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KatelynSmith Thesis

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  • THE PENNSYLVANIA STATE UNIVERSITY

    SCHREYER HONORS COLLEGE

    DEPARTMENT OF ENGINEERING SCIENCE AND MECHANICS

    EVALUATION OF THE CRITERIA FOR CRACK PROPAGATION

    IN THE PRESENCE OF HIGH SURFACE STRESSES

    KATELYN J. SMITH

    Spring 2010

    A thesis

    submitted in partial fulfillment

    of the requirements

    for a baccalaureate degree

    in Engineering Science

    with honors in Engineering Science

    Reviewed and approved* by the following:

    Albert E. Segall

    Professor of Engineering Science and Mechanics

    Thesis Supervisor

    Christine B. Masters

    Assistant Professor and Undergraduate Program

    Coordinator of Engineering Science and Mechanics

    Honors Adviser

    Judith A. Todd

    P. B. Breneman Department Head Chair

    Professor, Department of Engineering Science and Mechanics

    * Signatures are on file in the Schreyer Honors College and Engineering Science

    and Mechanics Office.

  • i

    ABSTRACT

    The petrochemical industry is largely dependent on the continuous operation of

    equipment to turn the maximum profit. Thus, there exists an intricate balance between the

    financial gains of the business and the structural integrity of the equipment. Numerous standards

    are therefore used to establish an appropriate degree of conservatism in the design process to

    ensure safe and reliable long-term operation.

    The standard under evaluation in this thesis research is the API 579-1/ASME FFS-1. The

    present methods for determining critical crack dimensions in this standard compare a calculated

    stress intensity value from the surface and depth locations with the material fracture toughness. In

    cases where high, residual stresses exist on the surface, this treatment may result in an overly

    conservative estimation of the likelihood of crack propagation due to the rapidly diminishing

    stresses as the crack face moves away from any high surface stresses. Since over-conservatism

    can be problematic, the goal of this project is to evaluate the critical crack assessment in the

    presence of a stress field that diminishes rapidly in the through-thickness direction.

    Through the use of finite element analysis and other analytical tools, the depth and

    surface locations of a semi-elliptical surface crack were examined in terms of stress intensity

    solutions. A comparison between the standard and the finite element analysis confirmed that the

    critical crack-like flaw assessment provides an additional degree of conservatism at the depth

    location of the flaw; this inherent conservatism is equivalent to a safety factor of approximately

    1.375 or higher for the cases reviewed. The surface location was unable to be properly assessed

    due to inconsistencies in the mesh applied at this location. Thus, further modeling work is

    necessary to accurately evaluate the level of conservatism at the surface location of the semi-

    elliptical crack.

  • ii

    TABLE OF CONTENTS

    LIST OF FIGURES ............................................................................................................... vi

    LIST OF TABLES ................................................................................................................. vii

    ACKNOWLEDGEMENTS ................................................................................................... viii

    Chapter 1 Introduction .......................................................................................................... 1

    1.1Equitment Evaluation ................................................................................................ 1 1.2 Objectives................................................................................................................. 2

    Chapter 2 Background .......................................................................................................... 3

    2.1 Pressure Vessel Stress Assessment ........................................................................... 3 2.1.1 Characterization of Stress .............................................................................. 3

    2.1.2 Origin of Stresses in Pressure Vessels ........................................................... 5 2.1.3 Stress Intensity Factor.................................................................................... 5

    2.2 API 579-1/ASME FFS-1Standard ............................................................................ 7 2.2.1 Backgound of the API 579-1/ASME FFS-1Standard..................................... 8

    2.2.2 Critcal Crack Calculation in the API 579-1/ASME FFS-1Standard .............. 9 2.3 Weight Function Application ................................................................................... 14

    Chapter 3 Modeling .............................................................................................................. 17

    3.1 Finite Element Analysis ........................................................................................... 17 3.2 Model Design ........................................................................................................... 18

    3.2.1 Crack Construction ........................................................................................ 20

    3.2.2 Loading Specifications .................................................................................. 21

    3.2.3 Material Selection .......................................................................................... 21 3.2.4 Additional Model Construction Parameters ................................................... 22

    Chapter 4 Results .................................................................................................................. 23

    4.1 Finite Element Analysis Results ............................................................................... 23 4.2 Analytical Work ....................................................................................................... 26

    4.2.1 API 579-1/ ASME FFS-1Analysis ................................................................. 26

    4.2.2 Weight Function Comparison ........................................................................ 29

    Chapter 5 Summary and Dicussions ..................................................................................... 31

    5.1 Dicussions ................................................................................................................ 31 5.1.1 Depth Stress Intensity Factor ......................................................................... 32

    5.1.2 Surface Stress Intensity Factor ....................................................................... 33

  • iii

    5.2 Summary .................................................................................................................. 34

    Chapter 6 Conclusions .......................................................................................................... 36

    Chapter 7 Future Work ......................................................................................................... 38

    7.1 Modeling Work ........................................................................................................ 38 7.2 Analytical Work ....................................................................................................... 39

    References.............................................................................................................................. 40

    Appendix A Overview of the Assessment Procedure ............................................................ 42

    Appendix B Stress Distributions ............................................................................................ 43

    Appendix C Weight Function Calculations ............................................................................ 45

  • iv

    LIST OF FIGURES

    Figure 2-1: A three-dimensional semi-elliptical external crack ............................................ 6

    Figure 2-2: Geometry with an external axial crack of semi-elliptical shape ........................... 7

    Figure 2-3: Failure Regions on the Failure Assessment Diagram. ......................................... 9

    Figure 2-4: Schematic Overview of the FAD Procedure ........................................................ 13

    Figure 3-1: Cut-Away View of Modeled Crack and Boundary Conditions. .......................... 22

    Figure 4-1: View of Model 2 Undeformed Stressed State .................................................... 24

    Figure 4-2: View of Model 1 Deformed Stressed State ......................................................... 25

    Figure 4-3: ABAQUS/CAE Stress Linearization Through-Thickness for Load 1 .................. 27

    Figure 4-4: ABAQUS/CAE Stress Linearization Through-Thickness for Load 2 .................. 27

    Figure 4-5: Polynomial Fit to Stress Normal to Crack for Load 1 .......................................... 28

    Figure 4-6: Polynomial Fit to Stress Normal to Crack for Load 2 .......................................... 28

  • v

    LIST OF TABLES

    Table 3-1: Dimensions of ABAQUS/CAE Model for Semi-elliptical Crack Growth. ........... 18

    Table 4-1: Stress Intensity Values for ABAQUS/CAE Model. ............................................. 24

    Table 4-2: Linear Approximation of Stress Intensity Values for ABAQUS/CAE Model ...... 26

    Table 4-3: Stress Intensity Values from API 579-1/ASME FFS-1Standard. ..............

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