Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

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Development of testing methods for measuring the effect of mode III loading at the crack tip on fracture toughness and fatigue crack growth rates, is necessary for the development of accurate damage assessment tools for analysis of the residual strength and lifetime of debonded sandwich composites. In the last chapters of the thesis, fatigue experiments were conducted on sandwich panels with a circular debond to validate the developed 3D numerical scheme. However, to fully examine the effectiveness of the developed numerical scheme and its limitations, more experiments on more complex geometries like curved structures and loading conditions, such as e.g. lateral pressure and in-plane compression, should be conducted. Additionally, since direct access to the crack in debonded panels is not possible, methods of measurement such as FBG sensors or ultra-sonics should be used instead of DIC technique to obtain better measurement of the debond growth. Different finite element based models have been put forward in this thesis based on fracture mechanics tools. However, all these models are limited to very simple, small geometries due to the need for high-density mesh at the crack tip (front) and practical/geometrical limitations regarding the generation of 3D finite element models in complex large geometries. In summary, in the case of damage assessment of large structures the global response of the structure should be accounted for in these models making the resulting finite element model extremely computationally heavy. An important step in further development of the devised damage assessment schemes is to develop new methodologies for the analysis of delamination/debonding in composite structures, taking into account the global response of the structures. The submodelling concept, as used in Chapters 2 and 3 of this thesis, can be used in a new way which allows for coupling between the global response of the structure and the local effects of the damage. As schematically described in Figure 6.1, an iterative procedure may be devised which couples the global complete model of structures and a detailed model of the damaged region as developed in this thesis. The global stiffness properties and behaviour of a structure, e.g. a wind turbine blade, can be determined using finite element modelling of the blade by shell elements as the first step. By reducing the stiffness of the elements in the damaged zone the effect of initial damage on the global behaviour of the structure can be estimated. The displacement boundary conditions of the 3D submodel, detailing the damaged zone of the structure, are subsequently updated on the basis of the results from the global shell finite element model by shell to solid submodelling technique, which is available in most of the commercial finite element software like ANSYS. The detailed analysis of the damaged zone can be conducted by means of state-ofthe-art fracture mechanics tools developed in this thesis. In the case of cyclic loading by determining the debond growth at the end of each cycle (cycle jump), the geometry of the debond can be updated in the global shell model accordingly. Finally, the global shell FE model of the blade can be reconstructed once again and the procedure is repeated for the next iteration, e.g. loading cycle. Thus, it is possible to overcome the scale limitations and couple the local scale effects of the debond damage with the global scale response of the blade. 134
As an alternative approach to modelling of the global structure using shell elements, which may be computationally expensive, beam cross sectional analysis, see e.g. Blasques et al. (2011), may be applied to analysis of the global response of the structure and extraction of interpolated boundary conditions in the cutting boundaries of the detailed submodel. Moreover, the cross section stiffness properties of the beam are then recomputed based on a 2D finite element representation of the cross section where the shape of the debonded area has been updated. Figure 6.1: Schematic presentation of the suggested multi-scale approach to the analysis of debond damaged sandwich structures. 135
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Residual Strength and Fatigue Lifet
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Published in Denmark by Technical U
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method to accelerate the simulation
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Synopsis Sandwich kompositter er i
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Contents Preface Executive Summary
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5 Face/core Interface Fatigue Crack
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H11 bimaterial constant H22 bimater
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These peculiar damage modes often r
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1.2 Overview of the Thesis In this
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Figure 1.3: Fracture modes, from Be
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In Equations (1.5) and (1.6) H11, H
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Figure 1.6: Schematic illustration
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The compact tension specimen (CT) i
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A Figure 1.10: CSB, DCB, TSD, DCB-U
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Chapter 2 Buckling Driven Face/Core
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(DIC) measurement system (ARAMIS 2M
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corresponds to the onset of debond
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Figure 2.7: Crack kinking into the
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(2.2) where and for plane str
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A modified version of the tilted sa
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previous observations of crack path
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of contact elements (CONTACT173 and
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the debond opening initially increa
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Table 2.4: Instability loads determ
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G (J/m2) 600 400 200 0 H100 IMP=0.1
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Hayman (2007) has described a damag
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Table 3.1: Panel test specimens. La
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sheet and the core thickness, respe
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Load (N) 250 200 150 100 50 0 H130
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Table 3.4: Parameters in the face/c
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was used to monitor 3D surface disp
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arising due to the junction between
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(a) Debonded face sheet Figure 3.16
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Figure 3.19 shows the determined en
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H130 MMB H130 Panel H250 MMB Interf
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3.6 Conclusion In this chapter the
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composite laminates under thermal c
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S S y( t ) y( t ) ( t ) 2 1 12 2
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listed in Table 4.1. The length and
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The strain energy release rate, G,
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high growth rate of G (see Figure 4
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4.4 Face/Core Fatigue Crack Growth
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Debond 310 mm Symmetry B. C. a Figu
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B 2 1/ 2 ( S44 S55 S45) (4.17) wh
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75), which illustrates possible ina
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Figure 4.18 (a) presents the deflec
Page 106 and 107: Debond radius (mm) 100 90 80 70 60
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Page 110 and 111: Chapter 5 Face/Core Interface Fatig
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Page 114 and 115: H250 Specimen H100 Specimen H45 Spe
Page 116 and 117: Figure 5.5: Test setup. Initially,
Page 118 and 119: H100 Specimen Fibre bridging Figure
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Page 122 and 123: Figure 5.14: Kinking of the crack i
Page 124 and 125: Crack length [mm] Crack length [mm]
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Page 128 and 129: should be taken into account for a
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Page 132 and 133: log (da/dN) (mm/cycle) 10 log G (J/
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Page 136 and 137: compared to the 65% efficiency obta
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Page 140 and 141: Load (kN) 2 1.5 1 0.5 0 Panel 1 Pan
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Page 144 and 145: G(J/m 2 ) 180 150 210 120 150 100 5
Page 146 and 147: 110 q=qG=0.4 Test #1 100 Test #2 Te
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Page 150 and 151: Chapter 6 Conclusion and Future Wor
Page 152 and 153: toughness using MMB fracture toughn
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Page 160 and 161: Bezazi A., Mahi A. E., Berthelot J.
Page 162 and 163: Kanny K. and Mahfuz H. (2005), Flex
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Page 168 and 169: Out-of-plane displacement (mm) 1 0.
Page 170 and 171: Out-of-plane displacement (mm) 3 2
Page 172 and 173: A.5 Out-of-plane deflection of Debo
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Page 176 and 177: (a) (b) Figure A.18: Out-of-plane d
Page 178 and 179: Load (kN) 250 200 150 100 50 250 30
Page 180 and 181: Displacement (mm) 4 3 2 1 0 8 9 (a)
Page 182 and 183: (a) (b) Figure B.11: Out-of-plane d
Page 184 and 185: (a) (b) Figure B.15: Out-of-plane d
Page 188: DTU Mechanical Engineering Section
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