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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 has two objectives. The first is to briefly introduce a readership in Engineering Design education to Half-Causation, which is a philosophical model for inventing and patenting. Following a brief introduction, Half-Causation will be illustrated using the well-known case of the centrifugal vacuum cleaner, which was invented by the British inventor Sir James Dyson in the late 1970s. The second objective is to present the outcome of a 3-hour workshop which took place at the University of Bristol in 2021, in which doctoral engineering students were introduced to Half-Causation, before being given an engineering design problem to solve using it. The problem was ‘how to reduce the probability of a cyclist unseating (flying over the handlebar) when braking at high speed.’ Instead of addressing the problem in the traditional terms of morphology or functionality, the participants were encouraged to focus on causal properties . The participants were divided into four teams, and they clearly got a reasonable grip on Half-Causation Branching and followed its instructions faithfully. The four teams developed 44 inventive concepts, albeit some of the same or similar inventive concepts were developed by multiple teams. The workshop ended with discussing the optimisation of the scope of sought patent protection, using Half-Causation Encapsulation . The paper concludes by recommending adding the Half-Causation tools to engineering curricula, both in terms of generating ideas and intellectual property, specifically patents.

Inventing and patenting using half-causation: new philosophical tools for engineering design education

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Predicting trans-laminar fracture using VCCT and in-situ CT scans

The increased usage of composite materials has raised the need for a more reliable damage detection strategy for structural health monitoring (SHM) systems. An important, and as yet unsolved, limitation of existing composite laminates is their susceptibility to impact damage. Low-velocity impact-induced damage is often hard to spot from the impacted side in a routine visual inspection, but it has a significant effect on the mechanical performance of laminates. This work investigates the possibility of embedding poly (vinylidene fluoride) (PVDF) as a sensor node for a passive SHM system in SE70 glass/epoxy laminates to monitor the damage while being subjected to an indentation test. The mechanical test results for the laminates both with and without embedded sensors indicate that the embedment of the PVDF sensors does not change the measured mechanical properties of the laminates. Acoustic emission (AE) signals obtained using the embedded PVDF sensor were compared with an identical PVDF sensor attached on the laminate’s surface. The results showed the possibility of successfully embedding PVDF sensors in composite laminates and the functionality of the PVDF sensors after the embedding procedure, i.e. their ability to withstand the composite curing temperature without experiencing degradation. However, the amplitude level of the AE signals obtained with the PVDF is lower than that of the commercial sensor, which is due to the low coupling factor and dielectric constant of the PVDF. Therefore, the development of two- and three-phase AE sensors (lead zirconate titanate (PZT)/epoxy and graphene nanoplatelets (GnPs)) were also explored to improve the sensitivity of the embedded sensor. The fabrication methods have been developed and the fabricated sensors showed better performance than PVDF sensors during the Hsu-Nielsen Source test. This is a continuing project, therefore further research will be conducted to implant the PZT nanocomposite sensors into composite laminate plates to examine the applicability of these novel sensors as flexible and light weight embedded sensor nodes for structural health monitoring systems.

High performance piezo electric nanocomposite sensor nodes for structural health monitoring

An optimised vascular network for the in-situ repair of complex composite structures has been presented. Vascular networks can be incorporated into a structure, such that during a damage event they become ruptured and provide access to the damage site for a liquid repair agent. The ability to access and repair damage in-situ is intended to mitigate the need for traditional invasive repair strategies. In this paper a number of novel vascular configurations were examined. Topology optimisation (TOPT) was carried out using 2D thermo-mechanical finite element analysis (FEA) and compared to conventional circular vasculature. Significant increases in vascule volume and surface area were achieved, whilst simultaneously decreasing stress concentrations observed in circular configurations.

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Michael R Wisnom

Papers

Inventing and patenting using half-causation: new philosophical tools for engineering design education

Predicting trans-laminar fracture using VCCT and in-situ CT scans

High performance piezo electric nanocomposite sensor nodes for structural health monitoring

Optimised vascular systems for the in-situ repair of composite T-joints

On the stiffness of intermingled discontinuous hybrid composites