Key Engineering Materials Vols. 334-335

Abstract: In this study, partially debonded spherical particles in a particulate composite are analyzed by three-dimensional finite element method to investigate their load carrying capacities, and the way to replace a debonded particle with an equivalent inclusion is examined. The variation in Young’s modulus and Poisson’s ratio of a composite with the debonded angle was evaluated for different particle arrangements and particle volume fractions, which in turn compared with the results derived from the equivalent inclusion method. Consequently, it was found that by replacing a debonded particle with an equivalent orthotropic one, the macroscopic behavior of the damaged composite could be reproduced so long as the interaction between neighboring particles is negligible.

Abstract: This paper discusses the results from a study that has been undertaken to develop a product from bamboo culm’s outer skin that was considered as wastes. Initially, by means of a simple process of gluing and pressing, bamboo panel composites were produced with Gigantochloa scortechinii bamboo skin wastes. Using existing sandwich technology, layered or sandwich panel were then produced using the bamboo skin wastes panel as the core material while plywood were used as the outer layer. The aim was to access the bending strength of the bamboo skin wastes panel, the sandwich panel and subsequently to compare them with common bamboo composites that utilized either strips or splits in their production process. Though the product has limited strength and is restricted to interior and non-structural components, it is still very well suitable for wall, doors, tabletops and shelves.

Abstract: In the present study, a method for reducing the domain of analysis is developed for the homogenization analysis of plain-woven laminates. Moreover, the method is applied to the quantitative prediction of elastic-viscoplastic deformation of plain-woven GFRP laminates. It is first shown that the internal structures of plain-woven laminates satisfy point-symmetry on the assumption that the laminates have the in-phase or out-of-phase laminate configuration of plain fabrics. The point-symmetry is then utilized for the boundary condition of unit cell problems, reducing the domain of analysis to 1/4 and 1/8 for the in-phase and out-of-phase laminate configurations, respectively. Using the present method combined with the nonlinear time-dependent homogenization theory, the elastic-viscoplastic behavior of plain-woven GFRP laminates under in-plane on- and off-axis loading is analyzed. In addition, the tensile tests of a plain-woven GFRP laminate at a constant strain rate are performed at a room temperature. Comparing the results of the present analysis with the experimental ones, it is shown that the analysis successfully predicts the in-plane elastic-viscoplastic behavior of the plain-woven GFRP laminate.

Abstract: In accordance with Helliger-Reissner variational principle, a semi-analytical Hamiltonian method has been developed for analyzing the process-induced stress analysis of composite wound vessel with metal liner during the curing process. A typical numerical result indicates that the process-induced thermo-stress fields involve complex variations of gradient and multi-peak values at different stages during the curing process.

Abstract: The ablation properties, thermal resistance and micro structural behavior of the phenolic resin (Resole) composites have been investigated in this research. Different materials, such as carbon fabrics, glass fabrics, also silica and zirconia powders have been used as reinforcements for synthesis of the composites. The specimens were prepared with three sets of compositions. The first set was produced with 37.5 wt% of Resole and 62.5 wt% of reinforcements. Another set of specimens were produced with 40wt% Resole, 40 wt% of silica and 20 wt% of zirconia. Also to achieve high insulation index in Resole/carbon fabrics composites a thin film of zirconia coated at the back side of the specimens. To explore the ablation characteristics of the composites in terms of insulation index, erosion rate and microscopic pattern of ablation, an oxyacetylene torch flame with heat flux of 10 Mw/m2 at approximately 2800°C was used. The ablation behavior and microstructure of the burnt-through specimens were also observed, using scanning electron microscopy. It was found from ablation test that the erosion rates of the Resole/carbon fabric specimens are 20% lower than the other specimens. Additionally the high insulation index of the Resole/carbon fabrics coated with zirconia indicates that these composites are the best ablative materials in the present study. It has been also reported that those specimens filled with zirconia have the highest insulation index. Although the erosion rate of the Resole/silica composites were 20% higher than the Resole/glass fabrics, but a 5mm depth hole (from 10 mm thickness of the whole specimen) was seen at the center of the Resole/glass Fabric specimens. SEM observations show that proper adhesion between reinforcements and matrix is important to achieve improved ablative properties, it was also reported many changes in diameter, shape and the surface of the carbon fibers through the ablated area. These changes can be reduced from surface to back side of the specimens.

Abstract: The full shape of constant fatigue life (CFL) diagram has been identified for three kinds of carbon fiber-reinforced plastic (CFRP) laminates of [45/90/-45/0]2s, [0/60/-60]2s and [0/90]3s lay-ups, respectively. First, the effects of mean stress on fatigue behavior are observed for the quasi-isotropic [45/90/-45/0]2s CFRP laminate. It is clearly observed that the CFL diagram turns out asymmetric and the peak of the CFL diagram is slightly offset to the right of the alternating stress axis. The CFL plots indicate that the alternating stress component of fatigue load becomes maximal at a critical stress ratio closely equal to the compressive strength to tensile one (i.e. a C/T strength ratio). It is also demonstrated that CFL curves for different constant values of life are linear in the range of a short life, but they turn quadratic in the range of a longer life. Similar features are observed for the other kinds of CFRP laminates. Then, to construct the asymmetric dissimilar shape of CFL diagrams, a new methodology is developed on the basis of the static strengths in tension and compression and the reference S-N relationship for the critical stress ratio. An efficient procedure for determining the reference S-N relationship for the critical stress ratio by using fatigue data for another stress ratio is also developed.

Abstract: Off-axis creep recovery behavior after complete unloading during creep at a constant stress is examined for a unidirectional T800H/3631 carbon/epoxy composite laminate at high temperature. Creep and creep recovery tests are performed on plain coupon specimens with four kinds of fiber orientations: 10, 30, 45 and 90°. It is observed that the creep strain appeared at a high stress does not completely recover after full removal of the creep stress, indicating that an irrecoverable creep strain has developed under the prior constant stress loading. Variable stress creep simulations are attempted using the modified kinematic hardening model for homogenized anisotropic inelastic composites in which an accelerated change in kinematic hardening over a certain range of viscoplastic strain is considered. Comparison with experimental results demonstrates that the proposed model can adequately describe the off-axis creep and creep recovery behaviors of the unidirectional composite system under constant and variable stress conditions.

Abstract: Off-axis tensile behavior of two grades of hybrid laminates, GLARE-2 and GLARE-3, made of unidirectional glass/epoxy laminae (GFRP) and aluminum (Al) alloy sheets is examined for various fiber orientations, and the differences in mechanical properties between the unidirectional and cross-ply hybrid laminates are quantified. The problem of accurately describing the nonlinear deformation of GLARE laminates under off-axis loading conditions is revisited, and the effect of local nonlinear deformation in the constituent GFRP layers on the overall inelastic behavior of GLARE laminates subjected to inclined tensile loading is elucidated by means of elastic-plastic finite element (FE) analyses.