Crack Paths 2006

due to the rotation of the tool; the side of the weld where translation and rotation speeds have

the same direction is called advancing side, while the side where they are opposite with each

other is the retreating side.

Several studies have been conducted on friction stir welded butt joints, demonstrating that

their strength is very similar to that of base material, and higher than the strength of the joints

obtained with traditional welding techniques [2,3,4].

In the last few years, increasing interest has been dedicated to overlap F S Wjoints in order to

verify their capabilities to replace riveted lap joints in aircraft structures, in fact rivet holes are

often preferential sites for crack nucleation and provide a path for propagation of multi-site

damage. Moreover, in riveted lap joints mastic is necessary in order to achieve pressure

tightness, that creates a potential site for crevice corrosion [2].

Therefore, assembling airframes using F S Wlap joints in principle could improve mechanical

performance reducing weight and extending inspection interval.

On the other hand, F S Wbutt joints are generally defect-free if welding process conditions

(travelling speed and sheet thickness) are properly tuned, while this is generally not the case

of overlap joints, where the welded region must be wider than in butt joints to have a correct

load transfer and oxides stirring and breaking is more difficult due to the relative orientation

of tool and interface.

Besides, two crack-like unwelded zones are present at overlap ends and whose shape can be

either straight or deflected of an angle up to 90° at the root of the welded zone (material

hooking, fig. 1b). This causes a net reduction of the cross section of the sheet and, in turn, of

the strength of the joint especially in fatigue, where the dimension of initial defects affects the

lifetime. The effective thickness in the region of material hooking is evaluated using an

Effective Sheet Thickness (EST) parameter. [3]

Another typical defect of F S Wlap joints is the plate thinning at the retreating side of the weld

due to material flow from retreating to advancing side during welding process. [4]

A big effort is being made for developing dedicate tool for overlap joints: different kind of tools (Skew-StirTM, Flared-TriflutTM, Re-StirTM, TrivexTM [4]) have been developed in order

to minimize material hooking and material softening in heat treatable aluminium alloys due to

dissolution and coarsening of the hardening precipitates [4].

With respect to a previous work devoted to the study and numerical simulation of the fatigue

behaviour of overlap F S Wjoints[1], this work is aimed at evaluating the effect of an initial

overload and of the presence of an antibending device on the lifetime.

M A T E R I A LN DE X P E R I M E N TMAELT H O D S

Specimen geometry

Figure 2a shows specimens’ geometry: the sheet thickness t is equal to 1,6 mm,the width b

20 m mand the overlap length l 20 mm,while the free (unclamped) length is 200 mm. Fig 2b

shows the topology of crack-like unwelded region on the advancing side, the crack tip is

parallel to the load direction, and this reduces stress intensity factor and potentially increases

joint lifetime. Anyway, due to material hooking, the ESTof the specimens on the advancing

side is equal to 1.3 mm,while the effective overlap length is about 5 mm.