This article presents a review of the major fatigue models and life time prediction methodologies for fibre-reinforced polymer composites, subjected to fatigue loadings. In this review, the fatigue models have been classified in three major categories: fatigue life models, which do not take into account the actual degradation mechanisms but use S-N curves or Goodman-type diagrams and introduce some sort of fatigue failure criterion; phenomenological models for residual stiffness/strength; and finally progressive damage models which use one or more damage variables related to measurable manifestations of damage (transverse matrix cracks, delamination size). Although this review does not pretend to be exhaustive, the most important models proposed during the last decades have been included, as well as the relevant equations upon which the respective models are based. This review article contains 141 references.

Vitrimers are covalently cross-linked polymeric materials that can be thermally processed in a liquid state without losing their network integrity. In this work, novel vitrimers based on the dynamic amine exchange reaction of vinylogous urea moieties are introduced. Following a systematic exploration of different vinylogous acyl compounds (urethanes, amides and urea), vinylogous urea clearly emerge as showing the fastest intrinsic exchange kinetics. The combination of these networks with a simple acid catalyst (0.5 mol% pTsOH) resulted in materials with good mechanical properties (Tg ~ 110°C, E ~ 2.2 GPa) and remarkably short relaxation times above Tg, in the order of a few seconds when heated. This attractive combination of properties made it possible to prepare vinylogous urea based composites as enduring prepregs. We demonstrate that the dynamic material properties give fully cured composites that still allow an efficient thermal fusion of multiple layers as well as thermoforming. Finally, we also demonstrate fiber-recycling by a simple chemical treatment.

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Today, fiber-reinforced polymer composites are a standard material in applications where a high stiffness and strength are required at minimal weight, such as aerospace structures, ultralight vehicles, or even flywheels for highly efficient power storage systems. Although fiber-reinforced polymer composites show many advantages compared to other materials, delamination between reinforcing plies remains a major problem limiting further breakthrough. Traditional solutions that have been proposed to toughen the interlaminar region between reinforcing plies have already reached their limit or have important disadvantages such as a high cost or the need for adapted production processes. Recently, electrospun nanofibers have been suggested as a more viable interlaminar toughening method. Although the expected benefits are numerous, the research on composite laminates enhanced with electrospun nanofibrous veils is still very limited. The work that has been done so far is almost exclusively focused on interlaminar fracture toughness tests with different kinds of nanofibers, where typically a trial and error approach has been used. A thorough understanding of the micromechanical fracture mechanisms and the parameters to obtain toughened composites has not been reported as of yet, but it is crucial to advance the research and design highly damage-resistant composites. This article provides such insight by analyzing the nanofiber toughening effect on three different levels for several nanofiber types. Only by combining the results from different levels, a thorough understanding can be obtained. These levels correspond to the hierarchical nature of a composite: the laminate, the interlaminar region, and the matrix resin. It is found that each level corresponds to certain mechanisms that result in a toughening effect. The bridging of microcracks by electrospun nanofibers is the main toughening mechanism resulting in damage resistance. Nevertheless, the way in which the nanofiber bridging mechanism expresses itself is different for each scale and dependent on parameters linked to a certain scale. The multiscale analysis of the toughening mechanisms reported in this paper is therefore crucial for understanding the behavior of nanofiber toughened composites, and as such allows for designing novel, damage-resistant, nanofiber-toughened materials.

This paper presents the experimental investigation on the progressive deformation behaviour of uni-directional pultruded composite tubes subjected to an axial impact load. Pultruded square and circular profiles with glass-polyester and glass-vinylester combinations were used for this study. Two types of triggering profiles were incorporated to investigate the effect of triggering on the energy absorption. All above combinations were investigated for three different impact velocities (9.3, 12.4 and 14m/s). The crushing peak and mean load characteristics of the composite tubes with different triggering profiles and their progressive failure modes are presented. To measure the impact velocity and the impact force, a contactless method using digital image correlation technique was adopted. The effects of the geometry profile, triggering, strain rate and the type of resin on energy absorption of the composite tubes were studied in detail.KEYWORDS: Specific energy absorption; Triggering mechanism; Crushing; Progressive failure; Composite tubes IntroductionA great deal of research and development has been carried out in the past decades to design safer automobiles. Out of the factors considered for safety criteria, the crashworthiness has attracted significant attention due to its multiple functions. The functions of the crashworthiness structures are to (i) absorb energy, (ii) keep the occupant compartments intact and (iii) ensure tolerable deceleration levels for driver and passengers during the crash event. To meet the above functions, the automobile industry is focused on the design architecture and materials used to produce crashworthiness. As a result, different forms of the energy absorbers [1,2] and combinations of high strength metal alloys are used for crashworthiness structures. The focus on new innovative materials which yield superior strength to weight ratio [3] has been increased in order to meet the future stringent crashworthiness norms and to enhance the fuel economy target.On the other hand, there is a considerable amount of experiments [4][5][6][7] conducted on composite material to assess the energy absorption. It is a well-known fact that one can

This work presents a straightforward strategy to introduce highly dynamic and adaptable cross-links into common epoxy resin formulations. For this, an oligomeric amine-based curing agent containing vinylogous urethane (VU) bonds was developed. This novel polyfunctional amine curing agent can be used as a drop-in solution for existing epoxy resin technologies, resulting in transparent, rigid and, at the same time, highly reprocessable catalyst-free epoxy vitrimers. The oligomeric VU curing agents are prepolymerised via a straightforward condensation reaction between acetoacetates extended with different classical amine monomers and epoxy hardeners. It is found that vitrimer properties can be readily introduced into these epoxy formulations by converting less than 50 mol% of the hardener's amine functionalities into dynamic vinylogous urethane bonds. In this way, epoxy vitrimers can be obtained with material properties comparable to ones of the VU-free epoxy formulations. In addition, remarkably short processing times are observed in the absence of any catalyst, and the material displayed very short stress relaxation times and good recyclability, actually representing the most performant VU-based vitrimers so far. Furthermore, a proof of concept for its use in obtaining glass fiber-reinforced epoxy composites is also presented.

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Delamination and brittle matrix fracture has since long been a problem of fibre reinforced composites. This paper investigates if polycaprolactone (PCL) nanofibre nonwovens can increase the interlaminar fracture toughness of resin transfer moulded glass fibre/epoxy laminates, without causing problems during impregnation and without negatively affecting other (mechanical) properties.The mode I fracture toughness was shown to be dependent on both the nanofibre content as well as on how the nanofibres were introduced into the laminates. Almost 100% improvement in fracture toughness could be achieved by electrospinning the PCL nanofibres on both sides of the glass fibre mats prior to impregnation. This led to a mode I fracture toughness of over 1200 J/m 2 . 2Tensile and dynamic mechanical properties of the toughened laminates were not affected by the PCL nanofibres. It could be concluded that even state of the art infusion resins with a high intrinsic fracture toughness can benefit significantly from nanofibre toughening.