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

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of this chapter, sandwich panels with a circular face/core debond exposed to cyclic loading are tested and simulated by use of the developed 3D fatigue crack growth finite element routine. 5.2 Face/Core Fatigue Crack Growth in Sandwich X- Joints Sandwich X-joints are widely applied to sandwich structures in order to connect panels which are attached perpendicularly to the face sheets of each other. An example of an application can be found in naval ships constructed of fibre composite sandwich materials, here among other locations an X-joint exists where the end bulkhead of the superstructure is attached to the deck, with an internal bulkhead placed in the same vertical plane below the deck. This joint will be subjected to alternating tensile and compressive loading in the vertical direction for respectively hogging and sagging bending deformation of the hull girder. When the core material is polymer structural foam, such joints are often strengthened by the insertion of a higher-density core material or core inserts of a stiffer material in the deck panel in the immediate region of the joint, see Hayman et al. (2007). The load transferred through X-joints can be tension or compression. Compressive load may lead to core indentation or crushing, whereas tensile loads may cause face/core debonding. Berggreen et al. (2007) proposed the Sandwich Tear Test (STT) specimen representing a debonded sandwich X-joint under tensile load, see Figure 5.1. In this section interface fatigue crack growth in sandwich X-joints is studied experimentally using a series of STT specimens. The STT specimens include variants with three different core densities and are tested under static and fatigue loading. Furthermore, the experimental results will be used to validate the numerical fatigue crack growth scheme presented in Chapter 4. Finally, a detailed analysis of the fatigue crack growth in sandwich X-joints will be presented and efficiency, accuracy and limitations of the proposed numerical scheme will be discussed. (a) Face/core debond Figure 5.1: Simplified geometry and boundary conditions for (a) sandwich X-joints and (b) Sandwich Tear Test (STT) specimen. 90 (b)
5.2.1 Experimental Study of the STT Specimens Static and fatigue tests were conducted on STT specimens to study the residual lifetime of debond damaged sandwich X-joints. Fifteen foam cored STT specimens with glass/polyester face sheets were manufactured for static and fatigue tests. The polyester resin is Polylite 413- 575, which is specially designed for vacuum injection due to its low viscosity. The sandwich faces consist of four DBLT quadraxial mats from Devold Amt, each of a thickness of 0.75 mm and a dry area weight of 850 g/m2 and the fibre directions relative to the longitudinal direction of the specimen [90,45,0,-45], where the -45 degree ply is placed closest to the core. The core materials applied include Divinycell PVC foams of the types H45, H100 and H250 with nominal densities of 45, 100 and 250 kg/m 3 , respectively. The core thickness is 50 mm. The properties of the core and face materials are given in Table 5.1. Face sheet material properties are obtained from the tests conducted on samples from the face sheet. Table 5.1: Face and core material properties. Material E (MPa) G (MPa) Face sheet 19400 7400 0.31 Core: H45 50 15 0.33 Core: H100 130 35 0.33 Core: H250 300 104 0.33 A selection of manufactured STT specimens is shown in Figure 5.2. All specimens were reinforced by wooden inserts at the ends to avoid crushing of the core when mounting them in the STT test rig. The debond defect was introduced during the manufacturing process by inserting a sheet of 0.025 mm thick Airtech release film on the core and sealing the edges with resin before vacuum injection. The panels were resin injected molded and cured with vacuum consolidation. The STT specimens were cut from the manufactured panels. The release film was placed along 480 mm of the specimen length so that the crack only propagates in one side, see Figure 5.2. The reason for not just testing simply a half part of the specimen where the crack propagates is that as the crack propagates the membrane forces in the face sheet becomes larger, generating harmful side forces on the testing machine actuator. The side loads can therefore be decreased by carrying the loads by the tension in the part of the face sheet which is not glued to the core. 91
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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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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
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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 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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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
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.
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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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