Numero 1 2007 - IIS

International Institute of Welding
Fracture analysis of
strength undermatched
welds of thin-walled
aluminium structures
using FITNET procedure (°)
1. Introduction
Driven by the demand for lighter and
cost-effective airframes as well as by the
close competition with the non-metallic
composite materials, the design of
metallic structures in the airframe fabrication
has experienced revolutionary
changes during the last decade. The well
established joining technique by rivets is
currently being replaced for some
airframe applications by welding using
novel welding technologies like laser
beam welding (LBW) and friction stir
welding (FSW). The adoption of these
welding processes provides savings in
structural weight and fabrication cost up
to about 15% [1]. The most widely used
metallic material in aircraft structures is
aluminium and was deemed to be
unweldable [2]. However, newly developed
aluminium alloys with silicon (Si)
and magnesium (Mg) as the main
alloying elements facilitate the use of
low heat input welding technologies to
manufacture crack and porosity free
welds with good mechanical properties
compared to the properties of the
conventional base material alloys.
(°) Doc. IIW-1709-05 (ex-doc. X-1577-05) recommended
for publication by Commission X
“Structural performances of welded joints -
Fracture avoidance”.
* GKSS Research Center, Institute for Materials
Research (Germany).
Summary
Stringer-to-skin joints in advanced
airframes of some airplanes are already
being produced using LBW with the use
of 12% Si containing wire, whereas for
the skin-to-skin joints, LBW and FSW
techniques are currently under consideration
in order to replace conventional
E. Seib *
M. Koçak *
The paper presents a methodology for the residual strength prediction
for the load carrying thin-walled components with highly strength undermatched
welds containing cracks. The analysis is based on the strength
Mismatch Option of the Fracture Module, being a part of the newly
developed fitness-for service (FFS) procedure FITNET. The Mismatch
Option of the FITNET Fracture Module allows for the account of weld
features like the weld tensile properties and weld geometry in the fracture
analysis of cracked welded components. The methodology described
was verified with the generated experimental results within this investigation.
The material used is an age-hardening aluminium alloy 6013 in
T6 temper condition used in recent airframe components. The welds in
the form of butt joints were produced using the CO 2 laser beam and friction
stir welding processes. The deformation and fracture behaviour as
well as the special features with respect to the FITNET FFS Procedure
were analyzed. The results have shown that using the presented methodology
along with newly proposed recommendations for the input data
selection, conservative predictions of the maximum load carrying
capacity of the large welded panels under tensile loading containing a
long crack in the weld can be obtained.
KEYWORDS: Aerospace; Aluminium alloys; Butt joints; Cracking;
Defects; Deformation; Elongation; Fractures; Friction stir welding; Friction
welding; Laser welding; Light metals; Mechanical properties; Mismatch;
Plastic deformation; Photon beam welding; Radiation welding; Reference
lists; Strength;Thermomechanically affected zone; Thin; Ultimate tensile
strength;Weld zone;Yield strength.
riveted lap joints. Current metallic
airframes of airplanes are designed to
satisfy the damage tolerance requirements
in terms of fatigue and residual
strength. The residual strength of a
structure is defined as the remaining
load carrying capacity in presence of one
Riv. Ital. Saldatura - n. 1 - Gennaio / Febbraio 2007
91