Inverse Fiber Reinforced Polymer/Metal-Hybrid Laminates for Structural Lightweight Applications

Abstract

Composite multi-material structures for the automotive industry are another step forward. This is because they contribute to a significant reduction in the weight of structural elements, and thus to energy savings and, consequently, lower emissivity of toxic gases. The paper presents research on a new multi-material system made of fiber-reinforced thermoplastics (FRP) combined with metal elements. To improve the adhesion between the metal insert and the fiber-reinforced plastic, an innovative combination of mechanical fit and adhesive was used. As a result, a targeted use of the excellent mechanical properties of the proposed structure was achieved. Additionally, the proposed method shows advantages in mass production processes of mass-optimized products with high stiffness and load-bearing capacity. The paper presents the results of a new material bending test.

1. Introduction

From scientific research reports and literature data it is well known that there is a need for new composite materials with good mechanical properties, high lightweight potential, and feasible stiffness-to-weight ratios for application in many industrial applications, especially in the automotive industry.

One solution is the combination of different materials to Fiber Metal Laminates (FMLs) [1,2]. FML is a material that consists of a combination of metal layer spacers and fiber-reinforced plastic layers [3]. They consist of common metal alloys such as aluminum, magnesium, titanium alloys and fiber-reinforced composites based on carbon, glass and aramid fibers [4]. These materials have many advantages such as fatigue resistance, excellent corrosion resistance, impact resistance and very good mechanical properties [5]. In aerospace engineering, the most commonly thermoset based fiber reinforced plastic/metal (FRP/M) are used. They have many disadvantages such as: long production time, low degrees of deformation, difficulty of recycling that hinder the usage of these materials in the various industry applications [6,7,8,9,10,11].

For the automotive industry, great potential for weight reduction is foreseen for future vehicle generations in the area of bodywork. These parts constitute the heaviest unit in terms of total vehicle weight (up to 40%). It is estimated that for every 100 kg of weight saved, fuel consumption is reduced by approx. 0.5 L per 100 km and thus CO₂ emissions by approx. 13 g/km. On the other hand, appropriate properties are offered by thermoplastic-based hybrid laminates that are currently only available as semi-finished products without fiber reinforcement (e.g., ALUCOBOND^®, BONDAL^®, LITECOR^®) [12,13]. Therefore, the mechanical properties are drastically limited. It seems that thermoplastic fiber-reinforced polymers (TFRP) with light metals in the form of metal/composite laminates are suitable for large scale production and to moderate the mass of an assembly or spare parts while increasing mechanical and functional properties. This aforementioned increase is considered not only by increasing the rigidity, the strength and toughness of the composite material, but also by improving the vibration resistance of the laminates. In addition, various construction solutions can be applied to meet unique customer and entrepreneurs’ requirements. In order to accomplish a cost-effective and mass-production-compatible structural part, an in-line production with continuous production is needed [14,15,16].

The other benefit is an application of an effective matrix component to an active metal/composite laminate that can create a multi-dimensional structure. This is a unique application with a great potential in the near future. The multidimensional shaping is stimulated by an outer excitation, where the matrix component interacts with dimensional changes [17].

To improve the generally understood properties of thermoplastic-based hybrid laminates, the main goal of this research work was to estimate mechanical properties of inverse hybrid laminates (IHL) with various fibers’ orientation and type, in order to ensure their appropriate usage for structural lightweight application in car production. In addition to their relatively short process time, thermoplastic-based inverse FRP/M-hybrid laminates, in comparison with commercially available thermoset-based composites, have greater degrees of deformation, favorable attenuation properties, a higher damage tolerance, and outstanding properties in terms of impact and fatigue resistance. The inverse hybrid composite materials prepared in this work were investigated by means of three-point bending (3-PB) test and the specimens’ fracture surface were observed by means of a Scanning Electron Microscope (SEM).

2. Materials and Methods

Many parameters can be chosen to characterize the inverse hybrid laminate material. They are type of material, number of layers, fiber orientation, and the interface between fiber-reinforced polymer and metal. Polyamide 6 polymer was used as the hybrid laminate composite matrix, whereas aluminum of 6061 grade with a thickness of 1 mm was utilized as the metal core layer. The reinforcement of the FRP/metal–hybrid laminate was composed by glass fiber reinforced prepregs (Ticona Celstran^® PA6-CF60) with a thickness of 0.15 mm and carbon fiber reinforced prepregs (Ticona Celstran^® PA6-GF60) with thickness of 0.29 mm. More detailed information about IHL composition is summarized in

Table 1. Structure of the investigated inverse hybrid laminates.

Metal	Aluminum 6061 Grade
Thickness	1.0 mm
Matrix	Polyamide 6 (PA 6)
Fiber volume content	60 %
Reinforcement fibers	Glass fiber (G)		Carbon fiber (C) + Glass fiber (G)
Variant	S1	S2	S3	S4
Layer structure	${[{\mathbf{0}}^{\mathit{G}}/{\mathbf{90}}^{\mathit{G}}/{\mathbf{0}}^{\mathit{G}}/\overline{\mathit{A}\mathit{l}\mathit{u}}]}_{\mathit{S}}$	${[{\mathbf{0}}_{\mathbf{3}}^{\mathit{G}}/\overline{\mathit{A}\mathit{l}\mathit{u}}]}_{\mathit{S}}$	${[{\mathbf{0}}_{\mathbf{4}}^{\mathit{C}}/{\mathbf{0}}_{\mathbf{1}}^{\mathit{G}}/\overline{\mathit{A}\mathit{l}\mathit{u}}]}_{\mathit{S}}$	${[{\mathbf{0}}^{\mathit{C}}/{\mathbf{90}}^{\mathit{C}}/{\mathbf{0}}^{\mathit{C}}/{\mathbf{90}}^{\mathit{C}}/{\mathbf{0}}^{\mathit{G}}/\overline{\mathit{A}\mathit{l}\mathit{u}}]}_{\mathit{S}}$
Thickness FRP	2 × 0.85 mm	2 × 0.9 mm	2 × 0.8 mm	2 × 0.85 mm
Thickness HL	2.7 mm	2.8 mm	2.6 mm	2.7 mm

.

Three-point bending test (Figure 1a) was performed according to DIN EN ISO 14125 using Universal Testing Machine Zwick Z100 with a traverse rate of 0.5 mm/min. Five flat specimens for each specification (variant S1–S4, see Table 1) and fiber direction (longitudinal and perpendicular to the main axis of samples) were used with the dimensions presented in Figure 1b. The flat samples were prepared and tested in two perpendicular directions, a longitudinal direction (0°) parallel to the main axis of the sample and a transverse direction (90°) perpendicular to the main specimen axis.

After mechanical testing, fracture surfaces of the samples were examined by SEM, Zeiss SEM EVO MA25 to find out about composite failure, e.g., cracks, delamination, and/or fiber buckling.

3. Results and Discussion

3.1. Mechanical Properties

The three-point bending test provides useful information about the flexural modulus of elasticity E_ƒ, the flexural strain ε_ƒ and the flexural stress σ_ƒ.

The E_f is determined from dependency (1):

$$E_f=\frac{{\sigma }_{f2}-{\sigma }_{f1 }}{{}_{\epsilon f2}-{ }_{\epsilon f1}}$$

(1)

where: ε_f1 and ε_f2 are the strain values for the corresponding normal stresses σ_f1 and σ_f2.

It is well known that the mechanical properties and the obtained results are strongly dependent on the specimen’s geometry, the strain rate, and the fibers’ type, orientation, and number of layers [18].

The flexural properties of the tested samples of composite materials are summarized in Figure 2, Figure 3 and Figure 4. Each column in the charts has an average value and scatter of the obtained results. In general, the FRP/metal-hybrid laminates indicate a high stiffness and flexural resistance [19]. In comparison with pure carbon fiber reinforced polymers the main advantage is the about 50% increase in mechanical properties in perpendicular direction to the fiber orientation, as regarded at the evaluation of the Interlaminar Shear Strength (ILSS) [20]. In this case, no significant difference between the single- and double-layer system can be found. The results of the 3-point bending testing in both perpendicular directions, 0° (E_f II) and 90° (E_f ⟂), presented in Figure 2 show a significant influence of the introduced fiber types and orientations on the Flexural modulus of the tested materials. The best results in 0° direction were measured for the variant 3 (S3) with E_ƒ = 71.2 ± 2.5 GPa. The lowest modulus is observed in variant 1 (S1) with E_ƒ equal to 13.3 ± 0.4 GPa.

The best result in 90° direction, however, was measured for the variant 4 (S4) with E_ƒ = 32.4 ± 1 GPa. The worst 3PB results in this direction is observed in variant 2 (S2) and 3 (S3) with E_ƒ of 5.83 and 6.69 GPa, respectively. Despite the best stiffness in the longitudinal direction of the variant 3 (S3), it seems that the most equal Flexural modulus properties toward both perpendicular directions possess composite material with variant 4 (S4).

The results of the flexural stress in both longitudinal and perpendicular directions are presented in Figure 3. The best results in longitudinal (0°) direction were measured for the variant 3 (S3) with σ_fB of 620 ± 23 MPa; however, unexpectedly in the perpendicular (90°) direction, the variant 1 (S1) with σ_fB of 430 ± 15 MPa had the highest value. The lowest flexural properties with a similar level were obtained in variant 1 (S1) with σ_fB 281 ± 20 MPa in longitudinal direction and variant 3 (S3) with σ_fB 71.1 ± 0.5 MPa in perpendicular direction. Note that the material with the variant 4 (S4) had relatively equal flexural stiffness values in both perpendicular directions, therefore a lower anisotropy in both directions, which is advantageous in some circumstances, e.g., in car body parts.

The results of the flexural strain properties in both perpendicular directions are presented in Figure 4. The highest flexural strain in longitudinal direction was obtained through the use of the variant 1 (S1) with ε_B of 3.3 ± 0.3% and in the perpendicular direction of the variant 3 (S3) with ε_B 4.5 ± 0.2%. Note that the lowest flexural strain is observed in the variant 3 (S3) with ε_B of 0.95% in longitudinal direction and for the variant 4 (S4) with ε_B of 2.4 ± 0.1% in 90° direction to the main axis. From the results presented here one can conclude that sample S1 (variant 1) had the most equal flexural strain of the composite materials, whereas S4 had inferior properties and lower difference of the flexural strain between both directions; however, with a relatively large scatter of these values.

Anisotropy reduction of the prepared laminated material offers vast advantages for the introduction of loads into hybrid structures. It seems that notch effects in joining areas can be markedly decreased by the usage of FRP/metal-hybrid laminates, instead of pure FRP.

In the case of the S3 composite material, some relation between stress and strain as a function of the specimen orientation (both perpendicular directions) can be observed, namely, the highest the flexural stress the lower the strain value. On the contrary, relatively equal flexural modulus, stress and strain properties independent of the specimens orientation were measured for the S4 material.

The results of dynamic and fatigue tests of the analyzed composite structures may also be a confirmation of the trends obtained in the presented results. Such a research work was performed as part of the project [20] where the bending fatigue test was executed. The test results presented there were performed on samples prepared in a twin technology, and in a configuration based on the same scheme presented in Table 1. A significant improvement in performance properties (e.g., the number of cycles to failure) was measured for samples with the S3 and S4 configuration in a longitudinal direction (0°). It is assumed, based on the 3-PB test results presented here, that for the S4 composite the fatigue properties in perpendicular (90°) direction will be similar.

3.2. SEM Observations

Microscopic examinations of the broken tested materials were carried out for observations of specimen failures resulted from the 3-point bending tests and presented in Figure 5 and Figure 6.

There are visible matrix damages in PA6/GF60% layers in the variant 1 (S1) caused by the tensile stress resulting from perpendicular loads to the main fiber’s direction. In the case of tensile load, parallel (0°) to the direction of fibers with a reinforced middle PA6/GF60% layer (S1), slight damage to the reinforcing fibers is observed (see Figure 5b,d). The glass fiber reinforced polymer (GFRP) layers of both the composite side subject to the stretching and compression and the carbon fiber reinforced polymer (CRFP) layers exhibited good adhesion to the matrix and delamination did not occur (Figure 5a,c).

Fractures observed in Figure 6, the variant 3 (S3) of samples in perpendicular (90°) direction present similar character to the variant 2 (S2) in the same direction. Due to the same fiber orientations in the C/GFRP material tensile stresses in bottom layers of the composites were transmitted only by GF6 matrix, leads to crack formation and propagation in the matrix from the outer surface to aluminum core. Both cracks of the PA6 matrix can be clearly observed in Figure 6a–c. Note that in this case delamination of the PA6 matrix and Al 6061 grade core has not been observed, and interphase between metal and polymer matrix of the composite has not been damaged. Notice that fibers were not taken part in tensile loads transmission during the 3-point bending tests, as for variant 3 (S3) in the longitudinal direction samples.

4. Conclusions

In the present study, the development of innovative inverse hybrid laminates made of an aluminum core and fiber-reinforced thermo-plastics cover layers is characterized. Relationship between the strength of mixed reinforcements is shown in Figure 7. From the results presented here it can be concluded that the flexural properties tested by means of 3-point bending testes of the novel composite materials can be strongly influenced by the usage of the inserted fiber types and orientation. Thus, composite material with variant 3 (S3) has the highest mechanical strength, both flexural modulus and flexural strength in longitudinal to the main axis direction. However, unexpectedly this material has the lowest flexural strain of 0.95% in longitudinal direction that is difficult to explain on this stage of the research work. On the contrary, the highest flexural strain was measured for this (S3) sample, up to 4.5% and SEM investigation revealed that in this direction no delamination, fiber pullout or breakage fracture mechanism took place but cracks propagation from outer surface to the aluminum core.

On the other hand, relatively equal static and fatigue mechanical properties in both perpendicular directions (the lowest anisotropy) were obtained in the case of variant 4 (S4) of the sample made of the carbon and glass fiber. Hence, depending on the fiber type and arrangement heavy-duty lightweight structures can be produced, whose properties can be specifically adapted to various car component requirements. Therefore, the presented results can be useful for the design and optimization of light inverse hybrid laminate fiber-reinforced thermoplastic composites not only for automotive industry, with high stiffness and resistance that meet the increasing demands for energy-efficient production of components with high power density.