PSI - Issue 13

Lukas Loh et al. / Procedia Structural Integrity 13 (2018) 1318–1323 Author name / Structural Integrity Procedia 00 (2018) 000–000

1320

3

has been introduced at the upper adherend, while the test velocity of all other series has been chosen above (viz. Tab. 1). It should be stated again, that a more precise description of test-control can be found in Loh and Marzi, (2018). In Fig.1 b), dimensions of MC-DCB specimens are illustrated. Within test, the elastic-plastic (crash optimized) adhe sive SikaPower R -498 has been investigated with a predefined nominal adhesive layer thickness of 0.3 mm. Specimens have been cured on a temperature of 175 ◦ C for 20 minutes. The loading was supplied by a biaxial tensional-torsional testing machine, as illustrated in Fig. 2. To measure the external loads acting on the specimen (viz. Fig. 1 a)), a multi-axial load cell has been used with capacities of ± 10 kN for axial forces, ± 4 kN for lateral forces and ± 245 Nm for all moments. At the lower clamping device, all rotational degrees of freedom has been locked ( θ = 0 , α = 0). Linear slides allowed unlocked linear motion in x-z-plane, which avoids lateral forces. At the upper adherend, the angle θ has been determined by an incremental shaft encoder. With GOM-Aramis TM stereo digital image correlation system, components u and w of crack opening displacement vector has been optically tracked.

α

a)

b)

1

F y

500

Upper adherend (1)

θ

1

M y

1

M x

y

1

M z M z

<

x

0.3

z

2

M x

<

2

<

θ =0

Adhesive S

2

F y M y

Lower adherend (2)

α =0

Fig. 1. MC-DCB specimen in unloaded state. a) Loading conditions of specimens with indicated integration path S . b) Specimen dimensions (all values given in mm).

Table 1. Test matrix of MC-DCB tests. Constant χ

Variable χ

90 ◦

90 ◦

90 ◦

85 ◦

75 ◦

60 ◦

45 ◦

30 ◦

15 ◦

0 ◦

Mode-ratio

χ I − III

Number of tests

3

1

1

3

3

3

3

3

3

3

5

Angular velocity ( ◦ / s) Lateral velocity (mm / s)

0.05 0.01 0.1

0.01 0.01 0.01 0.01 0.01 0.01 -

0.05

-

-

-

-

-

-

-

-

-

0.025 0.025

3. Experimental results and discussion

As previously mentioned, the manner of test-control varies in between tests of pure single-modes ( χ = 90 ◦ and χ = 0 ◦ ) and tests of constant mixed-mode ( χ = 85 ◦ to χ = 15 ◦ ). In case of constant mode-mixity, tests are conducted under J I -control to achieve the prescribed mixed-mode-ratio. Since controlling on external values such as J is very sensitive to outer disturbance, introduced angular-velocity has been chosen lower than in single mode tests, obtaining a more robust angle / displacement control (viz. Tab.1). To verify influence of performed angular-velocities on fracture behavior, Fig. 3 exemplarily depicts ERR of pure mode III tests under three di ff erent angular-velocities in order of one magnitude. Regarding the plateau-region of nearly constant ERR, which is denoted as critical ERR J c , crack propagation is assumed to be initiated here and no significant di ff erences could be detected in range of scatter. Consequently, a dependency of test-velocity on fracture behavior cannot be observed. Similar results has been obtained for di ff erent mode-mixities.