Mark Battley

Human-inspired robotic exoskeleton (HuREx) for lower limb rehabilitation

A robot exoskeleton which is inspired by the human musculoskeletal system has been developed for lower limb rehabilitation. The device was manufactured using a novel technique employing 3D printing and fiber reinforcement to make one-of-a-kind form fitting human-robot connections. Actuation of the exoskeleton is achieved using PMAs (pneumatic air muscles) and cable actuation to give the system inherent compliance while maintaining a very low mass. The entire system was modeled including a new hybrid model for PMAs. Simulation and experimental results for a force and impedance based trajectory tracking controller demonstrate the feasibility for using the HuREx system for gait and rehabilitation training.

Validation of an efficient method of assigning material properties in finite element analysis of pelvic bone.

Bone in the pelvis is a composite material with a complex anatomical structure that is difficult to model computationally. Rather than assigning material properties to increasingly smaller elements to capture detail in three-dimensional finite element (FE) models, properties can be assigned to Gauss points within larger elements. As part of a validation process, we compared experimental and analytical results from a composite beam under four-point load to FE models with material properties assigned to refined elements and Gauss points within larger elements. Both FE models accurately predicted deformation and the analytical predictions of internal shear stress.

Experimental method for dynamic residual strength characterisation of aircraft sandwich structures

This study investigates the effect of dynamic loading on the residual of sandwich structures used in aircraft interiors comprising glass fibre phenolic resin face sheets and Nomex® honeycomb core. A dynamic edgewise compression test method for residual strength testing of sandwich structures has been developed using a modified compressive Split Hopkinson Pressure Bar apparatus. Dynamic edgewise compression at strain rates of approximately 50 s−1 for undamaged specimens showed an average increase of 26% in compression strength compared with equivalent static edgewise compression tests. For low levels of indentation damage there was a 27% reduction in residual dynamic compressive strength compared with a 15% reduction in residual static compressive strength for equivalent prior damage. This new experimental method provides insights into the dynamic edgewise response of composite sandwich structures to aid in the design and development of future aeronautical structures.

Shear Strength Optimization of Reinforced Honeycomb Core Materials

This paper studies the performance of novel honeycomb materials with the cell walls made of natural fiber reinforced thermoplastics. It focuses on developing a methodology for the maximization of the specific transverse shear strength of reinforced honeycombs and prediction of the corresponding cell geometry. The load carrying capacity of wood fiber reinforced polypropylene honeycomb shows a substantial improvement as compared to that of a polypropylene honeycomb of the same density. Theoretical predictions agree well with the experimental results. Excellent performance of the reinforced polypropylene honeycombs indicates a promising future for such materials.

Wetdeck slamming loads on a developed catamaran hullform – experimental investigation

ABSTRACT Catamaran wetdeck slamming has been experimentally investigated using a servo hydraulic slam testing system. A series of controlled-speed water impacts was undertaken on a rigid catamaran bow section with two interchangeable centrebows. Entry into the body of water was at two fixed trim angles: 0° and 5°. The vertical velocity was varied from 3 to 5 m/s in 0.5 m/s increments. This study presents a new dataset of pressure distributions and slam forces on the arched wetdeck structure of catamaran vessels. The relationships between the peak force magnitudes, relative impact angle and vertical velocity are observed, with a small reduction in slam force for an amended centrebow. Limited pressure measurements along the archway were not found to be representative of wetdeck slamming loads.

Shear strength of sandwich core materials subjected to loading rates relevant to water slamming

Sandwich composite materials are widely used within the marine industry, particularly as hull panels. Water impact loads, known as slamming, can be very significant for these structures, particularly for high-speed craft. The transient nature of slamming loads means that the loads are applied very quickly, which can cause stress and strain rates that are high enough to affect the resulting strength of the core material, particularly for polymeric foams. The aim of this paper is to characterise the shear strength of cores at slamming relevant loading rates. Two testing approaches are used: a custom servo-hydraulic beam testing system and a drop-weight impact testing machine. Core materials studied included aramid honeycomb, cross-linked and linear polyvinyl chloride, polyethylene terephthalate and styrene acrylonitrile foams, representing a range of different levels of ductility and maximum elongation. For the moderate and high elongation core materials, there were significant increases in shear strength for dynamic loads; however, the strength of most of the materials does not appear to be sensitive to the exact loading rate.

Dynamic Damage Tolerance for Aircraft Sandwich Structures: Experiments and Modeling

Understanding of damage tolerance for aircraft interior sandwich structures is critical for design and certification purposes. This study investigates the effect of dynamic loading on the damage tolerance behavior of sandwich materials used in aircraft interiors, comprised of glass fibre phenolic resin face sheets and Nomex honeycomb core. These materials demonstrate strain dependent effects at loading rates relevant to aircraft design standards. A dynamic edgewise compression test method for residual strength testing of sandwich materials has been developed using a modified compressive Hopkinson pressure bar apparatus. Dynamic edgewise compression at strain rates of approximately 50 s for undamaged specimens showed an average increase in compression strength of 28 %, compared to equivalent edgewise compression tests at a quasi-static loading rate of 7.3e-5 s. Low velocity impact damage was replicated using a quasi-static indentation method to create damage in the sandwich material normal to the face sheet with a 25.4 mm hemispherical indentor. For low levels of indentation damage (0.22 mm residual dent) there was a significant reduction in residual dynamic compressive strength of 31 %. Static edgewise compression caused only a 15 % reduction in compressive strength for equivalent predamage. Common failure modes observed for dynamic failure of the sandwich structure were branching of cracks in the face sheet and fibre and matrix interface failure. This gives insights into the dynamic response of composite sandwich structures for the manufacture of future aeronautical structures with improved dynamic load considerations.

Experimental study on in-plane compressive response of irregular honeycombs

Compared with regular honeycombs, irregular honeycombs are more representative of real foams, and thus more suitable for the study of foam mechanics. In this paper, the deformation and failure progression in the irregular honeycombs are investigated by analysing the images captured in order to gain an improved understanding on foam failure. Irregular honeycombs with varying cell wall thickness, cell size and cell shape were manufactured using a three-dimensional printer and tested under compression. The behaviour of irregular honeycombs is found to be different from that of regular honeycombs. In irregular honeycombs, cell walls start to fracture at some point, initially at a low speed from multiple locations. The global stress reaches its maximum value shortly after the first fracture of cell walls. Only a few cell walls buckle in the specimens with cells of irregular shape. Fracture is more likely to occur to thin and long cell walls aligned within a medium angle (around 30 to 60°) to the compressive load. However, the susceptibility of a cell wall is to fracture is also affected by its neighbouring cell walls. Strong and stiff neighbouring cell walls could shield load away and protect it from breaking. Because of this, it is better to think of a weak spot as a region, rather than an individual cell or cell wall. Overall, the more uniform cell wall size and thickness are, the better the mechanical performance of cellular solids is.

An approach for characterising cellular polymeric foam structures using computed tomography

Global properties of foams depend on foam base materials and microstructures. Characterisation of foam microstructures is important for developing numerical foam models. In this study, the microstructures of four polymeric structural foams were imaged using a micro-CT scanner. Image processing and analysis methods were proposed to quantify the relative density, cell wall thickness and cell size of these foams from the captured CT images. Overall, the cells in these foams are fairly isotropic, and cell walls are rather straight. The measured average relative densities are in good agreement with the actual values. Relative density, cell size and cell wall thickness in these foams are found to vary along the thickness of foam panel direction. Cell walls in two of these foams are found to be filled with secondary pores. In addition, it is found that the average cell wall thickness measured from 2D images is around 1.4 times of that measured from 3D images, and the average cell size measured from 3D images is ...

Effects of Curvature on Slamming Loads

Much research has been directed at understanding and predicting water slamming loads for a range of geometries of varying rigidity and size. Analytical and numerical studies focused on slamming of cylindrical rigid bodies are present in literature but there are relatively few experimental studies useful for validation purposes, none of which methodically investigate a range of curvatures. Despite the current understanding of slamming loads and structural responses, high speed marine vehicles still experience slamming related failures in operation.In this study, nominally rigid, singly curved prismatic specimens of varying curvature are subjected to constant velocity water impacts relevant to those encountered by high performance offshore racing yachts and other high-speed craft.Peak impact forces of 14 to 52 kN were recorded while testing specimens with radii ranging from 0.300 to 5.000 m. Experimental peak impact force and event impulse are found to be significantly lower than predicted by numerical and small scale empirically derived methods. A modification is introduced which improves the empirical model.Copyright