2018 ncode User Group Meeting

Transcription

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

2 WholeLife Dr Andrew Halfpenny Director of Technology ncode Products

3 WholeLife Summary 20 WholeLife represents a unified approach to fatigue. From crack initiation to final failure. First release specifically targeting welded joints. Outstanding accuracy over current methods. Particularly well suited to lightweight structures and thick welds. New DesignLife option for welds

4 New WholeLife Glyph for Welds in DesignLife 21 A new Unified Theory of fatigue developed with Prof. G. Glinka, University of Waterloo, Canada More accurate modelling of the complete failure process that leads to better correlation with test Uses standard seam weld modelling Ability to analyse the detailed design of individual welds Multiple failure modes may be investigated Efficient, only critical locations are analysed Supports multiaxial time based loading Residual stresses in weld may be included

5 Fatigue Initiation & Crack Growth 22 Total Life = Crack Initiation + Crack Growth SN Analysis* EN Analysis LEFM Total Life Crack Initiation + Crack Growth

6 Idealisation of a crack growing through a plate 23 Progressive crack growth: sequence of successive initiation failures High stress at crack tip causes slip planes and progressive weakening of the grain Stress intensity increases as the crack grows so failure of each grain occurs more quickly Effective radius of crack tip grain size * *

7 Crack Growth Model 24 Crack growth rate is a function of the crack tip driving force Δ Stress Intensity K min K max K nxt Time Δ is a function of the stress intensity and R ratio (after Walker) Δ is a function of stress, geometry, crack length, and the residual stress field at the tip of the crack is the small crack correction 1

8 Universal Weight Function (UWF) Solutions 25 Y = f(geometry, stress profile) Transforms nominal stress into Stress Intensity (K) at the crack tip UWF applies stress profile explicitly of the geometry (i.e. use a single geometry for any number of stress distributions) UWF can deal with complex stress distributions such as residual stress fields and crack tip wake stresses

9 Cyclic Crack tip Plasticity Model 26 Crack tip opening stress ys Theoretical elastic stress Stress Intensity K 0 K 1 K 3 K 2 K max K min Time 2 x Multiaxial crack tip stress profile based on Creager Paris law for blunt cracks:

10 Cyclic Crack tip Plasticity Model 27 Crack tip opening stress ys Theoretical elastic stress Plastic energy needs redistributing Stress Intensity K 0 K 1 K 3 K 2 K max K min Time 2 x Loading Crack tip closing s 0 1 Unloading Crack tip plasticity is based on multiaxial Neuber Ramberg Osgood cyclic plasticity model with plastic redistribution: Stress 2 s 3 s Strain 2 2

11 Cyclic Crack tip Plasticity Model 28 Crack tip opening stress ys Theoretical elastic stress Plastic energy needs redistributing Stress Intensity K 0 K 1 K 3 K 2 K max K min Time 2 x Crack tip closing 2 r f Compression Crack tip plasticity is based on multiaxial Neuber Ramberg Osgood cyclic plasticity model with plastic redistribution: Stress s 1 1 = 3 s 1 1 = 3 Strain Stress 2 Strain Stress s 1 1 = 3 s 2 =0 Strain 2

12 Cyclic Crack tip Plasticity Model Crack retardation 29 Current Overload Cycle r 1 r 2 r f a s r Compressive wake from variable amplitude loading a a Compressive wake from constant amplitude loading

13 WholeLife Weld

14 Inputs to a WholeLife Weld Calculation 31 Applied load histories Through-thickness Kt profiles Bending Membrane Residual stress profile

15 Kt Profiles 32 Kt Stress profiles and Weight functions are used to calculate stress intensity factors DesignLife has default parameters based on simple weld geometry Stress multiplication factor Bending Membrane Fillet weld profile: Monahan CC (1995) Early fatigue cracks growth at welds. Computational Mechanics Publications, Southampton. Custom routine: CSV import of profiles Python scripting

16 Inputs to a WholeLife Weld Calculation 33 Applied load histories Through-thickness Kt profiles Bending Membrane Residual stress profile WholeLife material data

17 WholeLife Material Data 34 Strain-Life (EN) properties LEFM crack growth properties 0.01 Strain Amplitude (ue) R = 0.3 R = 0.2 R = Reversals to Failure Stress vs. Strain curve Stress Strain

18 Validation using SAE FD&E T Joint Test Data

19 Description of SAE FD&E Committee Total Life Project 36 SAE Fatigue Design & Evaluation (FD&E) Committee TOTAL LIFE FATIGUE PROJECT To validate the Glinka methodology life project

20 SAE Case Study specimen loading 37 Constant amplitude 24kN, R = kN, R = kN, R = kn, R = 1 Block load 24kN, variable amplitude, block load Random 24kN, variable amplitude, time history file

21 SAE Case Study stress distribution and residual stresses 38 Comparison - Welded to Machined FEM Stress Distributions Stress distribution from applied loads Residual stresses in the welded specimens

22 SAE Case Study 39 A36 T-Joint Test Results

23 SAE Case Study DesignLife analysis 40

24 Correlation with Test Comparison with Standard Solid Seam Weld Analysis 41

25 WholeLife vs Seam weld 42 The standard seam weld approach uses an SN curve that represents the fatigue behavior of a typical weld. Using this One Size Fits All approach, this SN curve is purposely conservative to deal with the worst case welds. WholeLife has used the actual weld geometry, and crack growth properties to predict the life. This reduces the need for conservatism, resulting in a more accurate life prediction. This methodology has been demonstrated by SAE to estimate life within a factor of

26 Demonstration

27 WholeLife Weld Demo 44

28 Summary 45 WholeLife represents a unified approach to fatigue Includes initiation and propagation stages Applicable to all fatigue analyses including welded joints Outstanding accuracy over current methods Particularly well suited to lightweight structures and thick welds We have correlated this method to the SAE test cases

29 Questions

30 Dr Andrew Halfpenny Director of Technology ncode Products WholeLife V???.pptx