Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
Residual Strength and Fatigue Lifetime of ... - Solid Mechanics Residual Strength and Fatigue Lifetime of ... - Solid Mechanics
A modified version of the tilted sandwich debond (TSD) specimen, shown in Figure 2.10, was used to determine the fracture toughness of the interface. The determined fracture toughness will be used later to determine the crack propagation load in the column specimens applying the finite element method. Finite element analysis of the modified TSD specimen was carried out in order to determine the appropriate tilt angle to match the mode-mixity phase angles for the tested columns. A 2D finite element model with a highly refined mesh in the crack tip region, smallest element size of 3.33 m, was developed in the commercial code, ANSYS version 11, using 8node iso-parametric elements (PLANE82), see Figure 2.12. The energy release rate (G) and the mode-mixity phase angle () were determined from relative nodal pair displacements along the crack flanks obtained from the finite element analysis using the CSDE method outlined in the introduction. The characteristic length h is arbitrarily chosen as the face sheet thickness. Figure 2.12: Finite element model used in analysis of the modified TSD specimen with neartip mesh refinement. The smallest element size is 3.33 m. The mode-mixity phase angle of each column specimen was extracted at a load corresponding to the onset of debond propagation (from the experiments) using finite element modelling (to be presented later). The extracted phase angles were exploited in finite element models of the TSD specimens to determine the matching tilt angle at a crack length of 50 mm for specimens with H45 core and 63.5 mm for specimens with H100 and H200 cores. The face sheets were 1.5 mm thick, and the core thickness 25 mm. A 12.7 mm thick steel bar of the same width and length as the sandwich specimen (25.4 x 180 mm) was used to reinforce the loaded face sheet. The material properties of the face sheets and cores in the TSD specimens are identical to those of the columns specimens. The resulting specifications for the TSD specimen including the calibrated tilt angle are given in Table 2.2. Table 2.2: TSD specimen dimensions and tilt angles. Core Initial crack length (mm) Phase angle, deg Tilt angle (), deg H45 50 -24 55 H100 63.5 -29 60 H200 63.5 -37 70 TSD specimens of a length of 180 mm and a width of 25.4 mm were cut from panels prepared with one face sheet only. Figure 2.13 shows the TSD test setup with an H100 TSD specimen 26
tilted 60. The bottom core surface of the specimen was bonded to a steel plate bolt connected to the test rig. Prior to bonding, the bonding surfaces were thoroughly sanded and cleaned with acetone to promote adhesion. Hysol EA-9309 aerospace epoxy paste adhesive was used for bonding. The steel bar contained a through-width hole near the end in order to allow pin load application. All tests were conducted at a rate of 1 mm/min, and three replicate specimens were tested. Figure 2.13: Modified TSD test setup. Figure 2.14 shows typical load vs. displacement curves for TSD specimens with H45, H100 and H200 foam cores. The load-displacement plots are fairly linear until the point of crack propagation, where the load suddenly drops. The load required to propagate the crack significantly increases as the core density is increased. Compared to conventional TSD specimens without steel reinforcement, see e.g. Li and Carlsson (2001), substantially larger loads are required to generate crack growth in the steel reinforced specimens, as a result of the large bending and shear stiffnesses of the steel reinforced upper face sheet. The crack propagation behaviour for the H45 specimens was rather unstable, with the crack suddenly growing 25-50 mm at each crack increment, which allowed only about three crack increments before the crack reached more than 70% of the total specimen length, where the test was stopped. For the specimens with H45 foam core, the crack propagated beneath the face/core interface, on the core side, Figure 2.15 (a), which is consistent with the observations from the column tests and the 27
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- Page 14 and 15: Contents Preface Executive Summary
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A modified version <strong>of</strong> the tilted s<strong>and</strong>wich debond (TSD) specimen, shown in Figure 2.10, was<br />
used to determine the fracture toughness <strong>of</strong> the interface. The determined fracture toughness will<br />
be used later to determine the crack propagation load in the column specimens applying the finite<br />
element method. Finite element analysis <strong>of</strong> the modified TSD specimen was carried out in order<br />
to determine the appropriate tilt angle to match the mode-mixity phase angles for the tested<br />
columns. A 2D finite element model with a highly refined mesh in the crack tip region, smallest<br />
element size <strong>of</strong> 3.33 m, was developed in the commercial code, ANSYS version 11, using 8node<br />
iso-parametric elements (PLANE82), see Figure 2.12. The energy release rate (G) <strong>and</strong> the<br />
mode-mixity phase angle () were determined from relative nodal pair displacements along the<br />
crack flanks obtained from the finite element analysis using the CSDE method outlined in the<br />
introduction. The characteristic length h is arbitrarily chosen as the face sheet thickness.<br />
Figure 2.12: Finite element model used in analysis <strong>of</strong> the modified TSD specimen with neartip<br />
mesh refinement. The smallest element size is 3.33 m.<br />
The mode-mixity phase angle <strong>of</strong> each column specimen was extracted at a load corresponding to<br />
the onset <strong>of</strong> debond propagation (from the experiments) using finite element modelling (to be<br />
presented later). The extracted phase angles were exploited in finite element models <strong>of</strong> the TSD<br />
specimens to determine the matching tilt angle at a crack length <strong>of</strong> 50 mm for specimens with<br />
H45 core <strong>and</strong> 63.5 mm for specimens with H100 <strong>and</strong> H200 cores. The face sheets were 1.5 mm<br />
thick, <strong>and</strong> the core thickness 25 mm. A 12.7 mm thick steel bar <strong>of</strong> the same width <strong>and</strong> length as<br />
the s<strong>and</strong>wich specimen (25.4 x 180 mm) was used to reinforce the loaded face sheet. The<br />
material properties <strong>of</strong> the face sheets <strong>and</strong> cores in the TSD specimens are identical to those <strong>of</strong> the<br />
columns specimens. The resulting specifications for the TSD specimen including the calibrated<br />
tilt angle are given in Table 2.2.<br />
Table 2.2: TSD specimen dimensions <strong>and</strong> tilt angles.<br />
Core Initial crack length (mm) Phase angle, deg Tilt angle (), deg<br />
H45 50 -24<br />
55<br />
H100 63.5 -29<br />
60<br />
H200 63.5 -37<br />
70<br />
TSD specimens <strong>of</strong> a length <strong>of</strong> 180 mm <strong>and</strong> a width <strong>of</strong> 25.4 mm were cut from panels prepared<br />
with one face sheet only. Figure 2.13 shows the TSD test setup with an H100 TSD specimen<br />
26
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