A quarter of a single bolt modelled with FE. The
figure shows the stress due to preloading.
A lighthouse with sweeping light. Foto: Else
Djurhuus, Fyrhistorisk Museum på Nakkehoved
Sandwich face/core disbond damage leading to
the loss of the rudder in an Airbus A310 aircraft.
Photo: Airbus Operations GmbH
Analysis and optimization of bolted L-flanged connections
In wind turbine towers the preferred design is
circular tubes that are connected by a bolted
flange joint. The design is typically that of an
L-flange resulting in an eccentrically loaded
bolted connection. The eccentricity results in
a non-linear relationship between the external
load on the tower and the tensile force in the
bolt. In the literature and also in standards, different
models are presented for this important
non-linear response. In this project, a simplified
expression for the non-linear force response is
presented based on finite element calculations
using contact analysis.
L-flange assemblies are, from a bolt point-ofview,
a bad design, and when this design is
selected anyway it is done due to constraint
on the possible layout. The important transfer
function between the external load and the
load in the bolt is shown to have two linear
asymptotes in the practical load spectrum of
the external load. The load curves can be approximated
by a Bézier curve. In order for the
analytical curve-fit to be sufficiently accurate,
numerical calculations must be performed. Overall
values for the asymptotes can be estimated
directly in many cases.
Different L-flange design changes are investigated
in the project. The best design improvement
is found by putting the bolt as close to
the tower wall as possible. Further significant
improvements are found by increasing the
flange thickness. Strength improvements up to
65 % relative to a standard design are found.
Replacing mercury support with roller bearings in lighthouses
Fresnel lenses occur in lighthouses from the
19th century. The Fresnel lenses greatly
increase the intensity and range of the light.
For rotating lights, it is important to maintain a
specific rotational speed, so that navigators can
identify the lighthouse and verify their position.
To achieve low friction, the light and the lens
are floating on a circular track of liquid mercury.
The required amount of mercury to make a lens
system weighing around 3 metric tons floating
is typically 20-30 liters.
Mercury is a deadly poison, so the environmental
and maritime authorities in the Nordic countries
require the mercury bearings substituted
by an environmentally acceptable solution,
and further to maintain the sweeping light, if
possible.
The core of the new solution is to replace the
mercury support with a modern roller bearing.
The relatively small bearing can actually support
far more weight than the big mercury bearing.
The typical mercury bearing has a diameter
of more than 2m whereas the spherical roller
bearing has a diameter of 150mm.
The specific design of the circular mercury
support and the shafting varies a lot from
lighthouse to lighthouse, so the replacement
arrangement must be adapted to the specific
design. Conservation authorities require the
new bearing arrangements mounted without
harming or destroying the existing components.
In principle, our solution makes it possible to go
back to mercury support, if required. DTU has
modified 16 lighthouses in cooperation with
Brdr. Petersens Maskinfabrik A/S in Gilleje until
now.
Damage assessment of sandwich disbond damages in aircraft components
Honeycomb sandwich structures, consisting of
a low-density core material reinforced with thin
or thicker stiff facings, are attractive for use in
aerospace applications due to their high specific
bending stiffness. A crucial factor determining
the integrity of a sandwich structure is the
bonding between the face sheet and core. The
face/core debonding (or “disbonding”) can be
instigated through a bird strike, hail strike, blunt
body impact or tool drop, as well as during the
manufacturing phase due to insufficient wetting
of face and core surfaces. The presence of
disbonds has led to several in-service failures.
A number of instances have occurred involving
component failure via this damage mechanism.
An example includes the loss of an Airbus A310
rudder, see the figure. The structure integrity
degradation can affect the continued operational
safety of the aircraft.
In a disbonded sandwich structure, the propensity
of the crack to propagate through the face/
core interface or kink into the core is driven by
the loading conditions. Therefore, the critical
energy required to separate face from core,
referred to as the fracture toughness must be
ascertained to aid in the design of sandwich
structural components. The interface fracture
toughness must be determined for a range
of mode-mixity values to serve as input into
numerical models of structural components in
the aircraft, as the load conditions may induce
mode conditions varying from mode-I to mode
II. The crack tip mode-mixity, expressed using
the phase angle, y, can be attributed to the
ratio of mode II to mode I loading.
The Lightweight Structures Group is taking part
in an international task group led by NASA and
supported by FAA, EASA and Airbus to develop
new design guidelines to assess the criticality
of disbond damages in aircraft sandwich structures.
This includes both development of new
damage characterization test methods, as well
as development of advanced fracture mechanics
based analysis models validated against fullscale
panel testing in the DTU Structural Lab.
Contact:
Niels L. Pedersen, e-mail: nlp@mek.dtu.dk
Contact:
Peder Klit, e-mail: klit@mek.dtu.dk
Contact:
Christian Berggreen, e-mail: cbe@mek.dtu.dk
Solid Mechanics 39