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Fatigue testing of composite-patch-reinforced ship structural parts

TWI contributed fatigue testing expertise to a European collaborative project addressing a recognised problem affecting large passenger liners.

Cruise ships occasionally encounter structural problems in the corner of the balcony openings of longitudinal bulkheads on passenger accommodation decks (Figure 1), which are prone to fatigue damage. The MOSAIC (Materials on-board: steel advancement and integrated composites) project, part-funded by the European Commission, was a three-year programme that brought together organisations from across Europe to develop solutions to this issue.

TWI’s contribution included a programme of large-scale fatigue testing to validate the innovative technological solutions developed by the MOSAIC project partners. TWI also provided input to the development of relevant design and manufacturing guidelines.

Lightweight strength

The solution developed by the project team was the application of a carbon epoxy/composite patch at the corner of the bulkhead openings. This approach avoided increasing the steel plate thickness or modifying the structural geometry of the corner of the opening, which would be expensive and add weight.

TWI’s main objective was to perform fatigue tests of large-scale composite subassemblies to failure, to determine their fatigue performance.

Figure 1*. Balcony corner openings at a cruise ship superstructure (general view and details of reinforcement)
Figure 1*. Balcony corner openings at a cruise ship superstructure (general view and details of reinforcement)
Figure 2. Technical drawing of the tested substructure with composite patches
Figure 2. Technical drawing of the tested substructure with composite patches

Test specifications

TWI designed, manufactured and tested three large-scale test specimens representative of a conventional corner opening (Figure 2). Specimens consisted of marine steel grade AH36 with two patches of carbon fibre applied on each side of the steel. Each specimen was instrumented with strain gauges and tested in air, at ambient temperature, in a servo-hydraulic test machine under constant amplitude axial loading, between -120kN and 210kN (Figure 3). This level of loading was chosen to correspond to the maximum stress levels developed in the real structure when the ship is sailing (hogging and sagging conditions). Prior to fatigue testing, the specimens were loaded under direct axial static loading in 30kN increments between maximum and minimum load.

Composite patch validated

Following fatigue testing, cracks in each of the specimens were found to have developed only in the steel plate, with no patch debonding or composite failure. The fatigue life of the patched corner opening substructure matched the behaviour of a prepared finite element model: approximately 56,000 cycles. This is twice the theoretical fatigue life obtained with BS76081, which is the code of practice for design and assessment of steel structures in the UK. In addition, the observed fatigue life from the test was found to be nine times longer than that assessed using the guidelines from Det Norske Veritas (DNV) classification society2.

References and credits

  1. BS7608: 2014: ‘Guide to fatigue design and assessment of steel structures’. British Standards Institute, March 2014. 
  2. Det Norske Veritas: ‘Fatigue assessment of ship structures’, Classification notes No.30.7, June 2010.

*Figure 1 drawing used by courtesy of FINCANTIERI S.p.A. – all rights reserved.

For more information, please email contactus@twi.co.uk

Figure 3. Test setup
Figure 3. Test setup
Avatar Emilie Buennagel Principal Project Leader - Fatigue Integrity

Emilie joined TWI in August 2010 after spending more than three years in the engineering and technology division of a large materials testing company in the UK. Previously she completed her MSc studies at the University of Orleans in France, including two work placements at Nexans in Germany.

Emilie brought to TWI experience of mechanicals testing and metallurgy, primarily for the aerospace industry. In her previous employment she was responsible for managing projects and developing new test programmes. At TWI she manages a range of projects, including failure investigations, fatigue assessments and resonance fatigue testing of full-scale pipe.

Among the research projects Emilie has been involved with has been a core research programme investigation comparing approaches to design fatigue assessment, with reference to a pressure vessel designed to an old standard. A published paper based on this research is available on the TWI website.

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