No abstract

Reinforced polymer-based composites are attractive lightweight materials for aircrafts, automobiles, and turbine blades, but still show strength and fracture toughness lower than traditional metals. An interesting approach to address this issue is to fabricate composites with structural features that absorb part of the elastic energy stored in the material during fracture through extrinsic and intrinsic toughening mechanisms behind and ahead of the crack tip, respectively. Inspired by the nacreous layer of mollusk shells, the fracture behavior of multiscale composites that combine intrinsic toughness from a brick-and-mortar structure connected through nanoscale mineral bridges and extrinsic toughness arising from a brittle-ductile laminate architecture at the millimeter scale are fabricated and investigated. Such a hierarchical toughening approach increases the dissipated energy by more than 30-fold during fracture with minimal loss in stiffness and strength. Using simple energy balance arguments and fracture mechanics concepts, guidelines are established for the design of nacre-like composites with a remarkable combination of stiffness, strength, and toughness. This demonstrates the possibility to controllably introduce toughening mechanisms at different length scales and to thus fabricate hierarchical composites with high fracture resistance in spite of the brittle nature of their main inorganic constituents.structures that can bear mechanical loads without damage, whereas high toughness is essential for safety since it ensures a graceful, noncatastrophic failure of the material in case the strength is exceeded. A major challenge arises from the fact that strength and toughness are mutually exclusive in most synthetic materials. [4] This is because both properties are intrinsically related to the ability of chemical bonds to either resist or facilitate stressinduced deformation. [4] Thus, gains in toughness are normally accompanied by a reduction in strength and viceversa. Remarkably, many natural materials are able to overcome this general rule by combining both strength and toughness in hierarchical composite architectures. As a result of a long evolutionary process driven by the need of structural protection or attack, such biological materials achieve outstanding properties using ordinary and abundant building blocks. [5] Replicating, in synthetic composites, the strengthening and toughening strategies that have evolved in these natural systems has been a major research goal in the field of bioinspired materials. [6] Such a research effort requires both understanding the design principles of biological materials and developing processing technologies that allow for their implementation in synthetic composites through bioinspired architectures with exquisite structural control.Studying the structure and mechanical behavior of natural materials at multiple length scales has enabled a better understanding of the design principles underlying the combined strength and toughness of biological composites. [7] Stre...

No abstract

Epoxy resins are widely used for different commercial applications, particularly in the aerospace industry as matrix carbon fibre reinforced polymers composite. This is due to their excellent properties, i.e., ease of processing, low cost, superior mechanical, thermal and electrical properties. However, a pure epoxy system possesses some inherent shortcomings, such as brittleness and low elongation after cure, limiting performance of the composite. Several approaches to toughen epoxy systems have been explored, of which formation of the interpenetrating polymer network (IPN) has gained increasing attention. This methodology usually results in better mechanical properties (e.g., fracture toughness) of the modified epoxy system. Ideally, IPNs result in a synergistic combination of desirable properties of two different polymers, i.e., improved toughness comes from the toughener while thermosets are responsible for high service temperature. Three main parameters influence the mechanical response of IPN toughened systems: (i) the chemical structure of the constituents, (ii) the toughener content and finally and (iii) the type and scale of the resulting morphology. Various synthesis routes exist for the creation of IPN giving different means of control of the IPN structure and also offering different processing routes for making composites. The aim of this review is to provide an overview of the current state-of-the-art on toughening of epoxy matrix system through formation of IPN structure, either by using thermoplastics or thermosets. Moreover, the potential of IPN based epoxy systems is explored for the formation of composites particularly for aerospace applications.