PSI - Issue 18

V. Dattoma et al. / Procedia Structural Integrity 18 (2019) 719–730 Author name / Structural Integrity Procedia 00 (2019) 000–000

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7

surfaces of the doubler and parts of the laminate distant from the hole. A pre-tension load was applied in the model in order to take into account the effect of the fastening system. The pretension is calculated with the equation (3) that gives initial bolt pretension “F t ” as a function of installation torque (T), nominal bolt diameter (d BOLT ) and torque coefficient (K T ) for which a value of 0,2 is commonly used. Referring to the HI-LOK™ manufacturer’s specification, values of 6,0791 [Nm] and 6,3373 [mm] was used for T and d BOLT respectively.

T K d T 

F

(3)



t

BOLT

Using this formula, a value of about 6 kN is obtained. However, the preload was varied in order to determine its influence on the joint stiffness, since real preload is not perfectly guaranteed. The constraints and loads applied for the model B are the same used for the Model A.

(a)

(b) Fig. 7. (a) Constraints and loads; (b) Preload of the fastening system.

4. Discussion of results 4.1. Experimental results

Main mechanical parameters (ultimate bearing strength F bu , offset bearing strength F by and bearing chord stiffness E b ) are evaluated at each required data point following indication of the standards and the common practice. In Fig. 8a, a scheduled table summarizes the experimental overview of bearing tests showing a general data variation (Ibrahim and Pettit, 2005).

F by [MPa]

E b [Mpa]

F bu [MPa]

Minimum

576.1

597.1

17863.0

Maximum

666.0

815.8

101146.0

Mean

629.3

723.9

34127.9

Std. Dev.

32.4

99.9

24042.8

COV(%)

5.1

13.8

70.4

(a) (b) Fig. 8. (a) Table of experimental results and (b) comparison of typical stress-strain curves of bearing tests.