Interlaminar Fracture Testing
of Composites
The threat of delamination arising from in-service loading has been
one of the factors in limiting the adoption of laminated composite
materials in greater volume for primary structure. While
other damage modes such as matrix cracks may occur first, delaminations
result in larger stiffness drops and reduction in load-bearing capabilities.
Delaminations may occur from interlaminar stresses arising from geometric
or material discontinuities from design features, such as an edge,
a hole, a dropped ply. However, they may also occur from matrix cracks
or from interlaminar stresses caused by structural loading, such
as in a curved laminate, or by foreign body impacts. The delamination,
once initiated, will grow under fatigue loads. During delamination
growth, the structural loads may be redistributed such that another
delamination occurs in another location. The delaminations may continue
to grow and accumulate until a structural failure occurs, such as
buckling or fibre failure. Alternatively, the delamination
may be arrested and the structure may maintain some integrity. Although
delamination may not cause total collapse of the load-bearing properties
of the component, it is usually a precursor to such an event. Therefore,
knowledge of the composite's resistance to interlaminar fracture
is useful not only for product development and material screening,
but a generic measurement of the interlaminar fracture toughness
of the composite is useful for establishing design allowables for
damage tolerance analyses of composite structures.
The equipment and fixtures at MERL allow us to conduct the following
preferred tests all under the conditions of fatigue, static, creep,
hot/wet cold dry, elevated temperature.
|
Mode I
Mode II
Mode III
Mixed Mode I/II
|
DCB
4ENF, ELS, ENF
ECT, SCB (modified)
MMB, FPS/ADCB, CLS, stabilised MMB |
