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

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Load (kN) 2 1.5 1 0.5 0 Panel 1 Panel 2 Crack propagation point 0 1 2 3 Displacement (mm) Figure 5.39: Axial piston displacement vs. load diagram from the static tests. To obtain stable crack growth 80% of the static crack propagation load was applied in the fatigue tests as maximum cyclic load with a loading ratio of R=0.1 and a frequency of 2 Hz. Load controlled fatigue tests were performed on the debonded panels and a DIC system was used to monitor the debond shape and the crack growth. Four images every second were captured from the surface of the panel every six hundred cycles by the DIC cameras. The debond front position was located using the out-of-plane displacement contour from the surface of the panels. Locating the debond front from the out-of-plane displacement of the debond is not as accurate as the method used in the previous section for the measurement of the crack growth in STT specimens, which was based on monitoring the crack tip and the strain concentration at the crack tip. However, since it was not possible to gain access to the crack tip due to closed debonding, the best possible option was to use the out-of-plane displacement contours and iso-surfaces to monitor the debond growth, as shown in Figure 5.40. Pre-cracking was performed to penetrate the resin accumulation at the predefined crack tip by loading the specimens up to 50-60% of the static crack propagation load by cyclic loading. Pre-cracking was stopped after approximately 5- 7.5 mm crack growth. Following the pre-cracking the specimens were loaded up to 100000 cycles until the debond reached the edges of the panels. Fatigue crack growth vs. cycle diagrams for the debonded specimens are presented in Figure 5.41. It is seen that initially the crack growth rate is large, but decreases as the crack propagates due to development of membrane forces resulting in a decreasing energy release rate at the crack tip. To investigate the crack growth paths after the panel tests, all tested panels were cut. Figure 5.43 illustrates the crack growth path in the sandwich panels at zero and ninety degrees, as shown in Figure 5.42, along the debond front. During the pre-cracking and initial loading cycles the crack 118
kinks into the core because of very low fracture toughness of the H45 core compared to the interface. It is observed that the crack continues to propagate in the core below the resin-rich cells after kinking. 121 mm 149 mm 16000 cycles 70000 cycles Figure 5.40: Out-of-plane deflection of the debond from DIC measurements. Diameter (mm) 160 150 140 130 120 110 100 0 20000 40000 60000 80000 100000 Figure 5.41: Fatigue crack growth vs. cycles. 119 Cycle Test #1 Test #2 Test #3
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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
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
Page 128 and 129: should be taken into account for a
Page 130 and 131: Figure 5.25: Kinking of the crack i
Page 132 and 133: log (da/dN) (mm/cycle) 10 log G (J/
Page 134 and 135: erroneous extrapolations in the tra
Page 136 and 137: compared to the 65% efficiency obta
Page 138 and 139: the case of uneven debond growth, t
Page 142 and 143: Figure 5.42: Zero and ninety degree
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
Page 148 and 149: panels were determined for differen
Page 150 and 151: Chapter 6 Conclusion and Future Wor
Page 152 and 153: toughness using MMB fracture toughn
Page 154 and 155: For the specimens with H250 core th
Page 156 and 157: Development of testing methods for
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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
Page 164 and 165: Ratcliffe J. and Cantwell W. J. (20
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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
Page 174 and 175: (a) Figure A.14: Out-of-plane defle
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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Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

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