# Fatigue Assessment of ** Weld Joints Using ANSYS, Verity & FE-Safe

## Presentation on theme: "Fatigue Assessment of ** Weld Joints Using ANSYS, Verity & FE-Safe"— Presentation transcript:

Fatigue Assessment of ** Weld Joints Using ANSYS, Verity & FE-Safe
Zhichao Wang Aditya Sakhalkar 5/8/2007 Part I: Key Points on Equivalent SS Method Part II: Example Application ** Verity is a weld fatigue assessment software developed by Battelle based on Structural Stress & Fracture Mechanics theory proposed by Dr. Dong et al, Battelle

Part I: Key Points on SS & Weld Assessments
Zhichao Wang Sr. Lead Engineer Emerson Climate Technology Inc. Background Why The Battelle’s Structure Stress Is Not Sensitive To FE Mesh Size? How Could Multiple Joint S~N Curves Be Reduced to A Single S~N Curve, the Master S~N Curve? Example Application (Part ii, Aditya)

Background

Fatigue Assessment – Welded Structures

Fatigue Assessment – Welded Structures
(i). Nominal Stress Method (BS,IIW) The nominal stress range is used to develop the S~N curves using samples with actual weld joint geometry. The life curves refer to particular weld details, there is no need for the user to attempt to quantify the local stress concentration effect of the weld detail itself. (ii). Hotspot Stress Approach (structural stress, geometric stress, BS, IIW, CEN,DNV) This procedure uses hot-spot stress range as a parameter. The S–N curves are obtained from tests of actual welded joints based on the hot-spot stress range rather than the nominal stress range. (iii). Local Notch Stress Method (ASME, BS, IIW). The notch stress approach attempts to include all sources of stress concentration in the stress used with the design S–N curve. Thus a single S–N curve may be sufficient for a given type of material. The problem is that the local geometry of the toe or root of a weld is highly variable. It may be hard to achieve consistent results.

Fatigue Assessment – Welded Structures

Structural Stress & Mesh Sensitivity

Is Not Sensitive To FE Mesh?
Why Structure Stress Is Not Sensitive To FE Mesh? (a) (b)

Weld Joint Categorization
Master S~N Curve Weld Joint Categorization Categorization of Weld Joints (BS 7608) Weld Joints (IIW) Most codes divide weld joints into different types

Multiple S~N Curves Due to Weld Joint Categorization
Example Design S~N curves for welded joints: (a) Steel weld joint S~N curves (BS 7608); (b) Weld joint type and S~N curves (IIW recommendations); (c) Aluminum weld joint S~N curves (IIW recommendations) (a) (b) (c) Multiple S~N curves provide flexibility for the selection of life curves and increase difficulty to select the proper one due to the variation of actual joints

How Could multiple joint S~N curves be merged into one - Master S~N curve?
Nominal Stress Method (BS) Hot Spot Stress Method (IIW) Weld Joints (IIW) Equivalent SS Method

Small Crack Propagation Behavior

A Unified Stress Intensity Factor (SIF) Formulation
t1/t=0.1

A Unified Stress Intensity Factor

A Unified Stress Intensity Factor

SIF Magnification Factor Mkn

Mkn for 1350 V Notch Specimen
a/t Conclusions: Mkn approaches unity as crack size a/t approaches 0.1 i.e., a/t=0.1 (short crack correction factor) Thus a/t=0.1 can be taken as a characteristic parameter beyond which the notch effect is negligible The difference between edge crack and elliptical crack solutions are not significant.

Modified Paris Law

Master S~N Curve

Master S~N Curve Nominal Stress Method Hot Spot Stress Method
Equivalent SS Method Weld Joints (IIW)

Summary The Structural Stress Method developed at Battelle is mesh insensitive that removes the uncertainty in the calculation of structural stress for weld joint fatigue assessment The nature of weld joint fatigue is considered through the introduction of Equivalent Structural Stress based on fracture mechanics, which enable most fatigue curves of weld joints merged into a narrow band, the Master S~N curve. Thus one S~N curve can be used for majority of weld joints Note: I believe that the weld joints have to be stress concentration dominant to achieve consistent results with test data.

References [1]. BS PD 5500: , BSI Standards, London, 2000.
[2]. European Standard for Unfired Pressure Vessels, EN 13445: 2002, BS EN 13445:2002, BSI, London, 2002 [3]. ASME Boiler and Pressure Vessel Code, Section VIII, Rules for construction of pressure vessels, Division 2- Alternative rules, ASME, 2003. [4]. Carl E. Jaske, FSRF for WPVP, Journal of Pressure Vessel Technology, AUGUST 2000, Vol. 122, [5]. S.J.Maddox, Review of fatigue assessment procedures…, Int. J. of Fatigue, Vol. 25, 12, 2003, [6]. Maddox S J: 'Fatigue aspects of pressure vessel design…, Spence J and Tooth A S, E & F N Spon, London, 1994. [7]. Harrison J D and Maddox S J: 'A critical examination of rules for the design of pressure vessels subject to fatigue loading' in Proc. 4th Int. Conf. on 'Pressure Vessel Technology', Mech E, London, 1980. [8]. Taylor N (Ed): 'Current practices for design against fatigue in pressure equipment', EPERC Bulletin No.6, European Commission, NL-1755ZG, Petten, The Netherlands, 2001. [9]. Dong, P., 2005, ‘‘A Robust Structural Stress Method for Fatigue Analysis of Offshore/Marine Structures’’, Journal of Offshore Mechanics and Arctic Engineering , Vol. 127, pp [10]. Dong, P., 2001, ‘‘A Structural Stress Definition and Numerical Implementation for Fatigue Evaluation of Welded Joints,’’ Int. J. Fatigue, 23/10, pp. 865–876. [11]. Dong, P., Hong, J. K, Osage, D., and Prager, M., ‘‘Assessment of ASME’s FSRF Rules for Pipe and Vessel Welds Using A New Structural Stress Method,’’ Welding In the World, Vol. 47, No. 1/2, 2003, pp. 31–43. [12] Dong, P., Hong, J. K., Osage, D., Prager, M., 2002, ‘‘Master S-N Curve Method for Fatigue Evaluation of Welded Components,’’ WRC Bulletin, No. 474, August. [13]. Lankford, J. , Fatigue of Eng Mater and Structures 5 (1982), pp [14]. R. Craig McClung, et al, Behavior of Small Fatigue Cracks, ASME, Vol. 19, Fatigue & Fracture, P153 [15]. Fricke W., 2001, ‘‘Recommended Hot-Spot Analysis Procedure for Structural Details of FPSO’s and Ships Based on Round-Robin FE Analysis,’’ ISOPE Proceedings, Stavanger, Norway, June.

Sr. Applied Mechanics Engineer Emerson Climate Technology Inc.
Part II: Example Application Aditya Sakhalkar Sr. Applied Mechanics Engineer Emerson Climate Technology Inc. Background ANSYS, Verity, Fe-Safe Weld Assessment Procedure ANSYS Preprocessing Verity Analysis Fe-Safe Analysis ANSYS Results Comparison of Analysis Results with the Test Results Conclusions

Background Resistance welded suction fitting
Three failures in the suction fitting weld during reliability testing Crack initiated at the weld toe (9 o’ clock) and propagated through the shell. Failure due to reverse bending fatigue.

Verity, Fe-Safe Weld analysis
ANSYS, Verity, Fe-Safe Weld Analysis Procedure Verity, Fe-Safe Weld analysis Ansys Preprocessing Ansys Post processing Meshing per Verity requirements Linear material properties Apply load Solution Verity Analysis (Eq. Structural Stress calculations along the weld line) Fe-Safe analysis (Weld life calculations using Master S-N curve) Import .rst file Post processing Verity result validation and comparison Reliability Testing Calibration Quantify loading relative to the test for FEA

Ansys Preprocessing- FE Model
Suction Fitting Shell Weld Line Element Type: Solid 186 Mesh Requirements Shell Elements: 3D mid-surface element with no through-thickness dimension Solid Element: Rectangular faces required along the through-thickness cut from the weld line Regular mesh along the through thickness cut from weld line Recommended elements: Hexahedral (brick) and Pentahedral (wedge) Tetrahedral elements can be used. Requires special handling. Linear material properties (E = 29,000 Ksi, ν = 0.29) Symmetry BC On sides Top & Bottom fixed Hex elements along weld line

ANSYS Results Maximum Stress Equivalent Stress s1 seq

Verity Procedures Structural Stresses calculated along the weld line at each node Fe-Safe builds a connectivity table and maps the stresses to elemental stresses. These stresses are inserted into the stress matrix to be used for fatigue evaluation

Fe-Safe Analysis – Weld life predictions using Master SN Curve
Fully reverse loading defined Fe-Safe calculates weld fatigue life based on the Battelle’s Master SN curve The weld life (log N) data at each node is written in the ANSYS .rst format

ANSYS Post processing – Weld Life Results
Minimum life at element # 4436, node # 4212

Comparison of Verity Predictions with Reliability Test Results
Conservative life estimate! Predicted Failure Location correlated well with the test failures!!

Conclusions ANSYS could be used for the fatigue assessment of welded joints in conjunction with Verity & Fe-Safe Verity provides conservative estimate of the weld life Good correlation with the predicted and test failure locations Verity & Fe-Safe provides a excellent weld design tool in conjunction with ANSYS

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