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

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Figure 3.19 shows the determined energy release rate vs. load curves for the panels with 100, 200 and 300 mm diameter. The fracture toughness values shown in Figure 3.19 were determined from the MMB tests at a -20 phase angle, which is close to the numerically determined phase angle at the experimental debond propagation load, described previously. It appears that the energy release rate increases significantly at a load level which can be associated with the buckling of the debond. The horizontal line in the diagrams shows the average fracture toughness of the interface of the panels for the determined phase angle. The point where the fracture toughness line and energy release rate curve intersect eachother indicates the debond propagation load. (a) (b) (c) 3.19: Energy release rate vs. load for panels with (a) PMI (b) H130 and (c) H250 core. Numerically predicted and experimentally determined propagation loads for the debonded panels are listed in Table 3.7. Due to large scatter in the measured interface fracture toughnesses, the crack propagation load was determined for a maximum, average and minimum measured fracture toughness level. It is seen that based on the average of the measured fracture toughnesses level, the FEA predictions are 7-46% higher than the experimental ones. For the minimum fracture toughness the deviation is between 3-33% and for the maximum 14-65%. This deviation may be partly due to differing crack tip details and crack growth mechanisms between the panels and the MMB specimens. The distortions in the debond crack tip are due to mechanical releasing of the debonds, making the debonds not perfectly circular, as well as some partial adhesions in the debonded area in the experiments as mentioned earlier, which are not taken into account in the finite element modelling. A further cause of inaccuracy in the predictions could be different debond growth mechanisms between the panels and the MMB fracture toughness characterisation tests. To investigate this issue some of the tested panels were cut, and debond propagation paths in the 0 debond front location where the debond starts to propagate were compared with the MMB specimens tested under mode I dominated loading. Figure 3.20 shows debond propagation paths in the panels and the MMB specimens. In the panels and the MMB specimens with H130 and PMI cores the debond kinks into the core and propagates beneath the face/core interface. However, in the panels and the MMB specimens with H250 core the debond propagates directly in the interface, which can be explained by the higher 56
fracture toughness of the H250 core compared to H130 and PMI cores. Fibre bridging was observed after around 4-5 mm of crack propagation in the panels with H250 core. The similarity between the debond propagation paths in the MMB specimens and the panels refutes the role of the different propagation paths in the inaccuracy of the determined debond propagation loads. Table 3.7: Numerical and experimental debond propagation loads. Panel Debond diameter (mm) H130 H250 PMI 57 Debond propagation load (kN) Experiments FE (min./ave./max.) Ratio FE/exp. 50 --- 270/304/324 --- 100 162.5 168/186/202 1.03/1.14/1.24 200 92.5 123/135/151 1.33/1.46/1.63 300 80 102/116/132 1.27/1.45/1.65 50 --- 314/328/341 --- 100 166 193/205/214 1.16/1.23/1.29 200 114.5 135/147/155 1.18/1.28/1.35 300 105 114/123/129 1.08/1.17/1.23 50 --- 181/186/193 --- 100 108.3 112/116/124 1.03/1.07/1.14 200 70.3 76/81/87 1.08/1.15/1.23 300 49 63/66/72 1.28/1.34/1.47
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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
Page 28 and 29: Figure 1.3: Fracture modes, from Be
Page 30 and 31: In Equations (1.5) and (1.6) H11, H
Page 32 and 33: Figure 1.6: Schematic illustration
Page 34 and 35: The compact tension specimen (CT) i
Page 36 and 37: A Figure 1.10: CSB, DCB, TSD, DCB-U
Page 38 and 39: Chapter 2 Buckling Driven Face/Core
Page 40 and 41: (DIC) measurement system (ARAMIS 2M
Page 42 and 43: corresponds to the onset of debond
Page 44 and 45: Figure 2.7: Crack kinking into the
Page 46 and 47: (2.2) where and for plane str
Page 48 and 49: A modified version of the tilted sa
Page 50 and 51: previous observations of crack path
Page 52 and 53: of contact elements (CONTACT173 and
Page 54 and 55: the debond opening initially increa
Page 56 and 57: Table 2.4: Instability loads determ
Page 58 and 59: G (J/m2) 600 400 200 0 H100 IMP=0.1
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Page 62 and 63: Hayman (2007) has described a damag
Page 64 and 65: Table 3.1: Panel test specimens. La
Page 66 and 67: sheet and the core thickness, respe
Page 68 and 69: Load (N) 250 200 150 100 50 0 H130
Page 70 and 71: Table 3.4: Parameters in the face/c
Page 72 and 73: was used to monitor 3D surface disp
Page 74 and 75: arising due to the junction between
Page 76 and 77: (a) Debonded face sheet Figure 3.16
Page 80 and 81: H130 MMB H130 Panel H250 MMB Interf
Page 82 and 83: 3.6 Conclusion In this chapter the
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Page 86 and 87: composite laminates under thermal c
Page 88 and 89: S S y( t ) y( t ) ( t ) 2 1 12 2
Page 90 and 91: listed in Table 4.1. The length and
Page 92 and 93: The strain energy release rate, G,
Page 94 and 95: high growth rate of G (see Figure 4
Page 96 and 97: 4.4 Face/Core Fatigue Crack Growth
Page 98 and 99: Debond 310 mm Symmetry B. C. a Figu
Page 100 and 101: B 2 1/ 2 ( S44 S55 S45) (4.17) wh
Page 102 and 103: 75), which illustrates possible ina
Page 104 and 105: Figure 4.18 (a) presents the deflec
Page 106 and 107: Debond radius (mm) 100 90 80 70 60
Page 108 and 109: using the cycle jump method, more t
Page 110 and 111: Chapter 5 Face/Core Interface Fatig
Page 112 and 113: of this chapter, sandwich panels wi
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
Page 120 and 121: propagation, the crack continues to
Page 122 and 123: Figure 5.14: Kinking of the crack i
Page 124 and 125: Crack length [mm] Crack length [mm]
Page 126 and 127: mode-mixity phase angle was chosen
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should be taken into account for a
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Figure 5.25: Kinking of the crack i
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log (da/dN) (mm/cycle) 10 log G (J/
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erroneous extrapolations in the tra
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compared to the 65% efficiency obta
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the case of uneven debond growth, t
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Load (kN) 2 1.5 1 0.5 0 Panel 1 Pan
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Figure 5.42: Zero and ninety degree
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G(J/m 2 ) 180 150 210 120 150 100 5
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110 q=qG=0.4 Test #1 100 Test #2 Te
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panels were determined for differen
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Chapter 6 Conclusion and Future Wor
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toughness using MMB fracture toughn
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For the specimens with H250 core th
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Development of testing methods for
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Bezazi A., Mahi A. E., Berthelot J.
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Kanny K. and Mahfuz H. (2005), Flex
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Ratcliffe J. and Cantwell W. J. (20
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Out-of-plane displacement (mm) 1 0.
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Out-of-plane displacement (mm) 3 2
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A.5 Out-of-plane deflection of Debo
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(a) Figure A.14: Out-of-plane defle
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(a) (b) Figure A.18: Out-of-plane d
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Load (kN) 250 200 150 100 50 250 30
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Displacement (mm) 4 3 2 1 0 8 9 (a)
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(a) (b) Figure B.11: Out-of-plane d
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(a) (b) Figure B.15: Out-of-plane d
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DTU Mechanical Engineering Section
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