aircraft structural integrity research at cranfield university · aircraft structural integrity...

Aircraft Structural IntegrityResearch at

Cranfield University

Xiang Zhang

Phil Irving

Niall Smyth

xiang.zhang@cranfield.ac.uk

Shrivenham

Cranfield

Location

2000+ employees (2007 data)

2971 students from over 100 countries(54%UK, 24% EU, 22% Overseas)

Cranfield specialises in

• Aerospace

• Defence

• Engineering

• Biotechnology

• Logistics & transport

• Management

• Manufacturing

• Materials

• Natural Resources

• Resilience

2007 data:

2000+ employees

3000 students (54%UK, 24% EU, 22% Overseas)

Commenced in 1946 as the Cranfield College ofAeronautics, a postgraduate institution” to developboth civil and military aviation”…

• Internationally renowned specialistinstitution in Science, Engineeringand Management

• Turnover £150M

• 2000 employees

• Postgraduate

• 5 Schools, 2 Sites

Our Graduates

Cranfield produces almostCranfield produces almost10% of the UK Engineering10% of the UK Engineeringpostgraduatespostgraduates –– more thanmore thanany other UK university.any other UK university.

* Financial Times* Financial Times

** 97.1% (source HESA)** 97.1% (source HESA)

Cranfield is the top UKCranfield is the top UKuniversity for Graduateuniversity for Graduateemployment. **employment. **

Rated in the top fiveEuropean ExecutiveBusiness Schools*

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

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; hybridmaterials

• Structural health monitoring – IVHM centre

Skin-stringer panel testing

Health Monitoring in Structures

Detection

Diagnosis

PrognosisUsage

DamageModel

• Improvements in Reliability

• Less down time

• Reduction maintenance costs

• Improvements in availability

Xiang Zhang

Cranfield University

17 Feb 2011

e-LSP Meeting

University of Bologna(16-18 Feb 2011)

Developing Prediction Methods

for crack growth in residual stress fields

Introduction

• Improvement in fatigue performance can be attributedto the compressive residual stresses in the surface ofshock peened metals

• In predictive models, influence of residual stresses canbe accounted as:

• “mean stress” (crack initiation)

• “effective stress intensity factor” (crack propagation)

• Use welding-induced residual stresses, as example, todemonstrate the analysis procedures

Content of this presentation

Prediction methods

Procedures for computing Kres using FEM

Current work on:

- Inverse method for evaluating weld residualstresses

- Weld metal intrinsic crack growth rates

Modelling strategy for LSP residual stresses

Welded structure & residual stress profiles

Courtesy Airbus

Methodology

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

Predicting fatigue crack growth life

- Superposition method

- Crack closure method

Superposition Method(crack growth in tensile residual stresses)

),( effeff RKfdN

da

resapp

resapp

effKK

KKR

max

min

appeff KK

“Crack Closure” approach(crack growth in compressive residual stresses)

)( effKfdN

da

eff appK U K

( )effU f R

resapp

resapp

effKK

KKR

max

min

(Newman’s eq. or FEM)

(Material “master” curve)

C(T) crack growth towards weld

Kres is the key parameter

for both methods

(for predicting crack growth life)

Example: a fusion weld

360

75

Same welds and same microstructural properties,

but crack positioned in different residual stresses.

M(T) crack growth from weld C(T) crack growth towards weld

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 andfinite element methods, Eng Fract Mech, 77 (2010) 2550–2566.]

Inputting residual stress in FE model

[Bao, Zhang, Yahaya. Evaluating stress intensity factors due to weld residual stresses by the weight function andfinite element methods, Eng Fract Mech, 77 (2010) 2550–2566.]

- Self-balance

- Equilibrium condition

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 andfinite element methods, Eng Fract Mech, 77 (2010) 2550–2566.]

Kres in an C(T)

-40 -30 -20 -10 0 10 20 30 40

-150

-100

-50

0

50

100

150

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

long

itud

inal

resi

dua

lst

ress

(MP

a)

distance from weld center x (mm)

Welding-induced residual stress

-10 0 10 20 30 40-15

-10

-5

0

5

10

Kre

s(M

Pa

m)

distance from weld center x (mm)

WFMFEM

FE calculated Kres: WFM vs. FEM

[Bao, Zhang, Yahaya. Evaluating stress intensity factors due to weld residual stresses by the weight function andfinite element methods, Eng Fract Mech, 77 (2010) 2550–2566.]

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 andfinite element methods, Eng Fract Mech, 77 (2010) 2550–2566.]

Treatment of Partial residual stresses

It is important to have a self-balanced residual stress field

-20 0 20 40-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

x (mm)-40 -20 0 20 40

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-40x (mm)

Case (1): balanced – full field

Case (2): unbalanced – solid line

[Bao, Zhang, Yahaya. Eng Fract Mech, 77 (2010) 2550–2566.]

crack

weld line

2W

Case (3): artificial balancing

by point forces

Case (4): artificial balancing

by distributed stress

Influence of partial residual stresses on Kres

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 residualstress 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

0 5 10 15 20 25 30 35 40

0.0

0.2

0.4

0.6

0.8

1.0

x (mm)

case 1case 2case 3case 4

Kre

s/

0a

-40 -20 0 20 40 60-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

Kre

s/

0a

case 1case 2case 3case 4

x (mm)

C(T)M(T)

Life Prediction: M(T) by superposition method

Constant amplitude load: Ds = 46.4 MPa, R = 0.1.

ESE(T): superposition & crack closure methods

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

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.

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

Inverse Method for Evaluating Residual StressesR Bao, X Zhang. An inverse method for evaluation of welding residual stresses via fatigue crack growth test data, Eng Fract Mech, 77(2010):3143-3156.

Inverse Method for Evaluating Residual Stresses

R Bao, X Zhang. An inverse method for evaluation of weldingresidual stresses via fatigue crack growth test data, Eng FractMech, 77(2010): 3143-3156.

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 ofwelding residual stresses via fatigue crack growth test data,Eng Fract Mech, 77(2010): 3143-3156.

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) 588–596

Evaluating weld metalintrinsic crack growth rates

X Zhang, R Bao. Determination of intrinsic crack growthproperties in welded material, Int J Fatigue, 33 (2011)588–596

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

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 stressesSlice 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?