Ian R Farrow

SILICOMB PEEK Kirigami cellular structures: mechanical response and energy dissipation through zero and negative stiffness

The work describes the manufacturing, testing and parametric analysis of cellular structures exhibiting zero Poisson?s ratio-type behaviour, together with zero and negative stiffness effects. The cellular structures are produced in flat panels and curved configurations, using a combination of rapid prototyping techniques and Kirigami (Origami and cutting) procedures for PEEK (Polyether Ether Ketone) thermoplastic composites. The curved cellular configurations show remarkable large deformation behaviours, with zero and negative stiffness regimes depending also on the strain rate applied. These unusual stiffness characteristics lead to a large increase of energy absorption during cyclic tests.

Fatigue life prediction under complex loading for XAS/914 CFRP incorporating a mechanical fastener

Abstract A method of predicting lifetime to failure for a carbon fibre reinforced composite system subjected to a complex load–time history has been developed. The prediction first requires the generation of a model to characterise the general fatigue response of the particular composite system. Once the models are derived they can be used to predict lifetimes to failure for a load–time history using a modified Miners damage summation rule and rainflow analysis. Variable amplitude fatigue testing of XAS/914 CFRP incorporating a mechanical fastener and use of the FALSTAFF load sequence allowed comparisons between predicted and actual lifetimes to failure and was useful in verifying the accuracy of the life prediction methodology employed. Two multidirectional CFRP laminates were studied, possessing matrix and fibre dominated properties respectively. The results from the life prediction models showed them to be accurate predictors of fatigue behaviour where matrix properties dominate but less so where fibre properties are the major influence on fatigue response.

Curved Kirigami SILICOMB cellular structures with zero Poisson’s ratio for large deformations and morphing

The work describes the design, manufacturing, and parametric modeling of a curved cellular structure (SILICOMB) with zero Poisson’s ratio produced using Kirigami techniques from polyetheretherketone films. The large deformation behavior of the cellular structure is evaluated using full-scale finite element methods and experimental tests performed on the cellular samples. Good agreement is observed between the mechanical behavior predicted by the finite element modeling and the three-point bending compression tests. Finite element simulations have also been used to perform a parametric analysis of the stiffness against the geometry of the cellular structures, showing a high degree of tailoring that these cellular structures could offer in terms of minimum relative density and maximum stiffness. The experimental results also show high levels of strain-dependent loss factors and low residual deformations after cyclic large deformation loading.

Composite flexible skin with large negative Poisson's ratio range: numerical and experimental analysis

This paper describes the manufacturing, characterization and parametric modeling of a novel fiber-reinforced composite flexible skin with in-plane negative Poisson’s ratio (auxetic) behavior. The elastic mechanical performance of the auxetic skin is evaluated using a three-dimensional analytical model based on the classical laminate theory (CLT) and Sun’s thick laminate theory. Good agreement is observed between in-plane Poisson’s ratios and Young’s moduli of the composite skin obtained by the theoretical model and the experimental results. A parametric analysis carried out with the validated model shows that significant changes in the in-plane negative Poisson’s ratio can be achieved through different combinations of matrix and fiber materials and stacking sequences. It is also possible to identify fiber-reinforced composite skin configurations with the same in-plane auxeticity but different orthotropic stiffness performance, or the same orthotropic stiffness performance but different in-plane auxeticity. The analysis presented in this work provides useful guidelines to develop and manufacture flexible skins with negative Poisson’s ratio for applications focused on morphing aircraft wing designs. (Some figures may appear in colour only in the online journal)

3D printed elastic honeycombs with graded density for tailorable energy absorption

This work describes the development and experimental analysis of hyperelastic honeycombs with graded densities, for the purpose of energy absorption. Hexagonal arrays are manufactured from thermoplastic polyurethane (TPU) via fused filament fabrication (FFF) 3D printing and the density graded by varying cell wall thickness though the structures. Manufactured samples are subject to static compression tests and their energy absorbing potential analysed via the formation of energy absorption diagrams. It is shown that by grading the density through the structure, the energy absorption profile of these structures can be manipulated such that a wide range of compression energies can be efficiently absorbed.

Kirigami stretchable strain sensors with enhanced piezoelectricity induced by topological electrodes

Rapid advances in sensing technologies are leading to the development of integrated wearable electronics for biomedical applications. Piezoelectric materials have great potential for implantable devices because of their self-powered sensing capacities. The soft and highly deformable surfaces of most tissues in the human body, however, restrict the wide use of piezoelectric materials, which feature low stretchability. Flexible piezoelectric polyvinylidene fluoride films that could conformably integrate with human bodies would have advantages in health monitoring. Here, a Kirigami technique with linear cut patterns has been employed to design a stretchable piezoelectric sensor with enhanced piezoelectricity. A parametric Finite Element Analysis study is first performed to investigate its mechanical behaviour, followed by experiments. An inter-segment electrode connection approach is proposed to further enhance the piezoelectric performance of the sensor. The voltage output shows superior performance with 2.6 times improvement compared to conventionally continuous electrodes. Dynamic tests with a range of frequencies and strains are performed to validate the sensor design. With its high performance in large strain measurements, the Kirigami-based sensing system shows promise in stretchable electronics for biomedical devices.

Design, Build and Test of a Semi-Monocoque Wing Structure

A time and cost effective laboratory exercise has been developed to provide large numbers of students with practical hands-on experience of a complete structural development cycle in the first year of their M.Eng. Aeronautical Engineering course. Teams of students are given a set of requirements to design, build and test a small-scale semi-monocoque wing box and centre section structure within a strict schedule. The exercise is assessed by a competitive team component based on structural performance and individual component based on a certification report. The exercise is well received by the students and provides a useful practical experience to relate to other more academic parts of the materials, structures and design courses. The prescribed form, although not covering the conceptual stage of design, provides an instructive layout for students to visualize a complete structural development process clearly despite the inherently large number of problems to be considered and solved.