David Vetturi

Developments in dynamic testing of rubber compounds: assessment of non-linear effects

Abstract In the present work, a test method to characterize the dynamic behaviour of rubber compounds by electrodynamic shaker (ES) in the frequency range of 10–1000 Hz was developed. Data of dynamic moduli of two different rubber compounds were determined through the analysis of the transmissibility of a suitably designed test system. The results were compared with those of dynamic moduli master curves obtained through frequency–temperature reduction of data measured by a commercial dynamic mechanical thermal analyser (DMTA), by scanning temperature at various frequencies in the range 0.3–30 Hz. Very good agreement of the data obtained by the two different aproaches were found, in spite of the different range of frequency explored by the two instruments, ES and DMTA, respectively. For one of the material examined, non-linear effects at low strain amplitudes were investigated by the two experimental methods considered.

Tailored One-Way and Two-Way Shape Memory Capabilities of Poly(ε-Caprolactone)-Based Systems for Biomedical Applications

This paper investigates the shape memory capabilities of semicrystalline networks, focusing the attention on poly(ε-caprolactone) (PCL) systems, a class of materials that allows to satisfy important requirements for their applications as biomedical devices, such as the good biocompatibility, the fast recovery of large “temporary” shape configurations, and the easy tailoring of the transformation temperatures. The materials were prepared with various crosslink densities and crosslinking methodologies; in particular, beside a thermal crosslinking based on reactive methacrylic end groups, a novel type of covalently crosslinked semicrystalline systems was prepared by a sol-gel approach from alkoxysilane-terminated PCL precursors, so as to avoid potentially toxic additives typically used for free-radical thermal curing. The materials were subjected to biological tests, to study their ability in sustaining cell adhesion and proliferation, and to thermal characterizations, to evaluate the possibility to tailor their melting and crystallization temperatures. The one-way shape memory (i.e., the possibility to set the material in a given configuration and to recover its pristine shape) and the two-way shape memory response (i.e., the triggered change between two distinguished shapes on the application of an on-off stimulus) were studied by applying optimized thermo-mechanical cyclic histories. The ability to fix the applied shape and to recover the original one on the application of heating (i.e., the one-way effect) was evaluated on tensile bars; further, to investigate a potential application as self-expandable stents, isothermal shape memory experiments were carried out also on tubular specimens, previously folded in a temporary compact configuration. The two-way response was studied through the application of a constant load and of a heating/cooling cycle from above melting to below the crystallization temperature, leading to a reversible elongation/contraction effect, involving maximum strain changes up to about 80%, whose extent may be controlled through the crosslink density.

Semi-Active Strategies for Racing Car Suspension Control

Quite a lot has been written on active suspension, the topic being suitable for theoretical studies. Unfortunately only a few applications has seen the road and even less went into production; by now very few road cars equipped with a real active suspension system are available on the market, and the interest to this kind of application seems to be reduced, perhaps because of high costs and restrictive regulations applied to the race car world. For this reason this paper presents the results of a study conducted on the vertical dynamics of a two wheel car model with a semi active suspension system, a possible alternative way to a fully active system to considerably improve suspension performance. Here the damping force of each suspension is obtained by modulating its damping factor according to opportune functions of the system state variables. This allows to reach several of the objectives achievable with a fully active suspension, without the need of a big amount of energy.

Development of a New Software for Racecar Suspension Kinematics

In the racecar design process the definition of suspension type and geometry is an important stage. A good design of the main parameters, in terms of static and dynamic angles in bump and steer, is the basis of a successful racecar quite often. This paper aims at introducing the development of a new software tool, called MLKrace, that can analyse the suspensions kinematics for a wide range of different layouts. There are today a lot of commercial tools that analyse or simulate suspension behaviour. In general their application is restricted to the classic double wishbone suspension and in this case the calculation of the kinematics is not so difficult. This layout is used on both ends of the majority of formula and sports race cars but it isn’t the only possible one. Figure 1 – Example of multilink suspension The MLKrace software is instead more suitable to analyse innovative suspension geometries, the mathematics being based on the resolution of the socalled Stewart platform, a method originally devised for parallel robot kinematics. Therefore full multilink or hybrid systems can be designed as easily as a double wishbone. In addition push rod, pull rod or outboard spring systems can be added and the driveshaft required float is computed. Multilink-McPherson hybrid suspensions can be designed as well. The flexibility is the great advantage of this new software. Apart from the mathematic model, a massive effort was devoted to the design of the Windows interface with the aim of making the designer’s job particularly easy and effective compared to outdated MS-Dos interfaces of most of the existing software. The output is shown in the form of 2D and 3D animations, a large number of predefined diagrams, numeric tables and regression functions. INTRODUCTION The suspension design process is an important point in the development of a new car or in the tuning of an existing car. As a matter of fact tyres can produce forces to generate high lateral and longitudinal acceleration only if they work in the proper way, where this means with a good contact area and angles. These angles are camber and toe. Figure 2 shows the effect of camber angle in cornering force. A general rule is that negative angles give more lateral force. But the optimum angle depends on the type of tyre and its application. The designer tries to keep it constant to have the best performance. In the design phase the suspension is conceived in a static configuration that defines the main parameters of the geometry. On the track, road bumps, load transfer and aerodynamic downforce produce a relative movement of the wheel. It is also important to study the variation of these parameters when the wheel is moving in bump and steer. A typical problem is to avoid “bump steer” or the toe change along the bump motion; this is a negative characteristic as it can upset the vehicle behaviour. Figure 2 – Camber effects on tyres The developed software “MLKrace” allows the designer to analyse the suspension system and to modify its characteristics for improvement. The choice of the suspension type is the first step in suspension design and MLKrace gives the opportunity to analyse different layouts to compare parameters, performance and overall dimensions. When the main layout is defined the design phase begins where the designer can move links and rods to obtain the best compromise for the specific car and application (road car, sportscar or formula). MLKrace aims to simplify the design process with the following features: • Modelling of the most common configurations • High quality and user-friendly interface • Fast and accurate calculation • Graphic representation of the suspension • Wide selection of displayed results Compared to other commercial suspension design software MLKrace is easy to use and complete and results are particularly accurate. SUSPENSION DESCRIPTION In the description of a new suspension the designer needs to input a lot of information. MLKrace’s main screen presents various options to define the suspension type and layout. The first parameter to set in the suspension is the layout. There are 5 possibilities: 1. Multilink: the most general type of suspension that gives the possibility to have virtual steering axis and a good control of angles. 2. Bottom multilink Top wishbone: a multilink where top arms are connected to form a wishbone. 3. Top Multilink – Bottom wishbone: a multilink where bottom arms are connected to form a wishbone. 4. Double wishbone: top and bottom arms are connected to form a wishbone. 5. McPherson: A strut geometry where the bottom links can be separated or connected to form a wishbone. 1 – Multilink 2 – Bottom Multilink 3 – Top Multilink 4 – Double wishbone

Evaluation of the shape memory performances of poly(ε-caprolactone)-based tubular devices for potential biomedical applications

The shape memory behavior of tubular specimens based on crosslinked poly(e-caprolactone) was investigated in order to evaluate their ability i) to restore their shape after being folded in a more compact one, and ii) to exert stress under external confinement (recovery stress). The specimens were prepared following different crosslinking methodologies and with different network densities, in order to tailor the material response in terms of transformation temperatures and recovery stress capabilities. The devices are able to fully recover their shape once heated close to the melting temperature and to exert moderate stresses, that may be controlled through thickness and crosslink density, and whose values were employed to develop a new testing apparatus for the measurement of radial dilation capabilities.

Tailored one-way and two-way shape memory response of poly(ε-caprolactone)-based systems for biomedical applications

A series of crosslinked poly(ε-caprolactone) (PCL) materials were obtained starting from linear, three- and four-arm star PCL functionalized with methacrylate end-groups, allowing to tune the melting temperature (Tm) on a range between 36 and 55°C. After deforming the specimens at 50% above Tm, the materials are seen to fully restore their original shape by heating them on a narrow region close to Tm; further, when the shape memory effect is triggered under fixed strain conditions, the materials are able to exert stress on a range between 0.2 and 7 MPa. The materials also display two-way shape memory features, reversibly moving between two shapes when cooled and heated under a fixed load. Finally, to investigate the application of the PCL materials as self-expandable stents, one-way shape memory experiments are currently carried out on tubular specimens.

Experimental kinematics of a special shape actuator

This paper investigates the kinematical behavior of a polymer based star-shaped actuator, able to produce mechanical work through the shape memory effect, that allows a significant shape variations on the application of an external stimulus. The adopted material is a semicrystalline network based on poly(e-caprolactone) crosslinked by thermal curing; the material was adopted due to its fast recovery process when heated close to the melting temperature and the high recovery degree, and, due to its good biocompatibility, it may suitable for biomedical application. The original, or “permanent”, material shape is that of a cylindrical annulus, which is set in a “temporary” configuration as a six spikes star. The temporary shape is fixed through a thermo-mechanical program, involving deformation above melting temperature and cooling under fixed strain and carried out by means of an ad-hoc designed fixture. By heating the deformed specimen above the melting temperature, the system is able to recover the original cylindrical shape realizing a motion and a mechanical power. This peculiar response, consisting in a progressive radial expansion activated by temperature, may be considered for application as self-expanding stenting device triggered by the human body temperature.The shape of the system, that changes during the transformation, can be described as a two dimensional temporal function that represents the mean line of the section of the cylindrical annulus (perpendicular to the height of the annulus). This temporal function is a combination of a circular function and of a modified rhodoneal function and, after a proper calibration through experimental tests, is used to evaluate the kinematics of the system. The function is able to describe adequately the shape evolution experimentally displayed by the samples, with a very good agreement at the starting and final instants of the transformation, while the accuracy during the transformation is acceptable for the proposed application.Copyright

How Geometrical Tolerances Affect the Measurement of Reciprocal Alignment of Two Different Assemblies: A Case Study

Often a designer has the problem to apply a suitable system of geometrical and dimensional tolerances to an assembly. The right solution is not unique, in fact it depends on the chosen parameters. If the tolerances have to be optimized, some important parameters have to be taken into account, e.g. the efficiency of each prescription, or if this last is reachable, or it can be verified and how much the realization costs. The authors opinion is that a statistical approach based on the Monte Carlo Method is very useful when the tolerances chains are complex. This paper shows an application of this method in order to verify the functional alignment between two assemblies and a critical analysis of the uncertainty in phase both of the component design and test. This study has been developed thanks to the strict requirements imposed by ESA (European Space Agency) on the components that Thales Alenia Space has to realize within the LISA Pathfinder experiment. The very critical aspect of this work is to reciprocally align two cylindrical elements of two different assemblies. The specifications require 100 μm as maximum linear displacement and 300 μrad as maximum angular displacement. Moreover this prescriptions have to be verified also when the two elements are independently moving. To be able to reach such strict accuracy level the components have been assembled in an ISO 100 class cleanroom and the work space was a 3D Coordinate-Measuring Machine (CMM). The cylindrical elements have a 10 mm diameter, so the value of the measurement uncertainty associated with the alignment check is fundamental. Starting from the different uncertainty sources, the measurability and verifiability of the alignment have been considered and evaluated. The overall uncertainty has been assessed by numerical simulations which have taken into account the dimensional, geometrical and form tolerances as well as the instrumental uncertainty of the 3D CMM. This estimation has been positively validated by a session of repeated measurements. Numerical simulations have also allowed performing a sensitivity analysis, in order to give information about which sources more contribute to the overall uncertainty.Copyright