PSI - Issue 22

V.M.G. Gomes et al. / Procedia Structural Integrity 22 (2019) 401–406 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

405

5

slopes in Fig 2(a), noting m values around 8, and near 5 for snug tight and preloaded bolted connections, respectively, instead of m =3 as suggested by EC3 Class 90. In addition, EC3 Class 90 slope showed to be more conservative for very high-cycle fatigue, however, for lower lives than 6.0E6 cycles, the EC3 S-N curve becomes unsafe. Preloaded bolts showed better fatigue behaviour than snug tight bolts for lower fatigue lives, not verifying large differences for higher fatigue lives. In addition, one verifies that decreasing the net section width from 80 to 70mm, the fatigue strength improved despite net cross-section stress range increasing. Multiple bolt connections showed better fatigue resistance than single bolt connections (assumed load transmission thru the first row of bolts); the drilling manufacturing process proved to be more harmful to fatigue behaviour than the punching process. Analyzing Fig 2(b), one verifies that for higher fatigue lives, at same stress ratio, the confidence bands of 3 times the standard deviation covers the fatigue data of preloaded and snug tight bolts. Moreover, the confidence bands of multiple bolts connection with punched holes until 3 times the standard deviation may not cover the fatigue data of the test specimens, with drilled holes along number of cycles fatigue range as shown. Therefore, the inferior limit of the confidence band from 2 times the standard deviation may be considered for S-N curve of single bolted joints in high cycle fatigue regime. 3.2. Fatigue Failure Modes Regarding the fatigue failure modes, two types of failure modes were verified: failures were verified thru cracking along net cross-section and or along the gross cross-section. Failures in the net cross-section occurred for all test fatigue specimens excepted for preloaded single bolt specimens tested in very high-cycle fatigue conditions (Fig 2(b)), presenting fatigue crack initiation close to the hole due to the clamping effect. In multiple connections, the failure always appeared in the first set of bolts (Fig 2(b)). Single connections with snug tight bolts showed a tendency of the crack position, moving away from the centre of hole, as the stress range decreases. The same tendency was verified when the stress ratio is increased, increasing the mean stress and decreasing the stress amplitude (Fig 2(a)).

S_P_R01_SN S_P_R01_PR M_P_R01_SN

Avg. S_P_R01_SN Avg. M_P_R01_SN

S_P_R01_SN

Avg. S_P_R01_SN

S_P_R01_SN_RA Avg. S_P_R01_PR

S_P_R01_PR S_P_R03_SN

M_D_R01_SN

S_P_R05_SN M_P_R01_SN M_D_R01_SN

Avg. M_P_R01_SN Avg. M_D_R01_SN

Avg+/- 1St.D S_P_R01_SN Avg+/- 3St.D S_P_R01_SN Avg+/- 2St.D M_P_R01_SN

Avg+/- 2St.D S_P_R01_SN Avg+/- 1St.D M_P_R01_SN Avg+/- 3St.D M_P_R01_SN

EC3 Class 90

700.00

700.00

Δσ NET [MPa]

Δσ NET [MPa]

(Net Cross-Sect ionRange St ress)

(Net Cross-Sect ionRange St ress)

70.00

70.00

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+04

1.00E+05

1.00E+06

1.00E+07

N f [cycles] (Number of cycles to fatigue failure)

N f [cycles] (Number of cycles to fatigue failure)

Fig. 2. Fatigue S-N data: (a) experimental data with mean S-N curve obtained experimentally; (b) data with confidence internals.

4. Concluding remarks This fatigue experimental campaign performed on bolted joints led to the following main conclusions: - Snug tight bolts show a significant fatigue resistance decrease with respect to the preloaded bolted joints; - The usage of multiple bolts led to an apparent fatigue resistance increase if the load transfer is assumed to occur exclusively thru the first row of bolts. If a load transfer scheme involving the joint action of the complete