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3. Material properties
3.1. CFRP
The fabrication of the CFRP box beam involved the use of unidi-
rectional CFRP prepreg produced by Toray Industries, Incorporated,
a commercial CFRP producer. The constituents of the prepreg were
121.1 C (250 F) curable epoxy and carbon fibre of type T700S, as
designated by Toray Industries, Incorporated. The CFRP laminate
has a thickness of 5 mm with cross-ply structure having 90% of
the fibres along the longitudinal axis of a beam and 10% along its
transverse direction.
A coupon test programme was performed to assess material
strengths. Coupons were extracted from a sheet laminate prepared
for testing purpose by maintaining the lay up and fabrication pro-
cess similar to those of the laminate in a box beam. This coupon
test is hereafter referred to as sheet coupon test. The test involved
tension and compression tests in two perpendicular directions, and
also an in-plane shear test to yield the respective strengths and
moduli. Nominal cross-section of the coupons was 8 mm 5 mm.
Also, in-plane Poisson’s ratio was tested. The nominal cross-section
of the coupons for this test was 12.5 mm 5 mm.
Afterward, coupons were also extracted from the undamaged
portion of the tested empty box beam. These coupons were tested
for tensile strength and modulus. This coupon test is hereafter re-
ferred to as box coupon test. Nominal cross-section of a box cou-
pon was 12.5 mm 5 mm. Material properties as referred from
the two coupon tests are presented in Table 1 with their usual
notations. Each property was obtained as an average of five tests.
However, in-plane shear strength was assumed by referring to
Mallick [14] for a high modulus and strength (HMS) carbon-epoxy
unidirectional composite.
Out-of-plane properties are not available from the test and are
rarely reported in the literature as well. They were suitably as-
sumed. Table 2 shows out-of-plane properties for the CFRP lami-
nate with their usual notations. A typical value of 62 MPa was
assumed for rT
3. Typical literature values for rC
3 were found higher
than rC
2 from the coupon test. It is not realistic to assume the out-
of-plane compressive strength more than the corresponding in-
plane strengths. Thus, rC
3 was taken same as rC
2. E3 was referred
from TORAY [15]. Out-of-plane strengths and moduli are often
considered as matrix-governed properties. A reasonably low value
of G23 for a graphite–polymer composite as mentioned by Hyer
[16] was taken for both G23 and G13. Interlaminar shear strengths
were specified corresponding to an assumed ultimate shear strainof 1%. In addition, Poisson’s ratios l23 and l13 were calculated from
classical lamination theory (CLT).
3.2. Finite element modelling
CFRP laminate was modelled as a 3D orthotropic material with
linearly elastic constitutive relationship. This has an advantage
that through-thickness normal stress and interlaminar shear stres-
ses are directly involved in beam response. The orientation of
material axis system (1–2–3) to the global loading axis system
(x–y–z) is different at different locations of a box beam as shown
in Fig. 2. Material properties were entered with proper orientations
for flange and web locations of a beam.
When a fibre passes across the corner of a box beam, it changes
its orientation from horizontal (z) axis in the flange region to ver-
tical (y) axis in the web region. The fibre bends gradually from 0 to
90 with respect to the z-axis. If the number of fibres in a cross-sec-
tion is sufficiently high, fibres of different orientations ranging
from 0 to 90 can be assumed to occur at the corner location of