Discontinuous Composites Research Articles

Manufacturing of a sandwich structure engineered wood with a rice husk flake core and teak veneer reinforced with glass/carbon fiber skin

The manufacturing of a sandwich structure engineered wood, constructed from a rice husk flake core and teak veneer as outside skins, was studied in this work. Epoxy adhesive was employed, while glass and carbon fiber, both short discontinuous and woven forms, were used as reinforcement. The impact strength, flexural properties, and dimensional stability of the samples after prolonged water immersion were measured. At the assigned reinforcement loadings, the rice husk flake/woven woods showed mechanical superiority over the rice husk flake/short discontinuous materials, regardless of the fiber type. The reason for the greater rice husk flake/woven interfacial adhesion and laminated woven strength, compared to the rice husk flake/short discontinuous composite was investigated. The samples constructed from teak veneer laminated with woven glass or carbon fiber skin and rice husk flake or rice husk flake/30% woven glass cores had greater mechanical properties. The high shear and tensile/compression stresses of woven glass or carbon fiber laminated onto teak veneer skins were confirmed. The sandwich structure engineered wood using woven glass and carbon fiber reinforcement exhibited good dimensional stability under prolonged water immersion. Carbon fiber was the better material candidate compared to woven glass in terms of manufacturing the sandwich engineered wood presented in this work.

Shear-lag model for discontinuous fiber-reinforced composites with a membrane-type imperfect interface

A shear-lag model is developed for discontinuous fiber-reinforced composites with a membrane-type imperfect interface, across which the displacement vector is continuous but the traction vector suffers a jump that is governed by the generalized Young–Laplace equation. Closed-form expressions are obtained for the stress fields in both the fiber-reinforced region and the pure matrix regions and for the shear stress on the interface from both the fiber and matrix sides. To illustrate the newly developed analytical model, a numerical analysis is provided by directly using the general formulas derived. The numerical results reveal that the fiber aspect ratio and the interface parameter can both have significant effects on the stress distributions in the composite.

Representative volume element generation and its size determination for discontinuous composites made from chopped prepregs

In the present study, the composites made from chopping prepregs in rectangular chips, called discontinuous composites derived from chopped prepreg (DCCP) were modeled for stiffness prediction. Standard mechanics approach with displacement boundary conditions is used for stiffness prediction. The minimum size of the representative volume element (RVE) for this material is determined. An algorithm based on the random sequential adsorption technique (RSA) has been exploited to generate the 3D RVEs. RVEs of five different aspect ratios of the cuboid-shaped inclusions are developed using this technique. The chips are completely randomly oriented with a constant volume fraction. A detailed study was carried out in each case by further generating the RVEs with different increasing sizes, and the smallest size of RVE with geometric periodicity was identified. Further, the characteristics of these RVEs have been investigated for different aspect ratios, varying the chip volume fraction for the influence of volume fraction on RVE size and the material properties. The efficacy of the developed approach is demonstrated through repetitiveness of the effective properties prediction for various scenarios studied. The resulting material showed isotropy in the plane where chips are distributed randomly while overall isotropy about 80% is seen in most of the RVEs studied.

Обзор экспериментальных исследований деформационного поведения алюмоматричных композиционных материалов, дискретно упрочненных карбидом кремния, в состоянии высокотемпературной сверхпластичности и при ползучести

Обзор экспериментальных исследований деформационного поведения алюмоматричных композиционных материалов, дискретно упрочненных карбидом кремния, в состоянии высокотемпературной сверхпластичности и при ползучести

The influence of 3D microstructural features on the elastic behaviour of tow-based discontinuous composites

Tow-based discontinuous composites consist of carbon-fibre tows dispersed in a polymeric matrix, resulting in a complex microstructure with inherent 3D features such as tow waviness, non-uniform tow thickness, and fibre content variations; however, most models for these materials are based on 2D formulations, or in very computationally-expensive FE models. This work proposes a computationally-efficient microstructure generator able to recreate the 3D features of tow-based discontinuous composites, coupled with an analytical stiffness model able to quantify the effect of tow waviness on the modulus of these materials, for the first time in the literature. The features of the microstructures generated are validated against 3D measurements obtained through micro-CT scans and optical micrographs. The results of the stiffness model are successfully validated against experimental results from the literature, showing that tow waviness can trigger a knock-down of over 20% on the stiffness of tow-based discontinuous composites. It is also shown that current testing standards can lead to uncertainties over 20% on the measurement of the Young’s modulus of tow-based discontinuous composites, and alternatives to define suitable experimental campaigns are proposed.

Impact of pregnancy on antidepressant treatment course: a population-based comparative cohort study in France

The prevention of relapses and the treatment of depression during pregnancy are difficult challenges. The maintenance of antidepressants in pregnancy with its concomitant risks to mother and child needs to be weighed against those associated with not treating the disease. This study aimed at quantifying the impact of the occurrence of pregnancy on the course of antidepressant treatment among newly treated women (< 6months). We performed a comparative observational cohort study using the nationwide French reimbursement healthcare system database. Women who conceived in 2014 and initiated an antidepressant at any time in the 6months before pregnancy were compared with nonpregnant women newly exposed to antidepressants with matching on age, antidepressant exposure, history of psychiatric disorders, and area of residence. The primary outcome was a composite of antidepressant discontinuation, switch to another antidepressant, and concomitant use of antidepressants. The secondary outcome was the resumption of antidepressant during follow-up. We used Cox marginal proportional hazards models to compare time to outcomes between pregnant and nonpregnant women. The pregnant cohort included 6593 women, and the comparison cohort 29,347 nonpregnant women. In the period following the first month of treatment, pregnant women were more likely to experience treatment modification, and especially to stop receiving it, compared with nonpregnant women (adjusted hazard ratio (aHR) 1.58; 95%CI, 1.51-1.62). Pregnant women who discontinued treatment had a 41% decreased incidence of antidepressant resumption compared with nonpregnant women (aHR 0.59; 95%CI, 0.56-0.62). Pregnancy was a determinant of antidepressant treatment modification, and especially of discontinuation.

Data-driven intelligent optimisation of discontinuous composites

Fibre composites, and especially aligned discontinuous composites (ADCs), offer enormous versatility in composition, microstructure, and performance, but are difficult to optimise, due to their inherent variability and myriad permutations of microstructural design variables. This work combines an accurate yet efficient virtual testing framework (VTF) with a data-driven intelligent Bayesian optimisation routine, to maximise the mechanical performance of ADCs for a number of single- and multi-objective design cases. The use of a surrogate model helps to minimise the number of optimisation iterations, and provides a more accurate insight into the expected performance of materials which feature significant variability. Results from the single-objective optimisation study show that a wide range of structural properties can be achieved using ADCs, with a maximum stiffness of 505 GPa, maximum ultimate strain of 3.94%, or a maximum ultimate strength of 1.92 GPa all possible. A moderate trade-off in performance can be achieved when considering multi-objective optimisation design cases, such as an optimal ultimate strength & ultimate strain combination of 982 MPa and 3.27%, or an optimal combination of 720 MPa yield strength & 1.91% pseudo-ductile strain.

Ultra-strong and stiff randomly-oriented discontinuous composites: Closing the gap to quasi-isotropic continuous-fibre laminates

Conventional randomly-oriented Tow Based Discontinuous Composites (TBDCs) are materials which combine good mechanical properties, lightweight and high manufacturability, and are therefore interesting for high-volume transport industries. This paper proposes, designs and successfully demonstrates a pathway to produce TBDCs with outstanding stiffness and tensile strength, by using ultra-thin tapes of (ultra-) high modulus carbon-fibres. Numerical models are used to explore the design space of discontinuous composite materials, in order to identify the optimal microstructural design to maximise stiffness and strength. Selected microstructures are manufactured and tested under tension; the experimental results show good agreement with the numerical predictions, and demonstrate a significant increase in the tensile strength and Young’s modulus of TBDCs by reducing the tow thickness and increasing the modulus of the fibres. Strength and stiffness increases of over 100% compared with the commercially available TBDC systems are achieved, resulting in mechanical properties that match the strength and overcome the stiffness of aerospace-graded continuous-fibre laminates.

Evaluating Adherence to Guideline-Directed C Difficile Infection Management Pre- and Postimplementation of an Electronic Order Set

Background: After publication of the Clostridioides difficile infection (CDI) guidelines by the Infectious Disease Society of America (IDSA) in early 2018, we identified that many prescribers at our institution continued to practice using the older guidelines. Objective: This study aimed to determine whether the implementation of an electronic order set for CDI would increase prescriber compliance to current IDSA recommendations for CDI management. Methods: This was a single-center, prospective cohort study of adult inpatients with a confirmed CDI. The study was conducted between March 1, 2018, and April 1, 2019. Patients were stratified into a preintervention and postintervention group before and after order set implementation. The primary outcome was a composite of appropriate CDI therapy selection and discontinuation of nonessential antimicrobials and acid-suppressive agents. The secondary outcome evaluated appropriate CDI therapy medications prescribed at hospital discharge. Results: Of the 149 patients included in this study, 96 were included in a preintervention group and 53 included in a postintervention group. The primary outcome was met in 45% of patients in the preintervention group and 66% of patients in the postintervention group (P = .01). The secondary outcome occurred in 86% of patients in the preintervention group and 100% of patients in the postintervention group (P = .02). Conclusion: Implementation of a CDI electronic order set and alert bundle was associated with enhanced prescriber adherence to guideline-directed therapy. Our results suggest that order sets not only improve inpatient compliance to guidelines but may also improve medication-related adherence to guideline recommendations upon discharge.

The influence of variability and defects on the mechanical performance of tailorable composites

Aligned hybrid-fibre discontinuous composites offer the ability to tailor their mechanical response through careful microstructural design. However, with tailorability comes microstructural complexity, which in turn leads to many sources of variability and defects. A virtual testing framework was further extended to investigate the influence of variability and defects on the mechanical performance of various aligned discontinuous composite material systems. This approach identified the most critical sources of variability as (i) fibre strength, (ii) the distance between fibre ends, or (iii) the level of fibre-type intermingling, depending on the material system. Fibre vacancy defects were shown to have the most significant influence on the strength and ductility of aligned discontinuous composites, although this sensitivity can be reduced through hybridisation of the fibre types.

Critically evaluating mechanics of structure genome-based micromechanics approach

The objective of this paper is to critically evaluate the accuracy and efficiency of a general-purpose micromechanics approach based on the Mechanics of Structure Genome (MSG), when it is applied to the constitutive modeling of 3D structures. The Generalized Method of Cell (GMC) is chosen as a reference method during efficiency evaluation. The predictions by Three-Dimensional Finite Element Analysis (3D FEA) are chosen as benchmarks during accuracy evaluation. Composites such as a continuous fiber-reinforced composite, a particle-reinforced composite, two discontinuous fiber-reinforced composites, and a woven composite are analyzed using MSG, GMC, and 3D FEA. During homogenization, MSG is found to be as accurate but much more efficient than 3D FEA, and despite high efficiency, GMC is found to sacrifice accuracy for efficiency. During dehomogenization, MSG is found to be as accurate as 3D FEA, but GMC is found not to be so accurate. The fidelity of MSG, when it is applied to the modeling of other structures (e.g., beams, plates, and shells), can be similarly evaluated.

Modelling of the Influence of Metal Connectors on the Resistance of Historical Dry-Stone Masonry Structures

ABSTRACT A commonly used method for increasing the resistance of historical structures, made of stone blocks with dry joints, is strengthening with the metal connectors. The numerical modelling of these structures is demanded due to a discontinuous nature and composition of the materials with significantly different properties. This paper presents a new numerical model of metal connectors developed on the basis of force-displacement curves experimentally obtained by testing two stone blocks connected with two different types of connectors — clamps or dowels. The model has been implemented into the 3D finite-discrete element code. The developed model can capture damage mechanisms of the structure both at the joint and the block level, coupled with the deformation and extracting of the connectors from the holes. The capabilities of the presented approach are demonstrated in the seismic analysis of the strengthened dry-stone arch.

Race, Ethnicity, and End-of-Life Care in Dialysis Patients in the United States.

End-of-life care is a prominent consideration in patients on maintenance dialysis, especially when death appears imminent and quality of life is poor. To date, examination of race- and ethnicity-associated disparities in end-of-life care for patients with ESRD has largely been restricted to comparisons of white and black patients. We performed a retrospective national study using United States Renal Data System files to determine whether end-of-life care in United States patients on dialysis is subject to racial or ethnic disparity. The primary outcome was a composite of discontinuation of dialysis and death in a nonhospital or hospice setting. Among 1,098,384 patients on dialysis dying between 2000 and 2014, the primary outcome was less likely in patients from any minority group compared with the non-Hispanic white population (10.9% versus 22.6%, P<0.001, respectively). We also observed similar significant disparities between any minority group and non-Hispanic whites for dialysis discontinuation (16.7% versus 31.2%), as well as hospice (10.3% versus 18.1%) and nonhospital death (34.4% versus 46.4%). After extensive covariate adjustment, the primary outcome was less likely in the combined minority group than in the non-Hispanic white population (adjusted odds ratio, 0.55; 95% confidence interval, 0.55 to 0.56; P<0.001). Individual minority groups (non-Hispanic Asian, non-Hispanic black, non-Hispanic Native American, and Hispanic) were significantly less likely than non-Hispanic whites to experience the primary outcome. This disparity was especially pronounced for non-Hispanic Native American and Hispanic subgroups. There appear to be substantial race- and ethnicity-based disparities in end-of-life care practices for United States patients receiving dialysis.

Development and assessment of modelling strategies to predict failure in tow-based discontinuous composites

Tow-Based Discontinuous Composites (TBDCs) are a growing class of materials that combine manufacturability, light-weight, and high performance. This study proposes multi-scale modelling approaches to predict the tensile strength and failure envelopes of tow-based discontinuous composites, by representing the actual composite (with randomly-oriented tows) as an equivalent ply-by-ply laminate. Several modelling approaches are considered for the different scales, including (i) a stochastic bi-linear shear-lag formulation accounting for the random location of tow-ends and matrix cracking, (ii) a novel failure criterion for a discontinuous uni-directional ply accounting for the interaction between tow pull-out and transverse failure, and (iii) a ply-discount method or a maximum strain energy criterion for the final failure of the composite. The model computes full failure envelopes for ply-by-ply laminates equivalent to TBDCs within minutes, and the results show good agreement with experimental data.

Evaluation of measurement method for carbon fiber length using an optical image scanner

Fiber length and its distribution are important factors to determine the mechanical properties of discontinuous carbon fiber-reinforced composites. For the efficient and easy measurement of fiber length, an optical image scanner was introduced into the measurement procedure. The scan image allowed to measure approximately 25,000 carbon fibers. The measured values () and mean value of these (<>) were evaluated by comparison with reference values () obtained using an optical microscope. were dependent on the angle between fiber and sub-scanning direction. Furthermore, the relative errors between <> and were less than 2.5% in case that were 0.226 mm or more. The sample standard deviations of were approximately constant and their values were ca. 0.010 mm. Therefore, it was found that the relative error of measurement value derived from the scan image was dependent on the measured fiber length.

Volume phase transition of a polymer gel induced by phase separation of mixed solvents of water and 2‐butoxyethanol

ABSTRACTThe occurrence of the volume phase transition triggered by the phase separation of mixed solvents is investigated using polyacrylamide gels in mixtures of water and 2‐butoxyethanol. When the water content of the mixtures is lower than that of the critical composition, the temperature‐dependent swelling curves show a steep change around the phase separation temperatures but remain continuous, although the solvent inside the gel undergoes a discontinuous composition change. This continuous change originates from the critical concentration fluctuation below the spinodal temperature. Hysteresis‐like behavior is also observed for the heating and cooling curves. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46366.

Simulating the effects of carbon nanotube continuity and interfacial bonding on composite strength and stiffness

Molecular dynamics simulations of carbon nanotube (CNT) composites, in which the CNTs are continuous across the periodic boundary, overestimate the experimentally measured mechanical properties of CNT composites along the fiber direction. Since the CNTs in these composites are much shorter than the composite dimensions, load must be transferred either directly between CNTs or through the matrix, a mechanism that is absent in simulations of effectively continuous CNTs. In this study, the elastic and fracture properties of high volume fraction discontinuous carbon nanotube/amorphous carbon composite systems were compared to those of otherwise equivalent continuous CNT composites using ReaxFF reactive molecular dynamics simulations. The simulation results quantify the dependence of composite mechanical properties on the number of nanotube-matrix interfacial covalent bonds. Furthermore, the mechanical impact of interfacial bonding was decomposed to reveal its effect on the properties of the CNTs, the interfacial layer of matrix, and the bulk matrix. For the composites with continuous reinforcement, it was found that any degree of interfacial bonding has a negative impact on axial tensile strength and stiffness. This is due to disruption of the structure of the CNTs and interfacial matrix layer by the interfacial bonds. For the discontinuous composites, the modulus was maximized between 4% and 7% interfacial bonding and the strength continued to increase up to the highest levels of interfacial bonding studied. Areas of low stress and voids were observed in the simulated discontinuous composites at the ends of the tubes, from which fracture was observed to initiate. Experimental carbon nanotube yarn composites were fabricated and tested. The experimental results illustrate the knockdown factors that reduce composite mechanical properties relative to those of the tubes themselves.

Virtual testing framework for hybrid aligned discontinuous composites

The inherent brittleness of conventional high-performance composites can be addressed by the use of discontinuous fibres or hybridisation of fibre-types. In this paper, we propose the first models in the literature to predict the stress-strain curve of hybrid discontinuous composites, with either a brick-and-mortar or an intermingled-fibre microstructure. The models consider a shear-lag stress-transfer between the hybrid reinforcement units, and show that this stress transfer becomes less efficient with hybridisation. The model for intermingled-fibre hybrids also considers stochastic fibre strengths and fibre fragmentation, and can therefore predict a brittle or pseudo-ductile response of hybrid discontinuous composites as observed experimentally, as well as hybrid effects. These models can be used to perform virtual testing and microstructural design of hybrid aligned discontinuous composites.

Exploring the pseudo-ductility of aligned hybrid discontinuous composites using controlled fibre-type arrangements

Pseudo-ductility presents a potential means for preventing catastrophic failure in composite materials; large deformations will prevent brittle fracture and provide warning before final failure. This work explores how the pseudo-ductility and strength of aligned hybrid discontinuous composites can be controlled by manipulating the arrangement of different fibre types. Aligned carbon/glass hybrid specimens with different fibre arrangements are manufactured and tested using a modification to the High Performance Discontinuous Fibre (HiPerDiF) method. Experimental results are complemented by an improved virtual testing framework, which accurately captures the fracture behaviour of a range of hybrid discontinuous composite microstructures. With a randomly intermingled fibre arrangement as a baseline, a 27% increase in strength and a 44% increase in pseudo-ductility can be achieved when low elongation fibres are completely isolated from one-another. Results demonstrate that the HiPerDiF method is the current state-of-the-art for maximising the degree of intermingling and hence the pseudo-ductility of hybrid composites.

Modelling hybrid effects on the stiffness of aligned discontinuous composites with hybrid fibre-types

Hybrid discontinuous composites offer the possibility to tailor the composite properties for specific applications, improve their manufacturability, and reduce cost by introducing cheaper fibres. However, the mechanical behaviour of hybrid composites often shows hybrid effects which cannot be modelled by the rule-of-mixtures and are therefore challenging to predict and explain. This paper presents models to calculate the Young's modulus of different discontinuous hybrid composites, which is affected by such hybrid effects. The models are based on shear-lag and consider two types of hybrid discontinuous architectures: (i) a deterministic “brick-and-mortar” architecture consisting of perfectly staggered platelets with two different Young's moduli and thicknesses, and (ii) a stochastic architecture of aligned fibres with two different Young's moduli and diameters, with randomly allocated fibre-ends and random or organised intermingling. The models show good agreement with numerical and experimental validations; their results show that hybrid interactions between different types of fibres or platelets reduce the Young's modulus of hybrid discontinuous composites, which justifies the negative hybrid effects observed.