Technical Field

The invention relates to fastener element made of a fibre-reinforced composite material. The fastener extends in a longitudinal direction for transmitting a load to a machine part, and is fixed at one of its axial end regions with the machine part, wherein the load is acting on the fastener element at the other axial end region in a load area. The fastener element comprises a base part extending in the longitudinal direction, where the load area is arranged, and has side walls that extend under an angle from the base part, having a height.

Background

Fastener elements are used in many applications to transfer a force acting on the element with a certain distance from the machine part into the same. Due to the fact that the load is acting at distance from the machine part onto the fastener element, a bending moment is exerted on the fastener element. Thus, the fastener element must be able to withstand a certain bending moment without failure, to securely transfer the load into the machine part. When the fastener element is made of a fiber-composite material, the bending moment can cause shear between the fibers, which limits the bending stiffness of the fastener element. It is an o b j e c t of the present invention to propose a fastener element of the kind defined above in which shear forces during load are reduced or even prevented and the fibres in the material of the fastener element are basically stressed only by tensile forces.

Summary of the invention

A s o l u t i o n according to the invention is characterized in that the side walls of the fastener element have a substantially constant height along a first extension of the fastener element, which first extension extends from the machine part to a transition region, wherein the load area is arranged along a second extension of the fastener element adjoining to the first extension, wherein the height of the side walls is reduced from the transition region to the axial end of the fastener element remote from the machine part and wherein at least the side walls are made of a composite material which comprises a number of continuous reinforcement fibers extending from the machine part to the axial end region remote from the machine part.

The continuous reinforcement fibers extend preferably completely from the machine part to the axial end region remote from the machine part.

The side walls comprise continuous fibers that run from the machine part in a longitudinal direction, along the first extension, which then, along the second extension, run down from the transition region to the level of the base part, where the load area is arranged.

Along the first extension, the fibers preferably run substantially parallel to the base part. The side walls and the base part can consist of a woven material made of or consisting of the continuous reinforcement fibers.

Alternatively, the side walls and the base part can consist of a plastic material in which the continuous reinforcement fibers are embedded.

A further alternative solution suggests that the side walls and the base part consist of a metallic material in which the continuous reinforcement fibers are embedded.

The continuous reinforcement fibers are preferably carbon fibers, glass fibers or aramid fibers.

The above-mentioned angle is preferably 90°.

The fastener element can further have a U-shaped form in a cross-section area perpendicular to the longitudinal direction.

Preferably, the reduction of the height takes place from the transition region to the axial end of the fastener element at least partially, preferably totally, in a linear manner.

The base part and the side walls have preferably a planar shape. The load area is preferably arranged between the transition region and the axial end of the fastener element. Preferably, the height of the side walls is zero in a final section of the second extension of the fastener element. In this case it can be provided that the load area is arranged within the second extension of the fastener element where the final section begins; alternatively, the load area can be arranged within the final section.

The fibers which are preferably employed are thus arranged in such a manner by the proposed geometry of the fastener element that shear in the fastener element is decreased or even prevented. Said fibers are thus placed such that they guide forces in tension rather than in shear. The fibers are thus arranged in such a manner that they are located in the load path as much as possible. Thus, the fastener element can be provided in a lightweight design that is capable of transmitting high forces and withstanding high bending moments.

Brief description of the drawing

The drawings show embodiments of the invention.

Fig. 1 shows in perspective view a fastener element, which is fixed at a machine part with one of its ends and which is loaded with a force at its other end,

Fig. 2 shows the side view of the fastener element according to Fig. 1, wherein the run of continuous reinforcement fibers in the side walls is illustrated for an embodiment of the invention and

Detailed description

In Fig. 1 a fastener element 1 is shown which is fixed with one of its axial end regions 3 at a machine part 2. In the other axial end region 4 a force F is acting on the fastener element 1 in a load area 5. The fastener element 1 has a longitudinal direction L; the total length of the fastener element 1 is L ₀ .

The fastener element 1 made of a composite material comprising continuous reinforcing fibers, and is designed to transmit a quite high force F from the load area 5 to the machine part 2. Thus, the fastener element 1 must have a high capability to carry bending moments.

The following general remarks should be given:

The normal equation for calculating the bending stiffness is to use the Euler- Bernoulli equation to calculate the deflection δ of a beam with length L under a load F as a function of the second moment of area I and the Young's modulus E:

F -ύ

3 - E -I

The Euler-Bernoulli equation assumes that shear is negligible. As a guideline this is possible if

^E - ^J « 1

K -L ² A G

Where G is the shear modulus and κ is a shear factor dependent on the poisson ratio and the geometry. A typical range of values for κ is 0.8 to 1 for isotropic materials. For isotropic materials, shear may be neglected if L > 16 A. In compact structures such as fasteners, this is not the case. Furthermore, composites are not an isotropic material. The interlamellar shear modulus can be many times smaller than the Young's modulus in the fiber direction. For example, while E/G is typically about 3 for isotropic materials, E/G can be in the order of 100 for fiber composite sheets. Consequently, the length to moment of area ratio I/L needs to be approximately 30 times bigger than for an isotropic material. The stiffness of fiber composite materials used in compact structures (small length/height ratio) is therefore limited by shear.

The following measures are therefore taken to avoid shear forces in the fastener element according to the invention:

The fastener element 1 has a base part 6 which is flat and forms the bottom of the fastener element 1. From the base part 6 two side walls 7 and 8 extend upwards under an angle a. This angle is 90° in the case of the depicted embodiment.

As can be seen from Fig. 1 the fastener element 1 can be divided into two sections:

Along a first extension i the walls 7, 8 of the fastener element 1 have a certain height H which is constant. This first extension Li abuts (at its left side) to the machine part 2.

Along a second extension L ₂ (abutting at the first extension Li) the height h of the walls 7, 8 of the fastener element 1 decreases. More specifically, the height of the walls 7, 8 is constant along the first extension Li to the end of the first extension L _{l 5} namely until a transition region 9 is reached, where the second extension L ₂ begins. Along the second extension L ₂ the height h is reduced in a linear way until it becomes zero; here a final section 11 of the base part 6 begins and extends to the axial end 10 of the fastener element 1.

In Fig. 2 the run of the reinforcement fibers 12 is depicted, i. e. the placement of fibers 12 in the side walls 7 and 8. As can be seen the fibers 12 are continuously running in this embodiment from the axial end region 3 of the side wall 7, 8 to the axial end region 4. The fibers 12 are embedded in the base material of the walls 7, 8 which can be e. g. plastic material.

Along the first extension Li the fibers are oriented parallel to the longitudinal direction L. In the depicted embodiment the fibers 12 are uniformly distributed along the height H of the side walls 7, 8.

From the transition region 9 the fibers run downwards and converge at the location where the height h becomes zero, thus creating a load path which maximizes tension in the continuous fibers and minimized shear.

The final section 11 may comprise no fibers; alternatively, the fibers 12 can run along this part of the fastener element as well. In a preferred embodiment of the invention the force F is applied as shown in Fig. 2, i. e. at that location where the height h of the side walls 7, 8 just becomes zero. Due to the shown concept the U-shaped structure of the base part 6 and the side walls 7, 8 is not extended beyond the load line of the force F. The end of the U-shaped profile is here at the edge of the load area 5, i. e. where the height h becomes zero.

The cross- section of the fastener element 1 is thus optimized to minimize shear.

Reference Numerals:

Fastener element

Machine part

Axial end region

Axial end region

Load area

Base part

Side wall

Side wall

Transition region

Axial end

Final section

Reinforcement fiber

Longitudinal direction

First extension

Second extension

Total length

Height of side walls along first extension Height of side walls along second extension

Load (force)

Angle