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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
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