Off-axis fatigue behavior of a plain-woven carbon/epoxy fabric laminate at room temperature and 100 °C has been studied. Tension–tension fatigue tests are first performed on plain coupon specimens with five kinds of fiber orientations θ =0, 15, 30, 45, 90° at a frequency of 10 Hz in respective temperatures. Test results show that the fatigue behavior of the carbon fabric laminate significantly depends on fiber orientation. The on-axis S – N relationships plotted on logarithmic scales are almost linear over the range of fatigue life up to 10 6  cycle. By contrast, the off-axis S – N relationships are characterized by their S-shape. The log–log plot of the off-axis fatigue data becomes steeper in an intermediate range of fatigue life than before, suggesting increased sensitivity to fatigue, and it subsequently reaches a plateau indicating an apparent fatigue limit. The fiber orientation dependence of the off-axis fatigue strength can substantially be removed by normalization with respect to a modified static strength. These features are common to the fatigue behaviors at room temperature and 100 °C. Additional fatigue tests with a frequency of 2 Hz demonstrate that the shape of the off-axis S – N curve at room temperature is significantly affected by load frequency, as a result of a different specimen heating. Comparison between the high-temperature fatigue data at 2 and 10 Hz reveals that the S-shape of the off-axis S – N curve is caused partly by an intrinsic rate dependence of the carbon fabric laminate. Applicability of a phenomenological fatigue model that considers the effect of fiber orientation and of multiaxial state of stress is also examined for description of the off-axis fatigue behavior at 10 Hz. The results illustrate that the fatigue model can be used for describing the off-axis fatigue behavior of the carbon fabric laminate at room temperature and 100 °C for a given frequency of fatigue loading.

Off-axis fatigue behavior of plain weave carbon/epoxy fabric laminates at room and high temperatures and its mechanical modeling

A difference between off-axis tensile and compressive strengths in a unidirectional carbon/epoxy laminate is examined at 100 °C for different fiber orientations and strain rates. By comparing their predictions with experimental results, the Tsai–Wu, Hoffman, Hashin–Rotem failure criteria that can distinguish between the off-axis strengths in tension and compression are evaluated for the accuracy of prediction of the off-axis strength differential (SD) effect and of the failure envelopes associated with off-axis loading at different strain rates. It is shown that the failure envelope associated with off-axis compression is unsuccessfully predicted by these failure criteria. The comparison suggests that the SD effects in the longitudinal, transverse and shear strengths should be taken into account for accurate prediction of the off-axis failure envelope. On the basis of this experimental implication, simple modifications to the representative failure criteria are attempted in which both the normal and shear SD effects are taken into account.

Off-axis strength differential effects in unidirectional carbon/epoxy laminates at different strain rates and predictions of associated failure envelopes

Off-axis fatigue behavior of a carbon/epoxy cross-ply laminate and predictions considering inelasticity and in situ strength of embedded plies

In this paper, a polymeric unidirectional carbon-fibre epoxy-resin composite is both experimentally and numerically investigated to study the nonlinear material behavior of impacted DCB (Double Cantilever Beam) specimens. For the impact analysis, the load and the displacement applied from pin onto end block as well as the crack energy release rate are measured and compared with the finite element analysis results. The energy release rate is a critical measure of the resistance to crack propagation, which can be estimated by the force and displacement at the crack tip. It is found that the energy release rate measured from impact tests on the specimens is well predicted by the suggested finite element model in this study.

Fracture behaviour of adhesively-bonded composite materials under impact loading

High-temperature stress relaxation behavior of symmetric angle-ply laminates made of unidirectional T800H/3631 carbon/epoxy composite is examined at relatively high strain levels. Stress relaxation tests are performed at 100 °C for 5 h on plain coupon specimens of three types of angle-ply laminates [±30] 3S , [±45] 3S and [±60] 3S under displacement controlled conditions. For each ply orientation, the stress relaxation tests are carried out at three different strain levels. Stress relaxation phenomena clearly appear in all kinds of angle-ply laminate. The average stress in the angle-ply laminates rapidly relaxes to approach a steady-state value associated with the strain sustained. The rate of stress relaxation in the angle-ply laminates substantially diminishes in the tested time range. Stress relaxation simulation of the angle-ply laminates is performed using the classical laminated plate theory and a phenomenological viscoplasticity model for individual plies. Material constants involved in the ply viscoplasticity model are identified on the basis of the off-axis creep behavior for a unidirectional laminate based on the same kind of carbon/epoxy prepregs. Excellent agreements between the predicted and observed results are obtained by taking into account a local recovery of total strain in the gauge-length part of specimens as well as a fiber rotation induced by deformation. The successful application of the ply viscoplasticity model confirms that the stress relaxation behavior is consistent with the creep behavior in the same composite system.

Application of a phenomenological viscoplasticity model to the stress relaxation behavior of unidirectional and angle-ply CFRP laminates at high temperature

Y. Masuko

Papers

Creep behavior of unidirectional and angle-ply T800H/3631 laminates at high temperature and simulations using a phenomenological viscoplasticity model