/metal laminate Camilo Zopp a, *, Axel Dittes b, Daisy Nestler a, Ingolf Scharf b, Lothar Kroll a, Thomas Lampke b a b
Chemnitz University of Technology, Lightweight Structures and Polymer Technology Group, Reichenhainer Strasse 31/33, 09126, Chemnitz, Germany Chemnitz University of Technology, Materials and Surface Engineering Group, Erfenschlager Strasse 73, 09125, Chemnitz, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Hybrid laminates Fiber metal laminates Fiber-reinforced thermoplastics Static properties Fatigue Bending
Thermoplastic hybrid laminates are a novel class of material compounds that offer the possibility for large-scale production routes. The laminates consist of an assembly of fiber-reinforced semi-finished layers and metallic sheets in an alternating arrangement. Through the targeted combination of the individual sub-components, a high performing lightweight material with tailor-made properties is achieved. The paper focuses on an advanced thermoplastic hybrid laminate of the CAPAAL type, which is made up of continuous carbon fiber-reinforced polyamide sheets, aluminum top sheets and with continuous glass fiberreinforced polyamide interlayers in between. The mechanical performance under quasi-static and 3-point bending fatigue, the corresponding damage as well as failure mechanisms are investigated. Under quasi-static bending, a comparatively high stiffness (52 GPa) and strength (645 MPa) in comparison to the commercially established thermosetting hybrid laminates were achieved while even at high strains no interfacial delamination was observed. The bending fatigue tests show that pure elastic loading of the laminate is below the fatigue limit and the laminate withstands one million load cycles without any measurable damages in the individual sub-components. Higher loads are leading to crack propagation inside the aluminum top sheet, whereas only the highest loads applied induced additional damage occurrence inside the thermoplastic matrix. For the first time, investigations on the mechanical performance of a CAPAAL type hybrid laminate under fatigue bending loads are presented. Hereby, in combination with the fast manufacturability of the laminate as a semi-finished product and its subsequent high formability, the suitability for e.g. automotive applications is demonstrated.
1. Introduction Due to their high lightweight construction potentials, hybrid lami nates based on continuous fiber-reinforced thermosetting plastics and aluminum as well as titanium alloys are commercially well established. Especially for aircraft applications, GLARE (GLAss fiber-REinforced epoxy/aluminum laminate), ARALL (ARamid fiber-reinforced epoxy/ ALuminum Laminate), CARALL (CARbon fiber-reinforced epoxy/ ALuminum Laminate) as well as TiGr (Titanium/Graphit Laminate) count among the best known hybrid laminates [1–4]. The beneficial approach is to combine the advantages of each single component and thereby to compensate individual weaknesses. For instance, fiber-reinforced plastics (FRP) show high failure stresses and enable excellent long-term fatigue behavior in consequence of the fiber bridging mechanism [2,5,6]. On the other side, metal layers are placed
at the outside of the hybrid laminate to protect the inner FRP layers e. g. from moisture absorption, UV radiation and additionally enable an improved energy absorption under impact loading [7,8]. Thermoplastic hybrid laminates are in general composed of contin uous FRP tapes and metallic interlayers and top sheets in an alternating arrangement. The thermoplastic matrix is typically made up of PA 6 (e. g. CAPAAL – CArbon fiber-reinforced PolyAmide/ALuminum laminate), TPU (e. g. CATPUAL– CArbon fiber-reinforced Thermoplastic Poly Urethane/ALuminum laminate), PA 6.6, Polypropylene (PP) or Poly etheretherketone (PEEK) [4,9–16]. Hybrid laminates based on thermoplastic FRP instead of thermoset ting FRP offer significant benefits and new fields of application can be obtained [9,10,17]. For example, the impact loading behavior and the recyclability are improved. Accounting to economic aspects, the op portunity for continuous, large-scale production routes and especially
* Corresponding author. E-mail address: [email protected] (C. Zopp). https://doi.org/10.1016/j.compositesb.2019.107043 Received 29 January 2019; Received in revised form 17 May 2019; Accepted 12 June 2019 Available online 12 June 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
C. Zopp et al.
Composites Part B 174 (2019) 107043
the possibility of subsequent forming and deep drawing processes after consolidation is given [18,19]. In particular, rapid production is feasible through the one-shot thermoforming process, whereby consolidation and forming takes place simultaneously [9,10,17]. The use of a ther moplastic matrix also allows tighter forming radii [20]. Forming tests were carried out, for example, on self-reinforced PP/Al composite compounds and on conventional CAPAAL [21,22]. Particularly with the hybrid laminate CAPAAL, no delamination perpendicular to the fiber direction under variation of the bending radii was observed. The fatigue properties of thermosetting hybrid laminates under tensile load and the crucial mechanisms of crack initiation, propagation and failure are well known due to investigations as part of the qualifi cation of GLARE and ARALL laminates [23]. However, only a few types of thermoplastic hybrid laminates are known and their mechanical performance, especially the fatigue behavior, was rarely investigated yet. The fundamental axial fatigue behavior of thermoplastic hybrid laminates was characterized e.g. for glass fiber and self-fiber-reinforced PP (GF-PP, SR-PP) hybrid laminates by Cort� es and Cantwell as well as Reyes and Kang [12,24]. The fatigue behavior of hybrid laminates under bending loads is not conclusively considered yet, only investigations on the thermosetting hybrid laminate ARALL are reported [25]. Referring to the state of art, investigations on the fatigue perfor mance of carbon fiber (CF) as well as mixed CF/GF-reinforced thermo plastic hybrid laminates have not been realized yet. However, these laminates are of particular interest for lightweight construction due to their high specific strength and stiffness of such laminates. This moti vates the focus on a novel CAPAAL type hybrid laminate of this contri bution. Aiming on suitability for large-scale production, a rapid manufacturing process for semi-finished products with excellent form ability for final shaping operations is presented. Thus, new industrial application potentials in the railway and the automotive sector can be raised. For the latter, generic demonstrators for the roof bow [17] and the wishbone [26] have been realized based on advanced CAPAAL. In relation to the load case of the roof bow, the paper presents in vestigations on the quasi-static and the bending fatigue properties. In comparison to the original CAPAAL hybrid laminate, an advanced lay-up is utilized in order to overcome the disadvantage of strong re sidual stresses as a result of the highly differing coefficients of thermal expansion (CTE) of aluminum and CFRP as well as to increase the reproducibility [27].
prevented and reproducibility is increased [10,17,20,31]. Additionally, the GFRP interlayers act as an insulator in between the electrochemi cally noble CF and the ignoble aluminum and whence contact corrosion is prevented [9,10]. Furthermore, an improved mechanical performance of the laminate is achieved compared to the utilization of unreinforced thermoplastic interlayers [32,33]. The individual unidirectional tapes of polyamide 6 were aligned with their fiber direction and compliant with the direction of rolling of the aluminum sheets in order to achieve high bending performance. No additional polymer interlayers or adhesion promoters were used. The pre-treatment of the aluminum sheets and the production of the selfdeveloped tapes is described in detail in section 2.2. 2.2. Preparation of sub-components The metallic top sheets of the hybrid laminate, each with a thickness of 0.5 mm, consist of EN AW 6082-T4. This alloy is distinguished by high ductility, good weldability, cold formability, good resistance to corro sion and secured availability. Furthermore, the alloy has a low spring back after forming and a good fatigue strength. For this reason, the aluminum alloy is also used in the automotive and railway industry. In order to achieve high adhesion towards the polyamide 6 based FRP, a suitable pretreatment of the aluminum is essential. In Refs. [31, 34], different approaches, e.g. mechanical blasting, anodic oxidation, brushing and etching were compared regarding the achievable joint strength. Blasting enables sufficient adhesion by mechanical inter locking, is economically cheap and offers additional advantages such as the removal of possible surface contamination and integrability into large-scale production and was hence applied as pre-treatment for the aluminum sheets [9,27,31]. Blasting was conducted at the bottom and the top side to avoid warpage owing to residual stresses. The surfaces were subsequently cleaned with compressed air and rinsed in iso propanol. In Ref. [20], the influence of the blasting abrasive and the process parameters on the roughness and the achievable adhesion is investigated. Considering these results, a special fused aluminum oxide (Co. Cerablast) with a size distribution between 125 μm and 180 μm was used. The blasting was performed under an angle of 45� using a pressure of 5 bar [20,27]. The roughness parameters Ra and Rz were determined before and after blasting by the profile method using a MarTalk device (Co. Mahr). The achieved Ra (Table 1) are comparable to other in vestigations illustrating the suitability for high achievable joint strengths [14]. The satisfying uniformity of the roughness feature dis tribution accounts from three-dimensional images (Fig. 2). For this, stripes light microscopy using a confocal-microscope Duo Vario (Co. Confovis) was conducted. The FRP tapes were produced by a self-developed fiber-foil-tape unit (FFTU). The exact process is described in Refs. [10,17,35]. Each ther moplastic tape was manufactured by combining two PA 6 foils (Cast-PA 6, Co. mf-Folien) accounting to 50 μm thickness each plus either an intermediary 50 K HT-carbon fiber roving (Panex 35, Co. Zoltek) or an E-glass fiber roving (TuFRov 4588, Co. PPG). The fiber sizings were applied by the supplier and are chemically unknown. The combination with polyamide 6 is particularly recommended. The average thickness of the pre-consolidated tapes is about 0.25 mm. The fiber volume content is about 54% in CFRP tapes and 49% in GFRP tapes. Quantification was performed by the thermogravimetric analysis according to DIN EN ISO 11358 [36]. The density was deter mined according to DIN EN ISO 1183 [37] using three specimens of the
2. Materials and manufacturing 2.1. Lay-up of the hybrid laminate A CAPAAL type hybrid laminate was manufactured using a 2/1 arrangement of the sub-components (Fig. 1). The 2/1 lay-up was chosen, since it generally offers greater formability than e.g. a 3/2 arrangement [28–30]. The laminate is composed of two aluminum top sheets (EN AW 6082-T4) and a FRP core unit, which consists of six unidirectional fiber-reinforced polyamide 6 based, pre-consolidated tapes (AL 6082/1 � GFRP/4 � CFRP/1 � GFRP/AL 6082). In this context, the continuous GFRP sheets are integrated as interlayers in between the aluminum- and the continuous CFRP layer. Thus, delamination is
Table 1 Surface roughness prior and after corundum blasting. Alloy EN AW 6082-T4
Fig. 1. Schematic 2/1 lay-up of the produced CAPAAL type hybrid laminate. 2
Prior blasting
After blasting
Rz [μm]
Ra [μm]
Rz [μm]
Ra [μm]
2.64 � 0.08
0.45 � 0.03
25.73 � 1.87
3.18 � 0.09
C. Zopp et al.
Composites Part B 174 (2019) 107043
time (20 min) is used for a polyetheretherketone based hybrid laminate in Ref. [39], whereby the heating and cooling times are unknown. 3. Experimental procedure 3.1. Residual stress X-ray diffractions (XRD) measurements were performed at the outwardly oriented surfaces of the aluminum top sheets in order to investigate the influence of the pre-treatment (corundum blasting, see section 2.2) and the effect of the consolidation process (see section 2.3). For this purpose, two specimens were studied under the following conditions: (a) as-delivered, (b) just-blasted, (c) blasted and annealed by applying the consolidation process without using the FRP tapes and (d) after consolidation into the hybrid laminate. The measurement areas were located at the zone of the maximum strain of the corresponding 3point bending tests (see sections 3.2 and 3.3). Three measurements were chosen in that area oriented perpendicular the final fiber direction. For the experimental studies a diffractometer D8 Discover (Co. Bruker AXS) equipped with a Co anode (40 kV, 40 mA, point focus, 1 mm collimator) was used. The aluminum phase (311) lattice planes was evaluated by applying the sin2 (ψ) method. The corresponding Young’s modulus and Poisson’s ratio were E {311} ¼ 69.3 GPa and η {311} ¼ 0.35, respectively.
Fig. 2. Surface topography of the aluminum sheet after corundum blasting.
dimensions 20 mm � 20 mm � 1 mm. Other properties of the thermo plastic tapes are presented in Ref. [35]. 2.3. Consolidation
3.2. Quasi-static 3-point bending test
The consolidation of the hybrid laminates was carried through a hot pressing process at a P300 P/M (Co. Collin). Further GFRP and CFRP composites were manufactured, consisting of 11 pre-consolidated tapes each and mixed CFRP/GFRP composites, which were produced from six tapes (1 � GFRP/4 � CFRP/1 � GFRP). Overall, unidirectional arrange ments of the tapes were applied for all the specimens. A mold tool with an effective press area of 260 mm � 260 mm was used to produce the composites. The utilized temperature-pressure-time regime (Fig. 3) is based on prior investigations including three funda mental steps [10]. First, heating of the tool and melting of the thermo plastic matrix take place. During the consolidation phase, the fibers are fully impregnated with the matrix while applying additional pressure. The last step is used to cool the hybrid laminate under defined and time-controlled increasing of the pressure. In comparison to Ref. [10], the adapted process, in particular the stepwise pressure increase during the consolidation and cooling steps is performed to obtain an improved impregnation and an increased reproducibility. The overall cycle time including heating, consolidation and cooling accounted to 29 min, wherein the consolidation phase endures 7 min. Compared hereto, the overall cycle time of 26.5 min utilized for the production of the PA 6 based CFRP/steel laminate is lower [38]. Nevertheless, in this study, fully consolidated CFRP tapes were used and whence a lower consolidation time (3.5 min) was needed. In contrast, the heating (70 min) and the consolidation (30 min) times for the pro duction of the thermoplastic SR-PP/Al laminate are higher and the cooling time is not provided [13]. Furthermore, a higher consolidation
Quasi-static 3-point bending tests were carried out on the manufac tured FRPs (GFRP, CFRP, mixed CFRP/GFRP) and the hybrid laminate in accordance to DIN EN ISO 14125 [40] using a Z 5.0 universal testing machine equipped with a bending test apparatus (Co. Zwick). Five specimens were tested respectively. The specimens were cut in a rect angular shape (100 mm � 15 mm) with water jet cutting and tested as soon as possible after finishing the manufacturing process to avoid sig nificant influences of the moisture uptake on the mechanical properties. An extra conditioning step was not performed. The long side of each specimen was chosen in accordance to the fiber direction. For testing, the support distance was 80 mm. The radii of the cylindrical bearings were 2 mm and the radii of the bending die was fixed with 5 mm. A cross-head displacement rate of 5 mm/min was used and the tempera ture was 23 � C. The deflection of the bending die was measured via an inductive position transducer, which was attached to the bottom of the specimen. The tests were either stopped if the force signal dropped below 80% of the maximum value or if the applied deflection reached 22 mm. As described in section 3.3, the 3-point fatigue bending tests were performed with an adapted geometry (140 mm � 40 mm). In order to guarantee comparability between the fatigue and quasi-static bending tests, additional quasi-static bending tests using this geometry and a support distance of 130 mm were performed. For this, the experimental set-up of the fatigue tests (see section 3.3) was used. The cross-head displacement rate was set to 5 mm/min. 3.3. 3-point bending fatigue test The bending fatigue tests were performed using a MTS Landmark (Co. MTS Systems) and a 3-point bending test device (Co. Grasse Zur). The radii of the cylindrical bearings were chosen with 5 mm and the experiments were carried out at a temperature of 23 � C. The hybrid laminate specimens were investigated without any initial notches or precracks and testing was conducted as soon as possible after consolidation in order to keep the influence of moisture absorption under control. In addition, the specimens were stored in sealed polyethylene plastic bags before testing. A sine-shaped deflection-controlled alternating excitation with a load ratio of R ¼ 1 was applied. This implies, in contrast to
Fig. 3. Pressure-temperature-time regime of the manufacturing process. 3
C. Zopp et al.
Composites Part B 174 (2019) 107043
unsymmetrical load ratios (0 � R < þ1), that possible non-symmetrical mechanical behavior caused by the material inhomogeneity of the hybrid laminates can be identified. Since no appropriated test device for the small standard geometry (DIN EN ISO 14125 [40]) was commer cially available, it was necessary to use an extended specimen geometry (140 mm � 40 mm) and to set the support distance to 130 mm. In this context, the experimental setup allows the monitoring of the tempera ture of the specimens during the bending fatigue tests with an infrared camera. The applied peak deflections were chosen, such that pure elastic as well as plastic material behavior in the laminates were examined. The applied displacement corresponds to the signal of the MTS testing ma chine. The excitation frequencies were chosen in dependence of the peak deflection in order to gain an equal average strain rate for each parameter set, cf. Table 2. The corresponding force signal was recorded to monitor the progressing laminate damage. Due to the high inertia of the bending device and the high load frequencies, a load cycle with a tenth of the actual load frequency was performed every 5000 normal load cycles and the peak forces in the reversal points were recorded and compared in-situ. The laminates were considered as damaged and the testing was automatically stopped, as soon as one out of the two current peak force values fell below 80% of the comparative initial value of the very first load cycle. The number of applied load cycles was limited to 1,000,000.
determined at different manufacturing steps and the results are given in Table 3. The orientation of the RS is given parallel and perpendicular to the rolling direction (RD), which is identical to the fiber direction (FD|| RD). Negative values indicate compressive stresses and vice versa. For each condition, the measured maximum and minimum values are given. Whereas the as-delivered aluminum sheets are almost free of residual stresses, the blasting process introduces strong compression stresses in the near-surface areas. For blasted aluminum sheets that were subjected to the consolidation process (see section 2.3) without adding the FRP tapes, almost no residual stresses were measured in the aluminum, which is probably a result of recovery effects (e. g. annihilation of dis locations) and partial recrystallization enforced through the applied elevated temperatures. In this context it is not predictable, if the residual stresses resulting from blasting have an influence on the total residual stress distribution of the hybrid laminate. It is unclear whether the compression stresses were reduced to zero before or after the form fit in between the aluminum sheets and the FRPs is formed during the consolidation. For the CAPAAL type hybrid laminate, medium tensile stresses (54 MPa up to 59 MPa) in alignment with the RD were observed at the aluminum top sheets. Perpendicular the RD, almost zero stresses or small compressive stresses were found. Residual tensile stresses in the top sheets were also reported for thermosetting hybrid laminates, such as GLARE and ARALL [42–44]. For example in CARALL hybrid lami nates, high tensile stresses of about 100 MPa were determined [44]. Since XRD measurements can be only performed at the outer surface of the aluminum, the residual stresses at the aluminum/GFRP interface are unknown. Indeed, the CFRP core unit is the origin of the residual stresses and it is thereby to expect that even higher tensile stresses are present in the aluminum next to the GFRP.
4. Results and discussion 4.1. Manufactured hybrid laminate The manufactured CAPAAL type hybrid laminates do not show macroscopic defects, pre-damage or delamination. The laminates have an average thickness of about 2.09 mm, which implies a metal volume content of about 48%. However, a theoretical thickness of 2.5 mm and thus a lower metal volume content (40%) are expected, when taking the thicknesses of the individual pre-materials into account. The differences are caused by the post-consolidation of the tapes, whereby a volume decrease is induced by further impregnation of the thermoplastic tapes. In principle, a complete overall impregnation is achieved by the applied consolidation process. As can be seen in the cross-sectional images (Fig. 4), a full interlock of the thermoplastic matrix and the inner aluminum surface, without any intermediary pores, was reached. Generally, the Aluminum/GFRP interface is characterized by a pure thermoplastic sub-area of a thickness between 4 μm and 30 μm, which is partly greater compared to GLARE with about 10 μm [41]. This ensures a high interfacial load transfer. Furthermore, sub-areas of uniformly distributed fibers as well as fiber agglomerations and pure thermoplastic zones are present, which result from the high viscosity of the thermoplastic PA 6 melt avoiding a homogenous fiber distribution even at high pressures. Additionally, the interfacial zones of the former FRP tapes are of pure polymeric kind accounting to a vertical extent in between 10 μm and 50 μm. The waviness of the FRP layers is a result of the FFTU tape production, whereby fibers are locally stacked during the movement through the gap of the calenders rolls. The applied pressure is not sufficient for the full impregnation of the tapes, nevertheless the wavy shape manifests during the consolidation into the hybrid laminates. Generally, strong residual stresses can be expected in consolidated aluminum/FRP hybrid laminates due to the highly differing CTE. For this reason, the residual stresses (RS) of the aluminum top sheets were
4.2. Quasi-static 3-point bending test The stress-strain-charts of the CFRP, the GFRP, the mixed CFRP/ GFRP composites and the unidirectional CAPAAL type hybrid laminate are given in Fig. 5. For the laminate and each composite five specimens were tested. Since the stress-strain charts show a good reproducibility only one representative chart is shown in the figure. The bending modulus and the maximum bending strengths were calculated each under the assumption of pure elastic deformation according to DIN EN ISO 14125 [40]. The bending moduli were determined using an evalu ation zone in between a strain of 0.05% and 0.25% for the FRPs and a strain of 0.05% and 0.15% for the hybrid laminate. The mechanical properties and the specific values are listed in Table 4. The bending modulus of the produced CFRP (85 GPa) is higher compared to the GFRP (36 GPa). Thus, the core unit of the hybrid laminate, which consists of a mixed CFRP/GFRP composite, shows an intermediate bending modulus (64 GPa). The bending modulus of the CAPAAL type laminate was calculated with means of the classical laminate theory [39,45]. For this, the experimental results of the GFRP and the CFRP as well as a Young’s modulus of 70 GPa of the aluminum alloy [46] were taken into account. In comparison, the calculated value (65 GPa) of the hybrid laminate is about 20% higher than the experi mentally determined bending modulus (52 GPa). The difference is caused by the manufacturing process (inhomogeneous fiber and matrix distributions, estimation of a uniform tape thickness, pressing temper ature). It is to assume, that interlaminar shear stresses cause plastic deformation in the pure thermoplastic interfacial regions (AL/GFRP, CFRP/GFRP, CFRP/CFRP, cf. section 4.1) during bending, which is not considered in the calculation. The bending modulus of the investigated hybrid laminate with GFRP interlayers exceeds the value of the classical CAPAAL laminate (43 GPa) without GFRP interlayers [9]. This results from the higher stiffness of the GFRP interlayers compared to the pure thermoplastic interlayers with a thickness of 100 μm, which are used in the conventional CAPAAL laminate. The specific bending modulus of the presented CAPAAL type hybrid laminate is 23.63 GPa � cm3/g and thus
Table 2 Set up parameters of the strain controlled alternating 3-point bending fatigue testing. Deflection amplitude [mm] Load frequency [Hz]
�0.5
�1
�1.25
�1.5
�2
15
7.5
6
5
3.75
4
C. Zopp et al.
Composites Part B 174 (2019) 107043
Fig. 4. Cross-sectional images of the hybrid laminate.
lower than the specific modulus of the thermosetting hybrid laminate CFRP/Al5052 (29.82 GPa � cm3/g, 3/2 lay-up, bidirectional) [47]. Maximum bending strengths of 956 MPa for the CFRP, 860 MPa for the GFRP and 828 MPa for the mixed CFRP/GFRP were achieved. The maximum bending strength of the CAPAAL type laminate can only serve as an estimate due to the plastic deformation induced in the aluminum during the bending tests. A lowered maximum strength of 645 MPa is achieved compared to the FRP composites. This is caused by the high aluminum content, which shows a lower strength itself. Nevertheless, the value is higher compared to the classical CAPAAL (608 MPa) [9]. The achieved bending strength is also higher in comparison to the steel based thermoplastic hybrid laminate CF-PA 6/HC220Y (529 MPa, 2/1 lay-up, unidirectional) [38]. Nevertheless, the bending strength is slightly lower than that of the thermosetting laminate CFRP/Al2024 (700 MPa, 2/1 lay-up, unidirectional) [48], which can be related to the higher strength of the Al2024 alloy, compared to the Al6082 alloy. The specific bending strength of the CAPAAL type hybrid laminate (295.83 MPa � cm3/g) is higher in comparison to thermosetting lami nate CFRP/Al5052 (222.7 MPa � cm3/g, 3/2 lay-up, bidirectional) [47]. The specific bending strength is almost equal to the thermoplastic laminate long-GF-PA 6.6/Al2024 (302.21 MPa � cm3/g, 2/1 lay-up) [14]. For the GFRP a bending strain at Fmax of about 3.7% was calculated, whereas a strain of only 1.2% applies for the CFRP, which agrees with the sustainable maximum strain of the carbon fibers. The CFRP shows an abrupt failure, whereas the GFRP is characterized by progressing failure in the individual sub-layers above strains of about 2.8%. This behavior corresponds to the sustainable maximum strain of the applied glass fi bers. For the hybrid laminate, the bending strain of 4.3% at the peak bending deflection, without damage in the fiber core, shows the good formability. The significantly higher strain tolerance of the glass fibers and the aluminum layers improves the damage tolerance as well as energy absorption capacity, compared to the CFRP. The bending tested CAPAAL type laminates show no damage and no delamination of the sub-components (Fig. 6). In contrast to in vestigations on laminates with long GF-PA 6.6/2024 or CF-PA 6/ HC220Y [14,38] no cracks were found in the aluminum. In this context,
Table 3 Residual stresses (RS) at the aluminum top sheets in dependence of the manufacturing process. Residual stress Parallel RD [MPa] Perpendicular RD [MPa]
Asdelivered 4.0 � 3 10.0 � 4 1.0 � 5 3.0 � 4
Justblasted 104 � 2 133 � 3 103 � 2 130 � 3
Blasted þ annealed
Consolidated
12 � 1 7�1 0�1 4�1
54 � 2 59 � 1 26 � 2 3�1
Fig. 5. Stress-strain charts for quasi-static 3-point bending of the produced CFRP, GFRP, the mixed CFRP/GFRP composite and the CAPAAL type hybrid laminate.
Table 4 Key properties of the CFRP, CFRP, mixed CFRP/GFRP and CAPAAL type hybrid laminate. Material Density [g/cm3] Thickness [mm] Bending modulus [GPa] Bending strength [MPa] Strain at Fmax [%] Specific bending modulus [GPa � cm3/g] Specific bending strength [MPa � cm3/g]
CFRP
GFRP
CFRP/GFRP
CAPAAL type
1.51 1.95 84.66 � 5.34 955.76 � 40.94 1.17 � 0.08 56.07 632.95
1.87 1.91 36.35 � 0.74 859.94 � 11.48 3.69 � 0.11 19.44 459.86
1.63 1.10 63.77 � 0.95 827.78 � 4.45 2.06 � 0.12 39.12 507.84
2.18 2.09 51.51 � 1.53 644.92 � 16.10 4.28 � 0.32 23.63 295.83
5
C. Zopp et al.
Composites Part B 174 (2019) 107043
Fig. 6. CAPAAL samples after quasi-static bending.
a high bending angle (β) of about 30� was measured.
tests were only stopped below one million load cycles (80 %-criterion default) for deflections of 2 mm. At all other deflections, specimens were tested for one million load cycles. Although the laminates tested with 1.25 mm and 1.5 mm run out one million load cycles, severe damages, as the failure of at least one aluminum sheet, were found after testing. For this reason, the recorded data (peak force values/number of load cycles) were evaluated applying an adapted 90% criterion, which is more meaningful and corresponds to complete cracking of at least one of the aluminum sheets. Following this, the number of sustained load cycles in dependence on the applied bending deflection is depicted in Fig. 8. The specimens tested at deflections of 0.5 mm and 1 mm sustained one million load cycles and none of the typical damage-patterns of hybrid laminates (fiber-matrix, matrix-matrix and metal-matrix delam ination as well as crack propagation inside the metal layer or fiber cracking [3,33]), were observed at cross-sectional images. A peak force of 65 N was measured for the fatigue test at a deflection of 1 mm, which corresponds to 16% of the maximum force (402 N) of the quasi-static bending test (cf. Fig. 7). In contrast, a force of only 47 N was applied
4.3. 3-point bending fatigue test Since an adapted specimen geometry was utilized for the fatigue tests, additional quasi-static 3-point bending tests were conducted using this specimen geometry. The force-deflection-charts (Fig. 7) are depicted for the range including the maximum force (402 N). At high deflections, the specimens slide through the supports without showing failure. Preliminary experimental fatigue tests were carried out monitoring the force-deflection hysteresis for different peak deflections. For de flections of 0.7 mm and above, a significant opening of the hysteresis was observed, which indicates a distinct energy consumption of the hybrid laminate due to plastic deformation of the aluminum. Hence this value is above the elastic limit. For all tested parameters (cf. Table 2), the in-situ observation of the specimens with an infrared camera showed that no significant increase in temperature occurred. For the manufactured hybrid laminates, the 3-point bending fatigue
Fig. 7. Force-deflection chart for the quasi-static 3-point bending test of the fatigue specimen geometry (dimensions: 140 mm � 40 mm, support distance: 130 mm). 6
C. Zopp et al.
Composites Part B 174 (2019) 107043
Fig. 8. Number of sustained load cycles in dependence on the applied bending deflection.
for 1 mm deflection in the quasi-static bending test. The difference is partly explained by the initial hardening of the aluminum that was observed in the fatigue tests. A distinct decrease in the peak force value below 90% of the initial value was observed during fatigue tests at higher deflections due to cracking of at least one of the aluminum sheets. Thus, the stiffness as
well as the measured peak force in the reversal points of the laminate are consequently lowered during fatigue bending. Although the tested laminates were symmetrically deflected (R ¼ 1), only one aluminum sheet was cracked at a deflection of 1.25 mm (Fig. 9a), which is reasoned by slight warpage of the consolidated laminate due to the unequally distributed residual stresses. Comparatively high numbers of load cycles
Fig. 9. a–f: Cross-sectional images of bending fatigue tested hybrid laminates at deflections of 1.25 mm, 1.5 mm and 2 mm. 7
C. Zopp et al.
Composites Part B 174 (2019) 107043
are sustained at a deflection of 1.25 mm and hence the fatigue limit is close to deflections of 1.25 mm. For laminates tested at a deflection of 1.5 mm, both aluminum sheets were completely cracked (Fig. 9b) and a few glass fibers located in front of the aluminum crack tip were found to be cracked (Fig. 9c). However, no delamination between aluminum and GFRP was observed. For deflections of 2 mm, the same damage-patterns (Fig. 9d and e) are observed as for deflections of 1.5 mm. In addition, delamination inside GFRP is found (Fig. 9f), which explains the stronger decrease in the measured peak force values of the laminate during fatigue testing. The applied force (127 N) is equal to 32% of the maximum force of the quasi-static bending test. The presented results show clearly that the prominent damage mechanism during fatigue bending of the CAPAAL type hybrid laminate is the cracking of the aluminum sheets. This is supported by the residual tensile stress (54 MPa up to 59 MPa) at the outside of the aluminum component, which is superimposed with the corresponding tensile stress, induced by bending. Even higher tensile stresses are to expect in the aluminum close to the GFRP interface (cf. section 4.1). It cannot be stated, whether the cracks initiate at the GFRP/aluminum interface or at the aluminum surface. The crack initiation is in both cases promoted by the residual tensile stresses and the fatigue strength of the hybrid laminate is consequently reduced [43,45,49]. Further, the roughening of the aluminum surface likely supports the crack initiation. To reverse the tensile stress in the aluminum into compressive stress different measures such as post-stretching are needed [42,43].
the CAPAAL type hybrid laminate, it is required to reverse the residual stresses in the aluminum, e.g. through subsequent stretching of the consolidated laminate. Further investigations on optimal processing steps for thermoplastic laminates are to be conducted. In this context, suitable measures may comprise the selection of an aluminum alloy of higher strength as well as using another interfacial design concept for the metal/FRP interface (e.g. coupling agents) to avoid roughening of the aluminum surface and to further increase the interfacial load transfer capability. The beneficial influence of using the GFRP in terlayers motivates further studies on even more finely graded lay-up concepts in order to decrease the pronounced mechanical property jumps in between the different sub-components. Acknowledgements This work was performed within the Federal Cluster of Excellence EXC 1075 “MERGE Technologies for Multifunctional Lightweight Structures” and supported by the German Research Foundation (DFG). Financial support is gratefully acknowledged. References € Çoban O. A review: fibre metal laminates, [1] Sinmazçelik T, Avcu E, Bora MO, background, bonding types and applied test methods. Mater and Des 2011;32: 3671–85. [2] Lalibert�e JF, Poon C, Stranznicky PV, Fahr A. Applications of fibre-metal-laminates. Polym Compos 2000;21:558–67. [3] Wu G, Yang J-M. The mechanical behaviour of GLARE laminates for aircraft structures. JOM 2005;57:72–9. [4] Guill�en JF, Cantwell WJ. The influence of cooling rate on the fracture properties of a glass reinforced/nylon fiber-metal laminate. Polym Compos 2002;23(5):839–51. [5] Alderliesten R. Fatigue and fracture of fibre metal laminates. Solid mechanics and its applications, vol 236. Springer International Publishing; 2017. [6] Wu HF, Wu LL. MIL-HDBK-5 design allowables for fibre/metal laminates: ARALL 2 and ARALL 3. J Mater Sci Lett 1994;13:582–5. [7] Vlot A, Gunnink JW. Fibre metal laminates. An introduction. Dordrecht: Springer ScienceþBusiness Media; 2001. [8] Botelho EC, Silva RA, Pardini LC, Rezende MC. A review on the development and properties of continuous fiber/epoxy/aluminum hybrid composites for aircraft structures. Mater Res 2006;9(3):247–56. [9] Zopp C, Nestler D, Tr€ oltzsch J, Trautmann M, Nendel S, Wagner G, et al. CATPUAL – an innovative and high-performance hybrid laminate with carbon fibrereinforced thermoplastic polyurethane. In: Hermann AS, editor. 21st symposium on composites. Trans Tech Publications; 2017. p. 294–301. [10] Zopp C, Nestler D, Buschner N, Mende C, Mauersberger S, Tr€ oltzsch J, et al. Influence of the cooling behaviour on mechanical properties of carbon fibrereinforced thermoplastic/metal laminates. Technol Lightweight Struct 2017;1(2): 32–42. [11] Cortes P, Cantwell WJ. The tensile and fatigue properties of carbon fiber-reinforced PEEK–titanium fiber-metal laminates. J Reinf Plast Compos 2004;23:1615–23. [12] Reyes G, Kang H. Mechanical behavior of lightweight thermoplastic fiber–metal laminates. J Mater Process Technol 2007;186:284–90. [13] Lee B-E, Park E-T, Kim J, et al. Analytical evaluation on uniaxial tensile deformation behavior of fiber metal laminate based on SRPP and its experimental confirmation. Composites, Part B 2014;67:154–9. [14] Kulkarni RR, Chawla KK, Vaidya UK, et al. Characterization of long fiber thermoplastic/metal laminates. J Mater Sci 2008;43:4391–8. [15] Compston P, Cantwell WJ, Jones C, et al. Impact perforation resistance and fracture mechanisms of a thermoplastic based fiber-metal laminate. J Mater Sci Lett 2001;20:597–9. [16] Reyes G, Cantwell WJ. The mechanical properties of fibre-metal laminates based on glass fibre reinforced polypropylene. Compos Sci Technol 2000;60:1085–94. [17] Nestler D, Trautmann M, Zopp C, Tr€ oltzsch J, Osiecki T, Nendel S, et al. Continuous film stacking and thermoforming process for hybrid CFRP/Aluminum laminates. Procedia CIRP 2017;66:107–12. [18] Rajabi A, Kadkhodayan M. An investigation into the deep drawing of fiber-metal laminates based on glass fiber reinforced polypropylene. Int J Eng IJE Trans C Asp 2014;27(3):349–58. [19] Rajabi A, Kadkhodayan M, Ghanei S. An investigation into the flexural and drawing behaviors of GFRP based fiber-metal laminate. Mech Adv Mater Struct 2017;25:805–12. [20] Wielage B, Nestler D, Jung H, et al. CAPAAL and CAPET – new materials of highstrength, high-stiff hybrid laminates. In: Fathi M, editor. Integrated Systems, design and technology 2010. Berlin: Springer; 2011. p. 23–36. [21] Mosse L, Compston P, Cantwell WJ, et al. The effect of process temperature on the formability of polypropylene based fibre–metal laminates. Composites, Part A 2005;36:1158–66.
5. Conclusion and outlook The paper investigates the quasi-static bending and the fatigue bending behavior of an advanced CAPAAL type thermoplastic hybrid laminate. The applied manufacturing approach is suitable for large-scale production routes, wherein the consolidation process yields complete impregnation without intermediary pores and for this an overall cycle time of 29 min was necessary. In comparison to the lay-up of the classical CAPAAL, the presented hybrid laminate contains GFRP interlayers in between the aluminum top sheets and the CFRP core unit. Is was shown by the performed quasistatic 3-point bending tests (DIN EN ISO 14125), that an increased bending modulus (52 GPa) and a higher bending strength (645 MPa) are achieved in comparison to the classical CAPAAL (43 GPa, 608 MPa). Additionally, the laminates were deflected to a maximum bending strain of 4.3% and no failure like interlaminar delamination and crack initia tion in the matrix, fibers or the aluminum was found. Together with the high bending angle (β ¼ 30� ) this implies a good formability for subse quent shaping operations. Future investigations should include the de pendency of the mechanical properties on different load directions, since a unidirectional fiber arrangement is used for the presented hybrid laminate. The 3-point fatigue bending tests were performed using alternating excitation (R ¼ 1), an adapted specimen geometry (130 mm support distance, 40 mm width). Different peak deflections of up to 2 mm were applied. It was found that, a deflection of 1 mm, which corresponds to slight plastic deformation in the hybrid laminate, is sustained for one million load cycles without damage being initiated. However, the load applied for 1 mm deflection accounted to only 65 N, which is rather low compared to the peak force (402 N) of the corresponding quasi-static bending test. Higher deflections cause damage during fatigue testing and the cracking of the aluminum top sheets was identified to be the predominant damage mechanism. Nevertheless, interlaminar delami nation was not observed. For the highest tested deflection, glass fibers were cracked and intralaminar delamination in the GFRP was found at the cross-sectional images. The cracking of the aluminum at comparably low loads is due to the residual tensile stresses (54 MPa–59 MPa) in the aluminum top sheets of the consolidated laminate. In order to improve the quasi-static and fatigue bending properties of 8
C. Zopp et al.
Composites Part B 174 (2019) 107043 [36] DIN EN ISO 11358: plastics - thermogravimetry (TG) of polymers - Part 1: general principles (ISO 11358-1:2014). [37] DIN EN ISO 1183: plastics - Methods for determining the density of non-cellular plastics - Part 1: immersion method, liquid pyknometer method and titration method (ISO 1183-1:2012). [38] Osiecki T, Gerstenberger C, Hackert A, et al. Metal/composite hybrids for lightweight applications. Mach Dyn Res 2015;39(4):117–23. [39] Cort� es P, Cantwell WJ. The prediction of tensile failure in titanium-based thermoplastic fibre–metal laminates. Compos Sci Technol 2006;66:2306–16. [40] DIN EN ISO 14125: fibre-reinforced plastic composites - determination of flexural properties (ISO 14125:1998 þ Cor.1:2001 þ Amd.1:2011). [41] Cook J, Donnellan ME. Tensile and interlaminar properties of GLARE laminates. Air Vehicle and Crew Systems Technology Department; 1991. [42] Krishnakumar S. Fiber metal laminates – the synthesis of metals and composites. Mater Manuf Process 1994;9(2):295–354. [43] Marissen R. Fatigue crack growth in ARALL. A hybrid aluminium-aramid composite material. Dissertation; 1988. [44] Xue J, Wang W-X, Takao Y, et al. Reduction of thermal residual stress in carbon fiber aluminum laminates using a thermal expansion clamp. Composites Part A 2011;42:986–92. [45] Khan SU, Alderliesten RC, Benedictus R. Post-stretching induced stress redistribution in fibre metal laminates. Compos Sci Technol 2009;69:396–405. [46] Aluminium Giesserei Hannover GmbH: Features of EN AW-6082. Product datasheet. [47] Dhaliwal GS, Newaz GM. Experimental and numerical investigation of flexural behavior of carbon fiber reinforced aluminum laminates. J Reinf Plast Compos 2016;35(12):945–56. [48] Ostapiuk M, Bienia�s J, Surowska B. Analysis of the bending and failure of fiber metal laminates based on glass and carbon fibers. Sci Eng Compos Mater 2018;25 (6):1095–106. [49] Chlupov� a A, Koz� ak V. In: Fatigue crack growth and delamination in fiber metal laminate (GLARE) during loading with positive mean stress. 18th international conference. Engineering Mechanics; 2012. p. 531–6.
[22] Kr€ ausel V, Graf A, Nestler D, et al. In: Forming of new thermoplastic based fibre metal laminates. 3rd Global Conference on Materials Science and Engineering (CMSE); 2014. p. 40–5. [23] Gunnink JW, Vlot A, De Vries J, et al. Glare technology development 1997-2000. Appl Compos Mater 2002;9:201–19. [24] Cort� es P, Cantwell WJ. The fracture properties of a fibre–metal laminate based on magnesium alloy. Composites Part B 2006;37:163–70. [25] Cook J, Donnellan M. Flexural fatigue behavior of ARALL laminates. Report No. NADC-90073-60. 1990. [26] Drebenstedt C, Zopp C, Hackert A, Kroll L. Wishbone made of hybrid aluminum foam sandwich. Lightweight Des 2018;11(2):36–40. [27] Nestler D, Jung H, Arnold S, et al. Thermoplastic hybrid laminates with varying metal component. Mat.-wiss. u. Werkstoff-techvol 45. Weinheim: WILEY-VCH; 2014. p. 531–6. 6. [28] Edwardson SP, French P, Dearden G, et al. Laser forming of fibre metal laminates. Lasers Eng 2005;15:233–55. [29] Sinke J. Manufacturing of GLARE parts and structures. Appl Compos Mater 2003; 10:293–305. [30] Mennecart T, Gies S, Khalifa NB, et al. Analysis of the influence of fibers on the formability of metal blanks in manufacturing processes for fiber metal laminates. J Mater Process Manuf Sci 2019;3(2). [31] Nestler D, Trautmann M, Nendel S, et al. Innovative hybrid laminates of aluminium alloy foils and fibre-reinforced thermoplastic layers. Mater Werkst 2016;47: 1121–31. [32] Vlot A, Gunnink JW. Fibre metal laminates. An introduction. Springer-Scineceþ Business Media, B.V; 2001. [33] Abdullah MR, Prawoto Y, Cantwell WJ. Interfacial fracture of the fibre-metal laminates based on fibre reinforced thermoplastics. Mater Des 2015;66:446–52. [34] Nestler D, Jung H, Trautmann M, Wagner G. Functionalized thermoplastic-based hybrid laminates. Lightweight Des 2015;4:20–5. [35] Stenbeck W, Schultze D, Kroll L, et al. Ready for large-scale production. Thermoplastic polyurethane composite sheet with continuous carbon fiber reinforcement. In: Kunststoffe international 08/2016. München: Hanser; 2016. p. 101–3.
9