Teunis Cornelis Bor

Review of current strategies to induce self-healing behaviour in fibre reinforced polymer based composites

Abstract This paper addresses the various strategies to induce self-healing behaviour in fibre reinforced polymer based composites. A distinction is made between the extrinsic and intrinsic healing strategies. These strategies can be applied at the level of the fibre, the fibre/matrix interface or at the level of the matrix. It is shown that the degree of healing depends on the type of damage and the testing mode used and examples are given both for extrinsic and for intrinsic healing systems. The conclusion is drawn that self-healing in fibre reinforced composites is possible yet unlikely to become a commercial reality in the near future.

Annealing of SnO2 thin films by ultra-short laser pulses

Post-deposition annealing by ultra-short laser pulses can modify the optical properties of SnO₂ thin films by means of thermal processing. Industrial grade SnO₂ films exhibited improved optical properties after picosecond laser irradiation, at the expense of a slightly increased sheet resistance [Proc. SPIE 8826, 88260I (2013)]. The figure of merit ϕ = T¹⁰ / R(sh) was increased up to 59% after laser processing. In this paper we study and discuss the causes of this improvement at the atomic scale, which explain the observed decrease of conductivity as well as the observed changes in the refractive index n and extinction coefficient k. It was concluded that the absorbed laser energy affected the optoelectronic properties preferentially in the top 100-200 nm region of the films by several mechanisms, including the modification of the stoichiometry, a slight desorption of dopant atoms (F), adsorption of hydrogen atoms from the atmosphere and the introduction of laser-induced defects, which affect the strain of the film.

Mechanical properties of Indonesian-made narrow dynamic compression plate.

Osteosynthesis plates are clinically used to fixate and position a fractured bone. They should have the ability to withstand cyclic loads produced by muscle contractions and total body weight. The very high demand for osteosynthesis plates in developing countries in general and in Indonesia in particular necessitates the utilisation of local products. In this paper, we investigated the mechanical properties, i.e. proportional limit and fatigue strength of Indonesian-made Narrow Dynamic Compression Plates (Narrow DCP) as one of the most frequently used osteosynthesis plates, in comparison to the European AO standard plate, and its relationship to geometry, micro structural features and surface defects of the plates. All Indonesian-made plates appeared to be weaker than the standard Narrow DCP because they consistently failed at lower stresses. Surface defects did not play a major role in this, although the polishing of the Indonesian Narrow DCP was found to be poor. The standard plate showed indications of cold deformation from the production process in contrast to the Indonesian plates, which might be the first reason for the differences in strength. This is confirmed by hardness measurements. A second reason could be the use of an inferior version of stainless steel. The Indonesian plates showed lower mechanical behaviour compared to the AO-plates. These findings could initiate the development of improved Indonesian manufactured DCP-plates with properties comparable to commonly used plates, such as the standard European AO-plates.

Optical and electrical properties of SnO2 thin films after ultra-short pulsed laser annealing

Ultra-short pulsed laser sources, with pulse durations in the ps and fs regime, are commonly exploited for cold ablation. However, operating ultra-short pulsed laser sources at fluence levels well below the ablation threshold allows for fast and selective thermal processing. The latter is especially advantageous for the processing of thin films. A precise control of the heat affected zone, as small as tens of nanometers, depending on the material and laser conditions, can be achieved. It enables the treatment of the upper section of thin films with negligible effects on the bulk of the film and no thermal damage of sensitive substrates below. By applying picosecond laser pulses, the optical and electrical properties of 900 nm thick SnO2 films, grown by an industrial CVD process on borofloat®-glass, were modified. The treated films showed a higher transmittance of light in the visible and near infra-red range, as well as a slightly increased electrical sheet resistance. Changes in optical properties are attributed to thermal annealing, as well as to the occurrence of Laser- Induced Periodic Surface Structures (LIPSSs) superimposed on the surface of the SnO2 film. The small increase of electrical resistance is attributed to the generation of laser induced defects introduced during the fast heating-quenching cycle of the film. These results can be used to further improve the performance of SnO2-based electrodes for solar cells and/or electronic devices.

Simulation of X-ray diffraction-line broadening due to dislocations in a model composite material

X-ray diffraction-line profiles of two-dimensional, plastically deformed model composite materials are calculated and analysed in detail. The composite consists of elastic reinforcements in a crystalline solid and is subjected to macroscopic shear. Slip occurs in the matrix only due to the collective motion of discrete dislocations on a single set of parallel slip planes. The results of dislocation dynamics computations are used as input for the calculation of the line profiles. The line profiles are computed directly using the kinematics approach, without making a priori assumptions on the dislocation distributions. Two steps are required. First, the full intensity distribution of a single crystal of composite material is calculated. Then, assuming a perfectly random orientation distribution of such single crystals, powder diffraction-line profiles are determined. Results will be presented for several orders of reflection and in different crystallographic directions. The broadening of the line profiles is shown to be not only determined by the density of dislocations, but also by their spatial distribution.

Matrix damage helaing in fiber reinforced composite materials containing embedded active and passive wires

Continuous fiber reinforced composite materials are susceptible to matrix cracking and delamination upon impact. Active and passive wires can be embedded within the composite material to support the healing behavior. Upon a local heating stimulus the wires, oriented mostly in the out-of-plane direction, can assist the buildup of compressive stresses to close matrix cracks and delaminations. Shape memory alloy wires, prestrained to contract upon heating, can actively close cracks in a damaged region during the heat treatment. A closing action can also be achieved in a passive sense, based on differences in thermal expansion of the composite material and the wires employed. The effects of the wire type, fraction and distribution on the closing and healing behavior of composite materials were studied for a simplified model case. The requirements for optimal matrix healing have been determined. The results are strongly dependent on the thermomechanical properties of the composite material and the wires. Small amounts of shape memory alloy wires are already sufficient to generate adequate compressive stresses at the healing temperature. Prestraining of the wires is not a prerequisite. Passive wires, such as glass and aramid fibers, can be employed in a similar way. A stitch type of wire distribution is more effective than a woven type of distribution. Experimental verification of the closing behavior of active and passive out-of-plane wires has been carried out through consolidation experiments of pre-impregnated composite layers. Active shape memory alloy wires were capable of generating sufficient compressive stresses in the out-of-plane direction to support closure and welding of separate composite layers. The fracture toughness obtained was comparable to that of a reference plate of the same material manufactured with a hot press. The active wires also coped better with size variations in the out-of-plane direction than passive wires based on glass and aramid fibers.