Interlaminar Progressive Failure Analysis of Composite Curved Aircraft structural components using...

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• Failure initiation and Propagation in a structure have to be appropriately predicted for any structure

• Local-Global issue: A very small Local failure could lead to a catastrophic Global collapse.

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Interlaminar Progressive Failure Analysis of Composite Curved Aircraft structural components

using MSC MARC

MSC Software India User Conference 2013

Raju


QuEST Global Engineering Diversified Global Engineering Services Company

• 4200+ professionals

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• Operating in 10 countries spread across APAC, Europe and NA

• Involved in high- technology product engineering and support

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POWER GENERATION OIL & GAS

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Equipment • Consumer Packaged Goods

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Composites Failure Mechanisms

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Delamination at Material and Geometric discontinuities

Inter-fiber failure modes

Delamination

Matrix Cracking

Fiber breakage under Tensile and Compressive load

Failure types in Composites • Matrix failure

• Debonding

• Fibre failure

• Interlaminar Failure


Curved composites & Delamination

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• Delamination is the primary failure mode for composites

• It reduces the strength and integrity of the structure, mainly through matrix cracking.

• Delamination is generally induced by Interlaminar tensile and shear stresses (ILTS & ILSS)

• These could be due to – Stiffness (elastic modulus)

mismatch between the layers, – Structural discontinuities, – Free edge effects around holes – Impact by a foreign object – Through-thickness stress in curved

composites


• Multiple failure at the bends of some aircraft structural components

• These were premature failures, identified with NDE techniques

• Significant amount of damage was observed with a number of components.

• The design of those components were based on Tensile and Shear strength

• Curved GFRP L-Bend specimens to be used.

• Objectives of the project: – An effective non-linear, orthotropic numerical model – Detailed distribution of stresses around the stress concentration area, just before

failure and a thorough Progressive failure analysis – Numerical model to be validated with detailed experimental investigation

Requirement of this project

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Material and Layup sequence

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L-Bend Layup 1 - CSM Layup 2 - CSM+DB Layup 3 - CSM+DB+UD

Layer 1 CSM 450gsm CSM 450gsm DB 611gsm

Layer 2 CSM 450gsm DB 611gsm CSM 450gsm

Layer 3 CSM 450gsm CSM 450gsm UD 451gsm

Layer 4 CSM 450gsm DB 611gsm DB 611gsm

Layer 5 CSM 450gsm CSM 450gsm CSM 450gsm

Layer 6 CSM 450gsm DB 611gsm UD451gsm

Layer 7 CSM 450gsm CSM 450gsm DB 611gsm

Thickness 4.62mm 4.62mm 4.62mm

• Fibre – Glass Fibre – Chopped Strand Mat (CSM) – Double Bias (DB) - +/- 450

– Uni-Directional Fibre UD)

• Resin – Vinylester • Hardener - MEKP

Chopped Strand Mat Double Bias

Uni-Directional


Material Properties used in the numerical model

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Material Properties Property CSM DB UD

E11 (MPa)b 9607 7872 23576 E22 (MPa)b 9607 7872 6560 E33 (MPa)c 6062 6673 6560 υ12 (MPa) c 0.347 0.62 0.369 υ23 (MPa) c 0.139 0.146 0.249 υ31 (MPa) c 0.108 0.133 0.087 G12 (MPa) c 2602 7157 2265 G23 (MPa) c 1847 2050 1847 G31 (MPa) c 1847 2050 2265 ILTS (MPa)a 9.5 10.5 10.5 ILSS (MPa)a 28 30 28

Flexure Modulus (MPa)b 1822 1169 4235 Flexure Strength (MPa)b 222 75 437

Fracture Toughness G1C (kJ/m2)a 0.68 1.04 0.84 Critical opening displacement (mm)a 0.05 0.05 0.05

a Literature [ Johnson, 1986; Dirand et al., 1996; Perrot et al., 2007; Park and Jang, 2004; Kitching et al., 1984] b Experimental testing c Using ‘Component Design and Analysis (CoDA)' software with resin and fibre properties


Specimen configuration details

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L-bend specimen geometry L-bend experimental setup


• Three different models for three layups were generated from MSC. PATRAN R2.1 and imported to MSC MARC – for Static analysis

• Three different layups, finer mesh around the corners. Each element thickness represents a layer in the laminate.

• Element 7 - Eight node isoparametric, arbitrary hexahedral element – Brick Element

• A single load case was defined with 200 fixed load steps.

FE Parameters

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Element orientations at the bend; Local co-ordinate system


• Three possible ways: – Virtual Crack Closure Technique (VCCT) – pre-existence of crack front – Cohesive Zone elements or Interface elements – stress based method for crack initiation

and fracture mechanics for crack progression – Used in current analysis – Strain Invariant Failure Theory (SIFT) – requires critical equivalent strain for both matrix

and fibre reinforcement phases.

• Available commercial Packages for delamination: – MSC MARC; ABAQUS

• Strength of MSC. MARC – Very powerful, general nonlinear versatile, stable, user friendly FEA package – Some benchmark studies with ABAQUS suggested that MARC is faster, stable and able

to handle complex non-linear problems; – Fracture mechanics in-built (Industries have no time to write own codes); – Easy and quick contact body definition; Adaptive meshing; – Multi-physics model capability; Non-linear material models; – Powerful failure analysis with multi-failure criteria; – Integrated pre/post-processing for ease of use;

Delamination prediction methods and available commercial packages; MSC. MARC preference

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• Failure initiation – Crack initiation based on the interaction of interlaminar tensile (ILTS) and shear stresses (ILSS), where the laminates are assumed to be free of defects.

• Damage onset is predicted using a quadratic stress criterion, allowing the mesh to split between the materials

• where σn and σt are the normal (ILTS) and tangential (ILSS) stresses and Sn and St, are the critical values of normal and tangential stresses

Failure Modelling – Crack Initiation – Stress based approach

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𝛔𝛔𝐧𝐧𝐒𝐒𝐧𝐧

𝟐𝟐+ 𝛔𝛔𝐭𝐭

𝐒𝐒𝐭𝐭

𝟐𝟐= 1


• Crack propagation is performed using Fracture Mechanics Criteria

• In the current numeric model, cohesive interface elements are introduced whenever delamination occurs.

• The effective traction is introduced as a function of the effective opening displacement and characterised by an initial reversible response, followed by an irreversible response as soon as a critical effective opening displacement has been reached, as shown in Figure.

Failure Modelling – Crack Propagation – Fracture Mechanics based approach

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Damage evolution curve for bilinear cohesive element


• The irreversible part is characterised by increasing damage, ranging from zero (onset of delamination) to one (full delamination). For maximum effective traction, tc corresponding to the critical effective opening displacement vc is expressed as

• Once the corresponding initiation criterion is reached, the specified damage

evolution law describes the rate the material stiffness degrades. • A scalar damage variable ‘D’ (𝟎𝟎 ≤ 𝑫𝑫 ≤ 𝟏𝟏) represents the overall damage in

the material and captures the combined effects of all active degradation mechanisms.

• When the overall damage variable reaches its limit Dmax at all material points, the cohesive element corresponding to complete fracture of the interface between layers can be removed and is considered as delamination propagation.

Failure Modelling – Crack Propagation….

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𝒕𝒕𝒄𝒄 =𝟐𝟐𝑮𝑮𝒄𝒄𝒗𝒗𝒎𝒎


Layup 1 Results

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L-Bend Layup 1 ILTS and ILSS distribution

Layup 1 - CSM CSM 450gsm CSM 450gsm CSM 450gsm CSM 450gsm CSM 450gsm CSM 450gsm CSM 450gsm


Layup 1 Crack Initiation & Progression

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Layup 1 - CSM CSM 450gsm CSM 450gsm CSM 450gsm CSM 450gsm CSM 450gsm CSM 450gsm CSM 450gsm


Layup 2 Results

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Layup 2 - CSM+DB CSM 450gsm DB 611gsm

CSM 450gsm DB 611gsm


CSM 450gsm



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Layup 2 - CSM+DB CSM 450gsm DB 611gsm



CSM 450gsm


Layup 3 Results

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Layup 3 - CSM+DB+UD

DB 611gsm CSM 450gsm UD 451gsm DB 611gsm

CSM 450gsm UD451gsm DB 611gsm



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Layup 3 - CSM+DB+UD

DB 611gsm CSM 450gsm UD 451gsm DB 611gsm

CSM 450gsm UD451gsm DB 611gsm


Through thickness ILTS Distribution

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0

3

6

9

12

0 0.66 1.32 1.98 2.64 3.3 3.96 4.62

ILTS

(MPa

)

Thickness (mm)

Layup 1

Layup 2

Layup 3

L-Bend Interlaminar Tensile Stress distribution just before first failure for all three layups


Through thickness ILSS Distribution

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-14

-10

-6

-2

2

0 0.66 1.32 1.98 2.64 3.3 3.96 4.62

ILSS

(MPa

)

Thickness (mm)

Layup 1 ILSS

Layup 2 ILSS

Layup 3 ILSS

L-Bend Interlaminar Shear Stress distribution just before first failure for all three layups


• Three layups of L-bend composite specimens (five each) were tested under displacement control up to a displacement of 100 mm.

• The load-carrying capacity, Failure initiation and progression were studied for each layup.

• With the detailed analysis, it was possible to estimate the reserve strength of the structure with the given amount of damage.

• The bending load in this project generated more interlaminar tensile stress than interlaminar shear stress.

• The failure initiation is a function of both the interlaminar tensile and shear stresses. Failure progression was done through Fracture mechanics approach.

• Numerical results were in good agreement with experimental data

Conclusion

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• A material and geometric non-linear numerical model for complex composite failure is developed.

• This numerical model is effectively validated by experimental investigation.

• Based on the analysis, the scope of the design for future projects changed from tensile/shear strength perspective to Interlaminar stress approach.

• This exercise has helped us, as well as the customer to think ‘out of the box’ to solve critical issues.

Business Value Decision and Achievement

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