Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
Residual Strength and Fatigue Lifetime of ... - Solid Mechanics Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
3.6 Conclusion In this chapter the compressive failure of foam cored sandwich panels containing a face/core circular debond was experimentally and numerically investigated. Sandwich panels with glass/polyester face sheets and H130, H250 and PMI foam cores were tested in a specially designed test rig. All debonded panels failed by the propagation of the debond to the edges of the panels. All intact panels with H130 and H250 cores 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 insert and the core and to a slight unintentional mismatch between the core and insert thicknesses. Intact panels with PMI core failed by a combination of shear crimping and global buckling. MMB characterisation tests were conducted to measure the fracture toughness of the face/core interface for a span of mode-mixity phase angles. Results showed increasing fracture toughness for increasing magnitude of the phase angle. A large scatter was observed in the fracture toughness results due to brittleness of the core material, different manufacturing defects and dissimilarity in crack propagation paths at the face/core interface. Instability and crack propagation loads of the panels were estimated based on geometrically nonlinear finite element analysis and linear elastic fracture mechanics. A numerical scheme similar to the one developed in Chapter 2, based on submodelling was used for the simulations. In some of the panels the FEA predictions are up to 46% higher than the experimental ones, which can be attributed to the large scatter in the measured fracture toughness using MMB fracture toughness results and differing crack tip details between the panels and the MMB specimens due to mechanical releasing of the debonded area. However, in most of the panels better agreement, up to 20% deviation, is observed between numerical and experimental results. To investigate the debond propagation path in the tested panels, some of the panels were cut and it was observed that the debond propagation path is similar between the panels and the MMB specimens. It was observed that in the panels and the MMB specimens with PMI core and panels with H130 core, the debond kinks into the core and propagates beneath the face/core interface. However, in the MMB specimens with H130 core two different crack growth paths were observed. In the H130 MMB specimens with the mode-mixity phase angle of 0° >> -20° (similar to the panels), the crack path was located below the face/core interface. However, when the magnitude of the mode-mixity phase angle was increased to -25° >> -65°, the crack path was directly in the face/core interface. In the panels and the MMB specimens with H250 core the debond propagates directly in the interface. This may be explained by the higher fracture toughness of the H250 core compared to H130 and PMI cores. Fibre bridging was observed after more than 4- 5 mm crack growth in the MMB specimens and panels with H250 core. The similarity between the debond propagation paths in the MMB specimens and the panels refutes the role of the different propagation paths in the inaccuracy of the determined debond propagation loads. 60
To examine the effect of initial imperfection magnitude on the behaviour of the panels, panels with different initial imperfection magnitudes were analysed. It was shown that the initial imperfection magnitude has no significant effect on the energy release rate. Regarding the modemixity the effect becomes less important at higher loads. Finally, based on experimental and numerical results, the strength reduction factor Rl was plotted against debond diameter. The plot is tentative due to the uncertainties regarding the intact strengths as well as the differences between test and analysis results. 61
- 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 84 and 85: This page is intentionally left bla
- 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 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
To examine the effect <strong>of</strong> initial imperfection magnitude on the behaviour <strong>of</strong> the panels, panels<br />
with different initial imperfection magnitudes were analysed. It was shown that the initial<br />
imperfection magnitude has no significant effect on the energy release rate. Regarding the modemixity<br />
the effect becomes less important at higher loads. Finally, based on experimental <strong>and</strong><br />
numerical results, the strength reduction factor Rl was plotted against debond diameter. The plot<br />
is tentative due to the uncertainties regarding the intact strengths as well as the differences<br />
between test <strong>and</strong> analysis results.<br />
61
Change language