PSI - Issue 79

Daniel Hofferberth et al. / Procedia Structural Integrity 79 (2026) 313–321

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pneumatic pressure to p clamp = 1 bar to prevent a plastic deformation of the specimens as well as the risk of fractures in the clamping area during fatigue tests. All fatigue tests were performed with sinusoidal wave form and constant amplitude loading in the range between 10 2 < N ≤ 10 6 load cycles. The test frequency was set, depending on the individual stress amplitude and estimated lifetime between f = 2 – 6 Hz. Each type of specimens was investigated under alternating loading (R = -1) and pulsation tensile loading (R = 0). End of test was set for failure or achieving N = 10 6 load cycles (run-out). The evaluation of the fatigue tests was performed by applying the Method of Maximum Likelihood “ MLM ” [Spindel, Störzel]. The parameters for describing the S-N curve are the slope k , if applicable the knee point N k and the endurable stress amplitude for a probability of survival of P S = 50 % at N = 10 6 load cycles. The scatter band T  of the S-N curves was determined according to the Gaussian log-normal distribution.

Thickness 4 mm

Fig. 1: (a) ISO 2740 specimen; (b) ISO 3928 (R = 30)

Detail

Thickness 4 mm

Fig. 2: (a) ISO 3928 (R = 0.9); (b) test setup

3. Numerical and analytical assessment of the bonded specimens For the fatigue strength evaluation, two relevant parameters need to be derived: 1) the local maximum stress resp. the stress concentration factor K t = σ max / σ n , a dimensionless value that quantifies the increase in stress compared to the nominal stress in the specimen net section, e.g., at geometric discontinuity, like notches or changes in shape, and 2) the highly stressed volume V 90% .