Journal of Composite Materials

Flexure-response and fracture behavior of unidirectional commingled-yarn-based carbon fiber (CF)/polyamide (PA) 6 composite were investigated using three-point edgewise and flatwise bend tests. Difference of the experimental flexure data from that expected was attributed to poorly bonded fiber/matrix interfaces and deviated fiber bundle orientations. For the fracture test, four kinds of notch direction were adopted: edgewise notches parallel (L) and transverse (T) to the major direction of fiber bundles, and flatwise notches parallel (ZT) and perpendicular (ZT) to this direction. Strength and absorbed fracture energy for L and ZL specimens exhibited high sensitivity to notching. ZL specimens exhibited the lowest values of the critical stress intensity factor, K _c which were slightly superior to those of unfilled PA6 matrix. T specimens showed the largest values of the strain energy release rate G_c which were mostly occupied by the plastic fracture energy: the large plastic energy coincided with generation of the crushing-mode failure on the compression side. Enlargement of the compression area was analyzed by means of the rigidity reduction resulting from the fracture progression.

Low-cycle and high-cycle bending fatigue experiments were conducted on 2-D woven carbon-carbon composites at room temperature to evaluate the possible evolution of damage within their complex microstructure. Since carbon-carbons are known to be weak in interlaminar shear, a 3-point bending geometry was chosen to produce high interlaminar shear stresses. This paper extends previous work by focusing on interlaminar shear fatigue, and by using a specially developed technique for the in-situ examination of distributed damage. For the low-cycle tests the load was cycled between 10% and 90% of the material's monotonic failure load; the range was 8% to 82% for the high-cycle experiments. A load-frame mounted optical microscope allowed the collection of digital images of the surface microstructure at various intervals up to 100,000 cycles for the low-cycle and up to 20 × 10⁶ cycles for the high-cycle experiments. Microcrack lengths and orientations were extracted from these images, which permitted defining a discrete damage function as a measure of the distributed microcrack damage. Within the uncertainty of the measurements, distributed fatigue damage evolution was not discovered. A slight increase in residual static strength was detected with no change in stiffness.

The structures of bamboos have been adapted to our natural environment over a long period of time. They are a composite material reinforced axially by fibers called bundle sheath. Furthermore, they have a hierarchical gradient structure, that is, a macroscopic gradient structure in culm (corresponding to trunk of wood) diameter, culm thickness and node length, and a microscopic one in the bundle sheath distribution. The macroscopic gradient structure leads to a constant surface stress at every height and the microscopic gradient structure provides a strength distribution in radial direction adapting to bending stress due to wind loads.

By introducing two concepts of the "equivalent residual lamina stiffness" and the "equivalent crack density" to eliminate the different stiffness degradation characteristics caused by transverse matrix cracks, the degradation of the three lamina stiffness components can be described by a same characteristic curve which is independent of the laminate layup. But the residual stress has some effect on the stiffness degradation, especially to the inplane Poisson's ratio of the laminate. Results show that for the same matrix crack density, the residual Poisson's ratio is always less than the residual shear modulus and Young's modulus. Due to the existence of the matrix crack saturation state, the degradation of the lamina stiffness has the low boundary value; this value seems to be independent of the laminate configuration and materials.

A three-dimensional nonlinear finite element code is established to analyze the effects of material nonlinearity on the state of stress and failure prediction near the stress concentrations of a pin-loaded laminated composite plate. Theoretically, stress singularities may occur at the interface between two layers of different ply orientation on the free edges. High magnitudes of stresses at the free edge of the ply interface are responsible for failure initiation at those locations. Therefore the state of stress near the edge of the hole and at the ply interface between two different layers is important and has been studied in detail. An existing nonlinear material model, for in-plane behaviour, has been modified to be applicable for three-dimensional cases. Nonlinear shear stress-strain behaviour of a unidirectional ply, in the x-y and z-x planes, is the source of nonlinearity. Three different configurations, namely, cross-ply [0₄/90₄] _s , and [90₄/0₄] _s and an angle-ply [+45₄/-45₄] _s , were considered. A failure analysis, using stress based failure criteria, is performed to predict the failure initiation load of different configurations. The effect of material nonlinearity on the predicted failure initiation load is studied. The results of prediction of failure initiation load by considering the material nonlinearity is in excellent agreement with experimental results. The results obtained from failure analysis emphasize that considering the material nonlinearity, especially for highly shear induced cases like [+45₄/−45₄] _s is important in failure analysis of composite laminates using stress based failure criteria.