2018 ncode User Group Meeting

Transcription

1 2018 ncode User Group Meeting February 28 March 1, 2018 Novi, MI USA

2 New Capabilities for Predicting the Durability of Welded Structures Jeff Mentley Applications Engineer

3 Observations on the Fatigue Behavior of Welds 3 The fatigue properties of a welded joint are quite different from those of the parent plate due to Weld geometry usually has Large stress concentrations factors Existing crack initiation sites High tensile residual stresses Non uniform material distribution Deposited weld metal Heat affected zone in parent metal S Life (cycles) Fatigue properties of welds are much lower than those of the parent metal Parent metal strength not reflected in the weld fatigue strength Weld fatigue behavior is primarily crack growth

4 Weld Fatigue Calculation Approaches 4 Crack Growth Local Notch Stress Structural Stress

5 The Fatigue Analysis Process using Structural Stresses 5 Typical fatigue process requires detailed material and geometry data Using structural stresses, weld is treated as a component Component fatigue curves represent the weld, not the weld material Material Properties Combine Wöhler circa 1850 Geometry (FEA) Damage Analysis Fatigue Life Load History

6 How a Structural Stress Approach is Applied 6 Load life curves from physical fatigue tests Finite element models of test specimens Structural stresses calculated from FE models With appropriate corrections load life curves collapse to single structural stress life scatter band The resulting structural stress curve represents the typical behavior of the welds in the specimens tested

7 Structural Stresses used in DesignLife 7 Spot welds Stresses calculated from forces and moments transferred through the connector t F z F y M y F x M x ΔS(θ) σ(f )cosθ σ(f )sinθ σ(f ) σ(m )sinθ σ(m )cosθ x y d x x y Shell Seam welds Structural stresses from nodal forces and moments along weld toe Solid Seam welds Through thickness integration (TTI) of actual FE stresses to obtain linearized stresses linearized peak T

8 Spot Welds 8 Nastran OP2, Optistruct OP2 & H3D Point to Point (P2P) Beam Element, CWELD CWELD ACM PENTA method Abaqus ODB Point to Point (P2P) ACM LS DYNA MAT SPOT WELD Beam Hex ANSYS Workbench Spot weld Support for Vibration Fatigue Curves for Steel, Aluminum, and Self Piercing Rivets New in 2018 *Rupp, A., Störzel, K. and Grubisic, V. (1995) Computer Aided Dimensioning of Spot-Welded Automotive Structures. SAE Technical Paper

9 Shell Seam Weld 9 Simple shell representation of weld Predefined failure locations Toes, root, throat Automatic bending ratio and thickness corrections Generic seam weld curves Nastran OP2, Optistruct OP2 & H3D Nodal Force and Moment Cubic Stresses Abaqus ODB, Ansys Nodal Force and Moment Support for Vibration Fatigue New in 2018

10 Solid Seam Weld 10 Applicable to broader range of applications and modeling Some things just can not be accurately modeled in shells Linearized stress at weld is not dependent on FE mesh density or element type 1 st & 2 nd order TET, PYR, WED & HEX Weld definition based on geometry, not FE model User controlled failure locations Uses same solution procedure and fatigue curves as shell seamweld Weld configuration file generators Hypermesh Ansys Workbench Abaqus CAE Support for Vibration Fatigue New in 2018

11 Stresses Calculated Directly by the Finite Element Model 11 Smax = 133 Smax = 142 Smax = 170 Smax = 252

12 Structural Stress from Through Thickness Integration 12 Smax = 117 Smax = 117 Smax = 109 Smax = 115

13 ncode DesignLife Process for Welded Solid Structures 13

14 Limitations of Structural Stress Approach 14 However useful, the structural stress approach is not perfect for all welds Weld fatigue is crack growth, and to analyze it more accurately we need to use crack growth Structural stress approaches either ignore the detailed weld geometry or group welds into relatively simple geometry classes Since the structural stress approach represents the fatigue behavior of a typical weld, it doesn t reflect improvements in weld geometry or welding procedure

15 Thick Welds Design the weld 15 Complex weld geometries where the standard methods don t give the correct answers and are too conservative

16 Automotive Lightweighting 16 Increasing use of different materials and manufacturing techniques means that more complex welded joint are being used in main stream automotive applications

17 Jeff Mentley Applications Engineer