Aircraft Structural Integrity Research at Cranfield University. Xiang Zhang Phil Irving Niall Smyth

Transcription

1 Aircraft Structural Integrity Research at Cranfield University Xiang Zhang Phil Irving Niall Smyth

2 Location Cranfield Shrivenham employees (2007 data) 2971 students from over 100 countries (54%UK, 24% EU, 22% Overseas)

3 Cranfield specialises in Aerospace Defence Engineering Biotechnology Logistics & transport Management Manufacturing Materials Natural Resources Resilience 2007 data: employees 3000 students (54%UK, 24% EU, 22% Overseas)

4 Commenced in 1946 as the Cranfield College of Aeronautics, a postgraduate institution to develop both civil and military aviation Internationally renowned specialist institution in Science, Engineering and Management Turnover 150M 2000 employees Postgraduate 5 Schools, 2 Sites

5 Our Graduates Cranfield produces almost 10% of the UK Engineering postgraduates more than any other UK university. Rated in the top five European Executive Business Schools* Cranfield is the top UK university for Graduate employment. ** * Financial Times ** 97.1% (source HESA)

6 Aerospace & Aviation facilities Commercial Airport Jetstream flying laboratory Vehicle test track & vehicle dynamics test rigs Unique large cabin evacuation simulators Wind, water and atmospheric icing tunnels Rigs for gas turbine performance trials Nanotechnology clean labs Simulation and Synthetic Environments labs Advanced materials & manufacturing labs High strain rate facility Ballistic ranges

7 Structural Integrity in Aircraft Research Approaches Issues on legacy aircraft - Assessment of structural integrity of damaged A/C - Life extension - sustainability! Future aircraft - structural integrity new materials & manufacturing processes - sustainability! - Polymer composite materials -GLARE - Joining - welding metallics; composite joints; hybrid materials Structural health monitoring IVHM centre

8 Skin-stringer panel testing

9 Health Monitoring in Structures Improvements in Reliability Less down time Detection Reduction maintenance costs Improvements in availability Diagnosis Damage Model Usage Prognosis

10 e-lsp Meeting University of Bologna (16-18 Feb 2011) Developing Prediction Methods for crack growth in residual stress fields Xiang Zhang Cranfield University 17 Feb 2011

11 Introduction Improvement in fatigue performance can be attributed to the compressive residual stresses in the surface of shock peened metals In predictive models, influence of residual stresses can be accounted as: mean stress (crack initiation) effective stress intensity factor (crack propagation) Use welding-induced residual stresses, as example, to demonstrate the analysis procedures

12 Content of this presentation Prediction methods Procedures for computing Kres using FEM Current work on: - Inverse method for evaluating weld residual stresses - Weld metal intrinsic crack growth rates Modelling strategy for LSP residual stresses

13 Welded structure & residual stress profiles Courtesy Airbus

14 Methodology Predicting fatigue crack growth life - Superposition method - Crack closure method Calculation of residual stress intensity factor Kres - FEA for standard test specimens: M(T), C(T), ESE(T) - Validation by established Weight Function (WF) solutions - Based on the validation, FEA procedures are established - For complex structures, use FEM

15 Superposition Method (crack growth in tensile residual stresses) da f ( K eff, R eff ) dn K eff K app Reff K app min K res K app max K res

16 Crack Closure approach (crack growth in compressive residual stresses) R eff K app min K res K app max K res U f ( R eff ) (Newman s eq. or FEM) K eff U K a p p C(T) crack growth towards weld da dn f ( K eff ) (Material master curve)

17 Kres is the key parameter for both methods (for predicting crack growth life)

18 Example: a fusion weld M(T) crack growth from weld C(T) crack growth towards weld Same welds and same microstructural properties, but crack positioned in different residual stresses.

19 Procedures for FE calculation of Kres Inputting residual stresses Computing Kres using commercial FE packages Validation by Weight Function (WF) solutions Dealing with partial residual stress field Effect of transverse residual stresses [Bao, Zhang, Yahaya. Evaluating stress intensity factors due to weld residual stresses by the weight function and finite element methods, Eng Fract Mech, 77 (2010) ]

20 Inputting residual stress in FE model - Self-balance - Equilibrium condition [Bao, Zhang, Yahaya. Evaluating stress intensity factors due to weld residual stresses by the weight function and finite element methods, Eng Fract Mech, 77 (2010) ]

21 Kres in an M(T) Welding-induced residual stress FE calculated Kres: WFM vs. FEM [Bao, Zhang, Yahaya. Evaluating stress intensity factors due to weld residual stresses by the weight function and finite element methods, Eng Fract Mech, 77 (2010) ]

22 Kres in an C(T) Welding-induced residual stress 150 FE calculated Kres: WFM vs. FEM initial residual stress distribution from M(T) residual stress before cutting the notch (FEA) redistributed residual stress after cutting the notch (FEA) meased residual stress for C(T) specimen 10 WFM FEM 50 Kres (MPa m ) longitudinal residual stress (MPa) distance from weld center x (mm) distance from weld center 20 x 30 (mm) [Bao, Zhang, Yahaya. Evaluating stress intensity factors due to weld residual stresses by the weight function and finite element methods, Eng Fract Mech, 77 (2010) ] 40

23 Procedures for FE calculation of Kres Inputting residual stresses Computing Kres using FE packages Validation by WFM Partial residual stress field Effect of transverse residual stresses [Bao, Zhang, Yahaya. Evaluating stress intensity factors due to weld residual stresses by the weight function and finite element methods, Eng Fract Mech, 77 (2010) ]

24 Treatment of Partial residual stresses weld line crack x (mm) x (mm) 20 2W Case (1): balanced full field Case (3): artificial balancing Case (2): unbalanced solid line by point forces Case (4): artificial balancing by distributed stress It is important to have a self-balanced residual stress field [Bao, Zhang, Yahaya. Eng Fract Mech, 77 (2010) ] 40

25 Influence of partial residual stresses on Kres 1.0 M(T) 0.8 case 1 case 2 case 3 case case 1 case 2 case 3 case C(T) 0.1 Kres / 0 a Kres / 0 a x (mm) x 40 (mm) Incomplete residual stress data have significant influence on Kres distribution. If an incomplete measured stress distribution is artificially balanced, then the calculated Kres by the FEM is acceptable in the region where the initial residual stress is known from the measurement e.g. for crack from known RS in M(T). If crack starts from un-known residual stress region, e.g. C(T), ESE(T), full field residual stress data is important 60

26 Life Prediction: M(T) by superposition method Constant amplitude load: Ds = 46.4 MPa, R = 0.1.

27 ESE(T): superposition & crack closure methods

28 Conclusions on Kres Calculation WFM is well established for simple specimen geometries; finite boundary correction is necessary FEM is versatile and robust for complex geometries; need care in: a) Residual stress balancing and equilibrium b) Treatment of partial residual stresses Tensile residual stresses Kres redistribution due to crack extension is accounted by WFM & FEM; No need for special modelling/treatment (Buckner principle) Compressive residual stresses a) WF integration needs smoothed residual stress profile b) Full field residual stress data is important

29 Conclusions on FCG life prediction Superposition method works for tensile residual stresses (positive Reff). For cracks initiating from compressive residual stresses, e.g. C(T) geometry, the crack closure approach gives better prediction.

30 Content of this presentation Prediction methods Procedures for computing Kres using FEM Current work on: - Inverse method for evaluating weld residual stresses - Weld metal intrinsic crack growth rates Modelling strategy for LSP residual stresses

31 Inverse Method for Evaluating Residual Stresses R Bao, X Zhang. An inverse method for evaluation of welding residual stresses via fatigue crack growth test data, Eng Fract Mech, 77(2010):

32 Inverse Method for Evaluating Residual Stresses R Bao, X Zhang. An inverse method for evaluation of welding residual stresses via fatigue crack growth test data, Eng Fract Mech, 77(2010):

33 Inverse Method for Evaluating Residual Stresses (a) FCG test data for fusion weld (b) Calculated residual stress field and comparison with measurement R Bao, X Zhang. An inverse method for evaluation of welding residual stresses via fatigue crack growth test data, Eng Fract Mech, 77(2010):

34 Evaluating weld metal intrinsic crack growth rates X Zhang, R Bao. Determination of intrinsic crack growth properties in welded material, Int J Fatigue, 33 (2011)

35 Evaluating weld metal intrinsic crack growth rates X Zhang, R Bao. Determination of intrinsic crack growth properties in welded material, Int J Fatigue, 33 (2011)

36 Content of this presentation Prediction methods Procedures for computing Kres using FEM Current work on: - Inverse method for evaluating weld residual stresses - Weld metal intrinsic crack growth rates Modelling strategy for LSP residual stresses

37 Modelling Crack Growth in LSP Metals issues for discussion Crack initiation prediction Unknown, being neglected, worth research effort Crack propagation life Kres is key parameter 3D cracks (surface flaws): Use weight function Kres in 3D residual stresses 2D cracks (through-thickness) in 3D residual stresses Slice synthesis model (WFM or FEM) to find Kres Partially measured residual stress field is an issue Can we find full field residual stress by calculation?

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