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Lightweight Design worldwide

, Volume 10, Issue 3, pp 34–39 | Cite as

Co-Bonding of Cured FRP Components and SMC by Compression Molding

  • Marc Fette
  • Nicole Stöß
  • Martin Hentschel
Production Thermoset-Overmolding
  • 278 Downloads

By using the “Thermoset-Overmolding“ technology, an innovative co-bonding process based on the pressing of uncured SMC on cured composite components, highly functional aircraft structures can be produced in an efficient way. Aim and purpose of the current work is to analyze this technology and to highlight its potentials.

A growing number of passengers can be recorded in air traffic. The average increase is about 5 % per year during the recent years. This trend cannot be compensated by building larger aircraft, but especially in conjunction with an increase of the productivity. The Airbus Group expects a doubling of the passenger aircraft until 2034, what is equivalent to a number of 36.800 aircraft within the next years. The growth in the field of cargo aircraft is about 65 %, thus circa 800 new aircraft are expected [1, 2, 3].

Compared to the increasing passenger numbers in the last 15 years, the fuel consumption is decreased per revenue passenger kilometer by 33 % [3]. This illustrates the desire for more fuel-efficient aircraft to saving costs during operational phase. Modern lightweight materials such as carbon fiber reinforced plastics are an enabler for reducing the weight. The Airbus A350 structure consists of more than 50 % fiber composites [4].

However, the trend for using more and more fiber reinforced plastic materials in the aircraft industry is contrary to a rising productivity due to higher production lead times in comparison to metals. In addition, higher costs for raw materials, semi-finished products and manufacturing processes as well as higher efforts for joining of composite components are further adverse aspects. As a consequence of this new and advanced manufacturing and joining methods fiber-reinforced plastics have to be optimized concerning shorter production lead times and cost-efficiency [5, 6, 7].

The current work deals with the analyses and the potentials of a new co-bonding process realized by hot-pressing of SMC on cured CFRP structures.

„Thermoset-Overmolding“ Technology

Currently, there are a lot of joining technologies necessary to connect complex FRP parts to get an integral design, to increase the complexity and the functionality. Consequently, manufacturing times are long and the production costs are high due to the need for different technologies and many production steps and a lot of knowledge related to each technology.

The motivation for the development of this new manufacturing process is the increase of the productivity due to the reduction of processing, post-processing, assembly and joining steps. Furthermore the geometric complexity and functionality of composite parts can be increased.

In addition manual work for the production of appropriate composite parts can be avoided and the reproducibility can be improved due to the ability of full automation.

The co-bonding process is divided into several steps, illustrated in Figure 1. First of all the structural composite component has to be manufactured. After complete curing the surface of this component can be prepared.
Figure 1

Process scheme for the Thermoset Overmolding of bionic or grid structures (© CTC)

The adaption of the matrix materials, the compression process parameters and the pre-treatment of the cured part surface are important to get an excellent adhesion. To get an optimum adhesion between the materials during the compression and curing process of the SMC mass a combination of specific and mechanical adhesion is required. The optimum pre-treatment of the surface for the cured FRP component shall be characterized by surface cleaning, methods for structuring of the surface and surface activation by an increase of the energy. Appropriate surface preparation methods should be processed in the right order and supplemented by design measures such as form-fit constructions to get an optimal connection.

The technology of pressing SMC on cured FRP components can be used in combination with different thermoset matrix material types such as epoxy resin, unsaturated polyester resin or vinyl ester resin. After preparation the surface of the FRP component the stacking and if it is required the preforming of the SMC material have to be performed. Then the whole stacking package, the cured composite part(s) and if necessary metallic elements such as inserts can be positioned into the compression mold. For avoiding stress cracking the cured FRP component(s) can be pre-heated to minimize the temperature deviation between the SMC material and the cured part during the compression molding procedure. After that the compression molding of the SMC and co-bonding take place. Finally, the whole co-bonded FRP part can be demolded and finished after curing cycle times of 3 to 5 minutes. The process parameters are depending primarily on the type of the SMC material and the design of the whole component, especially the SMC sections [8].

By the described co-bonding technology an increase of geometrical complexity and integral design for the composite components CFRP can be realized. In this regard, the creation of rib, grid or bionic stiffening structures, attachments and the direct integration of metal components as load introduction elements, the co-bonding of SMC promises an increase of the functional integration. Consequently the manufacturing technology obtains lower costs and increased productivity due to short cycle times, the elimination of production steps and less effort for finishing and assembly. Moreover, the opportunity of full automation and an optimized buy-to-fly ratio due to an excellent material usage by using SMC can create further economic and temporal benefits.

The technology components can be used in combination with different thermoset matrix material types.

Possible applications can be flat or curved composite shells and parts with pressed-on grid or bionic structures. Furthermore, complex fittings, attachments or other functional elements can be created by the SMC mass and pressed on structural FRP components. Finally, the SMC mass can be used as connector between FRP components and metallic elements by using this co-bonding technology.

Analyzing the Technical Feasibility

For analyzing the general feasibility of the manufacturing procedure and the potentials of the new co-bonding technology for aircraft components a methodical approach is necessary, illustrated in Figure 2. First of all the general requirements for the materials and manufacturing process have to be determined. Subsequently, potentials and risks can be analyzed, weighted and assessed in a theoretical way by methods such as brainstorming, paired comparison and utility analysis. Afterwards the fundamental manufacturability has to be validated by several manufacturing trails with different cured composite parts and SMC materials.
Figure 2

Methodical approach for analyzing the technical feasibility of the co-bonding method (© CTC)

These trials are including the analysis of different compression molding process parameters and their influences on the co-bonded component. Specimens that are made by a compression mold for the production of plates with different thicknesses can be used for that purpose. In addition, the influences of different surface treatment methods for the previous cured part on the bending and tensile behavior have to be determined due to their relevance on the whole co-bonding result. Consequently, the first experimental testing procedures for co-bonded specimen have to allow detailed conclusions regarding to the adhesion between SMC and cured FRP component. Therefore the first potential and risk analysis can be updated.

Finally, the SM C mass can be used as connector between FRP components and metallic elements by using this technology.

After that two testing methods are selected. On the one hand the determination of tensile lap-shear strength of rigid-to-rigid co-bonded assemblies according to DIN EN 1465 shall create conclusions for the adhesion between both materials types and the influences of the pre-treatment methods. Generally this destructive testing procedure is applied for structural adhesive bonding assemblies. On the other hand the bending behavior, the interlaminar connection between the co-bonded material layers and the interface between SMC and surface of the previous cured FRP part shall be analyzed by the three-point bending test to determine the apparent interlaminar shear strength according to DIN EN 2377, especially for FRP materials in aerospace industry.

Due to the potential use of this described technology for aircraft applications a qualified prepreg system for the cured components and two carbon fiber SMC materials are selected. To analyze the influences of the surface treatment the following methods or combination of appropriate methods in the listed order are selected regarding to the applicability in aviation industry:
  • ▸ Just surface cleaning by solvents and degreasers

  • ▸ Surface cleaning and atmospheric pressure plasma

  • ▸ Grinding and surface cleaning

  • ▸ Grinding, surface cleaning and atmospheric pressure plasma.

After the determination of the testing results the analysis of the failure mode of the specimens and the interfaces between both material types follows. The surfaces and the failure modes of the testing samples are analyzed by a scanning electron microscope (SEM). Finally a detailed potential and risk assessment for the general use and special applications can be performed.

Tensile Shear Behavior and Bending Properties

The tensile shear behavior of the samples was determined by the help of according to DIN EN 1465. The specimens were manufactured in several steps. At first plates with a quasi-isotropic laminate construction [0°/90°/+45°/-45°/90°/0°] out of HeyPly M21 from the Hexcel Corporation, a prepreg material for structural aircraft applications, were laid up. After applying a full vacuum the plates were cured in an autoclave process at 7 bar and 180 °C for 120 min. The surfaces of the plates are treated by the mentioned surface preparation methods. As SMC materials two structural carbon fiber reinforced semi-finished products are processed. The Polynt-SMCarbon 90 CF-3K, an epoxy based SMC material, and the Polynt-SMCarbon 24 CF 60–12K, a vinyl ester based SMC system.

The SMC area of the samples was created by pressing the uncured SMC on the cured FRP plate at 150 °C and 250 bar for 5 min. Additional steel plates in the cavity created the defined and required overlap area as Figure 3 shows. After the curing the co-bonded plates were demolded, the steel plates removed and the co-bonded plates cut to a width of 25 mm.
Figure 3

Schematic illustration of the construction of the semi-finished products and components for the manufacturing of the specimen consisting of SMC and CFRP plates (© CTC)

The results of the tensile tests show a different adhesion between the SMC and the prepreg depending on the SMC semi-finished product and the surface treatment of the cured prepreg before the co-bonding step. Figure 4 shows the tensile shear strength of the different samples. All surface treatments can increase the tensile shear strength of the samples, but not in such a significant way like expected before the experiments.
Figure 4

Results of the tensile shear tests (© CTC)

The best results concerning Polynt-SMCarbon 90 CF-3K achieves the combination of grinding and APP. When the vinyl ester based SMC is co-bonded on the cured prepreg the measured tensile shear strengths vary between 11.6 MPa for the just cleaned prepreg surface and 3.0 MPa for the additionally grinded an APP treated prepreg surface.

The higher strengths of the epoxy based SMC compared with the vinyl ester SMC are caused by a better adhesion between the SMC and the cured CFRP material. Furthermore the different thermal expansion between epoxy resin and vinyl ester can cause higher stresses in the interface and lead to an earlier failure of the interface. The Polynt-SMCarbon 90 CF-3K samples fail as a mix of adhesive and cohesive fails with low content of adhesive fails or as only cohesive fails of the SMC due to the good adhesion. The Polynt-SMCarbon 24 CF 60-12K samples break as a result of an adhesive failure or a mix of adhesive and cohesive failure with low content of cohesive fails.

This failure behavior of the interfaces underlines the better adhesion of Polynt-SMCarbon 90 CF-3K on the epoxy based prepreg. Furthermore the content of the cohesive fractured surface on the whole fractured surface increases with higher tensile shear strengths. Those analyses were performed with a SEM.

The samples for investigating the bending behavior of co-bonded structures were produced like the samples for the tensile shear tests. The only difference is the size of the cured prepreg plates placed into the cavity. The SMC was co-bonded on the whole surface. Afterwards the samples were cut according to DIN EN 2377. The results of the bending test show different apparent interlaminar shear strength between the SMC and the prepreg depending on the SMC semi-finished product and the surface treatment of the cured CFRP before the co-bonding step.

Figure 5 shows the apparent interlaminar shear strength of the different samples. For the samples without any surface treatment after the cleaning of the prepreg surface with solvents and degreasers no apparent interlaminar shear strength can be measured. The samples already fail during the cutting step. The same behavior can be observed for the Polynt-SMCarbon 24 CF 60-12K samples in combination with APP treatment of the prepreg surface.
Figure 5

Results of the bending tests for determining the apparent interlaminar shear strength (© CTC)

Grinding generated the lowest increase and the combination of grinding and APP treatment gained the best results, comparable to the tensile shear tests. Like the tensile shear tests show the higher apparent interlaminar shear strengths of the epoxy based SMC compared with the vinyl ester SMC are caused by a better adhesion between the SMC and the prepreg. The roughness and the surface energy of just cleaned prepreg surfaces are insufficient for generating bond lines in parts which are strained by bending stresses.

Summary and Outlook

The “Thermoset Overmolding” technology is a promising manufacturing procedure for functional, complex FRP parts in regard to cost-efficiency and production time-saving. By combining these material types in a highly efficient compression molding process functional and geometrically complex composite components can be manufactured in a cost-saving way.

Nevertheless, the general feasibility as well as the potentials and risks have to be analyzed and assessed for a use of this technology in aviation industry. The current work has been made by using a methodical approach for the determination of the technical feasibility of this new technology. The first manufacturing trails, several process parameter studies, the determination of the adhesive properties, the influences of surface treatment methods and the analysis of the interface between the co-bonded materials shows that the best results, especially regarding the adhesion, can be realized by using epoxy-based carbon fiber SMC material in combination with CFRP components. The used vinyl-ester-based SMC materials cannot meet the requirements of aircraft applications at the moment due to adverse adhesive properties and generally lower mechanical properties in comparison to epoxy-based SMC with chopped carbon fiber reinforcements.

Generally, the apparent interlaminar shear strength and the tensile lap-shear strength of co-bonded epoxy-based SMC and previous cured CFRP elements can be increased significantly through surface treatment. A combination of grinding, surface cleaning and atmospheric pressure plasma promises the best results due to an expansion, structuring and activation by an energy increase of the surface. However, the necessity for surface treatment before co-bonding depends on the load cases and the application field of the components.

In conclusion the co-bonding technology promises some technical and ecological benefits for a use in aviation industry. Nevertheless, more comprehensive and aerospace-specific material and process developments and tests have to be made. Furthermore, detailed cost analyses as well as reliable design and simulation methods are necessary for the further developments and the introduction of this manufacturing method into aviation industry.

Notes

Thanks

The authors would like to thank the Laboratory of Manufacturing Engineering (LaFT) and the Institute of Materials Technology from the Helmut Schmidt University / University of the Federal armed Forces of Germany in Hamburg for providing support and for the excellent cooperation.

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Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Marc Fette
    • 1
  • Nicole Stöß
    • 2
  • Martin Hentschel
    • 1
  1. 1.Composite Technology Center / CTC GmbH (An Airbus Company)StadeDeutschland
  2. 2.Polynt Composites Germany GmbHMiehlenDeutschland

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