Remko Akkerman

Prediction of springforward in continuous-fibre/polymer L-shaped parts

Continuous-fibre-reinforced thermoset laminates that are cured hot, exhibit deformations due to cure and thermal shrinkage. When L-shaped composite components are released from the mould after cooling to ambient temperature, a decrease of the enclosed angle is observed. This is referred to as the springforward phenomenon. The main cause of springforward, the mismatch in thermal expansion along and across the fibres, cannot explain totally the springforward found in experiments. Three other possible influences on springforward were investigated: an inhomogeneous fibre/matrix distribution through the thickness, an inhomogeneous heat distribution during the cure cycle and the difference in thermal expansion between the mould and the composite component. Finite element results show that the springforward is affected significantly by the amount of cure shrinkage and by the difference in thermal expansion between the mould and the composite component.

Prediction of the yarn trajectories on complex braided preforms

Braiding can be used to manufacture preforms for resin transfer moulding (RTM). With braiding, many yarns are used, non-geodesic yarn paths are possible, and the interlaced structure of braids provides typical mechanical properties such as high impact strength. Previously, several models were developed to predict the fibre angles on simple, stationary braided preforms, but not for complex, non-axisymmetric preforms. This paper presents a fast and efficient model to predict the fibre angles on complex biaxially braided preforms. The model is verified with experiments on two mandrels and the experimental and numerical results show good agreement.

A Review on the Mechanical Modeling of Composite Manufacturing Processes

The increased usage of fiber reinforced polymer composites in load bearing applications requires a detailed understanding of the process induced residual stresses and their effect on the shape distortions. This is utmost necessary in order to have more reliable composite manufacturing since the residual stresses alter the internal stress level of the composite part during the service life and the residual shape distortions may lead to not meeting the desired geometrical tolerances. The occurrence of residual stresses during the manufacturing process inherently contains diverse interactions between the involved physical phenomena mainly related to material flow, heat transfer and polymerization or crystallization. Development of numerical process models is required for virtual design and optimization of the composite manufacturing process which avoids the expensive trial-and-error based approaches. The process models as well as applications focusing on the prediction of residual stresses and shape distortions taking place in composite manufacturing are discussed in this study. The applications on both thermoset and thermoplastic based composites are reviewed in detail.

Inkjet-printing- and electroless-plating- based fabrication of RF circuit structures on high-frequency substrates

In this paper, a method to fabricate radio frequency (RF) circuit structures is described. This method involves inkjet printing of a silver nanoparticle-based ink on a functional substrate material to create the seed track (i.e., the seed layer), onto which copper is subsequently deposited by an electroless plating method, to obtain the desired thickness and conductivity of the RF structures. This process combination was validated by fabricating an S-band filter on a high-frequency substrate and comparing the RF performance of this filter with that of a filter fabricated using the conventional lithography-based method. The adhesion of the circuit structures to the substrate was qualitatively ascertained by the scotch tape test method. The performance of the inkjet-printed–electroless-plated filter was comparable to that of the conventional filter, thus proving the suitability of this novel method for practical RF applications.

On the properties of quasi-isotropic laminates

The three-dimensional thermoelastic properties of quasi-isotropic laminates are derived in closed form equations, in terms of the principal lamina properties. The equations describe both the inplane and the out-of-plane properties, applying the Classical Laminate Theory and the averaging method proposed by Goetschel and Radford, J Adv Mater 28 (1997) 37. A worked example illustrates the results and indicates the limitations when bending plays a significant role.

Mechanical Modelling of Pultrusion Process: 2D and 3D Numerical Approaches

The process induced variations such as residual stresses and distortions are a critical issue in pultrusion, since they affect the structural behavior as well as the mechanical properties and geometrical precision of the final product. In order to capture and investigate these variations, a mechanical analysis should be performed. In the present work, the two dimensional (2D) quasi-static plane strain mechanical model for the pultrusion of a thick square profile developed by the authors is further improved using generalized plane strain elements. In addition to that, a more advanced 3D thermo-chemical-mechanical analysis is carried out using 3D quadratic elements which is a novel application for the numerical modelling of the pultrusion process. It is found that the 2D mechanical models give relatively reasonable and accurate stress and displacement evolutions in the transverse direction as compared to the 3D model. Moreover, the generalized plane strain model predicts the longitudinal process induced stresses more similar to the ones calculated in the 3D model as compared with the plane strain model.

Tool‐Ply Friction In Composite Forming

Friction between fibre composite laminates and rigid tool materials is an important phenomenon in composite forming processes and the resulting product geometry, fibre orientations and fibre stresses. Pull‐out experiments are presented and compared with a novel meso mechanical model based on Reynolds’ equation for thin film lubrication. The film thickness is derived from this analysis, rather than postulated as in earlier publications. Good agreement is found between the experiments, the meso mechanical model results and earlier empirically determined master curves.

Manufacturing processes for braided composite materials

Braiding is not being used as a manufacturing process as much as would be expected based on its well-known advantages. Fabricating consistent, high-quality braided composite materials has been challenging. The range of available processes and selecting the right one for the right component has perplexed researchers and engineers. Herein, production processes and methods, as well as alternatives, for the production of two-dimensional and three-dimensional braided composites are discussed.

Modeling of Stress Development During Thermal Damage Healing in Fiber-reinforced Composite Materials Containing Embedded Shape Memory Alloy Wires:

Fiber-reinforced composite materials are susceptible to damage development through matrix cracking and delamination. This article concerns the use of shape memory alloy (SMA) wires embedded in a composite material to support healing of damage through a local heat treatment. The composite material contains a thermoplastic matrix that allows healing at elevated temperatures. The woven in SMA wires, oriented in the out-of-plane direction of the composite material, are used to close the delamination upon heating. Several case studies were performed. The influence of the SMA wire fraction, the degree of prestraining of the SMA wires, the glass transition temperature of the amorphous thermoplastic matrix, and the healing temperature have been considered. The delamination is compacted while heating up to the healing temperature. The thermal expansion coefficient of the thermoplastic matrix is much larger than that of the SMA wires causing a compressive thermal stress in the composite material upon heating. Prestraining of the SMA wires is not a priori required to obtain a compressive stress at the delamination interfaces at the healing temperature. After cooling to room temperature residual stresses occur in the composite material and SMA wires if the SMA wire composition at the start of the heat treatment differs from that at the end. The conditions under which no residual stresses develop have been determined. SMA wire fractions have been calculated to achieve a preset stress level in the composite material at the healing temperature and a stress-free state after healing. Finally, the use of high strength wires to replace SMA wires has been considered and shown to be a worthwhile alternative.

Analytical Modeling of Impact Resistance and Damage Tolerance of Laminated Composite Plates

The present study is concerned with a fully analytical model for predicting impact resistance and damage tolerance characteristics of laminated fiber reinforced composite plates. The energy balance for damage development during the impact process is established with the help of a localized deformation field, in which various effects such as the anisotropy of the delaminated sublaminates, elastic property degradation due to matrix cracking, multiple delaminations, and membrane deformation within the damage zone are taken into account. This results in upper and lower bounds of the delamination threshold load and a deflection-dependent critical load for stable delamination propagation. On the basis of the conservation of energy principle, simple relationships are found between the resulting damage area and the impact energy as well as the peak impact load. The global buckling load of damaged rectangular plates is derived from the energy-based stability criterion, and the residual ultimate strength is calculated using the von Karman postbuckling strength theory. The analytical solutions obtained turn out to be capable of providing highly accurate predictions compared with the standard impact and the compression-after-impact experiments of carbon-fiber-reinforced polyetherimide and aromatic polymer composites.