Exposure to extended periods of sub-Tg temperatures causes physical changes in the molecular structure of epoxy resins and epoxy-based materials to occur. These physical aging mechanisms include the reduction in free volume and changes to the molecular configuration. As a result, mechanical, thermodynamical, and physical properties are affected in ways that can compromise the reliability of epoxy-based engineering components and structures. In this review, the physical changes in the molecular structure of epoxies are described, and the influence of these changes on the bulk-level response is detailed. Specifically, the influence of physical aging on the quasistatic mechanical properties, viscoelasticity, fracture toughness, thermal expansion coefficient, volume relaxation, enthalpy relaxation, endothermic peak temperature, fictive temperature, and moisture/solvent absorption capability is reviewed. Also discussed are relationships between relaxation functions, crosslink density, composite reinforcement, and epoxy/copolymer blending and the physical aging response of epoxies. Finally, the concepts of thermal and mechanical rejuvenation are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

https://onlinelibrary.wiley.com/doi/pdf/10.1002/polb.22384

Physical aging of epoxy polymers and their composites

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1105&context=mechengfacpub

A peridynamic formulation for transient heat conduction in bodies with evolving discontinuities

Fibre-reinforced polymer composites typically feature low functional (e.g., electric and thermal conductivity) and structural (e.g. mechanical strength and fracture toughness) properties in the laminate’s thickness direction. In the event of lightning strikes, overheating, and impact by foreign objects, composite laminates may suffer wide spread structural damage. This research explores the synergistic physical interaction between two-dimensional nanostructured (graphene nano-platelets) and, zero- or one-dimensional conductive fillers (silver nanoparticles or silver nanowires, respectively) when both are dispersed in fibre–polymer laminates. The results reveal a synergistic improvement in the through-thickness thermal conductivity that is more than the additive improvements by each constituent. Specifically, the simultaneous inclusion of graphene nano-platelets and silver nanoparticles/nanowires at a combined loading of 1 vol% resulted in approximately 40% enhancement in the through-thickness thermal conductivity while the inclusion of graphene nano-platelets alone at the same loading resulted only in 9% improvement. Similarly, the through-thickness electrical conductivity of carbon fibre/epoxy laminates incorporating graphene nano-platelets together with silver nanoparticles/nanowires was notably higher (⩾70%) than can be achieved by graphene nano-platelets alone (∼55%). These results demonstrate that the presence of nano-reinforcements exhibiting varied phonon transport and electron transfer pathways, and geometric aspect ratios promote synergistic physical interactions. Small improvements were found in the mechanical properties, including tensile, flexural or compressive properties of the carbon fibre-reinforced laminates, due to the relatively low concentrations of the nano-fillers.

Improving the through-thickness thermal and electrical conductivity of carbon fibre/epoxy laminates by exploiting synergy between graphene and silver nano-inclusions

Global concerns over environmental issues have led to a tremendous growth in sustainable materials emerging from biobased plastics and their composites (biocomposites). This class of materials can be an alternative to traditional composite materials to reduce the carbon footprint and strain on the environment. Many studies and reviews have been focused on enhancing the mechanical performance of biocomposites with the aim for them to compete with traditional composites and expand their applications. However, the current scientific knowledge relating to the long-term durability performance of biocomposites is very limited in open access literature. Studies of the effects of different aging mechanisms when subjected to different service conditions and environments on the biocomposites' behaviours are needed. This review provides a focused discussion on the overview of the long-term durability performance and degradation behaviour under various aging environments (thermo-oxidative aging, accelerated weathering (ultraviolet aging), hydrolytic degradation, fatigue and creep, etc.) of the commercially important biobased-composites for the first time. Future perspectives and methods to improve the durability performance of biocomposites are also discussed in this review.

/pdf/studies-on-durability-of-sustainable-biobased-composites-a-1dn7e7e0b8.pdf

Studies on durability of sustainable biobased composites: a review

This paper presents a multiscale progressive failure approach for composite materials which allows predicting the failure of a laminate from the thermo-mechanical properties (behaviour and strength) of the individual unidirectional plies. This kind of approach is predictive for different laminate layups and takes also into account the effects of ply failures on the macroscopic behaviour and the final laminate failure. For the first time to our knowledge, the viscosity of the matrix was introduced into the model for a more realistic description of the mesoscopic behaviour, especially for the non-linearity under shear loading. The mesoscopic failure criterion is based on physical principles and makes a distinction between the fibre failure and the interfibre failure modes. In order to describe the progressive failure of a laminate (ply after ply), a progressive degradation model, based on thermodynamical damage models, was developed. The definition of the final failure is also improved in order to predict the final failure more accurately, especially for [± θ ] s laminates. Finally, some results (stress vs. strain laminate responses and macroscopic failure envelopes) are presented and compared with test data found in the literature.

A multiscale progressive failure approach for composite laminates based on thermodynamical viscoelastic and damage models

A coupled analysis of mechanical behaviour and ageing for polymer-matrix composites

Analyse multiéchelle des effets du vieillissement sur la tenue mécanique des composites à matrice organique

Analysis of how thermal aging affects the long-term mechanical behavior and strength of polymer–matrix composites

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A coupled analysis of mechanical behaviour and ageing for polymer-matrix composites

Analyse multiéchelle des effets du vieillissement sur la tenue mécanique des composites à matrice organique

Analysis of how thermal aging affects the long-term mechanical behavior and strength of polymer–matrix composites