Carbon fiber reinforced plastic (CFRP) do not oxidize or
rust and have a higher specific strength and stiffness
than conventional materials. Applications of CFRP are
being investigated, with a focus on applications as
aircraft materials that require strength and durability.
However, the superior mechanical properties of CFRP
laminate are limited to a strengthened direction (parallel
to fibers), and the strength of CFRP laminate is reduced
significantly in directions that are not strengthened
(interlaminar direction, for example.). CFRP laminate are
also susceptible to impact, with out-of-plane impacts
causing internal damage, such as peeling laminates, to
CFRP laminate. The design and product development of
CFRP laminate therefore incorporates damage tolerant
design, which takes into consideration the effects of
internal damage on the strength of the material.
Damage tolerant design must determine how resistant
a material is to interlaminar crack propagation, which is
done by fracture toughness testing.
For homogeneous isotropic materials, only fracture Mode
I (crack opening mode) is evaluated normally in fracture
toughness testing. Materials that are a composite of a
resinous matrix and fibers are anisotropic, and it is
important these materials are evaluated not just for
fracture Mode I, but also for fracture Mode II (crack sliding
mode), fracture Mode III (crack tearing mode), and mixed
mode fractures. Mixed-mode bending (MMB)
tests are used to evaluate fracture toughness in a mixed
mode that combines Mode I and Mode II. Features of
MMB testing are the mixed mode ratio (hereinafter
referred to as mode ratio) can be changed on subsequent
tests, and it is almost unchanged by crack propagation.
While the stress intensity factor K is often used to evaluate
the toughness of homogeneous isotropic materials, the
interlaminar fracture that occurs in anisotropic composite
materials is commonly evaluated using the energy release
rate G, which is proportional to the square of the stress
intensity factor K. 1)
MMB testing was performed in conformance with ASTM
D6671 and the total mixed-mode fracture toughness Gc
was determined at four different mode ratios.