Lifetime prediction of Tubular Composite Joints through multi-axial fatigue load experiments and VCCT

Tubular composite joints offer a non-welded alternative for offshore structures by bonding a composite wrap to steel Circular Hollow Section (CHS) members, eliminating weld-induced stress concentrations and significantly improving fatigue life. This enables steel weight and cost reductions and faster fabrication for jacket structures supporting large off-shore wind turbines. In service, these joints experience complex cyclic loads combining axial forces and bending moments, which can lead to interfacial debonding and delamination, necessitating damage-tolerant design. This paper presents one of the first experimental campaigns applying combined axial and bending loads on composite X-joints using a Hexapod system, enabling realistic offshore load simulation. Fatigue tests on 1/4-scale X90 specimens cover pure axial tension, out-of-plane bending, and combined cases. Two primary failure modes were observed: interfacial debonding under compressive strain and delamination under tensile in-plane strain. A numerical methodology based on the Virtual Crack Closure Technique (VCCT) and a stepwise crack-growth model incorporating non-linear crack retardation effects, rarely considered in composite joint fatigue modelling, was developed. Calibration of the Paris-law constant C revealed variations up to two orders of magnitude due to interface quality and manufacturing variability. Despite this, results demonstrate fatigue life extensions of up to 2000 times compared to welded joints. This work introduces a design philosophy leveraging crack retardation and interface friction effects to predict fatigue life, moving beyond conservative stress-based criteria towards damage-tolerant offshore design.