Introduction to Offshore Structures-PART-4

7/28/2019 Introduction to Offshore Structures-PART-4

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Types of Fracture


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Types of Fracture


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Types of Fracture


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Types of Fracture


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Types of Fracture


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Types of Fracture


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Types of Fracture


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Agents Inducing Fracture

External

Load - Static, Fluctuating

or Impact

Temperature Room

Temp., Low Temp. or

High Temp.

EnvironmentParticle

Impingement


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Agents Inducing Fracture

Internal

Material Grade,

chemistry, trace elements

Microstructure Phase

morphology, distribution

and nature,

Crystal structure,

Grain size, texture

Manufacturing Defects,

Crystal imperfections,segregation


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Concept of stress & Strain

Tensile stress ( = P / A)

Compressive stress ( = P / A)

Shear stress ( = P / A)

Shear strain = ( /2)

Normal strains cause changes in

dimensions and shear strains cause

changes in shape


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Concept of stress at a point

Biaxial Stress

A two-dimensional state of

stress in which only two

normal stresses are presentis called biaxial stress.

Uni-axial Stress

A one-dimensional state of

stress in which normal

stresses act along one

direction only is called auni-axial stress state

Tri-axial Stress

Tri-axial stress refers to a

condition where only

normal stresses act on anelement and all shear

stresses (xy, xz, and yz)are zero. An example of a

tri-axial stress state is

hydrostatic pressure acting

on a small elementsubmerged in a liquid.

Pure Shear StressPure shear refers to a

stress state in which an

element is subjected to

plane shearing stresses

only. Pure shear occurs in

elements of a circular

shaft under a torsion load

General state of stress

under combined loading

Plane Stress

Plane stress refers to a

condition where normal

and shear stresses acton a plane and where

z, xz, and yz are zero.An example of a plane

stress state is plate

subjected to combined

loading.


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Concept of Principal stress

Principal stresses:

As can be seen on Mohrs circle,

the principal normal stresses

occur on surfaces which have no

shear stress. Also, the maximum

shear stress is 90o away from the

maximum normal stress on

Mohrs circle so that it is on a

surface oriented 45o away fromthe surface on which the

maximum normal stress occurs.


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Failure Theories

This theory is approximately correct for cast iron and

brittle materials

According to this theory failure will occur when the

maximum principal stress in a system reaches the value

of the maximum stress at elastic limit in simple tension

Failure will occur if following condition is violated if :

Maximum Principal Stress (Elastic Limit or UTS / FoS)

Maximum Principal Stress theory (Rankine)

Principal stress is obtained from detailed stress analysis of the

component

Elastic limit or Ultimate Tensile strength is obtained from Tensile test of

the material from which the component is made


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Failure Theories

This is very relevant to ductile metals

It is conservative and relatively easy to apply

Determine shear stress (1 - 2) /2 ; (2 - 3) /2 ; (3 - 1)/2 Determine highest Maximum shear stress

Determine Yield strength ys of the material from Tensile test

Component will fail if the following condition is violated, if :

Highest shear stress ys

Maximum Shear Stress theory (Tresca)


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Failure Theories

Maximum Distortion Energy Theory (Von Mises)

Detailed studies have indicated that yielding is related to the shearenergy rather than the maximum shear stress It is more realistic. Total energy absorbed by the system consists

of two parts i) Energy towards changes in dimensions and

ii) Energy towards changes in shape (energy of distortion). In thistheory only distortion energy is considered

Determine the quantities : (1 - 2) ; (2 - 3) ; (3 - 1) Compute [(1 - 2)2 + (2 - 3)2 + (3 - 1)2 ]

Determine Yield strength ys of the material from Tensile test Component will fail if the following condition is violated, if :

[(1 - 2)2 + (2 - 3)2 + (3 - 1)2 ] ys


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Stress Concentration


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Design Process


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Failure Mode Assessment


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Documents

Introduction to Offshore Structures-PART-4