JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010 - 57

of up to 30-35 mm are either the perforation of the uncured top skin at regular intervals using specially-made spike guns, or the application of a thin adhesive skin with the purpose of sealing the honeycomb cells completely before application of the full skin. In Trimarine’s experience, neither method is satisfactory as both require extreme attention to detail and extra controls to ensure a completely successful result. As the tendency to use these core materials in larger and thicker hulls will increase in future, there is a need to review the techniques used as hull core thickness grows to 45-50 mm. The moisture vapour volume that needs to be evacuated from such large surfaces as the curing temperature increases requires that the laminates stay porous until the resin starts gelling. This can only be addressed with newer materials and modified processes. Such issues should not exist in production as there is less likeliness of using paper honeycomb cores or unidirectional prepregs. However, it is always advisable to cut and shape Nomex® honeycomb in a conditioned environment to reduce the moisture intake and always wear protective gloves to avoid contamination of the cell edges. correct flow of information between the design office and the shop floor. We say ‘between’ and not ‘from’ because the information flows both ways as the technical office requires feedback from the components construction. Weight controls Weight control is of utmost importance in one-off design. Quite often, the ballast configuration of a keel is not finalised until a large part of the structure, if not all of it, is completed. This allows designers to achieve exact results for displacement, draft and CG positioning. The methods employed can vary but, to be certain of an accurate result before it is too late to make any meaningful adjustments, it is critical to weigh everything that is incorporated into the structure at regular intervals. This is easier said than done as it requires a shop floor discipline that often goes against the local cultural current. In practice, Trimarine has found it worthwhile to have a responsible party from the design office that shares the burden of recording all weights with the workforce. This tends to alleviate the bureaucratic work usually unpopular with the shop floor. At the same time, it serves to permeate the workforce with the need to accurately report every relevant occurrence to the project managers. Planning and workforce organisation The importance of detailed and realistic planning is fundamental in the construction of one-off structures to organise the workflow and to always have a precise notion of the deadlines and time limits. In series production, having a detail process plan kept up-to-date and complete is perhaps even more important as it is the only real way to keep timing and cost under control. NDT controls Flaw detection in composites has become a necessary diagnostic and control system to ensure that any possible defects are eliminated at as an early stage of the process as possible. Portable equipment such as the Panametrics Epoch IV with B-Scan or the Epoch-III (Pulse-Echo, A-Scan) ultrasonic detectors are commonly used for such purposes. Voids, delamination, cracks, bubbles and porosity can be identified with ultrasonic detectors. Being a rapid and easy-to-use process, it must nevertheless be operated by an experienced technician if the results are to be trusted. It is particularly critical when scanning sandwich panels where probing must be done on both sides of the component, as it can sometimes fail to detect a disbond if the layers are in tight contact. Under such circumstances, there are other methods like laser shearography, a method based on holographic interferometry using vacuum or heat to stimulate the component surface being probed. Defects become apparent by analysing their nonhomogeneous deformation gradients. Both these procedures have their disadvantages in industrial testing and analysis, including speed of analysis and the level of user skill required. Thermal wave imaging technology is an emerging method that addresses both these issues, putting a fast and powerful NDT tool in the hands of less skilled operators. The system works by using impulse thermography. Heat is applied to the sample using a xenon flash lamp and the dissipation of heat is then tracked using a high-speed thermal imaging camera. A good Assembling sequence and assembling jigs In the construction of advanced one-off structures for the marine industry, many hours are occupied by control procedures both before and after assembly. This is an intrinsic cost to the process of one-off construction; the more complex the class rules, the more controls are needed. In series production, however, it is important to simplify the control procedures and hence greater attention and effort is put in the study and construction of positioning, alignment and assembly systems that have to be quick and easy to use by the operator. Having said that, it is probably a better choice to have an assembly control system that sacrifices a little precision but that can be used by a less trained operator, rather than a very precise and complex system that induces mistakes. Construction procedures Process procedures are common workshop elements in series production. It is more difficult to find a one-off boat yard that uses written process procedures as it is generally considered a waste of time to prepare what is in effect a large body of work only to produce one unit. Trimarine finds that written process procedures are, on the contrary, fundamental to the transfer of a No55 February - March 2010 / jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #55 - February/March 2010