Progressive damage modeling of adhesively bonded lap joints

A continuum damage model for simulating damage propagation of bonded joints is presented, introducing a linear softening damage process for the adhesive agent. Material models simulating anisotropic non-linear elastic behavior and distributed damage accumulation were used for the composite adherends as well. The proposed modeling procedure was applied to a series of lap joints accounting for adhesion either by means of secondary bonding or co-bonding. Stress analysis was performed using plane strain elements of a commercial finite element code allowing implementation of user defined constitutive equations. Numerical results for the different overlap lengths under investigation were in good agreement with experimental data in terms of joint strength and overall structural behavior.     

A new analytical model on predicting the elastic properties of 3D full five-directional braided composites based on a multiunit cell model

A new analytical model based on a multiunit cell model is proposed to predict the elastic properties of 3D full five-directional braided composites (F5DBC). The stiffness-volume averaging method is applied to predict the elastic properties of unit cell models in meso-scale and specimens in global-scale by using the multi-scale modeling procedures. The contribution of all unit cells to the elastic properties of specimen is considered in the analytical model. The predicted elastic properties are in good agreement with the available experimental data, demonstrating the applicability of the model. Also, the effects of the braiding angle and the fiber volume fraction on the elastic properties are discussed in detail. The elastic constants of each unit cell are analyzed and the effect of the number of yarn carriers on the mechanical properties is also investigated. Results indicate that it is convenient to apply the present analytical model to predict the elastic properties of 3D F5DBC due to high computational efficiency.     

Evaluation of thermal,mechanical, and electrical properties of PVDF/GNP binary and PVDF/PMMA/GNP ternary nanocomposites

Graphene nanoplatelet (GNP) was incorporated into poly(vinylidene fluoride) (PVDF) and PVDF/poly(methyl methacrylate) (PMMA) blend to achieve binary and ternary nanocomposites. GNP was more randomly dispersed in binary composites compared with ternary composites. GNP exhibited higher nucleation efficiency for PVDF crystallization in ternary composites than in binary composites. GNP addition induced PVDF crystals with higher stability; however, PMMA imparted opposite effect. The binary composite exhibited lower thermal expansion value than PVDF; the value further declined (up to 28.5% drop) in the ternary composites. The storage modulus of binary and ternary composites increased to 23.1% and 53.9% (at 25 °C), respectively, compared with PVDF. Electrical percolation threshold between 1 phr and 2 phr GNP loading was identified for the two composite systems; the ternary composites exhibited lower electrical resistivity at identical GNP loadings. Rheological data confirmed that the formation of GNP (pseudo)network structure was assisted in the ternary system.     

Nesting effect on the mode II fracture toughness of woven laminates

The mode II fracture toughness is evaluated for carbon fibre T700-epoxy reinforced woven laminates using the end notch flexure set-up. The analysed woven composites have a different tow size (3K/12K). Three different nesting/shifting configurations are applied to the plies at the fracture surface. Corrected Beam Theory with effective crack length method (CBTE) and Beam Theory including Bending rotations effects method (BTBE) are evaluated for obtaining mode II fracture toughness. During data post-processing, the importance of the bending angle of rotation and the test configuration is observed to be important. The results show that crack propagation under mode II is more stable if the matrix is evenly distributed on the surface. The nesting does not significantly affect mode II fracture toughness values, although a greater presence of matrix on the delaminated area increases its value.     

Uncertainty quantification for multiscale modeling of polymer nanocomposites with correlated parameters

We propose a stochastic multiscale method to quantify the correlated key-input parameters influencing the mechanical properties of polymer nanocomposites (PNCs). The variations of parameters at nano-, micro-, meso- and macro-scales are connected by a hierarchical multiscale approach. The first-order and total-effect sensitivity indices are determined first. The input parameters include the single-walled carbon nanotube (SWNT) length, the SWNT waviness, the agglomeration and volume fraction of SWNTs. Stochastic methods consistently predict that the key parameters for the Young’s modulus of the composite are the volume fraction followed by the averaged longitudinal modulus of equivalent fiber (EF), the SWNT length, and the averaged transverse modulus of the EF, respectively. The averaged longitudinal modulus of the EF is estimated to be the most important parameter with respect to the Poisson’s ratio followed by the volume fraction, the SWNT length, and the averaged transverse modulus of the EF, respectively. On the other hand, the agglomeration parameters have insignificant effect on both Young’s modulus and Poisson’s ratio compared to other parameters. The sensitivity analysis (SA) also reveals the correlation between the input parameters and its effect on the mechanical properties.     

Filling behavior,morphology evolution and crystallization behavior of microinjection molded poly(lactic acid)/hydroxyapatite nanocomposites

In this paper, the filling behavior, morphology evolution, crystallization behavior, thermal stability and mechanical property of poly(lactic acid) (PLA)/hydroxyapatite (HA) nanocomposite under microinjection molding conditions were systematically investigated. The comparison between micropart and macropart of PLA/HA nanocomposite was also conducted. Results showed that in the four stages occurring in the microinjection molding process, the mold cavity filling stage is an extremely rapid process and injection speed influences the filling behavior much more significantly than mold temperature. The remarkably enhanced shear force field generated under microinjection molding conditions proves to be beneficial to formation of highly oriented PLA matrix self-fibrillating structure, improvement of HA filler dispersion and enhancement of interfacial combination. Formation of such a highly oriented structure could lead to the remarkable difference in both crystallization behavior and mechanical property between micropart and macropart. The PLA/HA nanocomposite micropart possessed a significantly enhanced mechanical property and showed a good application prospect.     

New environmentally friendly composite laminates with epoxidized linseed oil (ELO) and slate fiber fabrics

This work focuses on the development of new composite laminates based on the use of epoxidized linseed oil (ELO) as matrix and reinforcement fabrics from slate fibers with different silane treatments. The curing behavior of the ELO resin is followed by differential scanning calorimetry (DSC) and the gelation is studied by oscillatory rheometry and gel-time. Composite laminates of ELO matrix and slate fabrics are manufactured by Resin Transfer Molding (RTM) and the mechanical properties of the composite laminates are tested in tensile, flexural and impact conditions. The effects of different silane coupling agents on fiber-matrix interface phenomena are studied by scanning electron microscopy (SEM). As in other siliceous fibers, silane treatment leads to improved mechanical performance but glycidyl silane treatment produces the optimum results as the interactions between silanized slate fiber and epoxidized linseed oil are remarkably improved as observed by scanning electron microscopy (SEM).     

Constructing General Orthogonal Fractional Factorial Split-Plot Designs

While the orthogonal design of split-plot fractional factorial experiments has received much attention already, there are still major voids in the literature. First, designs with one or more factors acting at more than two levels have not yet been considered. Second, published work on nonregular fractional factorial split-plot designs was either based only on Plackett–Burman designs, or on small nonregular designs with limited numbers of factors. In this article, we present a novel approach to designing general orthogonal fractional factorial split-plot designs. One key feature of our approach is that it can be used to construct two-level designs as well as designs involving one or more factors with more than two levels. Moreover, the approach can be used to create two-level designs that match or outperform alternative designs in the literature, and to create two-level designs that cannot be constructed using existing methodology. Our new approach involves the use of integer linear programming and mixed integer linear programming, and, for large design problems, it combines integer linear programming with variable neighborhood search. We demonstrate the usefulness of our approach by constructing two-level split-plot designs of 16–96 runs, an 81-run three-level split-plot design, and a 48-run mixed-level split-plot design. Supplementary materials for this article are available online.     

Enhancement of fracture toughness of hierarchical carbon fiber composites via improved adhesion between carbon nanotubes and carbon fibers

Hierarchical +1 composites consisting of carbon fibers with carbon nanotubes (CNTs) grown onto them and an epoxy matrix were processed, and the mode I fracture toughness of these composites was evaluated. The mode I fracture toughness of the initial batches of the hierarchical composites was lower than that of the baseline samples without CNTs. Hence, efforts to enhance the adhesion between carbon fibers and CNTs were made, resulting in enhanced adhesion. The enhanced adhesion was confirmed by Scotch tape tests and mode I fracture toughness tests followed by fractographic studies. The mode I fracture toughness of the hierarchical composites with enhanced adhesion was 51% and 89% higher than those of the baseline samples and hierarchical composites with poor adhesion, respectively. Moreover, fractographic studies of the fracture surfaces of the hierarchical composites with enhanced adhesion showed that CNTs were still attached to carbon fibers even after the mechanical tests.     

Manufacturing strategies for microvascular polymeric composites: A review

A microvascular network within a composite structure can significantly boost its performance. However, properties of microvascular network and host structure largely depend on the manufacturing method, used for vascularization. This paper presents a review on various manufacturing strategies that have been implemented so far to produce vascularized polymer composites. The ways by which polymer composites can be vascularized with isolated or interconnected networks are based on either by incorporating pre-made channels or removing pre-loaded solid performs from the cured laminates. Majority of the techniques were developed for healing and recovery of structural integrity after quasi-static fracture, but microvascular networks also showed promise for enhanced-damage visualization, self-cooling, and damage sensing applications. Each technique has its own merits and demerits but the manufacturing techniques that are not only compatible with current composite manufacturing, but also give the freedom to embed complex channels which can execute multi-functions synchronously still remains the main challenge.