Aircraft Structural IntegrityResearch at
Cranfield University
Xiang Zhang
Phil Irving
Niall Smyth
Shrivenham
Cranfield
Location
2000+ employees (2007 data)
2971 students from over 100 countries(54%UK, 24% EU, 22% Overseas)
Cranfield specialises in
• Aerospace
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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?