Patent Grant
7021856
This application is based on and claims the priority under 35 U.S.C. § 119 of German Patent Application 102 38 820.2, filed on Aug. 23, 2002, the entire disclosure of which is incorporated herein by reference.
The invention relates to a device, also referred to as splicing, for connecting thin-walled sheet metal end portions to each other in an overlapping contact surface area. At least one row of rivets subject to cyclical or dynamic loads is arranged in the overlapping area for splicing the two sheet metal end portions to one another.
At the present time rivet connections described above are the predominantly used splicing connections in aircraft construction.
In such conventional splicing connections an interlocking is achieved between the parts to be interconnected by a mechanical interlocking of geometric shapes to thereby provide an interlocking splice connection. In such connections it is necessary that the resistance of the inwardly facing walls of the rivet holes in the individual sheet metal portions and the shearing resistance of the rivets must be larger than the loads externally applied to the splice. Conventionally one or several rows of rivets are used in such splices of mutually overlapping sheet metal end portions, whereby full volume rivets, tight fit rivets, threaded rivets, or blind rivets are used. Typical examples for the connection of thin-walled structures are longitudinal and cross seams, as well as seams surrounding a repaired skin section. A multitude of rivet connections in an aircraft, particularly an aircraft body skin, is of basic importance for the flight characteristics of an aircraft. The rivets are individually dimensioned for the particular riveted splice taking into account the type of rivet, the size of the rivets, the spacing between the rivets and so forth, particularly paying attention to the local static and dynamic loads. In this connection it is an essential requirement that the splice has a high useful life and is substantially free of the need for inspections or requires only few inspections.
During the operation of an aircraft large areas or sections of the aircraft structure are subject to cyclical or dynamic tension loads. As a result, the components made of metallic materials are exposed to the potential danger of fatigue due to crack formations followed by crack progression or crack creeping. Individual cracks and particularly widespread fatigue damage caused by cracks can substantially reduce the strength characteristics of these metal components. These fatigue characteristics must be taken into account when inspection intervals are scheduled. In aircraft construction the thin-walled structures which have been optimized with regard to weight reduction are frequently subject to a high secondary bending load component, whereby a low crack resistance duration occurs which simultaneously requires a high inspection effort and expense. A secondary bending has been observed to occur when the load axis and the neutral phase are not identical in a structural component. For example, in the case of a splice interconnecting two overlapping sheet metal end portions the load axis and the neutral phase are staggered relative to each other.
In view of the foregoing it is the aim of the invention to achieve the following objects singly or in combination:
A splice between thin-walled structural components formed by at least one dynamically or cyclically loaded row of rivets is improved according to the invention by a further row of rivets positioned between an end edge of an end portion and the first mentioned row of rivets. The additional row of rivets is so constructed or provided with features that hold the two end portions together while simultaneously permitting a relative motion in the contact surface area between the two end portions, which are preferably sheet metal end portions.
Such an additional row of rivets constructed according to the invention has the advantage that particularly the conventional rivet row next to the additional rivet row is relieved at least partially of its high dynamic loads while the additional row of rivets is primarily exposed to a secondary bending load. The additional row of rivets extends preferably in parallel to the at least one conventional row of rivets. The reduction of the maximum tension load in the conventional rivet row or rows as achieved by the invention leads to an increased useful life with regard to crack formations. More specifically, the beginning of crack formations is reduced. Similarly, crack spreading following the formation of any crack is also reduced. Another advantage of these features according to the invention is seen in that the time intervals between inspections may be longer, thereby reducing the effort and expense for the inspection of such splice connections. This advantage is particularly important for riveted splices in aircraft because unscheduled dead times on the ground have been eliminated by eliminating additional inspections that were required heretofore.
Another advantage of the invention is seen in that additional methods can be employed for a targeted reduction of the locally effective maximum tension load. One such method involves work hardening. More specifically, a rivet hole is plastically deformed in the radial direction by widening the rivet hole for generating in the wall of the rivet hole tangentially effective residual compression stress which counteracts the effective tension load on the rivet hole. It is known from experiment that this work hardening is relatively ineffective in a structure subject to a large secondary bending load. However, it has been found that the work hardening of the rivet holes in combination with the invention can develop its full effectiveness in the conventional rivet row or rows because the additional rivet row according to the invention has deflected secondary bending loads from the conventional rivet rows by taking up such secondary bending loads itself. More specifically, secondary bending loads are now primarily effective only in the additional rivet row which neutralizes such bending loads by the limited relative movement between the sheet metal end portions.
A still further advantage of the invention is seen in that the flight characteristics of an aircraft have been improved by the teaching of the invention.
In order that the invention may be clearly understood, it will now be described in detail in connection with example embodiments thereof, with reference to the accompanying drawings, wherein:
Referring to
The fatigue strength that represents the duration between service initiation and the beginning of cracks at the edge of a rivet hole is influenced substantially by the locally occurring maximal secondary bending or rather tension.
σmax=Δσ1+σ2+Δσ3
wherein the first load situation involves a
The location of the maximal secondary bending in a multi-row longitudinal splice 103 is normally the outer rivet row 105 which is thus referred to as a fatigue critical rivet row. The initiation of a crack 106 takes place first at the edge of a rivet hole 104 in this row.
According to the invention the fatigue critical rivet row 6 is partially relieved of the above discussed loads by a rivet row 7 positioned according to the invention between the end edge 4A and the row 6, whereby the fatigue strength of the splice connection having a high secondary bending proportion is increased and the crack propagation is correspondingly reduced, that is improved. For this purpose the maximal tension in the critical initially outer rivet row 6 is reduced by reducing the secondary bending moment proportion Δσ3 to a minimum. This is achieved by the additional rivet row 7 which is primarily exposed only to the secondary bending proportion Δσ3. The initially critical outer rivet row 6 has now become the second rivet row which is exposed to a significantly reduced bending load.
By positioning the additional rivet row 7 preferably in parallel to and between the end edge 4A and the row 6, the additional rivet row 7 reduces the load to which the conventional rivet row 6 is exposed in a conventional rivet splice 103. The bending loads are also reduced in the rivet row 6′ and 6″ and this reduction in all three conventional rivet row 6, 6′ and 6″ leads to a prolonged duration between putting the structural component into service and the occurrence of a crack. Simultaneously the crack progression is reduced.
According to the invention the additional rivet row 7, features rivets 8 that provide a clamping force in the direction of the longitudinal axis 9 of a rivet shaft 10 to provide a positive interlocking to keep the sheet metal end portions 2 and 3 in contact with each other. Thus, a vertical displacement of the sheets 2 and 3 is prevented by this positive interlocking. However according to the invention, features are provided that permit a horizontal relative displacement or motion between the two portions 2 and 3. This horizontal relative motion is impeded only by friction, but not by a positive interlocking.
In the embodiment of
In the embodiment of the rivet 8′ of
In all embodiments the recess or groove 20 is so-dimensioned, that a sufficient play is permitted between the edges of the recess 20 and the rivet head 17 to permit the desired limited relative motion between the end portions 2 and 3.
Although the invention has been described with reference to specific example embodiments, it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims. It should also be understood that the present disclosure includes all possible combinations of any individual features recited in any of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102 38 820 | Aug 2002 | DE | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 2746579 | Gondek | May 1956 | A |
| 4394096 | Stevens | Jul 1983 | A |
| 5297760 | Hart-Smith | Mar 1994 | A |
| 6775895 | Koffi et al. | Aug 2004 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20040052581 A1 | Mar 2004 | US |
