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

ate, phase angle and debond propagation load in debond damaged sandwich structures subjected
to in-plane loading, and validated the results against experiments see e.g. Berggreen and
Simonsen (2005) and Sallam and Simitses (1985). A good starting point for detailed fracture
analysis of sandwich structures is specimens like columns and beams which can be modelled
using the 2D finite element models which has not been thoroughly examined in the literature.
In this chapter, as our starting point, failure of compression loaded sandwich columns with an
implanted through-width face/core debond is examined. Compression tests were conducted on
sandwich columns containing face/core debonds. The strains and out-of-plane displacements of
the debonded region were monitored using Digital Image Correlation (DIC) technique. Finite
element analysis and linear elastic fracture mechanics were employed to estimate the critical
instability load and compression strength of the columns. Tilted Sandwich Debond (TSD)
specimens were applied for determination of the fracture toughness of the interface in a modemixity
similar to tested sandwich columns. Energy release rate and mode-mixity were
determined and compared to fracture toughness data obtained from TSD tests, predicting
propagation loads. Instability loads of the columns were determined from the out-of-plane
displacements using the Southwell method. Results show that the finite element estimates of
debond propagation and instability loads are in overall agreement with experimental results. The
proximity of the debond propagation loads and the instability loads shows the importance of
instability in connection with the debond propagation of sandwich columns.
2.2 Experimental Setup
Sandwich panels consisting of 2 mm thick plain-woven E-glass/epoxy face sheets over 50 mm
thick Divinycell H45, H100 and H200 PVC foam cores were manufactured using vacuum
assisted resin transfer molding and cured at room temperature. A face/core debond was defined
by inserting strips of Teflon film, 30 m thick, between face and core in desired locations in the
panels. The widths of the Teflon strip were 25.4, 38.1 and 50.8 mm. The width defines the length
of the debond in the column specimens subsequently cut from the panels. It was observed that
the single Teflon layer insert used to define the face/core debond did not perfectly release the
bond between the face and core. To achieve a non-sticking, traction-free debond in the
specimens, the debond was mechanically released by wedging knives with very thin blades (0.35
and 0.43 mm thick). The width and length of the columns were 38 and 153 mm, respectively.
Figure 2.1 shows a column specimen cut from a panel. A test rig was designed and manufactured
for axial compression testing of the columns, see Figure 2.2 (a). The test rig includes four 25 mm
diameter solid steel rods to maintain alignment of the upper and lower plates of the test rig
during compressive loading. Linear bearings were attached to the upper plate to minimise
friction. Steel clamps of a width of 80 mm were attached to the upper and lower plates of the
fixture to clamp the columns. The test rig was inserted into an MTS 810 100 kN capacity servohydraulic
universal testing machine, see Figure 2.2 (b). A 2 MPixel digital image correlation
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