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

(3.15)
cr
0. 5 3 E f EcGc
where Ef and Ec are modulus of elasticity of the face sheets and core and Gc is the shear modulus
of the core. It is seen that wrinkling is likely to have influenced the strength of the A panels and
both wrinkling and crimping are likely to have influenced the strength of the C panels. The
observed behaviour raises an important question concerning the value of intact strength that
should be used in determining a local strength reduction factor Rl. Should this be based on the
compressive strength of a laminate measured in tests on small laminate samples in which all
types of buckling are prevented, or should it be based on the compressive strength actually
observed for the tested panel? Most types of buckling are dependent on the size of the panel and
its boundary conditions. Moreover, they are not affected by very local losses of stiffness. Thus, it
seems appropriate to base Rl on the basic compressive strength of the laminate and rather
perform separate checks of possible effects of buckling. An exception to this is local wrinkling of
the face sheet, which is independent of panel size and boundary conditions and in practice may
provide a modified material strength to replace the value measured in tests on small laminate
samples.
3.5: Measured and theoretical failure loads for intact panels.
Lay-up Measured failure
Theoretical failure loads (kN)
type load (kN) Compressive failure Shear crimping Face sheet wrinkling
A 325 448 642 409
B 357 502 1335 663
C 279 636 243 229
3.5 Panel Analysis
A 3D finite element model was developed in the commercial finite element code, ANSYS
version 11, using 8-node isoparametric elements (SOLID45). Geometrically non-linear analysis
of the debonded panels was performed with displacement controlled loading. The panels were
assumed to contain an initial imperfection in the form of a half-sinusoidal wave as determined
from eigenbuckling mode shapes. The magnitude of the initial imperfection is obtained from
DIC measurement of the test specimens shown in Figure 3.11. Because of geometry and loading
symmetry only a quarter of the panel was modelled. Symmetry boundary conditions were
applied to the symmetry planes. Due to the need for a high mesh density at the crack tip when
performing the fracture mechanics analysis, a submodelling technique was employed. The finite
element model and submodel are shown in Figure 3.16.
53