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

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110 q=qG=0.4 Test #1 100 Test #2 Test #3 0 20000 40000 60000 80000 100000 Cycle Figure 5.48: Debond diameter vs. cycles for the simulation with the control parameters qG=q=0.4. The number of simulated cycles and the computational efficiency of the simulations with different control parameters are listed in Table 5.5. By application of the cycle jump method up to 94% of the simulation time has been saved with fair accuracy. Increasing the control parameters leads to increasing computational efficiency up to 96%, but the accuracy of the simulations is considerably lower. Table 5.5: Computational efficiency of solutions with different control parameters. Control parameter qG=q Diameter (mm) 160 150 140 130 120 Simulation of debonded sandwich panels Number of simulated cycles Saved simulation cycles (%) 0.4 7121 92.879 0.45 6087 93.913 0. 5 5896 94.104 0.75 5051 94.949 1 3778 96.222 5.4 Conclusion In this chapter the accelerated fatigue crack growth simulation scheme developed in Chapter 4 was used to study interface fatigue crack growth in sandwich composites. Moreover, the accuracy and efficiency of the developed scheme were validated against fatigue experiments conducted on debond damaged sandwich beams and panels. 124
Sandwich Tear Test (STT) specimens with face/core debonding exposed to cyclic loading have been tested to study the fatigue behaviour of the interface cracked sandwich X-joints. Fatigue tests were performed on the STT specimens with H45, H100 and H250 PVC cores and glass/polyester face sheets. The following fatigue crack growth paths were observed during the experiments: For the specimens with H45 core, unstable crack growth took place initially. After the unstable propagation the crack propagated in the core underneath the resin-rich cell layer moving towards the interface. However, the crack never kinked into the interface due to low fracture toughness of the H45 core compared to the interface. For the specimens with H100 core, the crack propagated initially in the core and then kinked into the interface and continued to propagate in the interface. For the specimens with H250 core, the crack initially propagated in the core and kinked into the interface. The interface crack eventually kinked into the face sheet, resulting in large-scale fibre bridging. By application of the finite element method mode-mixity phase angles at the crack tip of the STT specimens were evaluated at different crack lengths using the maximum fatigue load amplitude. To characterise the fatigue response of the interface of the STT specimens, fatigue tests were performed on Mixed Mode Bending (MMB) specimens at similar mode-mixity phase angles. The da/dN vs. G relations measured by the MMB fatigue tests were used to simulate fatigue crack growth in the STT specimens by the finite element method. To accelerate the simulation, the cycle jump method was exploited. Control parameters were introduced to control the accuracy of the cycle jumps. Simulations with different control parameters and a convergence analysis were carried out to choose the most accurate and efficient control parameters. Simulations of the specimens with H100 core showed fair accuracy compared to the fatigue experiments. However, the simulation of H45 specimens was found to be less accurate due to unstable crack growth observed in the fatigue experiments of the H45 STT specimens. This inaccuracy can be attributed to the interface fatigue characterisation. Since the interface fatigue characterisation was only performed for the stable linear part of the crack growth rate diagram (the Paris regime), the resulting da/dN vs. G relation is not valid for unstable crack growth and produces incorrect results. The numerical scheme developed in Chapter 4 to simulate 3D fatigue crack growth in bimaterial interfaces was used to simulate fatigue crack growth in sandwich panels with a circular debond. Fatigue experiments were conducted on debonded sandwich panels to validate the numerical scheme. Sandwich panels with a circular face/core debond at the panel centre were exposed to cyclic loading. Fatigue tests were performed on the debonded panels with H45 PVC core and glass/polyester face sheets. It was observed that the debond initially kinked into the core and continued to propagate underneath the resin-rich cell layer of the core. Using the finite element method, the energy release rate and mode-mixity phase angle at the crack tip of the debonded 125
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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
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 140 and 141: Load (kN) 2 1.5 1 0.5 0 Panel 1 Pan
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 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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