Section 1 Introduction
Clasification Society 2024 - Version 9.40
Clasifications Register Guidance Information - Guidance Notes for the Classification of Special Service Craft – Version 1.0 Design Details, July 2013 - Chapter 1 General Introduction - Section 1 Introduction

Section 1 Introduction

Parent topic: Chapter 1 General Introduction

1.1 Purpose of the Guidance Notes

1.1.1 With the advent of novel and high speed craft types the need for good structural detail has never been more important. Most structural failures that occur in the service life of a craft result from fatigue. A considerable amount of published data exists for more conventional steel vessels, however, little or no such data exists for the novel or high speed craft constructed from aluminium alloy or composite materials. Lloyd’s Register (hereinafter referred to as LR) has amassed considerable knowledge in respect of structural detail which has demonstrated satisfactory service experience in resisting structural failure from fatigue. The details contained within these Guidance Notes are therefore provided to assist Builders and designers in identifying a minimum level of design detail which has provided acceptable levels of resistance to structural failure arising from fatigue.

1.1.2 It is not intended that the details contained within these Guidance Notes are the only solution to a particular structural design. Alternative structural details that have demonstrated satisfactory service experience will be acceptable.

1.2 General

1.2.1 Quality of detail design is fundamental if the craft's envisaged design life is to be attained. Experience over the past two decades, when structural concepts were revolutionised and aspects such as speed greatly increased, clearly indicated that areas of detail design became more prone to fatigue cracking with the traditional structural arrangements for details being used at that time.

1.2.2 Service experience has shown that provided the general concepts of a structural design are adequate then success or failure in structural terms will depend on the quality of detail design.

1.2.3 In general, the problems associated with aspects of detail design are those relating to fatigue ageing and failure.

1.2.4 Fatigue is failure under repeated loads. There are three phases in a fatigue fracture: crack initiation, crack propagation, and fracture. These phases are not completely separable. The process may be described as the formation of a crack, because of repeated local plasticity, its progression until a critical size is reached, where upon the structure fails. Fatigue accounts for a large percentage of all service failures.

1.2.5 A structural element can be subjected to various kinds of loading conditions, including fluctuating stress/strain, fluctuating temperature (thermal stress/strain), or any of these in a corrosive environment or at elevated temperatures. Most service failures occur as a result of tensile stresses.

1.2.6 Fatigue cracks generally initiate at high stress locations such as structural discontinuities, weld toes, matting in connections, etc. As these cracks propagate the ultimate load carrying capability of the structure is reduced until sufficient fatigue damage is accumulated for the structure to fail at normal working loads. Since fatigue cracks can be possible points of initiation for catastrophic failures or costly craft repairs, it is essential that fatigue is given more detailed consideration in the design of the structure.

1.2.7 Fatigue ageing of structural components is an accumulative process which is largely due to the environment and the loads experienced. An important realisation is that it is inevitable and where stress concentrations are present in association with significant magnitudes of stress variation then fatigue cracking will, in general, occur. Factors which influence performance, in that they affect the magnitude of stress ranges and provide stress concentrations, are as follows:

  1. The loading experienced.

  2. The quality of detail design.

  3. The selecting of the type and grade of material.

  4. The standard of workmanship in the craft construction.

  5. Corrosion rates and magnitudes (metallic structures).

  6. Erosion rates and magnitudes (composite structures).

1.2.8 Since the fatigue properties of higher tensile strength metallic materials are, in general, similar to those of the basic grade materials, the higher allowable stress magnitudes could entail a shorter fatigue life in standard details. Assuming that the fatigue life is a function of the stress range to the third power, it is clear that detail design requires special consideration to reduce the effects of stress concentrations. If higher tensile strength materials are incorporated and hence higher stress levels are accepted, then structural details, which would have been acceptable in mild steel structure manufactured from the basic grade material, might not be adequate.

1.2.9 The occurrence of cracking in craft is of prime concern from both a safety and maintenance point of view. Experience has shown that fatigue cracks in craft structures are normally of a self limiting nature. However, the existence of fatigue cracking may, if not repaired, render the structure susceptible to subsequent brittle or fast fracture. Thus both types of cracks are significant from a maintenance point of view. Fatigue cracks, if not repaired, may also initiate catastrophic failure as a consequence of the more extensive use of structural optimisation leading to a decrease in the level of structural redundancy.

Copyright 2022 Clasifications Register Group Limited, International Maritime Organization, International Labour Organization or Maritime and Coastguard Agency. All rights reserved. Clasifications Register Group Limited, its affiliates and subsidiaries and their respective officers, employees or agents are, individually and collectively, referred to in this clause as 'Clasifications Register'. Clasifications Register assumes no responsibility and shall not be liable to any person for any loss, damage or expense caused by reliance on the information or advice in this document or howsoever provided, unless that person has signed a contract with the relevant Clasifications Register entity for the provision of this information or advice and in that case any responsibility or liability is exclusively on the terms and conditions set out in that contract.