A novel methodology for the rapid identification of the water diffusion coefficients of composite materials

The paper presents a novel methodology for the rapid identification of the water diffusion coefficients of composite materials. The methodology consists in employing a numerical parametric Proper Generalized Decomposition (PGD) method allowing incorporating the diffusion coefficients among the number of degrees of freedom. Compared to classical identification schemes, often based on Finite Element Method (FEM) iterations, the proposed method allows achieving consistent CPU time gain. The method is general and can be applied when diffusion anomalies take place or when diffusion–reaction coupling must be taken into account, moreover can deal with anisotropic materials. However, for the scope of illustration, in the present case, it is applied to the simple case of “classical diffusion” (Fick’s model with constant boundary conditions) and concerns isotropic materials.     

Determining the mechanism controlling glass fibre strength loss during thermal recycling of waste composites

This paper presents an experimental investigation into the large reductions to the tensile fracture stress and the associated strength loss mechanism of E-glass fibres during thermal recycling. Fractographic analysis reveals the fracture process is controlled by surface flaws, irrespective of heat treatment temperature and duration. The fracture toughness is an important material property in order to understand possible changes in the strength–flaw relationship during heat treatment. Focussed ion beam (FIB) milling is used to artificially create a single nano-sized deep notch (between 30 and 1000 nm) in glass fibres. The strength loss, fracture toughness, fracture mirror constant and fracture mechanism observed for nano-notched and thermally recycled fibres are identical, indicating bulk property changes do not occur during thermal recycling. The study proves conclusively that surface flaw growth is the controlling mechanism reducing fibreglass strength during thermal recycling of waste polymer composites.     

Design of natural fiber composites utilizing interfacial crystallinity and affinity

Butanediol initiated poly(ε-caprolactone) (PCL) has recently been reported as a toughening agent for cationically curing cycloaliphatic epoxides providing plasticized thermosets with excellent properties (Lützen et al., 2013). In this contribution that promising toughening approach was applied for the first time for the development of novel natural fiber composites (NFC). NFCs based on conventional brittle thermosetting polymers often suffer from poor interfacial adhesion and stress cracking. Composites made up of the novel plasticized thermosets and woven flax fiber preserved the elastomer-like properties and increased tensile strength and elongation at break up to 60 MPa and 5%, respectively. Furthermore, PCL was shown not only to toughen the epoxide but also to modulate the affinity of the matrix to the fiber. In conclusion, improved interfacial adhesion and the resulting excellent mechanical properties of cationically curable NFCs were achieved by both interfacial crystallization and affinity.     

Effect of fibre wrinkling to the spring-in behaviour of L-shaped composite materials

To determine the amount of deformation resulting from fibre wrinkling at corner regions, a set of experiments have been conducted. As known in the conventional lay-up method, the prepregs are laid sequentially layer by layer on the mould surface. At the corner region of a female tool the radius decreases at the inner surface and the amount of wrinkles increase towards the top layer as the layers are laid up. In order to determine how much these wrinkles influence the dimensional stability of the manufactured parts, an alternative lay-up method is used. The amount of the wrinkles can be increased for the parts of same geometry by first stacking prepregs on a flat plate and then bending the whole stack to conform to the surface of the L-shaped mould. In this method, more wrinkling occurs on the inner surface of the corner regions as compared to the conventional lay-up procedure. It was found that fibre wrinkling decreases the spring-in values. The mechanism behind that observation is discussed with the help of a heuristic Finite Element Analysis (FEA). The conformation of the stacked prepregs on the mould was simulated by using FEA.     

Scaling of fracture response in Over-height Compact Tension tests

An experimental investigation into in-plane scaled Over-height Compact Tension (OCT) [45/90/−45/0]_4s carbon/epoxy laminates was carried out to study the scaling of fracture response. The dimensions of the baseline specimens were scaled up and down by a factor of 2. Interrupted tests were carried out for specimens of each size in which the tests were stopped after certain load drops in order to study the failure mechanisms. X-ray Computed Tomography (CT) scanning was applied after the interrupted tests to examine the damage development and its effect on the fracture response. The test results showed that the scaling of the initial propagation of fracture follows Linear Elastic Fracture Mechanics (LEFM), but the development of the damage process zone differs with specimen sizes. The OCT specimens were found to be not large enough to generate a self-similar damage zone during propagation, and so no conclusions could be drawn regarding the R-curve effect.     

N2 plasma-assisted grafting of fluorinated chains onto partially cured epoxy resins

Two routes for the grafting of fluorinated molecules to an epoxy resin were studied. The first one deals with the grafting of the liquid-state resin whereas the second one is focused on the grafting onto the solid-state resin. These grafting reactions were shown to be similar as studied through FTIR and XPS spectroscopies. However, it appears that the grafting onto the solid-state resin is limited by the curing advancement. N₂ plasma-activation was used to solve this drawback and enhanced the grafting yield. This grafting improvement was mainly explained in terms of the surface wetting improvement and the attachment of nitrogen containing groups at the surface of the treated resin.     

Evaluation of surface properties of epoxy–nanodiamonds composites

Epoxy–matrix reinforced with nanodiamond (ND) particles, with ND content up to 5 wt%, were synthesized. Characterization of NDs by field emission scanning electron microscopy (FE-SEM) and Raman spectroscopy was conducted, while composites were characterized through contact angle, nanoindentation, nanoscratch and scanning probe microscopy. The assessed properties evaluated were hardness, elastic modulus, contact angle, deformation mechanisms, creep deformation, coefficient of friction and adhesion, namely. Results showed that even small additions of ND lead to significant enhancement in the hardness and elastic modulus of epoxy matrix, while properties of composites present a switch in behavior after passing a concentration threshold; this threshold was identified and discussed.     

Environmental durability of adhesively bonded FRP/steel joints in civil engineering applications: State of the art

Over the past three decades, the strengthening and repair of existing civil engineering structures using FRP laminates has attracted a great deal of attention. With the advances in polymer science, adhesive bonding has become a common joining technology in these applications. Despite numerous studies that address the short-term behaviour of adhesively bonded FRP/steel joints, uncertainty with respect to long-term performance still remains. This knowledge gap is regarded as a critical barrier, hindering the widespread application of FRPs to strengthen and retrofit steel structures. This paper presents the state of the art in terms of the durability of FRP/steel joints used in civil engineering applications. Important influential factors relating to the durability of adhesively bonded joints are reviewed and different damage mechanisms are discussed. Moreover, related investigations of the combined environmental durability of these joints are critically reviewed and the findings are presented. The paper concludes with a discussion to motivate future research topics, while it is emphasised that the generalisation of the available results is questionable.     

Hygrothermal deformation of orthotropic nanoplates based on the state-space concept

This paper deals with the investigation of the effect of hygrothermal conditions on the bending of nanoplates using Levy type solution model employing the state-space concept. The nanoplates are assumed to be subjected to a hygrothermal environment. The two-unknown function plate theory is used to derive the governing differential equations on the basis of Eringen's nonlocal elasticity theory. The governing equations contain the small scale effect as well as hygrothermal and mechanical effects. These equations are converted into a set of first-order linear ordinary differential equations with constant coefficients. Analytical solution of bending response for nanoplates under combinations of simply supported, clamped and free boundary conditions is obtained. Comparison of the results with those being in the open literature is made. The influences played by small scale parameter, temperature rise, the degree of moisture concentration, boundary conditions, plate aspect ratio and side-to-thickness ratio are studied.     

Inter-ply stitching optimisation of highly drapeable multi-ply preforms

An efficient finite element model has been developed in Abaqus/Explicit to solve highly non-linear fabric forming problems, using a non-orthogonal constitutive relation and membrane elements to model bi-axial fabrics. 1D cable-spring elements have been defined to model localised inter-ply stitch-bonds, introduced to facilitate automated handling of multi-ply preforms. Forming simulation results indicate that stitch placement cannot be optimised intuitively to avoid forming defects. A genetic algorithm has been developed to optimise the stitch pattern, minimising shear deformation in multi-ply stitched preforms. The quality of the shear angle distribution has been assessed using a maximum value criterion (MAXVC) and a Weibull distribution quantile criterion (WBLQC). Both criteria are suitable for local stitch optimisation, producing acceptable solutions towards the global optimum. The convergence rate is higher for MAXVC, while WBLQC is more effective for finding a solution closer to the global optimum. The derived solutions show that optimised patterns of through-thickness stitches can improve the formability of multi-ply preforms compared with an unstitched reference case, as strain re-distribution homogenises the shear angles in each ply.