Effect of montmorillonite clay on flax fabric reinforced poly lactic acid composites with amphiphilic additives

Bio-composites (PF) were successfully prepared by reinforcing poly lactic acid (PLA) with woven flax fibers (F) in the presence of mandelic acid, benzilic acid, dicumyl peroxide (DCP) and zein as additives. To improve the mechanical properties of the bio-composites, montmorillonite clay (MMT) was also added. Characterizations of the bio-composites in presence and absence of MMT were performed by FTIR, DSC, TGA and DMTA. The interfacial adhesion between the fibers and the matrix was qualitatively assessed from SEM micrographs of fractured specimens. Intercalation of MMT with 1.4 nm basal spacing was observed in the PLA matrix leading to the increased modulus and water resistance of the bio-composites. Results indicated that mandelic acid and dicumyl peroxide acted as efficient additives for this system. This work provided us the exploratory idea of using MMT in presence of amphiphilic compounds as additives in bio-composites for possible applications.     

Dynamic strength of single lap joints with similar and dissimilar adherends

The increased use of adhesives for joining structural parts demands a thorough understanding of their load carrying capacity. The strength of the adhesive joints depends on several factors such as the joint geometry, adhesive type, adherend properties and also on the loading conditions. Particularly polymer based adhesives exhibit sensitivity to loading rate and therefore it is important to understand their behavior under impact like situations. The effect of similar versus dissimilar adherends on the dynamic strength of adhesive lap joints is addressed in this study. The dynamic strength is evaluated using the split-cylinder lap joint geometry in a split Hopkinson pressure bar setup. The commercial adhesive Araldite 2014 is used for preparing the joints. The adherend materials considered included steel and aluminum. The results of the study indicated that the dynamic strength of the lap joint is influenced by the adherend material and also by the adherent combination. Even in the case of joints with similar adherends, the strength was affected by the adherend type. The strength of steel–steel joints was higher than that for aluminum–aluminum joints. In the case of dissimilar adherends, the strength was lower than that of the case of similar adherends. The results of this study indicate that the combination of adherend material should also be accounted for while designing lap joints.     

A moving interface crack between two dissimilar functionally graded strips under plane deformation with integral equation methods

In this paper a finite interface crack with constant length (Yoffe-type crack) propagating along the interface between two dissimilar functionally graded strips with spatially varying elastic properties under in-plane loading is studied. By utilizing the Fourier transformation technique, the mixed boundary problem is reduced to a system of singular integral equations that are solved numerically. The influences of the geometric parameters, the graded parameter, the strip thickness and crack speed on the stress intensity factors and the probable kink angle are investigated. The numerical results show that the graded parameters, the thicknesses of the functionally graded strips and the crack speed have significant effects on the dynamic fracture behavior of functionally graded material structures.     

Study of critical buckling loads and modes of cross-ply laminated annular plates

Buckling of composite annular plates under uniform internal and external radial edge loads have been investigated using energy method. Trefftez rule is used in the stability equations. The symmetric buckling of symmetric cross-ply laminates is considered. In this paper, buckling behavior for the three laminates (90/0)_2s, (90/0₂/90)_s and (90₂/0₂)_s are studied. Influence of some parameters such as thickness, stacking sequence, type of supports and the ratio of hole to sheet radius on buckling loads and modes are investigated. The results of the energy method are compared with the results of numerical method. Based on the results, in the plates with clamped boundary conditions the symmetric buckling assumption is not accurate, contrary to other boundary conditions.     

Mechanical properties of wood–fiber reinforced polypropylene composites: Effect of a novel compatibilizer with isocyanate functional group

Natural fibers are increasingly being used as reinforcement in commercial thermoplastics due to their low cost, high specific properties and renewable nature. While the maleic anhydride modified polypropylene (MAPP) is most commonly used as compatibilizer to improve interfacial adhesion between hydrophilic wood–fibers and hydrophobic polypropylene, in this study, a novel compatibilizer (m-TMI-g-PP) with isocyanate functional group was synthesized by grafting m-isopropenyl-α,α-dimethylbenzyl-isocyanate (m-TMI) onto isotactic polypropylene (PP) in a twin screw extruder. The effect of filler concentration on the mechanical properties of wood–fiber filled composites, prepared by using m-TMI-g-PP as the compatibilizer, was investigated. The addition of the compatibilizer resulted in greater reinforcement of composites, as indicated by the improvement in mechanical properties. Tensile strength of composites so prepared increased by almost 45%, whereas 85% increase in flexural properties was observed. However the addition of wood–fibers resulted in a decrease in elongation at break and impact strength of the composites.     

Effect of alkali treatment on interfacial bonding in abaca fiber-reinforced composites

Abaca fibers demonstrate enormous potential as reinforcing agents in composite materials. In this study, abaca fibers were immersed in 5, 10 or 15 wt.% NaOH solutions for 2 h, and the effects of the alkali treatments on the mechanical characteristics and interfacial adhesion of the fibers in a model abaca fiber/epoxy composite system systematically evaluated. After 5 wt.% NaOH treatment, abaca fibers showed increased crystallinity, tensile strength and Young’s modulus compared to untreated fibers, and also improved interfacial shear strength with an epoxy. Stronger alkali treatments negatively impacted fiber stiffness and suitability for composite applications. Results suggest that mild alkali treatments (e.g. 5 wt.% NaOH for 2 h) are highly beneficial for the manufacture of abaca fiber-reinforced polymer composites.     

Safety analysis of building foundations over old goaf under additional stress from building load and seismic actions

Additional displacement of the building foundations over old goaf are prone to happen under the additional loads induced by new buildings, weakening-rock mass by mining and seismic actions, which will cause serious damage to the buildings. In order to analyze the safety of the building foundations safety over the old goaf, the structure characteristics of the strata over the old goaf was investigated and the instability conditions of overhanging rocks upon old goaf were also analyzed in this paper. The results indicate that the stability of overhanging rocks is remarkably decreased by the interactions of mining fractures, earthquake force and building load, in addition, the settlement of the foundations over old goaf is increased by the instability of overhanging rocks. According to the location of a new power plant in Yima Mine and its ambient conditions, we defined the influence scope of old goaf via resistivity tomography. Based on the seismic parameters of the construction site, a numerical FLAC^3d model of the building foundation under the seismic actions and building load was developed. The numerical results are obtained as follows: the foundation of the main power house meets the requirement of 6° seismic fortification intensity; however, under 7° seismic fortification intensity, the maximum differential settlement of foundation between the neighboring pillars is close to the maximum allowable value, while the seismic fortification intensity reaches 8°, but the safety requirements will not be satisfied.     

A Molecular-Level Interface Design Enabled High-Strength and High-Toughness Carbon Nanotube Buckypaper

Buckypaper (BP) assembled by carbon nanotubes (CNTs) is considered as a promising macroscopic material with excellent multifunctional properties. However, the poor interfacial stress-transfer ability between CNTs seriously limits its further applications. This paper proposes a novel approach to simultaneously improve both the tensile strength and the toughness of BPs, where a commercial poly(amido amine) (PAMAM) dendrimer is utilized to serve as an interfacial bridging agent, by realizing effective chemical cross-linking between CNTs. The result shows that the introduction of PAMAM can increase the tensile strength, Young's modulus, and toughness by 306%, 168%, 1007%, respectively, compared with that of pristine BPs based on weak van der Waals interactions. In addition, molecular dynamics (MD) simulation is employed to reveal the proposed interfacial enhancement. It is found that the elastic deformation, uncoiling, interlocking of PAMAM dendrimers, and chemical bonds between PAMAM dendrimers and CNTs contribute to the improvement of the macroscopic strength and toughness of BPs. More importantly, it is found that the formed amido bonds are very strong, and the breakage of chemical bonds occurs at C─N bonds within PAMAM themselves. This work provides a new route for interfacial structure design and an in-depth understanding of mechanical properties of BPs.