Aleksandar Subic

Feasible Build Orientations for Self-Supporting Fused Deposition Manufacture: A Novel Approach to Space-Filling Tesselated Geometries

Support material is often utilised in additive manufacture to enable geometries that are not otherwise self-supporting. Despite the associated opportunities for innovation, the use of support material also introduces a series of limitations: additional material cost, cost of removal of support material, potential contamination of biocompatible materials, and entrapment of support material within cellular structures. This work presents a strategy for minimising the use of support material by comparing the geometric limits of an additive manufacture process to the build angles that exist within a proposed geometry. This method generates a feasibility map of the feasible build orientations for a proposed geometry with a given process. The method is applied to polyhedra that are suitable for close packing to identify space-filling tessellated structures that can be self-supporting. The integrity of an FDM process is quantified, and using the associated feasibility map, self-supporting polyhedra are manufactured. These polyhedra are integrated with non-trivial geometries to achieve a reduction in consumed material of approximately 50%. Nomenclature

Flexible Learning Technologies and Distance Education: A Teaching and Learning Perspective

The number of distance education providers and learning options has increased rapidly in recent years with the emerging flexible learning technologies. The main challenge facing Australian and other universities aiming to deliver distance education programmes nationally or internationally is to find economical ways to encourage and enable effective learning at a distance and at the students own pace. It will be discussed in this paper that this challenge cannot be met by computer technology alone, but rather by integrating this technology within flexible learning approaches based on the universality of learning processes and teaching and learning approaches that encourage deep learning. The article focuses on the question of quality in distance education and how quality relates to student learning and flexible learning technologies used for this purpose. A particular student-centred model for flexible learning that is based on this paradigm of quality is presented. The implementation of this model is discussed in greater detail using the experiences of the RMIT Mechanical Engineering distance education programme offered in Singapore and Hong Kong.

Computer Aided Tolerancing (CAT) platform for the design of assemblies under external and internal forces

Due to the stochastic nature of manufacturing processes, the functionality of mechanical assemblies is subject to variation defined by tolerances and manufacturing process characteristics. In many assemblies, functionality is also dependent on external and internal forces. Numerous Computer Aided Tolerancing (CAT) tools have been proposed that address tolerance analysis problems in complex mechanical assemblies; however current tools do not accommodate a general class of problem where the functionality of a design is fundamentally dependent on the effects of external and internal forces. This research addresses the limitation of CAT tools to accommodate assemblies under loading by developing a tolerance analysis platform which integrates CAD, CAE and statistical analysis tools using Process Integration and Design Optimisation (PIDO) software capabilities. The platform extends the capabilities of traditional CAT tools by enabling tolerance analysis of assemblies in which assembly characteristics are dependent on external and internal forces. To demonstrate the capabilities of the developed platform, examples of tolerance analysis problems involving external forces (compliance) and internal forces (multi-body dynamics) are presented.

Modelling and analysis of alternative face guard designs for cricket using finite element modelling

In professional cricket, where bowlers can bowl balls that reach speeds of up to 160 km h-1, effective head protection is vital. Current head protection equipment typically consists of a helmet with a high impact grade polypropylene shell, a high density EPS liner, and a metal face guard. Most of the weight in existing helmets is attributed to the steel grill used as the face guard. We present a virtual design approach to the development and evaluation of new face guards made from alternative materials. In particular, we investigate a face guard design for cricket made from polycarbonate rather than steel using an explicit dynamic finite element analysis (FEA) approach. The FEA model developed for this purpose incorporates the headform, helmet, polycarbonate face guard and the impacting ball. ABAQUS CAE was used for FEA. HyperMesh and SolidWorks were used to develop the geometric model. This work identifies appropriate modelling and simulation strategies, and key design attributes for the development of new face guards using alternative materials. A preliminary study shows that by using polycarbonates it is possible to reduce the mass of the face guard by 20%, thus contributing to greater comfort of the players without compromising their safety. The key criteria for reduction of ball deceleration by at least 25% at each test site were satisfied, with deceleration reduction values ranging from 44% to 87% from those due to ball impact with the bare head.

Effects of seat structural dynamics on current ride comfort criteria

The ISO 2631-1 (1997) provides methodologies for assessment of the seated human body comfort in response to vibrations. The standard covers various conditions such as frequency content, direction and location of the transmission of the vibration to the human body. However, the effects of seat structural dynamics mode shapes and corresponding resonances have not been discussed. This study provides important knowledge about the effects of vehicle seat structural vibration modes on discomfort assessment. The occupied seat resonant frequencies and corresponding vibration modes were measured and comfort test was carried out based on the paired comparison test method. The results show that the ISO 2631-1 (1997) method significantly underestimates the vibration discomfort level around the occupied seat twisting resonant frequencies. This underestimation is mainly due to the ISO suggested location of the accelerometer pad on the seatback. The centre of the seatback is a nodal point at the seat twisting mode. Therefore, it underestimates the total vibration transferred to the occupant body from the seatback. Practitioner Summary: The effects of the vehicle seat structural dynamics have not been discussed in the human body vibration ISO . The results of this research show that the current measurement method suggested by ISO 2631-1 (1997) can significantly underestimate the vibration discomfort level at around the seat structural vibration mode.

Design for reliability of a vehicle transmission system

Robust vehicle design for improved reliability and quality has been one of the key areas of interest in the automotive industry for a number of years. Based on this approach, reliability is designed into the vehicle following a rigorous vehicle design and development process coupled with appropriate design methods, such as failure mode and effects analysis (FMEA), quality function deployment (QFD), design of experiments (DoE), the Taguchi method, and others. While significant progress has been achieved to date in improving the reliability and quality of vehicles, there is still some need to incorporate parts and subsystems with specific reliability functions within the vehicle system reliability model early in the design concept stage and throughout the vehicle design process so that the reliability of systems and subsystems can be designed-in with more confidence. A comprehensive design for reliability model for critical vehicle systems and subsystems coupled with the failure mode and effects analysis (FMEA) method could help identify and alleviate potential failure modes, and hence maximize vehicle reliability. This article proposes a design for reliability process that aims to achieve this particular outcome in the case of a mechanical vehicle transmission system. A detailed case study involving design for reliability of a vehicle transmission system is presented.

Comparative Life Cycle Assessment (LCA) of passenger seats and their impact on different vehicle models

The main purpose of Life Cycle Assessment (LCA) to date has been to evaluate life cycle impacts of different design solutions and materials for a car, its sub-systems and components. Considerable number of publications are available on LCA of automotive components. This research aims to extend the LCA approach by evaluating and comparing the effects of mass reduction of passenger seats for different vehicle models in order to provide strategic support for decision making in the development process and to validate the environmental benefits of design alternatives under investigation. For this purpose, the paper presents a comprehensive LCA of passenger seats with detailed consideration of alternative scenarios for the use phase for different vehicle models.

A fundamental model of quasi-static wheelchair biomechanics

The performance of a wheelchair system is a function of user anatomy, including arm segment lengths and muscle parameters, and wheelchair geometry, in particular, seat position relative to the wheel hub. To quantify performance, researchers have proposed a number of predictive models. In particular, the model proposed by Richter is extremely useful for providing initial analysis as it is simple to apply and provides insight into the peak and transient joint torques required to achieve a given angular velocity. The work presented in this paper identifies and corrects a critical error; specifically that the Richter model incorrectly predicts that shoulder torque is due to an anteflexing muscle moment. This identified error was confirmed analytically, graphically and numerically. The authors have developed a corrected, fundamental model which identifies that the shoulder anteflexes only in the first half of the push phase and retroflexes in the second half. The fundamental model has been extended by the authors to obtain novel data on joint and net power as a function of push progress. These outcomes indicate that shoulder power is positive in the first half of the push phase (concentrically contracting anteflexors) and negative in the second half (eccentrically contracting retroflexors). As the eccentric contraction introduces adverse negative power, these considerations are essential when optimising wheelchair design in terms of the users musculoskeletal system. The proposed fundamental model was applied to assess the effect of vertical seat position on joint torques and power. Increasing the seat height increases the peak positive (concentric) shoulder and elbow torques while reducing the associated (eccentric) peak negative torque. Furthermore, the transition from positive to negative shoulder torque (as well as from positive to negative power) occurs later in the push phase with increasing seat height. These outcomes will aid in the optimisation of manual wheelchair propulsion biomechanics by minimising adverse negative muscle power, and allow joint torques to be manipulated as required to minimise injury or aid in rehabilitation.

Analysis of the Nominal Load Effects on Gear Load Capacity Using the Finite-Element Method:

This research investigates the effects of the nominal load value on load distribution of simultaneously meshing gear teeth pairs, and on the involute gear load capacity. The research results presented in this article confirm that the nominal load value has a significant influence on the gear load capacity calculations. However, this influence is generally neglected in standard gear calculations, which can result in oversized gear dimensions. This can lead to inadequate gear designs in practice due to increased demand for reduced gear size and weight in modern machinery. The article provides a detailed description of the iterative numerical method developed in this research to support the modelling and analysis of load distribution in meshed gears using the finite-element method.

Region-based geometric modelling of human airways and arterial vessels.

Accurate geometric models of human airways and arterial vessels play a critical role in the analysis of air and blood flows in human bodies. The generic geometric modeling methods become invalid when the model consists of bronchioles and very small vessels. This paper presents a new region-based method to reconstruct the airway tree and arterial vessels from point clouds obtained from CT or MR images. A novel layer-by-layer searching algorithm is developed to identify the bifurcation points and branches in the airway tree. The surface patches on each branch are constructed according to the number of points on the branch other than by applying a single tolerance for the entire reconstruction process regardless of the number of elemental points. The distortion problem occurred at small bronchus and vessels are thus solved.