i
Other Reeds Professional titles Reeds Sea Transport 6th edition by Patrick M Alderton ISBN 9781408131428 This book gives a complete picture of the Maritime Transport Industry. Now in its sixth edition, it includes new data and statistics, new advice on safety, a review of ship types including the growth in tonnage and the increase in container ship sizes, as well as the effect of the ‘depression’ over recent years, all of which make essential reading for professionals as well as students on courses concerned with Shipping Ports and Transport. Reeds Marine Distance Tables 15th edition by Miranda Delmar-Morgan ISBN 9781472948977 These tables, which give worldwide coverage, are particularly useful to ship owners and brokers for voyage estimating. Using this one reference, anyone can calculate the shortest or most economical distances between all the major ports in the world. Reeds Maritime Meteorology 4th edition by Maurice Cornish and Elaine Ives ISBN 9781472964151 This book is written primarily for serving and trainee deck officers and those studying for certificates of competency in merchant ships. It also forms an invaluable reference for other professional seafarers, for fishermen and yachtsmen. It provides descriptions of the elements and forces which contribute to maritime meteorology and the principles which govern them. Reeds Marine Engineering Series Volumes 1–15 Fifteen volumes on syllabus subjects for the Department of Transport Marine Engineers’ Certificates of Competence including worked examples and typical exam questions. Visit www.adlardcoles.com for more details.
REEDS Bloomsbury Publishing Plc 50 Bedford Square, London, WC1B 3DP, UK BLOOMSBURY, REEDS, and the Reeds logo are trademarks of Bloomsbury Publishing Plc First published in Great Britain by Waterline Books, 1998 Second edition published 2007 by Adlard Coles Nautical This third edition published 2019 by Reeds Copyright Š Thomas Ask, 1998, 2007 Copyright Š Thomas Ask and Valerio De Rossi, 2019 Thomas Ask and Valerio De Rossi have asserted their right under the Copyright, Designs and Patents Act, 1988, to be identified as Authors of this work All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage or retrieval system, without prior permission in writing from the publishers Bloomsbury Publishing Plc does not have any control over, or responsibility for, any third-party websites referred to or in this book. All internet addresses given in this book were correct at the time of going to press. The author and publisher regret any inconvenience caused if addresses have changed or sites have ceased to exist, but can accept no responsibility for any such changes A catalogue record for this book is available from the British Library Library of Congress Cataloguing-in-Publication data has been applied for. ISBN: PB: 978-1-4729-6012-2; eBook: 978-1-4729-6011-5 Typeset in Myriad Pro by Deanta Global Publishing Services, Chennai, India To find out more about our authors and books visit www.bloomsbury.com and sign up for our newsletters
CONTENTS Preface to the third edition vii Acknowledgements viii Introduction ix
PRIMERS 1 1 Vessel loads 1 2 Material mechanics 6 3 Stress concentration 16 4 Fatigue 19 5 Buckling 22 6 Corrosion 25 7 Failure modes and analysis 35 8 Wood 41 9 Plastics 48 10 Metals 51 11 Composite materials 60 12 Fluid mechanics 68 13 Combustion and fuels 77 14 Hydraulics and pneumatics 80 15 Non-destructive testing 85
DESIGN AND APPLICATION GUIDES 16 17 18 19 20 21 22 23
100
Hulls 100 Propellers 119 Fasteners 127 Engine systems 132 Noise and vibration 143 Heating, ventilation, air conditioning and refrigeration systems 147 Firefighting 154 Mechanical systems 161
TECHNIQUES 175 24 Survey techniques 25 Survey format 26 Survey notes • Inspecting materials • Through-hulls, sea cocks and valves
175 184 190 190 207
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Sacrificial anodes Hull-to-deck and other FRP connections Mounted equipment and fasteners Grab rails, toerails and stanchions Drainage Hatches and windows Pumps Steering systems Propellers Shafts and drives Rudders Engine installation Fuel systems Ground tackle Potable water and sanitation Gas cooking system (stove and oven) Heating, ventilation, air conditioning and refrigeration (HVACR) Electrical systems Navigation equipment Communication and radio equipment Safety equipment Crew safety Keels Mast and rigging Sails and canvases
CLASSIFICATION SURVEYING 27 28 29 30 31
Overview of classification surveying Sea trials Classification societies and IMO conventions Risk Assessment and safety management Marine surveyor liability
211 213 214 221 222 223 224 225 226 228 230 231 236 237 238 240 240 244 246 246 247 248 251 253 256
257 257 283 291 296 305
APPENDIX 311 Survey checklist Index
311 315
Preface to the third edition I wrote the first edition of this book nearly 25 years ago. I wished to write a book that was founded on engineering principles rather than anecdotes. In the marine surveying profession, what you learn under your fingernails is more important than what you read in a book. However, for those who strive to excel in their profession, they can gain deep, textured insights by studying engineering theory and experientially rooted techniques. Recognising that a careful eye and practical experience are the distinguishing capabilities of an expert marine surveyor, this writing is intended to come alongside those practical abilities. It is also intended as a humble adjunct to published standards and testing methods. In this third edition, I am pleased to invite the participation of Valerio De Rossi, MSc, D.Prof, CMarTech, FIMarEST, AFNI, who has decades of experience as a ship’s master and marine manager. He wrote the Classification Surveying section and added to the Firefighting chapter. I am pleased that we can share his expertise and passion for marine safety in this new edition. TA
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Acknowledgements I remember receiving my thick three-ring binder containing American Boat and Yacht Council (ABYC) Standards many decades ago. I was impressed by the care and thoughtfulness of this document. Over the years, I have enjoyed membership of various professional maritime organisations and have had helpful visits with classification and testing organisations. While industry standards change with time, the foundations described in this book are only helpful when they point to these standards. It is for these reasons, I wish to acknowledge and thank all the organisations dedicated to maritime safety for their contributions to this book. I also wish to thank my co-author, Valerio De Rossi, for agreeing to expand this book into classification surveying. I met Valerio during our graduate studies and was impressed by his intellect and expertise. Many thanks to the editors at Bloomsbury Publishing for their care with this project and their commitment to excellence. It is a great delight to entrust this writing to gifted editors. I wish to specifically thank Senior Editor, Clara Jump, for her energetic and professional management of the project. I also wish to thank Jenny Clark, Commissioning Editor, for her enthusiastic shepherding of the book at the early stages. Writing is a solitary endeavor and I would like to thank my family for their patience and support of my late-night forays. My children, Eric and Elayna, were not yet born when I started the first edition. Now they are grown up, out of the house and have probably heard too much about the maritime world. My dear wife, Beth, has always been patient with my musings and to her I am foremost and always most thankful. Thomas Ask I must start by thanking my co-author, Thomas Ask, for inviting me to be part of the 3rd edition of this book. It has been a real pleasure working with him, and I hope that my contribution will add value to this project. I would also like to thank my wife, Federica, and my daughter, Domiziana, for being patient with me during my writing, often late at night and in the weekend, and for their advice on the cover of the book. Lastly, I wish to join Thomas in thanking the team at Bloomsbury Publishing. The support provided by the team was second to none; from keeping us on track with the schedule, designing the cover to helping with editing and proofreading. For all of this, my sincere thank you. Valerio De Rossi
viii
Introduction Our desire to walk off the land and slip on to the water has thrust boats into a cherished category of mechanical things. Boats have allowed us to harvest the seas, explore new lands, escape from oppression and find serenity. We rely on boats to take us into environments where we cannot survive alone, therefore they draw from us an intense trust. This utter dependence requires us to appreciate their design and integrity. A marine survey will ensure a boat or ship is in good condition and will confirm that the vessel’s inventory is sufficient to handle emergencies. The aim of this book is to provide a solid foundation of knowledge upon which experience and skill can be developed. It is much more important to understand likely causes for troubles and common areas from which they originate than to list frequent problems experienced on a given vessel design. For example, among all the words on theory and technique, the reader will find that most problems relate to fatigue, corrosion (along with its ‘wooden’ cousin, rot) and overloading at stress concentrations. The marine surveyor has two principal responsibilities to the client. They are: 1. evaluation of the structural integrity of the vessel 2. evaluation of the safety equipment This is a very short description, but these two key points must always be kept in mind. Structural integrity is perhaps a vague description, but it includes all the items that keep a vessel safely afloat – such as the hull and deck materials, ribs, bulkheads/web frames, stringers/longitudinals, engine bed, chain plates, mast steps, standing rigging and all fasteners that keep them connected. Safety equipment is dictated by governmental or classifications statute and also offered as a recommendation by professional organisations dedicated to marine safety. Upon identification of problems, marine surveyors must decide whether the problem is serious, and then consider the best course of repair so that the cracks in the corner of the fibreglass settee do not rise in importance to the level of separated bulkheads or a loose keel. The survey requirements for classed ships are explicitly stated in classification societies’ documentation. A variety of surveys are required to retain class. These include the annual survey with its examination of hull, machinery, firefighting equipment, inert gas systems and any additional equipment covered by the
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class notation. Additional surveys include investigations of the hull bottom, propeller shafts, boiler and special surveys. Special surveys can include an evaluation of a wide range of features, including thickness measurements, tank inspections, hatch covers, holds, integral and independent tanks, decks, deck erections such as hatch coamings, deckhouses and superstructures. Special surveys may also require the examination of steam turbines, steam engines, internal combustion engines, and a variety of mechanical and electrical equipment. The survey requirements are usually a function of the ship’s age, service and classifications. For survey requirements, reference must be made to appropriate classification society documents and vessel classification certificate. Marine surveying is not a glamorous profession. Headfirst contortions into malodorous bilges and the inadvertent burying of hands into piles of decaying rodents can be part of the business. However, a love of boats, a desire to ensure the safety of their passengers and crew, along with personal integrity, are sufficient ingredients to motivate the surveyor into the unpleasant, nevervisited corners of a vessel. Some areas will accurately be listed as ‘inaccessible for inspection’, but this is an admission of defeat for the surveyor that should rarely be necessary. However, even the most careful surveyor cannot always identify and prevent problems related to fatigue, crevice corrosion, metallurgical problems and inaccessible areas, and the burden of addressing these types of problem lies fully with the naval architect or engineer. The designer must provide preventative maintenance scheduling for all critical components, and this scheduling can be based on operating environments (eg salt water versus fresh water, pleasure versus workboats, etc), but it is impossible for a surveyor or owner to always know when such things as the keel bolts, standing rigging, hull-to-deck fasteners and bedding need to be replaced. The replacement schedules for these items are based on engineering analysis and experimentation, neither of which are the responsibility of the marine surveyor. The International Association of Classification Societies (IACS) recognises the limitation of class surveyors as follows: ‘It must also be emphasized that a class surveyor may only go on board a vessel once in a twelve month period for the annual survey. At that time it is neither possible, nor expected that the surveyor scrutinize the entire structure of the vessel or its machinery. The survey involves a sampling, for which guidelines exist based upon empirical experience which may indicate those parts of the vessel or its machinery that may be subject to corrosion, or are exposed to the highest incidence of stress, or may be likely to exhibit signs of fatigue or damage.’ (Source: The International Association of Classification Societies)
Introduction
xi
But just as providing poor preventative maintenance specifications is a denial of responsibility for the naval architect, it is equally wrong for the marine surveyor to draw no conclusions about the vessel’s condition. For example, the survey can register a long list of cracked features in the deck, but is the deck seaworthy? Are the repairs of these cracks required, recommended or cosmetic? That is, the surveyor must come to conclusions on behalf of the client. Deep cracks in the coamings and some gel coat cracks in the hull do not mean the vessel is a dangerous wreck. In the same way, a gleaming sailing yacht with a poorly supported, misaligned keel is a disaster waiting to happen.
PRIMERS
1
VESSEL LOADS
Understanding the loads that a vessel encounters is critical in gaining insight into potential structural problems. Vessel loading is carefully considered during the design process, but the marine surveyor also needs to have a thorough comprehension of the topic. A hull structural panel consists of plating over transverse and longitudinal supports, and a variety of terms are used to describe structural elements. In this book, the fore/aft elements will be referred to as longitudinals (eg stringers and girders). Athwartship elements that run transversely to the longitudinals will be referred to as transverses. Traditionally, hulls use a transverse framing system with a longitudinal keel, closely spaced transverse ribs or frames, a series of longitudinals and plating. This transverse framing system is the most common framing system for small vessels. In this set-up, the keel, stem and stern are assembled first, and then a series of curved ribs are attached to the keel. The ribs are then tied together by longitudinals that curve to the shape of the hull, and separate beams attached to the top of the ribs are used to produce the strong box-like frame of the hull. These beams also support the deck. Athwartship bulkheads are added for additional strength. On larger craft, these bulkheads provide watertight compartments with gasketed and watertight doors for reserve damage buoyancy. Bulkheads also impede the spread of fire and smoke. Larger craft will often have longitudinal bulkheads as well. Longitudinal framing is used on larger ships that require higher hull strength. This framing technique is the opposite of transverse framing in that the longitudinal elements are closely spaced while the transverse, deep web frames are widely spaced. However, these massive deep web frames interrupt cargo space and therefore a combination of techniques is sometimes used to provide large cargo areas. Small craft will have unsupported decks. The decks rely upon the strength of the cored deck and an occasional bulkhead for strength and rigidity. The
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The steel used in this ship became brittle at cold temperatures.
hull of FRP (fibre reinforced plastic) craft often includes an integrated structural support system that provides hull strength and rigidity. Vessels experience several constant baseline loads: • • • •
live loads: weight of personnel and equipment dead loads: weight of vessel equipment loads: large equipment, cranes and masts tank loads: weight and hydrostatic pressure of liquids
In addition to these loads, the forces arising from sea conditions need to be considered and are generally more significant than the constant baseline loads presented previously. Sea loads can be more difficult to quantify because they are a function of wave action. These loads include wave slamming and slapping, bow submergence, passing waves and heeling. The hull’s longitudinal girder loading under various sea conditions is considered important on large vessels. Longitudinal loading is produced by the buoyant and gravitational forces upon the hull. The loading conditions must be considered in still water as well as in hogging and sagging waves (see Figs 1 and 2). Hogging and sagging waves can produce high flexural loading upon the longitudinal supports. A vessel encounters other environmental loading in the forms of wind, ice and snow. In addition to providing additional loading, all of these conditions
VESSEL LOADS
3
Fig 1  Hogging wave: excess weight at bow and stern; excess buoyancy amidships.
Fig 2  Sagging wave: excess weight amidships; excess buoyancy at bow and stern.
This photo shows an example of hogging (Source: Eirik Hustvedt).
can decrease stability by either their heeling force or additional topside weight. Structural design for small craft is dictated by the classification societies, ISO Standards (ISO12215), engineering calculations, testing or data derived from service history of similar craft. In addition to structural loads on the hull, careful consideration must be given to predicting the other loads that are carried by every part of the vessel. For example, how strong should a filter bracket mounted on the engine pan rail be? After evaluating the acceleration generated by engine vibrations and the weight of the full filters, the answer is: the filter must be strong enough that a heavy mechanic can stand on it while servicing the turbocharger on
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Reeds Marine Surveying
Submerged Bow
Waterline
Fig 3 Bow flare impact load.
An example of bow impact (Source: NOAA).
top of the engine. How strong should a fire extinguisher bracket adjacent to the companionway be? Strong enough to support one end of a wet clothes line. The designer and the surveyor should expect all of these ‘unexpected’ applications.
VESSEL LOADS
Wave Height
Design Waterline
Passing Wave
Design Waterline
Heeling
Fig 4  This figure demonstrates the relationship between design waterline, heeling and the effect of a passing wave.
Heeling caused by high winds and light ballast. This vessel is turning back to its protected harbour (Source: Eirik Hustvedt).
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2
MATERIAL MECHANICS
Understanding the nature of solid materials is critical when conducting a marine survey, and no checklist or rules of thumb replace an attentive eye guided by intelligent insight. Although technologies such as radiography and ultrasound are helpful in evaluating suspect areas, they still require wise application and discernment. When a force is applied to an object it will either accelerate to some sort of equilibrium speed or, if restrained, the object will move internally by compressing (or stretching). The way an object moves internally is the key to understanding a material’s behaviour under load. Every type of material has a unique way of stretching or compressing. The relationship between the force and the movement of the material defines how it can be used. A 10lb (4.5kg) weight hung on a rubber cord would stretch the cord a long way. If the same 10lb (4.5kg) weight were hung on a 1in (2.5cm)-diameter steel rod, there would be no noticeable stretch (even though it actually stretches 0.0000004in per in [0.00001mm per mm] length of the steel more on this later). Neither of these materials would break, but it is obvious which one should be used to support an overhead hoist. Although a force can only produce tension, compression or shear, it is convenient to think of loads being applied to an object in five ways: tension, compression, shear, torsion and flexure (see Fig 5). Tension is simply a force that pulls apart an object’s molecules, while compression is the opposite of tension: the squeezing of an object’s molecules. Most materials have a higher compressive strength than tensile strength. Shear is a set of opposing forces in the same plane. For example, if the bottom of a plate is rigidly attached to a surface and then the top of the plate is pushed parallel to the surface, a shear force will be applied to the plate. The shear force occurs when the force at the bottom of the plate resists the force on top of the plate. These opposing forces are transmitted as a shear force in the plate. If the plate were made from a stack of papers, the shear force would readily allow the papers to slide across one another. The ability to bear a shearing stress is the feature that defines solids.
6
MATERIAL MECHANICS
7
Solids resist shearing forces, while fluids (gases and liquids) will flow until the shearing stresses are gone. Torsion is a twisting force. For example, a rod that is held rigid at one end while a rotating force is applied at the other end will experience a torsional force. Flexural loading is a convenient way to describe the unique loads produced on a flexed object, such as a hogging hull. If a bar is supported at both ends and a load is applied in the centre, the material on the same side of the centreline as the load will be in compression, whereas the material on the opposite side of the load will be in tension. In addition, the stress varies with the distance from the centreline. The outer surface has the highest stress and the centreline will have zero stress.
Tension Pulls molecules apart
Torsion
Compression Squeezes molecules together
Shear Slides molecules over each other
Flexure
Fig 5  Loading conditions.
In addition to the reaction of a material to a load, another important characteristic of a material is its behaviour as it is about to break. Some materials are brittle and hardly stretch before they break. Glass is a good example of a brittle material. Other materials, such as wood, are ductile and stretch noticeably before they break. This ductility characteristic determines the existence of common warning signs (bending or stretching) prior to imminent failure.
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A force’s effect on a material is dependent on the size of the area upon which the force is applied. The force per area is called stress and is simply equal to force divided by area. s = F/A Where, s = stress F = force A = area Strain is the same as stretch or compression, but refers specifically to the movement per unit length. Stress-strain diagrams have been developed for most materials by taking a sample of the material and applying force while monitoring its length. The force is continually increased until the sample breaks (see Fig 6). Most materials in boat/ship construction are linearly elastic. That is, over a certain range of stresses the strain is directly proportional to the stress (Hooke’s Law). Moreover, when the stress is removed, the materials return to their original unstressed dimensions. The slope of the stress-strain line is called the modulus of elasticity (or Young’s modulus) and is equal to the difference in stress divided by the difference in strain (see Table 1). This number indicates the rigidity of a material. E = s /e Where, E = modulus of elasticity s = stress e = strain What happens to a material as the stress is continually increased? The yield strength or elastic limit is reached, beyond which the material continues to stretch but will not completely recover if the stress is removed. This permanent stretching (called yielding or plastic strain) at stresses above the elastic limit occurs in what is called the plastic region. As the stress is further increased, the material finally breaks at a load called the ultimate strength. Where does the material for the stretching come from? In the elastic region, the stretching comes from the increased distance between atoms. In the plastic region, groups of atoms (crystals or grains) slip or deform to allow the stretch. As the material strains in one axis, it also strains in the perpendicular axes. This intuitive relationship is denoted by the ratio of strains called the Poisson’s ratio.
MATERIAL MECHANICS
9
Table 1 Moduli of elasticity Material
Tensile at 70°F (21°C) x 1,000,000psi
GPa
Steel
30
206
Stainless steel
30
206
Brass/bronze
16
110
Aluminium
10
69
Grey cast iron
20
138
Monel®
26
180
Titanium
16.5
114
Magnesium
6.5
45
Osmium
80
552
Nylon
0.4
2.8
Polyethylene
0.014 to 0.18
0.096 to 1.24
Rubber
0.0006 to 0.50
0.004 to 3.4
Plate glass
10
69
Wood
1.5
4.6
Concrete
3
9.2
Consider some of these practical implications of stress and strain:
•
•
•
Nearly all product designs are based on the yield strength. That is, all considerations of safety factors, fatigue strength and impact loading are based on a value that will allow the material to recover completely from the applied stress. Notable exceptions are armour and crash cushions where the energy of the impact is absorbed by taking the material up to or beyond its ultimate strength. The modulus of elasticity determines the flexibility of a material and is independent of its strength. However, an object’s shape is the most important determinant of its flexibility. For example, a thin stick blows readily in the wind whereas a thick trunk made of the same wood does not move at all. The relationship between shape and rigidity is presented in more detail later. The area under the stress-strain curve indicates a material’s toughness or energy absorption ability. The larger the area, the more energy the material can absorb before it breaks.
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Reeds Marine Surveying Unlike metals, typical composites do not have the same stress-strain characteristics in all axes (known as anisotropic), therefore the modulus of elasticity and yield strength will be different depending on orientation. If a load is pulling in the same direction as the fibre orientation, it will be much stiffer and stronger than if it is pulling perpendicular to the fibre orientation. With a perpendicular load, the fibre strength is unused and only the resin offers resistance to the load. Metals are isotropic materials and have the same stress-strain relationships in tension, compression and shear.
A material’s hardness is another characteristic that has practical importance in determining a part’s strength and wear characteristics. Hardness is defined as the resistance to penetration and is directly related to the material strength. Metal hardness is usually changed by various heat-treating processes that modify the grain structure. Yield Strength Ultimate Strength Fracture Stress
Elastic Region
Strain
Plastic Region
Yield Strength, Ultimate Strength, Fracture
Stress
Modulus of Elasticity
Strain
Fig 6 Stress-strain diagrams for ductile (top) and brittle materials (below).
MATERIAL MECHANICS
11
Thermal expansion Thermal expansion is linearly related to the temperature and is caused by an increase in the vibratory movement of atoms. The amount of expansion varies by material and is measured by the thermal expansion coefficient. Brass has a thermal expansion that is 50 per cent more than steel and aluminium is twice that of steel (shown in Table 2).
Table 2 Coefficients of thermal expansion at 70ºF (21ºC) Expansion coefficient in/in/°F
mm/mm/°C
Steel
0.000006
0.000011
Stainless steel
0.000009
0.000017
Brass/bronze
0.000009
0.000017
Aluminium
0.000012
0.000022
Cast iron
0.000005
0.000009
Monel®
0.000008
0.000014
Magnesium
0.000014
0.000025
Graphite
0.000003
0.000005
high density
0.000066
0.000119
Polyvinyl chloride*
0.000031
0.000056
Polycarbonate*
0.000040
0.000072
Nylon*
0.000056
0.000101
Plate glass
0.000005
0.000009
Material
Polyethylene*
* Varies widely based on specific composition.
If two materials expand into each other, either because they have different thermal expansion coefficients or they are at widely different temperatures, they develop stresses directly related to the strain they produce. That is, the strain produces stress rather than the other way round. If a column or pipe is anchored at both ends and therefore axially constrained, the compressive stress generated by thermal expansion becomes high and is simply calculated by:
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Alternatively, one can consider the thermal expansion or contraction as described by: L2 = L1 a ΔT Where, L1 = initial length L2 = change in length after heating or cooling a = coefficient of thermal expansion ΔT = temperature change Thermal expansion exerts tremendous stresses and therefore the routing of anything that gets hot, such as the engine exhaust, needs to be closely inspected. Expansive thermal forces produce high stresses when two materials with dissimilar thermal expansions are entrapped. However, the effect of linear expansion on the exhaust system is usually the most important consideration on a vessel. Thermal expansion can tear out pipe anchors and turbochargers. These powerful expansive forces are accommodated by including bends in the piping or flexible joints.
Relationship between shape and stiffness Strength and stiffness come from two different features. Strength is a property of a material, while stiffness is a property of the shape of the material. A piece of sheet metal is strong enough to withstand a high level of stress before breaking, yet it easily flexes and buckles. To stiffen the assembly, the pieces of sheet metal could be layered upon one another to produce a thicker assembly. This thick assembly of sheets of metal would be rigid and strong, but very heavy and expensive. Marine vessels require more sophisticated solutions than thick plates. In metal construction, the hull and deck plates are stiffened with longitudinal (stringers/longitudinals) and transverse structures (ribs/bulkheads/web frames). In FRP (fibre reinforced plastics) construction, a sandwich assembly combines a thick layer of low-density material (eg balsa wood, foam or metal honeycomb
MATERIAL MECHANICS
13
material) bonded between an inner and outer FRP laminate. The resulting sandwich composite is lighter, less costly and nearly as strong and as rigid as an equally thick laminate (see Fig 7). Load Thin section gives higher maximum stress.
Load
Tensile stress on top. Stress increases with increased distance from neutral axis.
Neutral Axis (no stress) Thick section gives lower maximum stress.
Plates loaded in exure.
Compressive stress on bottom. Stress increases with increased distance from neutral axis. Cross-section of plate showing distribution of stresses when loaded in exure.
Fig 7 Relationship between stress and thickness.
Consider one everyday example of sandwich construction: corrugated cardboard. Corrugated cardboard is made entirely of paper with a wavy (corrugated) section of paper sandwiched between the outer layers of flat cardboard. The corrugated cardboard is made rigid because the light, weak corrugations locate the flat cardboard, which does not stretch easily, away from the neutral axis. Imagine how floppy the cardboard would be if the corrugated section was flattened out and all the paper layers were laid on top of one another. Even if the flattened paper were replaced by steel, it would still not be as rigid as the corrugated form. This increased rigidity and strength is only apparent when the composite is bent. In pure tension it has virtually no effect – that is, the flattened paper would be as strong as the corrugated paper if it were pulled at the ends. These relationships between rigidity and shape are quantified by the moment of inertia. Moment of inertia is specifically related to the geometry of the cross-section (referred to in this context as the ‘section’) under load. In the case of flexural loading (a load that produces only a moment, not an axial or torsional force), the stress produced in the part is inversely proportional to the moment of inertia:
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Reeds Marine Surveying s = Mc/I Where, s = flexural stress M = moment on section equal to the force times distance c=d istance from neutral axis. Usually the only value of concern is the maximum c value, which is half the thickness of the section I = moment of inertia
The higher the moment of inertia, the lower the stress produced under a given load. If a part is designed so that the material is far away from its centre, or neutral axis, it will have a higher moment of inertia than if all the material is bunched up around the centre. In fact, the moment of inertia increases as a cubic function of the distance of the material from the neutral axis. For a rectangular cross-section, the formula for I is: I = bh3/12 Where, b = width of section h = thickness (or depth) of section For a solid rod cross-section, the formula for I is: I = p r4/4 Where, r = radius of rod A tube has a much higher moment of inertia than a solid rod of the same mass because the mass of the tube is located further from the centre. Unlike a tube, an I-beam has most of its material oriented along only one plane. Therefore, it has to be oriented correctly with respect to the intended load to take advantage of its shape (see Fig 8). Section modulus is often used to describe the properties of a structural shape. Section modulus is equal to the moment of inertia divided by the distance from the centre of the shape to its furthest edge (c), such as the radius or half the height (h) in the case of a rectangular shape: Section modulus (S) = I/c
MATERIAL MECHANICS
1.00in
0.88in
25.4mm 0.80in
22.3mm
20.3mm
Tube
Rod
15
Neutral Axis Rectangular beam
I-beam
Fig 8  Moment of inertia of different shapes.
Flexural loading is the most common loading for most structures, such as the hull and deck. Very few items are under pure tension, compression or shear. Some items (such as bolts) are designed to be under pure tension and shear only; however, these are the exception. Consequently, understanding flexural behaviour is essential for predicting potential failure mechanisms. Flexural behaviour does not consider another phenomenon that will make parts fail at stress well below their yield stress. This phenomenon is called buckling and is a common failure mechanism for long, slender objects. It is presented in the chapter on Buckling, and must be considered by marine surveyors. Buckling behaviour, a well-understood phenomenon, is carefully considered in classed-vessel design calculations.
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STRESS CONCENTRATION
All fractures start from sharp notches, and these notches can be large cracks or microscopic fissures. When assessing a vessel, a marine surveyor’s focus should immediately be drawn to sharp corners and crevices. Crevices are a frequent site for corrosion while cracks typically originate at sharp corners. A sudden change in an object’s dimensions – such as a sharp corner, notch, hole or crack – will result in an increase in stress called a stress concentration. The mathematical analyses of these stress concentrations are so complex that their amplifying values are obtained experimentally. The graph in Fig 9 relates an object’s shape to the corresponding stress concentration. The relationship between shape and stress concentration illustrates the importance of gradually blending mating surfaces. Internal stress concentrations developed in the voids and cracks of normally produced materials inspired the development of materials fabricated from tiny particles to be so much stronger than the normal, parent form. Metals and ceramics crushed into particles of less than four millionths of an inch (two hundred thousandths of a millimetre) in diameter are being used to fabricate very strong objects. When the ‘nanopowders’ are compacted they can be as much as ten times stronger than their parent material.
Stress concentration factors When assessing a vessel, the marine surveyor needs to have an understanding of how loads are distributed. With this feel for load distribution, special attention then needs to be paid to sources of stress concentrations that could be exposed to heavy or cyclic loads. Understanding where stress concentrations are produced is important in understanding where failures are most likely to originate.
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STRESS CONCENTRATION
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r/d = 0.05 r/d = 0.1 r
r/d = 0.2
K
r/d = 0.5 r/d = 1.0
D
h
0.29in
h/r
Fig 9  Stress concentration factors (K) for bending of flat bar with geometry portrayed above.
When a crack is formed, the ends of the crack produce a stress concentration. It is accepted practice to drill small holes at each end of a crack to reduce the stress concentration in this area and to stop the crack’s progress. This drilling is done before repairs are made. In some applications, small metal cracks in non-critical areas are treated in this manner and no additional repairs are necessary.
Closely spaced through-hull fittings and their associated stress concentrations weaken this transom.
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Ductile materials are less sensitive to stress concentrations than brittle materials. As a ductile material is loaded, it can deform to redistribute the load over a broader area. Even though the brittle form of a material can often handle higher stresses, a ductile material can distribute stress – and therefore actually handle a higher load.
These naturally developed wood sections used in boatbuilding have large fillets and provide reduced stress concentrations.
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FATIGUE
As a material is heavily loaded and unloaded, its maximum strength decreases. This gradual weakening is called fatigue, a name derived from an early misunderstanding of this phenomenon. The metal was thought to have become ‘tired’ over time. It is now understood that fatigue is caused by tiny movements between grains that cause cracks to propagate through the grain boundaries. One solution to fatigue is to over-design an object – that is, use massive structures and plates to ensure longevity. However, understanding the rate of decline of a material’s strength is a more environmentally and costsensitive solution. Generally, if the stresses experienced by an object under cyclic loading are less than one-half of the yield stress, the object will never fail due to fatigue. Therefore, highly efficient designs are produced only when the anticipated loading and fatigue characteristics are fully understood. When watching the constant twisting of hull and rigging, one can only wonder, ‘How long can it take that?’ The onus for determining the fatigue life lies with the designer, who can consider test data, acoustic emissions and other advanced techniques that are unavailable to the surveyor in evaluating the initiation of fatigue. However, there are some visual clues that may help diagnose imminent failure. Fatigue failures often start at sources of stress concentration or at the surface of an object where the stresses are usually highest. Small surface cracks may be the first sign of fatigue – then again, there may be no visible cracking. They may be too small to see, start from inside an object, or be obscured. For example, the stress concentration at the corner of a shaft keyway is a common location for fatigue cracking, but this area cannot be observed unless the shafted assembly is completely torn apart. Inspecting a broken object subject to fatigue failure will always show two different surface characteristics: 1) the fatigue zone where cracks slowly propagated into the object, and 2) the instantaneous zone where the crack accelerated quickly through the object. The relationship between the size of the fatigue zone and the instantaneous zone indicates the relative loading of an object. The instantaneous zone is produced when the effective area (total area minus fatigue zone) is too small to support a load. Therefore, if the instantaneous
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zone is small, the object was not heavily loaded. However, if the instantaneous zone is large, the object was heavily loaded. The fatigue zone is identified by beach marks, which are small ridges on the broken surface of a material. These beach marks are produced by the crack propagation and plot the crack progression. Due to abrasion within the crack, older fatigue zone areas are smoother than new ones and can help identify the failure origin.
Fatigue strength
Stress (S) (kpsi)
Although fatigue strength is not directly related to any other material property, it is most closely related to tensile strength. Corrosion, galling and other surface defects reduce a material’s resistance to fatigue more than can be associated to stress concentrations alone. Consequently, fatigue strength is usually determined experimentally. The fatigue properties of a material are based on testing specimens at different stress levels (S) and measuring the number of cycles to failure (N) to produce an S-N curve. Because real objects do not behave like test specimens, the design strength of a part is reduced, based upon: 1) whether the part is subject to flexural or axial loading, 2) the part size, 3) the surface condition (eg polished versus forged), 4) operating temperature and 5) safety factor (see Fig 10). Metals that are repeatedly yielded become work hardened. This makes the metal harder and more brittle. When a paperclip or wire is bent back and forth until it finally breaks, it has been made more brittle by the process and finally snaps.
70 60 50
Medium Carbon Steel
40
Endurance Limit
30 20
Cast Iron
10 10 000
50 000 20 000
200 000 100 000
1000 000 5 000 000 500 000 2 000 000
Fig 10  Number of cycles to failure (N): S-N curve for steel and iron. Note: These relationships vary widely based upon different materials. Polymers have the most unpredictable relationship between fatigue strength and cycles.
FATIGUE
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Fretting fatigue Fretting fatigue is a special form of fatigue that is initiated by the small vibrations of parts that are pressed together. The vibrations of the parts cause surfaces to crack. These cracks produce stress concentrations that accelerate the fatigue of the parts. Fretting fatigue often occurs at pressed joints that are not intended to move. It can be readily reduced in these cases by decreasing the clearance between the fastener and the through hole or by using higher strength and correspondingly higher tightening torques. Impact loading may result from oversized holes due to design or wear, and impact is highly undesirable because of the stress amplification that it produces.
Creep and thermal relaxation Creep is a term used to describe the gradual stretching of a material under a load. Although it is most common in unreinforced plastics, it happens to some degree in metals also. Creep is due to slippage of a material’s crystals or grains along their boundaries. This generally is not a concern except with highly loaded plastics that require dimensional stability and materials subject to high temperature. Thermal relaxation describes creep that is accelerated by high temperature.
Challenges for the marine surveyor Both fatigue and creep occur at stresses lower than the yield strength of a material. This means, for example, that a propeller shaft or shroud can break under normal loads after many years of reliable service. Furthermore, there will be no indicators of incipient fatigue failure – very troubling for the marine surveyor. Corrosion of any sort greatly decreases fatigue strength. In fact, a part that has had pitting corrosion machined off will have much higher fatigue strength, even with the large loss of material, compared to the original part with the pitting. The solution to eliminating fatigue failures is preventative maintenance. Many objects are designed for an infinite number of cycles, but this evaluation can only be done by the designer – not the marine surveyor. Fatigue life can only be determined by experimentation. A preventative maintenance schedule of critical components must be offered by the naval architect/marine engineer to properly ensure their reliability.
Covering the latest in marine surveying technology, this Reeds Professional handbook is aimed at students of marine surveying, professional marine surveyors, boatyard operators, and technically-minded boat owners.
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