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

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Chapter 6 Conclusion and Future Work 6.1 Face/Core Debond Propagation in Sandwich Structures under Static Loading In the first chapters of this thesis a methodology for the estimation of face/core debond propagation load in sandwich structures under static loading was developed and validated against experiments. The developed finite element scheme involves three overall steps: 1) Generating a global model of cracked structures, and estimating the global response of the structures with a coarse mesh around the crack tip. 2) Generating a sub model of the crack tip (front) with a very fine mesh, interpolating the boundary conditions in the cutting boundaries of the submodel and solving the detailed finite element model of the debond front for the interpolated boundary conditions. 3) Extracting the energy release rate and mode-mixity at the crack tip from the submodel using the Crack Surface Displacement Extrapolation (CSDE) method (Berggreen et al, 2005). By application of the developed scheme, debond initiation loads were predicted in debonded sandwich columns and panels. Initially, the compressive failure of foam cored sandwich columns containing a face/core debond was investigated using the developed scheme. Compression tests were performed on sandwich columns to validate the finite element model. Sandwich columns with glass/epoxy face sheets and H45, H100 and H200 PVC foam cores with different debond lengths were tested under static compressive loading. It was observed that most of the debonded columns failed by unstable debond propagation at the face/core interface towards the column ends. However, face sheet compression failure was observed in all columns with H200 core and smallest debond length, 128
due to the proximity of the debond propagation load and the compression failure load of the face sheets. Bifurcation type buckling of the debonded face sheet was not observed and the debond opening occurred gradually, which can be attributed to large initial imperfections. Slight kinking of the debond into the core was observed in the columns with a low-density H45 and H100 core. Modified Tilted Sandwich Debond (TSD) specimens were tested under different tilt angles to measure the fracture toughness of the interface at the calculated mode-mixity phase angles for the column specimens associated with the debond propagation. The measured interface fracture toughness was used to determine crack propagation loads from the finite element model of the columns. Instability and crack propagation loads of the columns were predicted on the basis of a geometrically non-linear finite element analysis and linear elastic fracture mechanics. Fair agreement was achieved for the comparison of the measured outof-plane deflection, instability, and debond propagation loads from the experiments and the finite element analysis. For most of the investigated column specimens, it was shown that the instability and debond propagation loads are very reasonable estimates of the ultimate failure load, unless the other failure mechanisms occur prior to buckling instability. To examine the accuracy of the developed scheme in case of sandwich panels, debond propagation in sandwich panels with a circular debond at the centre was modelled. To validate the finite element model of the debonded panels, intact and debonded sandwich panels with glass/polyester face sheets and H130, H250 and PMI foam cores were tested under static inplane compressive loading. The following damage mechanisms were observed during the experiments: 1) All debonded panels failed by the propagation of the debond to the edges of the panels. 2) All intact panels with H130 and H250 core failed by the compressive failure of a face sheet very close to the wooden inserts, which can be attributed to additional peeling stresses arising due to the junction between the wooden insert and the core and to a slight unintentional mismatch between the core and the thicknesses of the insert. 3) Intact panels with PMI core failed by a combination of shear crimping and global buckling. This time instead of using the TSD specimen, characterisation tests were performed on Mixed Mode Bending (MMB) specimens to measure the fracture toughness of the face/core interface for a span of mode-mixity phase angles. As expected it was shown that the fracture toughness is increasing with increasing magnitude of the mode-mixity phase angle. The obtained fracture toughness data was used to determine the crack propagation load in the debonded sandwich panels. Instability and crack propagation loads of the panels were estimated on the basis of geometrically non-linear finite element analysis and linear elastic fracture mechanics. It was shown that the FEA predictions in few cases are much higher than the experimental ones (a maximum deviation of 46%), which can be attributed to the large scatter in the measured fracture 129
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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
Page 100 and 101: B 2 1/ 2 ( S44 S55 S45) (4.17) wh
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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
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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
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 146 and 147: 110 q=qG=0.4 Test #1 100 Test #2 Te
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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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