The invention relates to a load-introducing armature adapted to be embedded between layers of fibre-reinforced plastic matrix to form a laminated structural element .

It is the purpose of the invention to improve the intro¬ duction of load into fibre-reinforced plastic structures. Such plastic structures may be manufactured to have very high weight-specific strength values and/or rigidity values (E-module values ) . In many cases difficulties ar is e in constructing and manufacturing a load- introducing armature which by exploiting the high weight- specific strength value of the fibre composition yields a satis factory result as far as weight and strength is concerned. A typical example is the wing spar of an aeroplane where the heavy loads substantially comprise pressure or tensile loads in the main direction of the fibre-reinforcement .

In such wing spars the load- introducing means comprise a number of thin plates , preferably of metal such as steel/aluminum intercalated between layers of laminate. As the glueing capacity between plastic laminate and plain metal surfaces often is insecure and always diffi¬ cult to verify and control it is the purpose of the invention to improve the cohesion between armature plates and adjacent layers of fibre-reinforced plastic matrix, the invention being characterized in that the portion of each armature embedded between plastic layers is provided with a plurality of holes having larger opening width than depth. Through these holes plastic matrix layers on either side of the armature plate are mutually connected by pegs consisting of or amalgamatable with the matrix material and completely filling the holes in the arma-

ture. When the armature is exposed to load these pegs are subject to shearing stress. Due to the large opening width of the holes in relation to their depth the cross- sectional area of the pegs when subject to shearing stress will be very large, both ends of each peg carrying load. In this way the shearing load acting on the pegs will be very moderate even near the breaking load of the armature.

The provision of a plurality of holes in each armature plate in accordance with the invention in addition gives the possibility to bring about, by a reduction of the spacing between the holes in the direction away from the point of load-introduction on the armature, a gradual transition between the superior stiffness of the armature (as a rule steel or hard aluminum alloy) and the lower stiffness of the adjacent laminate. By a suitable dimen¬ sioning of the total hole area in relation to the surrounding mater-ial of the armature it is thus easy to gradually increase the resilience of the metal so that the armature near the transition zone between the armature and the laminate has a stiffness in close agree¬ ment with that of the laminate. Thus the armature may be specifically adapted to a certain laminate or vice versa.

The characteristic properties of an armature and a lami¬ nated structural element according to the invention will appear from the attached claims.

Hereafter the invention will be described in greater detail by reference to the attached drawings in which

Fig. 1 is a plan view of an armature plate according to the invention and

Fig. 2 is a sectioned side view of a laminated structural element comprising five armature plates according to Fig. 1 e mbedded betw een laye r s o f f ibre- reinforced plastic matrix.

An armature 1 comprising a plate of steel or hard alumi¬ num alloy having a thickness of about 2 mm is intended to be combined in the way shown in Fig. 2 with layers 4 , 5 of fibre-reinforced plastic matrix to constitute a laminated structural element 3. Part 7 of armature 1 which in the laminated structure element is positioned outside the plastic matrix layers 4, 5 is provided with a coupling hol e 2 adapted to receive a mounting bolt or the like .

The portion 6 of the armature 1 embedded within the lami¬ nated structural element 3 is provided with through-holes 8 having larger opening width than depth .

Suitably, holes 8 are provided in a pattern having decreasing spacing between the holes in the direction away from the coupling hole 2 .

Hereby a continously increasing degree of resilience in the direction away from the coupling hole 2 towards the embedded end of the armature is achieved. An additional improvement in res ilience, to conform more closely to that of the fibre-reinforced plastic layers , is obtained by the staggering of the holes in adjacent rows , as shown in the drawing.

As shown in Fig. 2 , it is advantageous that in such armatures which in the laminated element 3 are positioned closest to the surface, the edge portion 9 at the end opposite to the coupling hole 2 may be bevelled. The pur¬ pose of this is to permit positioning of the reinforcing

fibres during assembly or lamination in such a way that abrupt changes in the direction of the reinforcing fibres within the laminate and thereby a deterioration of the strength properties of the laminate are avoided.

When the armature 1 i s combined with layers 4, 5 of the plastic matrix to form the element 3 according to Fig. 2 it is necessary in accordance with the invention that the matrix material or a material amalgamatable therewith forms the pegs 10 which connect the embedded plastic matrix layers 5 through the holes 8 in the armature 1 for the introduction of load via the armature to the element 3 with both ends of each peg 10 being subj ect to shearing stress. Obviously, each hole 8 in each of the five armature plates according to Fig. 2 is filled with such a peg 10. While pegs 10 may be constituted of portions of the material of adjacent plastic matrix layers 4, 5 pressed into the holes 8, it is preferred, for obtaining a complete filling of holes 8, to form the pegs 10 of a plastic material amalgamatable with the ^* matrix material and introduced into holes 8 during assembly prior to the lamination .

As shown in Fig. 2 the end surfaces of the three inter¬ mediate armature plates are straight. To avoid abrupt changes of the direction of extension of the reinforcing fibres in adjacent plastic matrix layers 5 and thereby to avoid deterioration of the strength properties of the laminate, wedges 11 are inserted in continuation to the free end surfaces of these armatures 1 to extend between adjacent fibre-reinforced plastic matrix layers 5 and having their base in conforming contact with the end surfaces of the armature plates 1 in question.

In order to counteract any cleaving tendency of the finally lamintated structural element under the action of

stresses acting in directions different from the stresses predominantly taken into consideration, the finished laminate with embedded armature or armatures 1 may be enclosed by a protective collar 12 as shown in Fig. 2.

A laminated structural element of the type shown in Fig. 2 having five plates of steel embedded between outer layers 4 and inner layers 5 of fibre-reinforced plastic matrix was exposed to a test load causing the metal to crack in the mounting hole at a load in excess of 20 tons while no fault could be observed as far as the coherence of the laminated part is concerned.