Ship Stresses

SHIP STRESSES Rishi Piravin Anmol Agilan Arief  Immanuel

What is stress and strain? •

STRESS – Is defined as the load put on a piece of material or a structure

•

STRAIN – Is defined as the permanent deformity or weaness caused !y e"cessi#e stress

What is stress and strain? •

STRESS – Is defined as the load put on a piece of material or a structure

•

STRAIN – Is defined as the permanent deformity or weaness caused !y e"cessi#e stress

There are $ main types of stress% •Tensile

& Tensionin' •(ompressi#e & (ompression •Shear

Tensioning

tendency to pull the material apart

Compression tendency to crush the material or to buckle

Shear is the effect of two forces actin' in opposite directions and alon' parallel lines)

Shear 

Mechanical Properties of Metals

Elasticity

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The case with which a metal may be bent or molded into a given shape.

Brittleness - The opposite of plasticity, lack or  

elasticity

Malleability - The property possessed by a metal of becoming permanently flattened or stretched.

Hardness - The property of a metal to resist wear and abrasion. atig!e

$!ctility

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" metal s!b#ected to contin!ally varying loads may event!ally s!ffer from fatig!e. - "bility to be drawn o!t lengthwise, the amo!nt of the e%tension meas!res the d!ctility.

Brittle ract!re * &ow temperat!res at or near free'ing   points, *&oad on material relatively light, *$efects or fa!lts in a weld *(nternal stresses within material ie. welding may initiate a fract!re.

The ship at sea or lying in still water is constantly being s!b#ected to a wide variety of stresses and strains, which res!lt from the action of forces from o!tside and within the ship. These forces may initially be classified into)

STATI( +,R(ES -.NA/I( +,R(ES

STATIC FORCES These are due to •

•

Internal forces resultin' from structural wei'ht0 car'o and machinery wei'ht) E"ternal static forces includin' the hydrostatic pressure of the water on the hull)

S t a t i cf o r c e

Vessel at rest

DYNAMIC  FORCES FORCES They result from • • •

The ship1s motion at sea) The action of wind and wa#es) The effects of operatin' machinery)

Dynamic Forces •

• •

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A ship is free to mo#e a!out si" de'rees of freedom) Three linear and three rotational) 2arious forces actin' on ship are constantly #aryin' in de'ree and fre3uency) This mo#ement of the ship introduces dynamic forces which result in stresses on the ship1s structure)

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Hea#e is the motion of the ship when the ship ha#e !ein' up !y a wa#e or sea)

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Sway is the swin' of a mast or !ow of a ship from side to side as the #essel pro'resses in a hea#y sea)

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Sur'e is the mo#ement forward as the !ow of a ship rises and dips when it encounter wa#es which are stron' enou'h to life it) The ship sur'e up the side of a wa#e0 often shippin' water as it passes throu'h the crest0 then dips down the other side of the wa#e)

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•

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Pitch is the motion of a ship in risin' the crest of a wa#e then descendin' into the followin' trou'h) Roll is the motion of a ship from side to side as she mo#es throu'h the water) .aw is where the !ow of a ship falls away or sways erratically from side to side as the #essel mo#es throu'h the water)

+orces produce stresses in the ship1s structure which may !e di#ided into two cate'ories%

•4lo!al

•5ocal

ship

stress – affects the whole ship

stress6 affects a particular part of a

O!!IN! •

Ho''in' is when the ship !ends upwards lon'itudinally) This occurs when there is more wei'ht concentrated at the ends due to une#en car'o distri!ution or when the #essel rides a wa#e crest in its middle0 causin' e"cessi#e !uoyancy)

SA!!IN! •

Sa''in' is the re#erse of ho''in' when the ship !ends lon'itudinally in the downwards direction) This occurs when there is more wei'ht concentrated in the mid len'th of the #essel due to une#en car'o distri!ution or when the #essel rides a wa#e trou'h in its middle causin' e"cessi#e !uoyancy at the ends)

RAC"IN! •

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When a ship is rollin'0 the accelerations on the ship1s structure are lia!le to cause distortion in the trans#erse section) 4reatest effect is under li'ht ship condition)

 TORSION •

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A ship tra#ersin' a wa#e train at an'le will !e su!7ect to ri'htin' moments of opposite directions at its ends) The hull is su!7ect to a twistin' moment and the structure is in torsion)

STRESSES D#E TO $ATER PRESS#RE •

*ater press!re acts perpendic!lar to the s!rface and increases with depth

STRESSES D#E TO DRY%DOC"IN! • Tends

to set the eel upwards) • -ue to the up6thrust of the eel !locs) • Tendency for the ship1s sides to !ul'e outwards) • 8il'es tend to sa')

PANTIN! •

This is a stress0 which occurs at the ends of a #essel due to #ariations in water pressure on the shell platin' as the #essel pitches in a seaway) The effect is accentuated at the !ow when main' headway DMS-DO

St r e s s e sc a us e dbyl oc a l i z edl oa di ng 5ocali9ed hea#y loads may 'i#e rise to locali9ed distortion of the trans#erse section) Such local loads may !e the machinery :/ain en'ine; in the en'ine room or the loadin' of concentrated ore in the holds)

Po!nding d!e to heavy pitching as the whole ship is lifted in a seaway may be s!b#ect the forepart to severe impact from the sea. (t gives greatest effect when in light condition. "bo!t + from forward strengthened.

Pounding Effect: 0.25L of the bow/stern

$eck opening creates areas of high local stress d!e to lack of contin!ity of tr!ct!re