PSI - Issue 25

R.M.D. Machado et al. / Procedia Structural Integrity 25 (2020) 71–78 Machado et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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and  xy (b) stress distributions for the adhesive Araldite O . Irrespectively of L O ,  y is lower than  xy stresses, apart from the bond edges (near x/L O =0 and 1) and the step transitions ( x/L O ≈0.33 and 0.67). Without the step effect, the  y stresses’ behavior is similar to that of SLJ , i.e. peaking at the edges and with compressive regions at the inner bondline, due to the adherends’ bending.  xy stresses present high variations between each step. However, within each individual step, one may find a similar behavior to SLJ. Indeed,  xy peak stresses in each step occur at the step edges, while in the middle the adhesive is lightly loaded. This behavior is due to the variation of the adherends’ axial deformation that occurs in each step. Hence, the combined effect of two adherends, with opposite axial deformations between the adhesive, results in  xy peak stresses at the edges of each step. ® 2015 as a function of L

a)

b)

3

5

2

4

1

3

σ y / τ avg

τ xy / τ avg

0

2

-1

1

-2

0

0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

x / L O 12.5 mm 25 mm 37.5 mm 50 mm

x / L O 12.5 mm 25 mm 37.5 mm 50 mm

Fig. 3 –  y (a) and  xy (b) stress distributions at the adhesive mid-thickness as a function of L O for the adhesive Araldite ® 2015.

 y stresses are highly dependent on L O . In fact,  y peak stresses at the step edges (either tensile or compressive) tend to increase with L O . This behavior is widely acknowledged for typical lap geometries, and it occurs because the longitudinal strain differential between the two adherends in a given step also increases with L O . By increasing L O ,  xy peak stresses also increase due to higher adheren ds’ axial deformation of each step.

4.3. Experimental strength

Fig. 4 (a) presents the experimental P m as function of L O . It is clear that P m is highly dependent on L O . Actually, regarding L O , the percentile increases between 12.5 and 50 mm was by 227.6%. For L O =12.5 mm the average P m was by 5.2 kN. Whereas for L O =25 mm, an increase of 101.3% was found, since P m was by 10.6 kN. Regarding L O =37.5 mm, the average P m was by 14.6 kN, displaying an increase of 38.8% comparing with the previous L O . Finally, for L O =50 mm the average P m was by 17.2 kN, showing an increase of 17.3% comparing with L O =37.5 mm.

a)

b)

25

20

20

15

15

10

P m [kN]

10

P m [kN]

5

5

0

0

12.5

25

37.5

50

0

0

12.5

25

37.5

50

L O [mm]

L O [mm]

SLJ

DLJ

Stepped-lap joint

Fig. 4 – Experimental results of P m as a function of L O (a) and experimental P m comparison between the SLJ, DLJ and stepped-lap joint.