University of Alexandria Faculty of Engineering
Dept. of Naval Architecture & Marine Engineering Instructor: H. W. Leheta
CHAPTER 8 SHIP STRUCTURAL FAILURE Ships are one of the largest mobile structures built by man, and both their size and the requirement for mobility exert strong influences on the structural arrangement and design. Ship structures are likely to be subjected to various types of loads and deformations arising from the routine to the extreme to the accidental. The mission of the ship structural designer is to design a ship that can withstand such demands throughout its expected lifetime. 8.1 The
structural design process
It is characterized by the following key steps: (a) Development of the initial configuration and scantlings. (b) Analysis of the t he performance of the assumed design. (c) Comparison with performance criteria. criteria. (d) Redesign to effect an improvement. (e) Repeat in order to approach an optimum. 8.2
Levels Levels of response
The geometrical arrangement and resulting stress or deflection response patterns of typical ship structures are such that it is usually convenient to divide the structure and the associated response into three components, which are labeled primary, secondary and tertiary, see Figure below.
Levels of response
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University of Alexandria Faculty of Engineering
Dept. of Naval Architecture & Marine Engineering Instructor: H. W. Leheta
Primary: It is the response of the entire hull, when bending and twisting as a beam, under the external longitudinal distribution of vertical, lateral and twisting loads. Secondary: The response of a single panel of stiffened plating. Example: panel of bottom structure between two adjacent transverse bulkheads. Tertiary: The response of an individual plate between stiffeners, which is a part of the secondary panel. 8.3
The nature of ship st ructur al failure
Structural designers must avoid structural failure. In order to do so, the naval architect must be aware of the possible modes of failure and the methods of predicting their occurrence. Structural failure may occur in different degrees of severity. There may be small cracks or deformations in minor structural members that do not jeopardize the basic ability of the structure to perform its function. The severest failure is total catastrophic collapse of the structure resulting in the loss of the ship. There are other several modes of failure that may reduce the load-carrying ability of individual members or parts of the structure but due to redundancy do not lead to total collapse. Such failures can be detected and repaired before endangering the ship. Collision and grounding also cause structural failure. 8.4 Lim it
st ate analysis
A limit state is any condition in which an entire structure or a structural member fails to perform its function and becomes unfit for one of its intended roles, due to one or more loads and/or load effects. Four types of limit states are considered for steel structures, namely: • serviceability limit state (SLS); • ultimate limit state (ULS); • fatigue limit state (FLS); and • accidental limit state (ALS). The serviceability limit states (SLS) involve the deterioration or loss of some function of the structure or member during normal operations. Some SLS design considerations: • local damage which reduces the durability of the structure or affects the efficiency of structural elements; • unacceptable deformations which affect the efficient use of structural elements or the functioning of equipment relying on them; • excessive vibration or noise which can cause discomfort to people or affect the proper functioning of equipment; and • deformations and deflections which may spoil the aesthetic appearance of the structure.
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University of Alexandria Faculty of Engineering
Dept. of Naval Architecture & Marine Engineering Instructor: H. W. Leheta
The ultimate or collapse limit states (ULS), in which the structure or member has failed in its primary load carrying role, i.e. loss of structural stiffness and strength. Some ULS design considerations: • loss of equilibrium in part or of entire structure, often considered as a rigid body (e.g. overturning or capsizing); • attainment of the maximum resistance of structural regions, members or connections by gross yielding, rupture or fracture; and • instability in part or of the entire structure resulting from buckling and plastic collapse of plating, stiffened panels and support members. FLS represents fatigue crack occurrence of structural details due to stress concentration and damage accumulation (crack growth) under the action of repeated loading. ALS represents excessive structural damage as a consequence of accidents, e.g. collision, grounding, explosion and fire, and freak waves, which affect the safety of the structure, environment and personnel. The level of seriousness of a limit state corresponds approximately to the level or extent of structure which has failed: overall, substructure or local. A limit state can be expressed as follows: γ ⋅ Q ≤ QL where Q = load or load effect γ = safety factor QL = limit value 8.5 Modes
of ship st ructural failure
Four principal mechanisms are recognized as causing most of the cases of ship structure failure, aside from accidents (e.g. collision and grounding). These modes of failure are as follows: • Excessive tensile or compressive yield. • Buckling due to compressive or shear instability. • Fatigue cracking. • Brittle fracture
8.5.1 Excessive tensile or compressive yield It occurs when the stress in a structural member exceeds a level that results in a permanent plastic deformation of the material of which the member is constructed. The stress level is termed the material yield stress. At a somewhat higher stress, termed the ultimate stress, fracture of the material occurs, see Figure. Point ‘a’ is the proportional limit, ‘b’ is the elastic limit after which a permanent plastic deformation is produced, ‘c’ is the upper yield point, and ‘d’ is the lower yield point. From ‘d’ to ‘f’, strain-hardening occurs. Point ‘g’ is the ultimate strength. Distortion energy theory (Von Mises criterion) is usually used here to obtain the equivalent stress to compare with the material strength.
(
σ e = σ x 2 + σ y 2 − σ x σ y + 3τ 2
1
)
2
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University of Alexandria Faculty of Engineering
Dept. of Naval Architecture & Marine Engineering Instructor: H. W. Leheta
Stress-strain relationship
8.5.2 Buckling or instability failure Instability failure of a structural member loaded in compression may occur at a stress level that is substantially lower than the material yield stress. The load at which instability or buckling occurs is a function of member geometry and material modulus of elasticity rather than material strength. The most common example of an instability failure is the buckling of a simple column under a compressive load that equals or exceeds the Euler critical buckling load . In contrast to the column, however, exceeding this load by a small margin will not necessarily result in complete collapse of the plate but only in an elastic deflection of the central portion of the plate away from its initial plane. After removal of the load, the plate will return to its original undeformed configuration. The ultimate load that may be carried by a buckled plate is determined by the onset of yielding at some point in the plate material or in the stiffeners, in case of a stiffened panel. The critical buckling load for a plate depends on plate thickness, lateral dimensions, edge support conditions and modulus of elasticity of the material. The following buckling limit states may occur in structures: 1- For unstiffened rectangular plates: (a) Serviceability limit state: to avoid elastic and plastic buckling. (b) Ultimate limit state: to avoid collapse.
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University of Alexandria Faculty of Engineering
Dept. of Naval Architecture & Marine Engineering Instructor: H. W. Leheta
Unstiffened plate 2- Stiffeners and effective plating (a) Column buckling (compressive load only): collapse or ultimate limit state is considered.
Column (b) Beam-column buckling (compressive and lateral loads): collapse or ultimate limit state is considered.
Beam-column
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University of Alexandria Faculty of Engineering
Dept. of Naval Architecture & Marine Engineering Instructor: H. W. Leheta
(c) Torsional flexural buckling.
Torsional-flexural buckling 3- Stiffened panels: (a) Serviceability limit state. (b) Ultimate limit state.
Loads on stiffened Panel
Load-deflection curve for plates and stiffened panels
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University of Alexandria Faculty of Engineering
Dept. of Naval Architecture & Marine Engineering Instructor: H. W. Leheta
4- Hull girder: Collapse: ultimate strength of hull is reached.
8.5.3 Fatigue fracture In steel and other metals, a fluctuating stress can initiate microscopic cracks which gradually increase in size until, after a large number of cycles, the cracks have become so large that fracture occurs.
Cyclic stress FLS design and analysis should be undertaken for every suspect source of fatigue cracking which includes welded joints and local areas of stress concentrations. Two types of FLS design approaches are typically considered for steel structures, namely: • S-N curve approach, • fracture mechanics approach. S-N curve: it is obtained by experiments on specific materials
S-N curve 98
University of Alexandria Faculty of Engineering
⎛ C ⎞ SN = ⎜ ⎟ ⎝ N ⎠
1
Dept. of Naval Architecture & Marine Engineering Instructor: H. W. Leheta
m
or
log N = Log C – m . log SN
SN = stress range for failure at N cycles. N = number of cycles to failure. m = negative or inverse log-log slope of curve. S = endurance limit or fatigue limit or fatigue threshold. C = the life intercept of the S-N curve. ∞
log S slope = m
Log N Log C
The following are sources of cyclic stresses in ships: 1) Wave induced loads: especially bending of hull girder. 2) Alternation between loaded and ballasting conditions: e.g in tankers. 3) Mechanical sources: e.g engine and propeller. Fatigue damage computations for ship structures are made for large sizes and special types, such as tankers, bulk carriers, large containerships. Fatigue cracks usually occur in poorly designed brackets and other details, requiring repair at times of overhaul. Cracks in longitudinal strength members can be readily detected and repaired before the safety of the ship is threatened. Fatigue damage criteria must be applied in the design of offshore structures. The structural design criteria for the FLS are usually based on the cumulative fatigue damage of a structure under repeated fluctuation of loading, as measured by the Palmgren-Miner cumulative damage accumulation rule: B
ni
∑ N ≤ D
D=
allow
i =1
i
where ni and N i denote the number of stress cycles in stress block i , and the number of cycles until failure of at the i -th constant amplitude stress range block. Dallow is the allowable limit that is defined in design codes. Fracture mechanics this is characterized by the Paris equation: da dN
m
= C (ΔK )
where
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University of Alexandria Faculty of Engineering
da dN
Dept. of Naval Architecture & Marine Engineering Instructor: H. W. Leheta
is the crack growth rate per cycle and ΔK = K max − K min
a is the crack size and N is the number of stress cycles. K max and K min are the maximum and minimum values of the stress intensity factor, at the upper and lower limit stresses during a cyclic loading. Material properties C and m may be found from design codes for typical materials used for marine structures. Stress intensity factors may be obtained from handbooks for simplified geometries and loads. 8.5.4 Brittle fracture The term “brittle fracture” refers to the fact that below a certain temperature, the ultimate tensile strength of steel diminishes sharply. The value of this transition temperature depends almost entirely on the chemical composition of the steel and the metallurgical processes by which it is made. For ship structures, a good quality steel is absolutely necessary, and in most cases sufficient, to avoid brittle fracture. Brittle fracture was found to play a major role in the failure of many of the emergency cargo ships (Liberty sister ships) built during World War II. This was due to the relatively new techniques of welding construction employed in building t he ships. The development of new design details that avoided the occurrence of notches and other stress concentrations is one solution to the problem, the major solution being the selection of high quality steel. Hence, brittle fracture is controlled by detail design and material selection.
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