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108
Novel Form-Fitting (FF) Tolerance-Tolerant (TT)
blind rivets developments and test results
1. State of the art aerospace standards permit a permanent
Fig 2
1.1 Blind rivets “setting” of up to .04D, where D is the nominal
The load carrying ability of a riveted joint diameter of the rivet. [3]
depends on the diameter of the rivet, the
sheet thickness’, the rivet materials and the 1.1.1 Conventional blind rivets -
material(s) of the sheets and whether the joint Setting operation
is force- or form-fit or both. Generally, with the During the setting operation, the mandrel
exception of rivets with interference-fit hole upsets the shank and, before it folds to form
preparation, blind rivet joints are force-fit. This the blind head, its walls thicken such, in the-
Force vs. Displacement diagram of a
is also borne out by the results of recent ory, the gaps between mandrel, shank and
stainless steel tensile shear specimen
research projects, Fig.1, [1, 2] work piece are bridged. After separation of the
(Source: University of Rostock)
mandrel, all components experience spring-
Fig 1
back: mandrel, shank and the work piece. The
lengths of the mandrel and shank increase, overcome. The shear force and the bearing
their diameters decrease, the inside and out- pressure experienced by the rivet are much
side diameters of the shank become smaller higher than if the force had been applied
and the same applies to the diameter of the with the parts – shank/work piece(s) and
work piece hole(s). Because of the different shank/mandrel - already in contact. This can
cross-sections, stiffness’ and yield strengths, decrease the fatigue life of joints by a factor
the dimensional changes are generally largest of 10 to 15 at R = 0.1, [4].
in the shank and the mandrel and smaller in
the work pieces. Should the gaps between 2. Reasons for developing the new
mandrel, shank and work piece(s) be bridged FF® TT® rivets
during the setting operation, they will again The aim was to enable the production of
appear after the separation of the mandrel. form-fit joints, necessary for a current
application, without requiring the very
1.2 Behaviour of (blind) rivet joints expensive hole preparation as practised by
Micrograph of state of the art high
under quasi static loads the aerospace industry, and at the same
strength blind rivet joint showing gaps
After frictional locking is overcome, there is time providing a comparatively low-priced
relative movement of the sheets to take up rivet for making joints with vastly improved
Any gap in a riveted joint, either between slack. This bridging of gaps can occur in several load carrying characteristics.
mandrel and shank or between shank and steps. Fig.2 shows a force vs. displacement
work piece shows up as slack in a tensile shear diagram for a lock bolt tensile single-shear 2.1 Design concept of the new
test, once frictional locking is overcome. And specimen in 2 + 2mm stainless steel sheet [2]. rivets [5]
this value is always much lower than the value Frictional locking is initially overcome at The concept of the new rivets is based on
for the ultimate strength. approximately 2.4kN tensile load. The gaps are the bridging of gaps by a direct positive
Except in the case of structures in which the bridged in two steps. The stiffness of the joint radial expansion of the shank. Towards this
joints are subjected to either static shear loads, increases after each phase and reaches a maxi- end, a splined or ribbed mandrel is
cyclic tensile shear (R = 0.1) or cyclic mum after all gaps have been bridged, Fig.2. employed, its circumscribing diameter being
compressive shear loads (R = 1.0) with load larger than the inner diameter of the shank.
amplitudes lower than the frictional lock value, 1.3 Lock bolts On making contact with the walls of the
it is of great importance that the joints have Joints made with lock bolts and bolts are work piece hole(s), the “excess” material of
form-fit. Otherwise cyclic loading will over- both force-fit and therefore not suitable for the shank “flows” into the valleys between
come frictional locking and destroy the joint in the transmission of higher shear loads. In adjacent splines, ensuring form-fit both
a very short time. such cases, the use of fitting bolts and/or between work pieces and shank as well as
Attempts have all along been made to fitting keys is necessary. between shank and mandrel.
improve the hole-filling capabilities of the
rivets. An internet search, with the term 1.4 Behaviour of riveted joints 3. FEM - based investigations [6]
“hole-fill rivets”, yields hits in which numerous under cyclic loads A 2D - Finite Element Method (FEM) –
manufacturers and suppliers offer “rivets with With normal load build up, the gaps are feasibility study was carried out. In a
“very good hole-fill properties”. However, even bridged quickly after frictional locking is second step, 3D-FEM parameter
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