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Repeated wave impact is a critical loading condition for racing yacht design. A test that was initially used to compare materials has been further developed to provide quantitative results and input for numerical modelling.
1- PETER DAVIES, 2- DOMINIQUE CHOQUEUSE, 3- BENOIT BIGOURDAN , RESEARCH ENGINEERS, IFREMER MATERIALS & STRUCTURES GROUP, IFREMER BREST CENTRE 4- HERVÉ DEVAUX, 5- STEVEN ROBERT , DIRECTOR AND ENGINEER, HDS(Published on August-September 2010 - JEC Magazine #59)
The Materials and Structures group at Ifremer, the French Ocean Research Institute, has been developing tests to simulate the loading of composite structures at sea for many years. HDS is a small company based in Brest which has developed expertise in the design and calculation of composite structures, particularly racing yachts.
The sandwich materials used in racing yachts are very stiff and lightweight, but sensitive to impact damage. Standard test methods generally use rigid impacters to evaluate the impact response of sandwich materials, but these impacters produce a localized damage in the form of a perforation of the impacted facing. Wave impact resistance is one of the factors that limit the reliability of racing yachts, and the damage observed is generally more diffuse and involves core crushing. Figure 1 shows an example of impact damage on a multi-hull forward beam.
There is a need for a test that simulates wave impact and induces damage due to distributed pressure loading rather than localized indentation. Modern ocean racing multi-hulls such as Groupama 3 and America’s Cup yachts such as BMW Oracle (Figure 2) can be subjected to pressure loads well in excess of 20 tonnes/m², so the test used should be able to apply similar loadings. HDS and Ifremer have worked with both teams in recent years.
The first tests (Figure 3a) performed for PRB in 1995  simply involved dropping an 18-kg medicine ball onto a panel placed either directly on the floor or on sand. This test set-up was progressively improved over the years. The support conditions are now better defined and extensive instrumentation has been added (Figure 3b).
The following parameters are now measured:
All these data are recorded and stored for subsequent analysis.
Several test programmes were implemented, the details of which can be found in [2,3]. These revealed a range of damage mechanisms (Figure 4) that depend on core type, facing thickness and panel geometry.
These mechanisms are more or less favoured by the panel geometry. For example, a thicker core is more susceptible to shear failure. Given these differences, it is essential to develop modelling tools that will allow the test results to be analysed correctly and used for design.
Numerical modelling was therefore performed. Figure 5 shows an example of a finite element model of an impact test. One quarter of the medicine ball and panel are represented. This is a dynamic model using ADINA.
Quadratic shell elements and an isotropic linear elastic material were used for the medicine ball envelope. Quadratic 3-D elements and a non-linear Mohr-Coulomb material were used for the sand in the ball. The sandwich skins were represented by quadratic multilayered shell elements and an orthotropic linear elastic material and the core was modelled by quadratic 3-D elements and an orthotropic (honeycomb) or isotropic (foam) material (Figure 5a).
Provided the medicine ball is correctly modelled, a reasonable correlation between test results and simulations can be obtained (Figure 5b). More complex material models (elasto-plastic or viscoelastic) can be used to simulate the response of foam cores.
Modelling work is continuing in order to integrate material response in the structural models used at HDS to dimension racing yachts. Other aspects of wave impact are also being studied, including the influence of stiffeners on structural response and the effects of repeated impact on damage development.