JEC COMPOSITES MAGAZINE - Issue #59 - August/September 2010 - 36

FEATURE

“Marine”

g…/… Validation of marine structures - Testing appraoches due to the specific geometry of the section: the final failure was initiated in flexure on the thickest section of the sole and propagated inside the cells. It is important to note that the failure is not governed by the compressive failure of the transverse partitions. The force/displacement curve shows that a significant loss of rigidity occurs at 480 kN. The final failure was observed at 770 kN.

Non-destructive tests based on modal analysis
Non-destructive methods based on modal analysis offer many advantages, both for measuring dynamic properties and for estimating the global rigidity of structures. Three examples of structural tests are presented on keel, mast and cable rigging elements. With the optimization of the IMOCA Open 60 racing monohull, a resonance phenomenon appeared on the keel. The problem results from a coupling effect between the vibration behaviours in flexion and torsion, and occurs at high speed when the torsional rigidity of the keel is much lower than the flexural rigidity and the ballast weight. Measurements during sailing showed that the vibration modes concerned are the first modes in flexure and torsion. The dynamic behaviour of ballast keels was investigated using a vibration analysis protocol. This test is now recommended by the IMOCA class rules. The objectives are as follows: - Validating the first natural frequencies in flexion and torsion in relation to the theoretical values of the design office. Non-coupled values of these frequencies are expected in order to ensure a good dynamic behaviour of the keel. - Implementing periodic controls. Structural modifications of the keel or a loss of rigidity due to damage can change the natural frequency values. For example, a higher ballast weight compared to the initial keel configuration leads to lower natural frequencies. As shown in Figure 3, these vibration measurements were performed using accelerometers and video analysis. Similarly, NCD developed a measuring protocol for masts. The aim
Fig. 3: Keel test - Determination of natural frequencies

is to validate the dynamic signatures of new masts with respect to calculated values from the design office and to perform periodic controls. The measurements are especially useful to validate a construction for standardized production. The test protocol includes two sets of measurements in longitudinal and transverse configurations: - simply supported: the mast is set on cradles, - free/free: the mast is hanging from elastic cables. The events are recorded using accelerometers and the signals are analysed by FFT (Fast Fourrier Transform) and analytical models. Based on the signals and natural frequencies measured on the mast, two parameters are estimated: - global flexural rigidities between the limit conditions of the test, - dynamic coefficients of damping as indicators of response time.

Focus
NCD is an independent consulting firm that was created in Brest, France in 2007. NCD specializes in composite materials engineering. Its services include composite design, quality control and surveys, research on the damage tolerance of composite materials, and non-destructive testing of structures using modal analysis techniques. Established in Brest, France since 2002, FMC specializes in the manufacture of composite parts and structures. The company offers a number of manufacturing processes, from Class 1 autoclave prepregs to infusion moulding and including RTM, LRI, RFI, and compression moulding. Its main customers are in the defence, oceanography, nuclear energy and racing yacht sectors.

Fig. 4: Mast test – Dynamic signature in bending

jec composites magazine / No59 August - September 2010

JEC COMPOSITES MAGAZINE - Issue #59 - August/September 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #59 - August/September 2010