Fatigue Strength Assessment of the Pin Connection in the TetraSpar Concept

Abstract

The fatigue strength of the pin connection is assessed in the research project presented in this thesis. The fatigue strength is quantied by an estimated fatigue lifetime, which is the expected time to failure of the most critical point in the connection. The lifetime is estimated based on the yearly accumulated fatigue damage. A computer model of the structure has been constructed, with which time signals of the internal forces in the structure are calculated, based on the environmental loads. The pin connection was not included in this model, so internal forces in the steel tubular members have been translated manually to forces in the pin connection. The connection was decomposed in its basic components and nine potential critical locations have been identied. Stress concentrations are expected at these locations, which make them sensitive to fatigue. The internal forces were combined with the dimensions of the components, so that nominal stresses could be calculated. The hotspot stress at the potential critical locations was estimated with stress concentration factors (SCFs), which are found in literature. Relationships between the internal force and the hotspot stresses are derived in this way, and time signals of the hotspot stresses are obtained. Most of the identied locations could be assessed with hand calculations, except for two locations. They are analyzed with nite element software (ANSYS). The time signals of the hotspot stresses have been used to calculate the expected fatigue damage with the Palmgren-Miner rule. The damage is calculated for all wave directions and based on real wave statistics.