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, Volume 10, Issue 3, pp 28–33 | Cite as

Manufacturing Principles for Z-Pin Reinforced FRP Composite Laminates in the Case of Bolted Joints

  • Benjamin Schornstein
  • Robert Staschko
  • Normen Fuchs
  • Nikolai Glück
Construction FRP composite laminates
  • 205 Downloads

Pre-loading of bolted FRP joints leads to creep and relaxation processes. Those effects are causing a continuous pre-load loss. A newly developed production method allows a reduction of these effects by using thin CFRP pins. Those pins reinforce the composite laminate in thickness direction.

Bolted joints are the most common way to connect different materials in constructional design. Combinations of components based on metal and fiber reinforced plastics (FRP) are currently assembled using bolted joints characterized as bearing-type shear joints (SL). Alternatively they have to be realized with auxiliary constructions, e.g. using metallic inserts. SL joints transmit operating loads due to a punctiform or line-shaped surface contact of the bolt to the borehole. Therefore the bolt is decisively stressed by shear loading and the sheets are exposed to bearing stress between the shank of the bolt and the borehole.

An alternative is provided by slip-resistant pre-loaded (AP) joints. In the case of AP joints, the operating loads are transferred by frictional forces acting between the sheets. The frictional forces are determined by the pre-load of the bolt and the surface conditions of the components, Figure 1.
Figure 1

Transmission principle of operating loads in different connection types [1] (© Fraunhofer IGP)

Motivation

The bearable load transmission in operation FB of an AP joint is generally influenced by the tensile stress of the connected components. The tensile stress is depending on the pre-load FV, the coefficient of friction μ in the interstice as well as the form tolerances of the sheets. The permitted pre-load is determined by the screw and its strength class as well as the material strength of the joining parts.

In the case of AP joints, in particular when FRP is used as a joining part, the question of maintaining the required pre-load over service life time is essential. VDI 2230-1 standardizes the dimensioning and design of high duty bolted joints [2]. Accordingly three mechanisms are relevant for a pre-load decrease in bolted joints:
  • ▸ exceeding of the interfaced surface tension of the sheet material

  • ▸ embedding of the joint by leveling roughness peaks of the surface structure under the screw and nut head, in the thread and between the sheets

  • ▸ material, temperature and load-dependent creep and relaxation processes of the sheets [3, 4].

The effect of pre-load losses of bolted joints due to embedding as well as creeping and relaxation mechanisms are shown in Figure 2 represented by the joint diagram. The bolt load FS is plotted over the bolt elongation fSM and the compression of the joining parts fPM. Starting with the assembly pre-load FM (1), embedding leads to an immediate and rapid reduction represented by FZ (2).
Figure 2

Influence of embedding as well as creeping and relaxation mechanisms on the pre-load of bolted joints [4] (© Fraunhofer IGP)

Depending on the application case FRP components are produced with polymer thermoset or thermoplastic matrices alternatively. In the case of mechanical stress both types of matrices will be subject to significant creep and relaxation mechanisms. That leads to an additional pre-load reduction (3). To ensure the functionality of AP joints over service life time, the knowledge of the amount of occurring pre-load losses is necessary.

State-of-the-Art

Currently screwing of FRP components is requiring a complex production process, for example the use of metallic inserts. This causes an extensive number of process steps as well as a cost-intensive and time-consuming production. In particular, the draping of the semi-finished fiber products, the borehole rework and the compensation of production tolerances are representing a decisive cost factor. Due to those facts an alternative production method for AP joints in hybrid structures is required.

Currently FRP with Z-pins are only feasible in automated serial production.

In the production of FRP components, reinforcing layers and fabrics are placed in the fiber plane (X-Y plane). Those component reinforcements are absorbing all initiated operational loads. Perpendicular to the fiber plane (Z plane direction / material thickness direction), only the matrix will transfer operational loads. Possibilities to reinforce the FRP in Z plane direction by material based modifications are hardly known.

Commonly FRP components will be reinforced perpendicular to the fiber plane by procedures such as tufting, suturing or Z-pin insertion [5]. Z-pins are thin FRP or metal pins, which support the composite laminate in the thickness direction. Usually, their diameter is 0.2 to 1 mm and the volume fraction of the pins is 0.5 to 4 % [6,7]. Due to their orthogonal orientation relative to the fiber plane interlaminar shear strength, fracture toughness, the resistance to impact damage as well as the general tensile strength will be increased. Previous tests and applications in aviation and motorsports proved Z-pinning based on carbon-fiber reinforced (CFR) pins to be very suitable. The insertion of Z-pins reduces the matrix material percentage in the composite laminate. This should lead to a reduction of creeping effects. Figure 3 illustrates the insertion of Z-pins in a FRP component.
Figure 3

Orientation of Z-pins in a FRP composite laminate (© Fraunhofer IGP)

There are several approaches to insert Z- pins into composite laminates. All of them are focusing on the reinforcement of very cost-intensive prepregs. Prepregs are pre-impregnated layers or fabrics that are cured in an autoclave under pressure at elevated temperatures. Usually, the Z-pins are packaged in carrier foams and are inserted into the prepregs either mechanically (not commonly used in industrial application) or by using ultrasonic (commonly used) [5, 7, 8].

Although these methods are partially automated, a satisfactory result cannot always be obtained due to a defective placement of the composite laminates with Z-pins as well as a strongly inclined orientation (10 to 15 °) of the pins. Alternatively the Z-pins can be inserted using a reusable two-part die [8]. However, it can be problematic to remove the composite laminate from the die without any damage and to apply the pins exactly on the carrier plate.

The above-mentioned production process is the basis for the implementation of AP joints in FRP component assembly processes.

Although these methods are partially automated, a satisfactory result cannot always be obtained due to a defective placement of the composite laminates with Z-pins as well as a strongly inclined orientation (10 to 15 °) of the pins. Alternatively the Z-pins can be inserted using a reusable two-part die [8]. However, it can be problematic to remove the composite laminate from the die without any damage and to apply the pins exactly on the carrier plate.

In summary, it can be stated that previous approaches of Z-pin insertion are used exclusively in semi or fully automated serial production, which necessitates cost intensive manufacturing equipment. An adaption is not feasible for small quantities where composite laminates are commonly produced using infusion processes.

Approach

In a recent research project, a new production method was developed, that allows a flexible insertion of Z-pins into dry fiber layers. In this case Z-pins, placed on a carrier plate, are used to reinforce composite laminates in Z-direction, Figure 4. Compared to metallic inserts the production time can be reduced significantly by using these Z-pin plates. This is caused by the simplicity of the production process and the substantially reduced equipment costs. For the first time Z-pins can be used for infusion processes by utilizing the developed carrier plate method. Using this method the Z-pins can be pressed manually, mechanically, or automatically into dry fiber layers before the actual infusion is executed, Figure 4.
Figure 4

Carrier plates with 3 mm (top left) and 6 mm (top right) CFRP-Z-pins; pneumatic gripping and pressing-in of the carrier plates into dry fiber layers (bottom) (© Fraunhofer IGP)

Manufacturing Requirements

On the contrary to existing Z-pin insertion methods several unknown parameters had to be considered in the developed carrier plate process, e.g. process-induced orientation deviation of the Z-pins, the insertion method, the insertion force and the pinhead shape.

It was assumed that the required force for inserting the Z-pins into the fiber layer is decisively influenced by the shape of the pin head. The production of Z-pins with a diagonally sliced head or a tapered head can be realized. However, the effort differs significantly. Taking this into consideration, investigations were carried out to compare different slicing angles of the pin head. Object of investigation had been diagonally sliced CFRP-Z pins (Ø 0.5 mm) in combination with glass fiber semi-finished products, in which the slicing angle of the pin head was varied. The test specimen has been a square arrangement of 4x4 Z-pins. The applied insertion force maxima and resulting pin deflections (orientation) are plotted in Figure 5.
Figure 5

Influence of the Z-pin slicing angle on the insertion force and the pin deflection (© Fraunhofer IGP)

If the slicing angle increases, the insertion force is decreasing whereas the pin deflection is increasing initially. On the one hand, the lower insertion force leads to minor requirements related to insertion technology. On the other hand, this effect indicates lower fiber damage within the semi-finished product that should be reinforced in the Z-plane direction. The diagram also shows that the Z-pins inclination in the composite laminate decreases again from an angle of 35 °. With that in mind, Z-pins with a slicing angle of at least 45 ° are preferred.

Prerequisite for the production of AP joints is the dimensional accuracy of the FRP components at the joint with regard to plan parallelism and component thickness. In contrast to diagonally sliced Z-pins, tapered ones show significant advantages. This was proved by measuring the surface geometry using a coordinate measuring machine, Figure 6.
Figure 6

Thickening of the FRP surface by using different Z-pin head shapes (in mm) (© Fraunhofer IGP)

The required force for inserting the Z-pins into the fiber layer is influenced by the shape of the pin head.

As shown in figure 6, the Z-pin specimens with a tapered pin head do have significantly less thickening than those with diagonally sliced Z-pins. This is caused by separate fiber layers, which remain non-pinned after the carrier plates have been inserted into the dry fibers and can only be pinned by the negative pressure in the infusion process. Tapered Z-pins are easier to insert than diagonally sliced ones. This results in a lower thickening. As an additional result, tapered Z-pin heads generate significantly lower transverse forces during insertion, which allows a manufacturing with a maximum reproducibility. However, the thickening is more influenced by the process itself. By using a pressing tool with a high stiffness, similar to hot pressing, the Z-pins are completely pressed into the composite laminate during the curing process. Thus, a thickening can almost be avoided.

From a production-technological point of view, the FRP components produced by the optimized process comply with the defined requirements for AP joints. Thus, the above-mentioned production process is the basis for the implementation of AP joints in FRP component assembly processes.

Summary and Outlook

The aim of the project is the demonstration of a reduced pre-load loss of bolted joints by the use of FRP semi-finished products, which were reinforced by using CFRP pins, so-called Z-Pins, in composite laminate thickness direction. For this purpose, the project covered basic manufacturing-related questions concerning the production of Z-pin reinforced FRP components:
  • ▸ the development of a novel manufacturing process allows the Z-pin reinforcement in prepreg processing as well as in infusion processes

  • ▸ the quality related questions of the reinforcement (geometry tolerances, thickening and reproducibility) could be answered satisfactorily and are decisively depending on manufacturing parameters

  • ▸ the process forces and requirements for insertion technology can also be adjusted by selecting the right Z-pin head design

Taking the preceding points into account, the possibility of using AP joints in order to avoid bearing stress caused fiber damage in FRP components now has to be determined. Therefore, joining specific requirements have to be fulfilled:
  • ▸ reproducible joining properties of the FRP components (form tolerances, etc.)

  • ▸ high interfaced surface tension capabilities of the sheet material

  • ▸ small pre-load losses due to matrix induced creep and relaxation mechanisms

Investigations to determine the influence of Z-pin reinforced FRP composite laminates with regard to resilience and damage characteristics will follow in the mentioned research project. Secondly the influence of the Z-pin reinforced composite laminates on the load bearing capacity of bolted joints will be investigated. Therefore, idealized creep tests on reinforced composite laminates as well as practical creep tests on bolted joints will be carried out on FRP-metal structures. Based on this, quasi-static sliding load tests will follow. This will expose the dependence between creep duration and occurring of failure.

Notes

Thanks

The research project (AiF 18811BR) is funded and managed by the Research Association EFB e. V. It is promoted by the consortium of Industrial Research Associations “Otto von Guericke” e. V. (AiF), due to a program for the promotion of industrial collective research and development (IGF) by the German Federal Ministry of Economics and Energy on the basis of a decision of the German Bundestag. We would like to sincerely thank for funding the research project. For the content wise supporting of the project and the provision of material, we would like to thank the companies of the project-accompanying committee.

References

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    Duchardt, M.: Bewertung des Vorspannkraftverlustes in Schraubenverbindungen unter kombinierter mechanischer und thermischer Beanspruchung, Aachen, Shaker, 2013Google Scholar
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    Chang, P.; Mouritz, A.P.; Cox, B.N.: Properties and failure mechanisms of z-pinned laminates in monotonic and cyclic tension [online]; 2006Google Scholar
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    Mouritz, A.P.: Design dilemma for z-pinned composite structures [online]Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Benjamin Schornstein
    • 1
  • Robert Staschko
    • 1
  • Normen Fuchs
    • 1
  • Nikolai Glück
    • 1
  1. 1.Fraunhofer-Research Institution for Large Structures in Production EngineeringRostockDeutschland

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