Fatigue Life Analysis of an Oil Injector of a Diesel Engine

Transcription

1 Project -July 2004 On Fatigue Life Analysis of an Oil Injector of a Diesel Engine Tutors: Prof. Dr.-Ing. Joachim W. Bergmann Prof. Dr.-Ing. Habil. C. Könke Dipl.-Ing. Rayk Thumser Dipl. -Ing. Torsten Luther Dipl. -Ing. Diemar Srihari Kurukuri Kiran Kumar Mandapati Dhiraj Patil

2 Contents: Introduction FE Based Evaluation of Notch Factor. FE-based Evaluation of the Plastic Limit Load. S-N- curve prediction for RL=0 based on different methods (local approach and analytical strength assessment). Comparison to experimental results.

3 Introduction: -Injector is subjected to high pressure to low pressure loading during its operation. This type of loading is called fatigue loading. -Fatigue is the process by which the material is stressed repeatedly or in some cyclical manner. -As geometry of injection system is complicated, fatigue design of these components is a challenging job. - Calculation of fatigue life of fuel injector is very complicated because of its sharp notch.

4 Concerning the FE Analysis of notch factor the following points are discussed: Modeling Meshing Material Parameters Boundary Conditions

5 Modeling: actual specimen simplified specimen

6 1/8 th Model 1/16 th Model

7 Meshing:

8 Element Type: Solid95:The element is defined by 20 nodes having three degrees of freedom per node: translation in the nodal x, y and z direction.

9 Material Parameters: Component is made up of 42CrMo4 (ASIS 4140) It is assumed to be follow linear elastic isotropic material law Following material properties are to be used during analysis: Young s Modulus MPa Poisson s ratio 0.3

10 Boundary Conditions:

11 Results and Discussions: Stress Distributions

12 Results and Discussions: 1/Element size vs. Notch factor(kt) N o t c h f a c t o r ( K t ) /Element size Path Plot 1/Element size vs. notch factor

13 Plastic Limit Load Calculation: FE Calculation of plastic limit load 1) Modelling and Meshing 2) loading and Boundary conditions 3) Material Properties

14 Modelling and Meshing: Both 1/8 and 1/16 Models are Used. Coarser and Uniform Meshing.

15 Loading and Boundary Conditions: Symmetric Boundary Conditions. A Load of 2000MPa.

16 Material Properties: Bilinear Kinematic Hardening (BKIN) material law is used Young s Modulus Poisson s ratio Yield strength Tangent modulus MPa MPa 0 During this analysis load is divided into time steps

17 Results and Discussion: Stress Distribution: Whole component is plasticized

18 Kp Calculation: Plastic Notch factor K = P L L P F Models Lp Lf Kp 1/16th /8th

19 Normalised Curve: P f ε f P ε v is Yield Pressure is Yield Strain is Pressure at a point is equivalent strain 1 [ ( ) ( ) ( ) ( ) ( ) ( ) ]2 1 ε = ε + ε ε = ε 2(1 ) x ε V y + εy ε z + εz ε x + ε 2 xy + ε 2 yz + ε + ν 2 xz V, tot V, el V, pl

20 Life Prediction of injection component Fatigue Fatigue Tests SN Curve Main Features of SN Curve 1) Greater the stress fewer the cycles 2) Endurance Limit

21 Factors affecting the fatigue: 1) Type of loading 2) Size of the component 3) Mean Stress 4) Loading frequency 5) Amplitude and 6) Surface finish

22 SN Curve predication of Injection Component: Safe life prediction methodologies Nominal stress life approach Local stress life approach Difference between local strain and nominal stress life Method Calculations for nominal approach Calculations for strain life approach Conclusions

23 Safe life Methodologies: Nominal stress life approach Local strain life approach Differences in life calculated by both

24 Nominal stress life approach: Nominal cross-sectional area Calculation for SN curve Synthetic or measured SN curve Decision for nominal cross-sectional area Empirically based Method Used for notch as well as un-notched specimen Local stress =Kt*nominal stress Modifications in values

25 Local strain approach: Known as low cycle fatigue method Life expectance Magnitude of fluctuating strain Stress analysis by linear elastic material behavior Local plastification by Neuber s Rule PSWT damage parameter- Mean strain Instead of stress life curve- strain life curve Strain life curve combination of plastic and elastic life curve

26 Difference between local strain and nominal stress life Method: Use to predict crack initiation life not total life Strain life curve is use rather than most familiar SN curve Local cycle stress history is analytical synthesized, and tracked on a cycle by cycle basis. Residual stress resulting from overloads or under-loads, or from manufacturing processes such as cold working or interference fit fasteners, are explicitly accounted for

27 Calculation for Nominal approach: σ D [ MPa] σ P [ MPa] σ B [ MPa] R z 2µm = R z = 40µ m K [ cycles] N DB [ bar] S P [ ] * * S B bar Table-9: Load cycle data from nominal approach

28 Conclusions: Local approach vs. experimental data

29 Nominal approach vs. experimental data:

30 Nominal approach vs. experimental data :

31 Comparison of experimental data with predicated curves:

32 Conclusion: Prediction for fatigue life is done by nominal as well as local approach. During this study, it is observed that predicted values obtained by nominal approach are more close to experimental values than local approach. As experimental values are more reliable than predicted values, nominal approach is better than local approach for this special case.

33 Thank you very much for your attention!