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

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corresponds to the onset of debond propagation. Figure 2.3 (b) shows that the critical propagation load increases as the core density is increased. Figure 2.4 shows DIC images of outof-plane displacement in a column with an H45 core and a 50.8 mm debond just before and after debond propagation. During the compression tests the DIC measurements furthermore revealed that opening of the debond was not perfectly symmetric, see Figure 2.4 (a). This can be attributed to a slight misalignment of the fibres in the face sheets and lack of perfectly uniform load introduction at the ends of the columns. Figure 2.5 shows DIC images of initial out-of-plane imperfection of two columns with H100 cores and 50.8 mm debonds, released using the thin (0.35 mm) and thicker (0.43 mm) blades, respectively. The inital imperfection amplitudes are approximately 0.25 and 0.51 mm. A Photron APX-RS high-speed camera was used to track the debond propagation at a frame rate of 1000 images per second. Figure 2.6 shows the debond 1 ms before and right after the debond propagation. A slight opening of the debond is seen before propagation. Slight crack kinking into the core, resulting in the crack propagating just beneath the interface on the core side, was observed in most of the column specimens with H45 core. Some specimens with H100 core displayed this failure mode as well, see Figure 2.7. The fracture toughness of the H45 core (150 J/m 2 , see Table 2.1) is likely less than that of the face/core interface, which could explain the observed crack propagation path. A detailed kinking analysis, similar to what was presented in Li and Carlsson (1999), must be carried out to investigate this further. This is, however, out of the scope of this study. All columns with H200 core and 25.4 mm debond failed by compression failure of the face sheet above the debond location, see Figure 2.8. This can be explained by the proximity between the debond propagation load of the debonded face sheet and the compression failure load of the face sheet, which can be calculated from the compression strength and the cross section area of the face sheet, see Table 2.1. Face compression failure was also observed for one of the columns with H100 core and 25.4 mm debond length. The H200 column specimens with 38.1 and 50.8 mm debonds failed by debond propagation although kinking was not observed, thus, promoting crack propagation directly in the face/core glue interface. Moreover, the observed crack propagation rate was less for the H200 specimens, indicating a tough interface. (a) (b) Figure 2.4: Debond opening (a) prior to propagation (b) after propagation for a column with H100 core and 50.8 mm debond length from DIC measurements. 20
(a) (b) Figure 2.5: Initial imperfections in columns with H100 core and 50.8 mm debond where the debond was released using (a) a thin blade (0.35 mm) (b) a thicker blade (0.43 mm). Figure 2.6: High-speed images, which show the debond in a column with H45 core and 50.8 mm debond length 1 ms before propagation and right after propagation has taken place. 21
Page 1: Residual Strength and Fatigue Lifet
Page 4 and 5: Published in Denmark by Technical U
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Page 8 and 9: method to accelerate the simulation
Page 10 and 11: Synopsis Sandwich kompositter er i
Page 14 and 15: Contents Preface Executive Summary
Page 16 and 17: 5 Face/core Interface Fatigue Crack
Page 20 and 21: H11 bimaterial constant H22 bimater
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 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 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 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,
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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
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B 2 1/ 2 ( S44 S55 S45) (4.17) wh
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75), which illustrates possible ina
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Figure 4.18 (a) presents the deflec
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Debond radius (mm) 100 90 80 70 60
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using the cycle jump method, more t
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Chapter 5 Face/Core Interface Fatig
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of this chapter, sandwich panels wi
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H250 Specimen H100 Specimen H45 Spe
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Figure 5.5: Test setup. Initially,
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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
Page 188:
DTU Mechanical Engineering Section
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Residual Strength and Fatigue Lifetime of ... - Solid Mechanics

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