Section 1 Identification of critical areas
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Clasifications Register Guidance Information - Guidance Notes for the Classification of Special Service Craft – Version 1.0 Design Details, July 2013 - Chapter 5 Detail Design Improvement for Composite Construction - Section 1 Identification of critical areas

Section 1 Identification of critical areas

Parent topic: Chapter 5 Detail Design Improvement for Composite Construction

1.1 General

1.1.1 LR has applied direct calculation procedures in the structural appraisal and approval of new buildings and in various investigations on special service craft of composite construction. Through these procedures and the wealth of information collected on the LR fleet database, a number of locations have been identified where good design, workmanship and alignment during construction are particularly important. These are usually locations where high stress variations can be experienced during the lifetime of the craft. These are referred to as critical locations and are highlighted in this Chapter.

1.1.2 This Chapter identifies the critical areas within various structural elements of the hull structure and transverse bulkheads.

1.1.3 In Ch 5, 2 Structural details the structural detail design improvements that can be applied to increase the fatigue life of the structural components are provided. These detail improvements are intended to give the designer guidance for meeting the design criteria for structural detail components.

1.1.4 The application of 2 and 3-dimensional finite element analyses techniques to the hull structure enables the global and local capabilities of the hull structure to withstand static and dynamic loadings to be assessed. Such analyses will enable those high stress locations and joints within the craft to be readily identified. Such locations will then, by their very nature, be at risk to fatigue damage unless appropriate measures are taken at the design stage and subsequently during construction.

1.1.5 Extensive ‘in-service’ experience of the performance of existing craft structures, already provide an awareness of those critical locations which merit particular attention either due to stress or alignment difficulties.

1.2 Critical areas

1.2.1 Stress concentrations occur in both the primary and secondary structures of all craft and are identified during the design process by such means as finite element calculations. The designer will modify the detail to alleviate the stress concentration either by redesign or increase in scantlings. However, even after modification that area will still, in general, be exposed throughout the life of the craft to stresses higher than in surrounding areas.

1.2.2 At the design appraisal stage, a plan of the structure should, where appropriate, be prepared by the Builder or designer indicating these regions, and consideration can then be given, by the production team, into the appropriate methods of construction and the tolerances to be applied in order to remain within the assigned design parameters.

1.3 Misalignment during construction

1.3.1 The very nature of composite construction involves the manufacture of the material at the same time as the product and therefore, the alignment of the moulds and formers is one of the major considerations. The bonded interface between structural components in sub-assembly areas, prefabrication stages must also be carefully controlled to ensure accurate alignment and to achieve a satisfactory bond.

1.3.2 The most critical type of joint is the bonded ‘tee joint’ where it is subjected to high magnitudes of tensile and shear stresses. Particular attention must also be given to the transition between different types of stiffener members i.e. top-hat to plate laminates.

1.3.3 It can readily be seen that the combination of stress concentration and misalignment is to be avoided if the fatigue strength is to be satisfactory during the service life of the craft.

1.4 Fatigue considerations

1.4.1 The bottom shell area of high speed craft is subjected to the highest cyclic loading throughout the life of such craft.

1.4.2 The fatigue cracks in bottom shell laminates in way of internal hard spots, and in way of longitudinal end-connections, has been well documented. Constructional details in way of these areas, designed to increase fatigue life, are now incorporated by many Builders as standard. It is, therefore, important that due consideration be given to these details at the design stage to reduce the risk of fatigue cracking during service.

1.4.3 Detailed recommendations are detailed herein for the critical areas, see Ch 5, 2 Structural details.

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