Using ncode DesignLife for Fatigue of Welds

Transcription

1 Using ncode DesignLife for Fatigue of Welds Jeff Mentley HBM Prenscia October 5-6,

2 Agenda 3 1. Overview of the Fatigue of Welds 2. Approaches for the Analysis of Fatigue of Seamwelds 3. Using Structural Stresses for Fatigue Analysis 1.DesignLife Hotspot Stress Recovery 4. Shell Element Approach in ncode DesignLife 1.Combined fillet and overlap 5. Solid Element Approach in ncode DesignLife 6. New Weld features 7. Questions 3 October 5-6,

3 Overview of fatigue of welds 4 October 5-6,

4 Observations on the Fatigue Behavior of Welds 5 The fatigue properties of a welded joint are completely different from those of the parent plate due to Fairly sharp and ill controlled geometric features cause Large stress concentrations factors Existing crack initiation sites Defects such as slag inclusions High tensile residual stresses All cycles become tensile Nonuniform material distribution Deposited weld metal Heat affected zone in parent metal Fatigue behavior is primarily crack growth Fatigue properties of welds are much lower than those of the parent metal Parent metal strength not reflected in the weld fatigue strength October 5-6,

5 Approaches for the analysis of fatigue of seamwelds October 5-6,

6 Weld Fatigue Calculation Strategies 7 Crack Growth Local Approaches Structural Stress Approaches Nominal stress Hot spot stress Structural stresses from nodal forces Through thickness integration October 5-6,

7 Definition of Hot Spot Stress 8 The stress at the weld toe obtained by the linear extrapolation of the stress distribution in the neighborhood of the weld but being far enough not to be affected by the local features of the weld 0.4 t 1.0 t Surface Stress Extrapolation October 5-6,

8 Structural Stress from Nodal Forces and Moments 9 A stress resulting from the factoring of the forces and moments in the analyzed cross section by its section properties Eliminates the impact of the stress riser at the weld toe and provides a structural stress, and bending ratio T 2 s = m + b = P/A + Mc/I r T 1 P M October 5-6,

9 Through Thickness Integration 10 Linearize the stress profile through the thickness of the plate Eliminates the impact of the stress riser at the weld toe and provides a structural stress, and bending ratio linearized peak T October 5-6,

10 Using Structural Stresses for Fatigue Analysis October 5-6,

11 Structural Stresses 12 Hot spot stress Structural stresses from nodal forces Through thickness integration Material Properties Combine Geometry (FEA) Damage Analysis Fatigue Life Load History October 5-6,

12 Structural Stress in a Component Stress Life Approach 13 Wöhler circa 1850 October 5-6,

13 Predicting the Life of Welds using Weld Design Codes 14 BSI, IIW, Eurocode Choosing a weld class or classes for the joint considering: Joint geometry Loading direction and mode Failure location to be considered Type of weld (full penetration etc.) Stress Nominal Hotspot October 5-6,

14 Hot Spot Structural Stress Calculation ( new in ncode DesignLife 12.1 ) 15 Uses solid seamweld configuration file Stress recovery locations independent of mesh Supports IIW reference points defined by thickness ratio (type a hot spot) and actual distance (type b hot spot) Handles both linear and parabolic extrapolations Mesh density corrections Allows user input ratios or distances total stress structural stress F hot spot reference points F October 5-6,

15 Seamweld Modeling with Shell Elements October 5-6,

16 Shell Element Seamwelds 17 Sheets and welds modelled predominantly with 4 node shells Shell elements on the mid surfaces Small radii not modelled Input to ncode DesignLife is group of weld throat elements Structural stresses on attached elements used for fatigue calculation October 5-6,

17 Shell Weld Configurations 18 Fillet Overlap October 5-6,

18 CombinedFilletOverlap ( new in ncode DesignLife 12.1 ) 19 a1 fillet a2 fillet and overlap a3 overlap User specified transition angles October 5-6,

19 Structural stress from nodal forces and moments 20 Membrane and bending stresses normal to weld are calculated for weld toe and weld root elements Nodal forces and moments are collected at weld toe nodes and shared in proportion to element edge length Line forces and moments are averaged to mid point of edge and translated to local co ordinate system Forces and moments are converted into stress normal to weld toe for both surfaces of the shell f m x' y' top, normal 6 t 2 t October 5-6,

20 ncode DesignLife Process for Welded Shell Structures 21 October 5-6,

21 Seamweld Modeling with Solid Elements October 5-6,

22 Solid vs Shell Weld Modeling 23 October 5-6,

23 Through Thickness Integration 24 Linearize the stress profile through the thickness of the plate Eliminates the impact of the stress riser at the weld toe and provides a structural stress, and bending ratio linearized peak T October 5-6,

24 Stress Integration Method 25 ASME Boiler & Pressure Vessel Code VIII Division 2 The linearized stress is a 2D tensor, S tt, S ee, S et October 5-6,

25 2016 ncode User Group Meeting Stresses Calculated Directly by the Finite Element Model Smax = 133 Smax = 142 Smax = 170 Smax = October 5-6,

26 Structural Stress from Stress Integration 27 Smax = 117 Smax = 117 Smax = 109 Smax = 115 October 5-6,

27 Defining Solid Welds for ncode DesignLife 28 Utilities available for Hypermesh, Ansys Workbench, and Abaqus CAE Edges and surfaces are graphically selected to define the weld lines plus the direction and number of normal vectors Use specified number of locations for weld life calculations Locations are independent of FE mesh October 5-6,

28 ncode DesignLife Process for Welded Solid Structures 29 October 5-6,

29 New Weld features 30 Hotspot stress extrapolation (12.1) Uses solid seamweld definition file Input to IIW, BS7608 etc. Combined Fillet and Overlap (12.1) Removes need to manually group weld types Seamweld using vibration loading (12.1) Enabling PSD & swept sine loading Generic seamweld configuration using midedge Useful for butt welds Spotweld using vibration loading (13.0) Enabling PSD & swept sine loading October 5-6,

30 Summary 31 The nature of welds does not lend itself to the classic parent metal fatigue analysis Fracture mechanics, local notch stresses and structural stresses are all potential approaches Structural stresses from nodal forces and moments are an effective approach for seamweld analysis in shell models New Combined Fillet and Overlap shell weld configuration will simplify modeling Structural stresses from stress linearization are an effective approach for seamweld analysis in solid models 31 October 5-6,

31 Questions? 32 October 5-6,

32 Jeff Mentley Connect with us on: linkedin.com/company/hbm ncode +ncode October 5-6,