How Inspectors Can Use Fracture Mechanics Roger Cordes

8/8/2019 How Inspectors Can Use Fracture Mechanics Roger Cordes

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How Inspectors Can UseFracture Mechanicsby Roger Cordes, Ph.D., P.E.


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Roger Cordes, Ph.D., P.E.

Involved with a wide variety of solid mechanics projectsfor SES Floating Production Systems and Fitness-For-Service groups.

Generally focused on solid mechanics analyses includingstrength and fatigue assessments.

Involved in contact problems, damage evolution,dynamic/impact simulations, post-buckling behavior,thermal behavior, and crack modeling.

Crack assessment projects at SES include prediction ofcrack growth rates and component life, maximumacceptable initial flaw size studies, and forensic work.

B.S.M.E from the University of Arkansas and a Ph.D.from Northwestern University. Licensed ProfessionalEngineer in Texas.


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Purpose of Presentation: Discussing howfracture models can be used in conjunctionwith inspections

Overview Typical analyses Basic fracture mechanics Material toughness and critical crack sizes Fatigue crack growth and ECAs Inputs needed and safety factors

Sources API 579 Fitness for Service British Standard 7910 Guide to Methods for

Assessing the Acceptability of Flaws in MetallicStructures

API 1104 Welding of Pipelines and RelatedFacilities


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Overview

Relating fracture mechanics to inspections

Typical analyses

Basic fracture mechanics

Material toughness and critical crack sizes

Fatigue crack growth and ECAs

Inputs needed and safety factors


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Typical Analysis

A Stress Analysis of a Pipe,Vessel, Component

Stresses

Can it survive theloading?

How long is it predictedto last?

(Strength Check) (Fatigue Check)


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How Fracture Mechanics Fits In

A Stress Analysis of a Pipe,Vessel, Component

Stresses

How big a crack cansurvive the loading?

How fast will a crackgrow?

How big an initialcrack is OK?

(Flaw tolerance/Toughness Check)

(Inspection Interval Check)


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Nomenclature

Inspectors find indications.If the indication is larger than a pre-defined

acceptable size, it is a defect.

If the indication is sharp, it is a crack.

Crack


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The Basics of Fracture Mechanics (1)

s

If you pull on something with avoid or particle in it, you get a

stress concentration factor(SCF).

If you flatten that void,you get a crack with astress intensity factor

(K).

s


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The stress field at the tip of the crack can bewritten in the following form:

s is the far field stress,

Y is a function of the geometry,

ais the crack length, and

Kis the stress intensity factor.

a

r

s

The Basics of Fracture Mechanics (2)

aYK s

MATERIAL

MATERIAL

KK

KK

2 Options

Failure not predicted

Failure predicted


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Fracture Toughnesses

Material

Approximate Values

Charpy

(ft-lbs)

CTOD (mm) KMATERIAL(ksi*in.)

Glass


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Surface Crack Example

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

Crack Length, inches

CrackDepth,

in

ches

.

Case A

Case B

Case C

Wall Thickness


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Fatigue Crack Growth

Cracks mainly grow from the stress rangeduring cyclic loading.

High

Stress

LowStress

StressRange


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Crack Growth

OD Surface Crack Depth Growth versus Time

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

0.0 3.0 6.0 9.0 12.0 15.0

Time, years

CrackDepth,m

m

Initial Height = 1.0 mm

Initial Height = 1.5 mm


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Crack Growth Over Time

Final crack

width, 2Cf

Initial crack

width, 2CiInitial crackdepth, ai

Final crack

depth, af

ECA=Engineering Critical AssessmentCommonly used to determine the initialcracksizes that are OK based on the expectedloading over the life of the component.


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ECA Results

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

Crack Width, 2C (inches)

CrackDepth,a(inches)

.

Proposed Criteria

ECA Result


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Crack Locations and Orientations

Acceptable crack criteria can be define for

Circumferential cracks driven by axial stress

Axial cracks driven by hoop stress

Internal Surface Crack

External Surface Crack

Wall

Thickness

Embedded

Crack

Inner

Radius


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Interaction Rules

If cracks are too close together, they

are treated as one crack.


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API 579 Fitness for Service and

British Standard 7910

Failure assessment diagram (FAD) approach

is used for the ECA (API 579) and BS7910.

FAD based on

K

Plastic

Collapse

FAD


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Inputs for Analysis

Significant inputs driving the solution:

Geometry

SCF

Primary Cyclic Loading

Crack Growth Constants

Target Life

Initial Crack Size

Material Toughness

Other inputs

Residual Stress

Maximum Primary Stress

Yield/Ultimate Strengths


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Safety Factors

API 579 utilizes safety factors on three items:

Applied Loading

Fracture Toughness

Crack Size


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How Fracture Mechanics Fits In

Before Putting aComponent in Operation

Do Fracture MechanicsEvaluation

Determine Allowable FlawSizes for Life of Vessel

Set Inspection Criteria

If You Inspect and Find aCrack During Operation

Do Fracture MechanicsEvaluation

See How Big Crack Can Get

Or

See How Fast Crack Will Grow


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Questions ???

Roger Cordes, Ph.D., P.E.Stress Engineering Services, Inc.

13800 Westfair East DriveHouston, Texas 77041-1101

Phone: 281-955-2900Fax: 281-955-2638

Email: [email protected] Site: www.stress.com
mailto:[email protected]://www.stress.com/http://www.stress.com/mailto:[email protected]

How Inspectors Can Use Fracture Mechanics Roger Cordes

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