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American Society for Testing and Materials

Biocomposites reinforced with natural fibers: 2000–2010

Double-network (DN) gels have drawn much attention as an innovative material having both high water content (ca. 90 wt%) and high mechanical strength and toughness. DN gels are characterized by a special network structure consisting of two types of polymer components with opposite physical natures: the minor component is abundantly cross-linked polyelectrolytes (rigid skeleton) and the major component comprises of poorly cross-linked neutral polymers (ductile substance). The former and the latter components are referred to as the first network and the second network, respectively, since the synthesis should be done in this order to realize high mechanical strength. For DN gels synthesized under suitable conditions (choice of polymers, feed compositions, atmosphere for reaction, etc.), they possess hardness (elastic modulus of 0.1–1.0 MPa), strength (failure tensile nominal stress 1–10 MPa, strain 1000–2000%; failure compressive nominal stress 20–60 MPa, strain 90–95%), and toughness (tearing fracture energy of 100∼1000 J m−2). These excellent mechanical performances are comparable to that of rubbers and soft load-bearing bio-tissues. The mechanical behaviors of DN gels are inconsistent with general mechanisms that enhance the toughness of soft polymeric materials. Thus, DN gels present an interesting and challenging problem in polymer mechanics. Extensive experimental and theoretical studies have shown that the toughening of DN gel is based on a local yielding mechanism, which has some common features with other brittle and ductile nano-composite materials, such as bones and dentins.

Why are double network hydrogels so tough

: An overview of the virtual crack closure technique is presented. The approach used is discussed, the history summarized, and insight into its applications provided. Equations for two-dimensional quadrilateral elements with linear and quadratic shape functions are given. Formula for applying the technique in conjuction with three-dimensional solid elements as well as plate/shell elements are also provided. Necessary modifications for the use of the method with geometrically nonlinear finite element analysis and corrections required for elements at the crack tip with different lengths and widths are discussed. The problems associated with cracks or delaminations propagating between different materials are mentioned briefly, as well as a strategy to minimize these problems. Due to an increased interest in using a fracture mechanics based approach to assess the damage tolerance of composite structures in the design phase and during certification, the engineering problems selected as examples and given as references focus on the application of the technique to components made of composite materials.

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Virtual crack closure technique: History, approach, and applications

An engineering solution for mesh size effects in the simulation of delamination using cohesive zone models

This paper presents an experimental investigation into the effect of scarfed ply drops on the low velocity impact response of asymmetrically tapered, multi-directional, carbon/epoxy laminates, in comparison with that of conventional ply drops. Impact load was applied to the flat side of the tapered region at an energy level of 12 J. The effect of ply scarfing on the internal damage evolution during impact was then investigated using X-ray Computed Tomography (CT) scanning. It was found that the scarfed ply drops reduced stress concentrations at the terminated plies and effectively prevented fibre breakage compared to non-scarfed conventional ply drops. Furthermore, the laminates with scarfed ply drops exhibited less delamination and a lower degree of internal matrix cracking than those with conventional ply drops, and thus were deemed more damage resistant despite having near identical external damage. 

Michael R Wisnom

Papers

Improved impact damage resistance of tapered composite laminates using a ply scarfing technique

Residual Tensile Strength of Adhesively Bonded Double Lap Joints after Transverse Impact

Advanced experimental techniques for determination of the tensile and compressive failure strain of carbon/epoxy composites

IV ECCOMAS Thematic Conference on the Mechanical Response of Composites

Polybenzimidazole carbon fibre composite – Utilising a high performance thermoplastic