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

More documents

Recommendations

Info

Chapter 5 Face/Core Interface Fatigue Crack Propagation in Sandwich Structures 5.1 Background Fatigue behaviour of sandwich structures has been of great interest to researchers recently. Sandwich specimens with and without initial damages have been fatigue tested and analysed by various authors. Fatigue analysis of undamaged sandwich beams has typically been carried out by beam bending tests to investigate the fatigue response of foam cores subjected to shear loading. Shenoi et al. (1995) conducted flexural fatigue tests on sandwich composites with glass aramid/epoxy face sheets and cross link foam. They used a ten-point configuration with simply supported ends approximating a uniformly distributed load over the span of the beam. Burman et al. (1997) analysed the fatigue response of H100 PVC and Rohacell WF51 foams by four-point bending tests on undamaged sandwich beams. Kanny and Mahfuz (2002, 2005) studied the fatigue behaviour of sandwich beams exposed to flexural loading with different loading frequencies. They found that by increasing the loading frequency, the crack growth rates in the tested sandwich beams decrease. Kulkarni et al. (2003) studied fatigue crack growth in foam cored sandwich composites exposed to flexural cyclic loading in a modified three-point bending test rig. It was observed that the first visible damage was face/core debonding in the centre of the sandwich beams. Zenkert et al. (2011) studied the failure mode shift from core shear failure to face sheet tensile failure, as a function of load amplitude in GFRP/foam cored sandwich beams. Bezazi and co-authors (2007 and 2009) investigated experimentally and analytically the fatigue behaviour of sandwich composites in a three-point bending test rig. Mahi et al. (2004) studied the flexural behaviour of sandwich composites exposed to cyclic loading in a three-point bending test rig. He proposed a damage accumulation model for the sandwich specimens and used the model to analyse the fatigue life of sandwich composites. Quispitupa and Shafigh (2006) conducted fatigue tests on sandwich beams via three-point bending. They observed both global mode I and mode II cracking in the face/core interface of the specimens. In the case of debond 88
damaged sandwich composites subjected to cyclic loading, a limited number studies can be found in the literature. Fatigue experiments have been carried out by Shipsha et al. (1999, 2000, 2003) on debond damaged sandwich beams to determine stress-life S-N diagrams, crack growth rates and indentify fatigue crack growth mechanisms. They evaluated face/core interface crack growth rates under global mode I and II loading by use of the Double Cantilever Beam (DCB) and Cracked Sandwich Beams (CSB), respectively. Additionally, he studied the fatigue behaviour of foam cored sandwich beams in the presence and absence of initial damage under shear loading in a specially designed four-point bending test rig. Burman et al. (1997, 2000) also conducted four-point bending tests on debond damaged sandwich beams. They tested sandwich beams in a modified four-point bending test rig with different loading amplitudes. At higher load amplitudes the failure mode was core shear failure, but at smaller load amplitudes the failure mode was governed by tensile failure of the face sheet. Bozhevolnaya and co-authors (2009) conducted three-point bending fatigue tests on sandwich beams with peel stoppers. They reported that even though the peel stoppers have no significant effect on the fatigue life of the sandwich beams, they may prevent cracks from propagating in the face/core interface. Berkowits and Johnson (2005) carried out fatigue tests on double cantilever beams (DCB) of honeycomb core and carbon/epoxy face sheets. They used the compliance of the DCB specimen to determine the crack length and the crack growth rates. Liu and Holmes (2007) investigated fatigue crack propagation in thin-foil Ni-base and honeycomb core sandwich structures. Edge-notched honeycomb core sandwich panels were tested under tension-tension and tension-compression fatigue loading. All the above-mentioned studies are either purely experimental, or the proposed numerical or analytical methods for modelling the fatigue behaviour of sandwich structures are limited to a specific loading condition or geometry (e.g. beams). Thus, a more general approach is desirable. In Chapter 4 a general scheme for an accelerated simulation of fatigue crack growth in bimaterial interfaces was proposed. The main idea behind the proposed method is that once a specific interface has been characterised under cyclic loading, the extracted crack growth rates vs. energy release rates for different explicit mode-mixity phase angles can be utilised to simulate fatigue crack growth, and thus fatigue lifetime, of any structural component with an arbitrary loading condition as long as a similar face/core interface exists. In this chapter the proposed numerical scheme is applied to analysis of face/core interface fatigue crack growth in foam cored sandwich components. Moreover, the proposed numerical scheme will be validated against fatigue tests conducted on debonded sandwich beams and panels. In the first part of this chapter face/core fatigue crack growth in sandwich X-joints is studied experimentally. Furthermore, the face/core interface of the X-joints is characterised under cyclic loading. The obtained fatigue crack growth rates data is subsequently used as input for the 2D fatigue crack growth finite element routine developed in the previous chapter. In the second part 89
Page 1:
Residual Strength and Fatigue Lifet
Page 4 and 5:
Published in Denmark by Technical U
Page 6 and 7:
This page is intentionally left bla
Page 8 and 9:
method to accelerate the simulation
Page 10 and 11:
Synopsis Sandwich kompositter er i
Page 12 and 13:
Page 14 and 15:
Contents Preface Executive Summary
Page 16 and 17:
5 Face/core Interface Fatigue Crack
Page 18 and 19:
Page 20 and 21:
H11 bimaterial constant H22 bimater
Page 22 and 23:
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 42 and 43:
corresponds to the onset of debond
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 60 and 61: This page is intentionally left bla
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,
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 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 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
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
Page 166 and 167:
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
show all

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

Create successful ePaper yourself

Delete template?

Save as template?