COMPOSITE CONSTRUCTION


A study of the effect of slamming on sandwich panels


A


NYONE who has seen recent footage of the Velux 5 Oceans Race - or even sailed


around the buoys in a local fleet race - can attest to the fact that sailing yachts live in a world that is far from static. Similarly, sports fisher motorboats and patrol craft crash through waves on a daily basis. The current scantling codes used in the


design of these vessels are based on the application of uniformly distributed quasi- static pressures on shell panels – a loading scenario that is far from reality. High Modulus has been involved with


the composite structural design of sailing and motor yachts of all shapes and sizes for over 20 years. The company knows that it is not sufficient for these vessels simply to be designed to meet a rule – rather it is necessary that they are fit for purpose, and this means taking into consideration the actual conditions that the yacht will face. Sandwich panels are widely used within


the marine industry as the primary hull shell structure, yet core shear can be experienced when vessels are subjected to high slamming loads. For this reason, High Modulus set about determining how dynamic and high strain-rate loading affects various sandwich core materials, and how this compares with the behavioural assumptions of uniformly distributed quasi-static pressure. All core materials are not created equal.


Core materials come in different densities, are manufactured using different processes, and are made from different raw materials. But which cores perform best in rough seas? And how big a difference does it make?


The approach The core shear characteristics of four different core materials were compared using both traditional ASTM standard test methods and new developmental procedures that are capable of assessing the dynamic properties of core materials. The core materials tested during this


project were: S89 Balsa, NHOX80 partial over expanded Nomex, Airex C70.130 cross- linked foam, and Airex R63.140 linear foam.


Plate shear results. Applied shear load vs elongation for a linear foam (left) and balsa cores.


Fastenings for panels and structures T


HERE is a growing call for composite panels and structures in the marine


industry, with the composite materials utilising carbon fibre, glass reinforced plastic, glassfibre, thermoplastic, wood, and alloy outer skins. These skins are typically bonded to honeycomb block, foam, or solid core materials. A fastener for composite materials has been


introduced by BigHead, which has been a supplier of thin-head fasteners for 40 years. In partnership with Caparo Vehicle Products,


56


the 316 stainless steel TwinDisk uses BigHead weld technology to provide a rapid and efficient fastening point anywhere in a structure. BigBond adhesives, also produced by


BigHead, complete the system, and it is thought that the two part FS acrylic version gives the maximum strength. TwinDisk functions with two heads or disks,


joined by an internally-threaded collar or a threaded stud. To affix the TwinDisk, a hole is drilled into the required panel, adhesive is applied, and the whole mechanism is pushed


The plate shear test set-up.


determine the relationships between shear and bending stresses during testing, and ultimately were used to determine core shear yield and panel failure.


The results The plate shear test gives an indication of dynamic performance. Some core materials failed due to brittle fracture when loaded, whilst others yielded only when they had elongated more than 50% of their original thickness. The cores that failed due to brittle fracture may achieve higher initial shear strength; the cores which elongated absorbed more energy. The results from the beam testing at


Standard specimen and beam tests were


performed using plate shear and four-point testing. Load-def lection slope, maximum and yield shear stress were compared for the different core materials and loading rates. To compare static uniformly loaded panels


to those panels subjected to slamming loads, a static test rig and a unique servo-hydraulic slam testing system were used. The surface strains at the panel centre and centre of the long edges, and the panel def lection at the centre, were recorded during the panel testing. The strains and def lections were used to


quasi-static and dynamic rates showed that medium and high elongation core materials such as cross-linked and linear foams had an increase in shear strength of 50%-60% over their static performance when tested at high strain rates. Low-elongation core materials, such as Balsa and Nomex, had little change in shear strength performance. The same phenomenon occurred when the


results for the static panels were compared to the dynamic panels. In other words, some foam cores increase in strength the faster you hit them – these core materials will absorb more energy without fracture and survive the tough conditions. Consequently they are more suited for the hull bottom panels of yachts.


into the cavity. The bottom disk glues to the bottom skin, and the top disk to the top skin, which can be rebated or surface mounted. Tensile, shear, and lateral loads are


efficiently transferred to both skins. Available in male and female versions, the male versions are supplied with adjustable top heads to fit depths from 5mm to over 100mm. Male and female threads are M4 to M16,


with a range of head diameters at 20mm, 30mm, and 40mm. Other sizes can also be supplied.


SHIP & BOAT INTERNATIONAL MAY/JUNE 2007


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