Key Engineering Materials Vols. 334-335

Abstract: The high performance of poly p-phenylene-2,6-benzobisoxazole (PBO) fibre in mechanical properties and environmental resistance provides great potential applications as reinforcement fibres for composites. However, poor adhesion between PBO fibre and matrix is found because of the chemically inactive and/or relatively smooth surface of the reinforcement fibre. Here, we report the surface modification of PBO fibres by O2 and NH3 plasma treatments. The surface energy and roughness are increased for both sized and extracted fibres after plasma treatments. The sized fibre shows marginal improvement in interfacial adhesion and no change in fibre tensile strength because of the barrier effect of the sizing layer. For the extracted fibre, the plasma treatments cause reduction in both interfacial adhesion and the tensile strength, which is sensitive to plasma treatment conditions as revealed by a bimodal Weibull statistical distribution analysis. The fibre surface roughness is increased and more surface flaws are induced, which could result in coarse interface structures when the treated fibre surface has no adequate wetting and functional groups.

Abstract: In this paper, a coupled thermal-mechanical-damage model, Material Failure Process Analysis for Thermo code (abbreviated as MFPA-thermo), was applied to investigate the formation, extension and coalescence of cracks in FRCs, caused by the thermal mismatch of the matrix and the particles under uniform temperature variations. The effects of the thermal mismatch between the matrix and fibers on the stress distribution and crack development were also numerically studied. The influences of the material heterogeneity, the failure patterns of FRCs at varied temperatures are simulated and compared with the experimental results in the present paper. The results show that the mechanisms of thermal damage and fracture of the composite remarkedably depend on the difference between the coefficients of thermal expansion of the fibers and the matrix on a meso-scale. Meanwhile, the simulations indicate that the thermal cracking of the FRCs at uniform varied temperatures is an evolution process from diffused damage, nucleation, and finally linkage of cracks.

Abstract: To predict the mechanical properties of composite materials by using computer is complicated, because it is difficult to model directly by ordinary FEM. A calculation method by using the mesh superposition method and periodic boundary condition has been proposed in order to obtain the equivalent mechanical properties of composite materials easily. The numerical results by proposed method have shown good agreement with ones by the traditional procedure. The proposed method is efficient for the materials with complicated structure like woven fabric composites etc.

Abstract: Biaxial compressive tests of plain concrete specimens suffering 0、25、50 and 75 cycles of freezing and thawing respectively in 3.5% sodium chloride solution imitated seawater were completed under four lateral constant stress rates. The tests were completed by using the large static-dynamic triaxial test system for concrete in the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology. The biaxial compressive strength and deformation characteristics of concrete confined by lateral stress after suffering corrosive cycles of freezing and thawing were studied in a systematic way. On the basis of the tests, the failure criterion of concrete subjected to constant lateral compressive stress after suffering corrosive cycles of freezing and thawing was established in the stress-strain system. The conclusions can provide the testing and theoretical basis for deeper study of the concrete construction subjected to biaxial compressive combined loads in northern cold region or influenced by corrosive cycles of freezing and thawing nearby the coastal and offshore area.

Abstract: The objective of this study was to manufacture and investigate the properties of barrier films made from microfibril-reinforced polymer composites (MFCs) based on blends of linear low density polyethylene (PE) and poly(ethylene terephthalate) (PET). Blends containing 30 wt% of PET were prepared and extruded into strands. These were then subjected to cold drawing and compression moulded into thin films. The permeability of the films to oxygen gas was measured using a permeation test chamber and the effect of the draw ratio on the size of the reinforcing fibrils was also observed. Scanning electron microscope (SEM) revealed substantial differences in the structures of neat and MFC films. A model developed for predicting the permeability of filled polymer composite films was modified and compared to the test results. The draw ratios of the MFCs did not appear to have a significant effect on oxygen permeation, despite altering the dimensions of the reinforcing fibrils. The MFC films were found to be half as permeable as the neat polymer films.

Abstract: Large shear deformation of plain woven composite sheets and corresponding failure mechanism are investigated by bias extension test. Digital image correlation analysis was conducted on a series of photos taken during the test. Four typical phases were identified, and a theoretical model of the large deformation is proposed from energy point of view. Numerical simulations have also been carried out, but it will be reported in a subsequent paper.

Abstract: To make an expansion of composite consumption ever widening with use at demanding and safety critical applications, it is important to establish the evaluation technique of the strength, the safety, etc. To reach the goal with a useful reliable technique, we have developed a numerical simulation program on damage development of composites based on damage mechanics. The following points are the keys to simulate the mechanical behavior; (1) The generation technique of FE model with yarns and matrix, (2) The classification of anisotropic damage mode based on damage mechanics, (3) Multi-scale modeling by Mesh superposition method. In this paper, the numerical technique and the results of multi-scale analysis of woven composites are described.

Abstract: Recently, author had presented that impact force history of composite laminates subjected to low-velocity impact could be well analyzed using linearized contact law instead of the modified Hertzian contact law. If the linearized contact law concept is applied in impact response analysis, the impact problem can be transformed as a general structural analysis problem, so general purpose FEM software can be used in this kind of impact response analysis. In the present study it will be shown that impact damage, specially delamination area, as well as impact response can be well analyzed using the linearized contact law concept. In order to accurately predict delamination area, geometrical nonlinear analysis considering large deflection effect of plate has been performed and thermal stress analysis to consider thermal residual strain induced in curing process has been performed. Also, a proper failure criterion for delamination estimation has been used. In this failure criterion, in-situ strength values, obtained through matrix crack onset analysis have been used. Finally, analytically predicted delamination areas have been compared with experimental results. It shows that this analytical procedure can well predict delamination area of composite laminates subjected to the low-velocity impact.

Abstract: The usefulness of rotational moulding (rotomoulding) as a polymer processing technique is often limited by the selection of polymers, which in most cases happens to be polyethylene (PE). In the present study, PE polyethylene was blended with wollastonite microfibres and maleated polyethylene (as a coupling agent) with the purpose of developing an improved material for rotational moulding. The incorporation of wollastonite fibres without any coupling agent improved the tensile strength, but showed a reduction in impact strength. As expected, the most significant enhancement due to wollastonite was in the tensile modulus.. The addition of a coupling agent improved both the impact strength and the processability, especially when wollastonite was coated with aminosilane. Scanning electron microscopy revealed good adhesion between the coated fibre reinforcement and the polyethylene matrix at the fracture surface.

Abstract: In this work, we report our study of the microstructure and mechanical properties of pure Ti, Ti-11Al, Ti-47Al and Ti-6Al-4V nanostructure coatings on AZ91D Mg alloys prepared using DC magnetron sputtering. The fine structures of the coating (see fig. 1 for Ti-6Al-4V coating) are presented. The hardness of the coatings were tested on a CSEM nano-hardness tester with a Berkovich diamond intender. Hardness values were obtained from analyses using the Oliver-Pharr scheme. The hardness of the nanostructure coating increases from about 3.5GPa up to about 6.8GPa when the aluminium concentration increases from zero to 47at.%. Maximum hardness value was found on the Ti-6Al-4V coating, about 7.3GPa. The elastic modulus of the pure Ti coating has the lowest modulus and strain burst “pop-in” has been observed on Ti-47Al coating.