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

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Table 3.4: Parameters in the face/core interface fracture toughness function, Equation (3.14). Core GIC (J/m 2 ) Average fit GIC (J/m 2 ) Lower bound GIC (J/m 2 ) Upper bound k H130 450 280 620 0.45 H250 500 350 660 0.55 PMI 115 80 150 0.55 Figure 3.6 illustrates the typical crack path observed in the MMB specimens with PMI core. As it is seen, the crack grows below resin-rich cells in the core for all measured mode-mixities. Figure 3.6: Crack path for an MMB specimen with PMI core. For specimens with H130 core and mode-mixity phase angle of 0° >> -20°, the crack path was located below the face/core interface, see Figure 3.7. However, for the mode-mixity phase angle of -25° >> -65°, as shown in Figure 3.8, the crack path was in the actual face/core interface. As mentioned earlier, a toughening mechanism due to the increased negative mode-mixity is observed in this core material as well. This trend in the fracture toughness vs. mode-mixity was previously reported for other debonded sandwich materials tested at controlled mode-mixities, see Berggreen et al. (2005). Figure 3.7: Crack path for an MMB specimen with H130 core below the face/ core interface. 48
Figure 3.8: Crack path for an MMB specimen with H130 core in the face/core interface. For specimens with H250 core, the crack propagated in the interface for all measured modemixities as shown in Figure 3.9. In these specimens the fracture toughness increased with increasing magnitude of the negative mode-mixity phase angle at the crack tip similar to the specimens with PMI and H130 cores. Additionally, it was observed that in longer crack lengths (4mm), fibre bridging started to emerge, which can be attributed to the CSM layer placed in the face/core interface during the manufacturing process of the MMB specimens. The fibre bridging enhances the fracture toughness by creating a large fracture process zone and, thus, the modemixity might lose its validity. Since the fracture experiments are focused on fracture initiation and not propagation, no analysis for fibre bridging is presented in this study. 3.4 Panel Tests Figure 3.9: Crack path for an MMB specimen with H250 core. Figure 3.10 shows the test rig designed to introduce a uniform in-plane compressive load to the edges of either plane or singly curved sandwich panels. The test rig was inserted into a four- column Instron 8508 servo-hydraulic testing machine with a maximum capacity of 5 MN. However, a 1 kN Instron load cell was used for the tests to increase the accuracy of the load measurements. A 4 Mpix Digital Image Correlation (DIC) measurement system (ARAMIS 4M) 49
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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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Page 20 and 21: H11 bimaterial constant H22 bimater
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Page 24 and 25: These peculiar damage modes often r
Page 26 and 27: 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
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 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 78 and 79: Figure 3.19 shows the determined en
Page 80 and 81: H130 MMB H130 Panel H250 MMB Interf
Page 82 and 83: 3.6 Conclusion In this chapter the
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
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propagation, the crack continues to
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Figure 5.14: Kinking of the crack i
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Crack length [mm] Crack length [mm]
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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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Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

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