Eugenio Dragoni

Intrinsic static strength of friction interfaces augmented with anaerobic adhesives

The paper is focused on the static strength of friction joints supplemented with anaerobic adhesives. Through torsional tests on the annular interface between tubular steel specimens, the friction and the adhesive contributions to the overall strength are determined as a function of the clamping force. The analysis shows no interaction of the component strengths, with the frictional strength proportional to and the adhesive strength independent of the clamping force as if the underlying mechanisms were acting separately. This cumulative performance, intrinsic of the interface in the homogeneous test conditions attained, should be exploited to the full when designing efficient real joints based on this hybrid technology.

Axisymmetric Mechanical Analysis of Ceramic Heads for Total Hip Replacement

An axisymmetric, mechanical analysis of conical press-fit ceramic heads is performed. The head strength and its fracture modes are assessed experimentally. The stress field is examined by finite element, strain gauge and photoelastic methods. An alternative head design, characterized by a cylindrical engagement with the stem, is analysed with the same techniques and its merits are explored.

Mechatronic Design of a Shape Memory Alloy Actuator for Automotive Tumble Flaps: A Case Study

The continuous advance in mechatronics has long attracted researchers toward the development of new highly integrated actuators for automotive applications where reduced space and low weight are common constraints. In this context, Shape Memory Alloys (SMAs) offer many peculiar characteristics that make this technology very attractive for the construction of miniature mechatronic actuators. This paper presents the design, the prototype fabrication, and the functional testing of a case study, where an SMA binary actuator is used for automotive tumble flaps. The innovative solid-state actuation system is proposed as an alternative to electromagnetic and pneumatic effectors, traditionally used to drive the tumble shaft of an air intake manifold for internal combustion engines. Original features of the linear actuator involve the mechanical architecture and the control structure. On the mechanical side, two contrasting sets of SMA springs are used to actively generate the net actuating force during both out-stroke and in-stroke. On the control side, a current feedback is exploited for sensorless real-time monitoring of the working temperature of the SMA springs during electrical supply. Major shortcomings of the proposed solution are a low response time and a power consumption higher than pneumatic and electromagnetic counterparts.

Effect of Bondline Thickness on the Static Strength of Structural Adhesives Under Nearly-Homogeneous Shear Stresses

An usual experimental observation retrieved in the technical literature is that the strength of an adhesive joint decreases by increasing the adhesive layer thickness. This well-known behaviour is still not completely understood. All works found in the literature consider a complex stress state in the adhesive with mode mixing, stress concentrations on the midplane, and stress singularities at the interface occurring at the same time. This paper aims at estimating the effect of the adhesive thickness on its intrinsic static shear strength and evaluate whether this strength can explain the behaviour of a real bonded joint. A nearly uniform shear stress distribution is obtained through an ad-hoc tubular butt joint subject to pure torsion. A standard single lap joint is considered as a benchmark, due to its complex and singular stress field into the adhesive. The experimental campaign is focused on two brittle adhesives: a modified methacrylate and high-strength epoxy. Four levels and three levels of the adhesive thickness were considered in the tubular butt joint and in the single lap joint, respectively, all in the range between 0.05 and 0.4 mm. The effect of the adhesive thickness on the static strength of the adhesive is investigated by considering the type of failure and by comparing the structural stresses in the tubular butt joint with the ones on the midplane of the adhesive layer in the single lap joint. Moreover, the stress intensity factor in the single lap joint is calculated.

Effect of Thread Pitch and Frictional Coefficient on the Stress Concentration in Metric Nut-Bolt Connections

By means of the finite element method, this paper establishes how much the stress state within standard metric nut-bolt connections is affected by variations of the thread pitch and of the frictional coefficient. Following a validated simplified approach, the actual three-dimensional geometry of each connection is replaced by an axisymmetric model which recreates the outline of the joint on an appropriate meridional section. The numerical data prove that, for prescribed nominal thread diameter and bolt load, The peak stress in the screw monotonically increases as the pitch decreases. Further, as far as complete sticking between nut and bolt is not achieved, the stress level linearly increases with the coefficient of friction, the rate of variations being higher at the lowest pitches.

Cumulative static strength of tightened joints bonded with anaerobic adhesives

Abstract This paper deals with the static strength of mechanically tightened joints augmented with anaerobic adhesives. Tests were performed on a variety of specimens distinguished by fairly homogeneous (annular butt joint) or realistically inhomogeneous (threaded, cylindrical and double lap joints) working conditions over the bondline. For each geometry, the strength of the dry (unbonded) joint was compared with the strength of the joint bonded with an assortment of anaerobics. The results indicate that the strength of all joints (bonded and unbonded) increases with the contact force. When strong adhesives (retainers) are used, the overall strength approximately equals the sum of friction and adhesive strengths, taken separately. When weak adhesives (threadlockers) are adopted, the superimposition of effects grossly overestimates the measured strength. A micromechanical model is proposed that explains the observed macroscopic behaviour.

Analysis and Design of Hollow Helical Springs for Shape Memory Actuators

Shape memory alloys (SMAs) can be exploited successfully to reduce the complexity and the weight of actuators, but the main drawbacks that limit the use of SMAs are the low bandwidth, poor energetic efficiency, and unsatisfactory stroke. This article contributes to enhancing the mechanical, thermal, and electric performances of SMA actuators by providing analysis and design equations for helical springs with hollow round section. By emptying the inefficient material from its center, the hollow section features a lower mass, lower cooling time, and lower heating energy than its solid counterpart for given strength, stiffness, and deflection. The advantages of the hollow construction over solid springs are presented and discussed by means of dimensionless functions. A step-by-step procedure leading to the optimal design of hollow springs with minimum energy consumption is finally proposed.

Conceptual Design and Simulation of a Compact Shape Memory Actuator for Rotary Motion

This work describes the conceptual design, the modelling, the optimization, the detail design and the virtual testing of a shape memory actuator purposely conceived to maximize torque and angular stroke while limiting overall size and electric consumption. The chosen design, achieved by means of a Quality Function Deployment approach, features a fully modular concept in which an arbitrary number of identical modules are assembled to produce the desired angular stroke and output torque. The basic module contains shape memory springs that actuate the device and also a conventional spring that reduces the torque ripple. Following the concept generation stage, a thermo-electromechanical model is developed and a numerical optimization performed, aimed at minimizing the electrical consumption of the actuator. Finally, the device is designed in detail and the actuator is tested virtually. Thanks to the proposed modular construction and the use of a conventional balancing spring, the device shows better performances than known rotary shape memory actuators in terms of rotation, torque and customization.

Theoretical analysis of an unpressurized elastomeric O-ring seal inserted into a rectangular groove

Abstract Based on linear elasticity, a theoretical model is developed which is able to describe the mechanical behaviour of an unpressurized, elastomeric O-ring seal inserted into a rectangular groove. First, the analytical tools employed to handle the unilateral contact problem and to improve the pressure profile regularity are discussed in detail. Next, an assessment of the model is performed for a laterally free seal, for which a variety of studies exist in the literature. Finally, the model is applied to the analysis of a seal installed into several grooves of different width, and the results retrieved are compared with the few data available. The model response to a perturbation of Poisson ratio of the elastomer is also critically examined.

Modeling of Wire-on-Drum Shape Memory Actuators for Linear and Rotary Motion

The article presents the analytical model of a linear/rotary solid-state actuator formed by a shape memory wire wound over a cylindrical drum. The model assumes a bilinear stress-strain behavior of the wire in the martensitic state (low temperature) and a linear elastic response in the austenitic state (high temperature). Based on simple equilibrium conditions, the model calculates the stress and strain distributions in the wire when subjected to a constant external backup force and undergoing frictional sliding forces at the contact with the drum. Closed-form expressions are supplied for the stroke produced by whatever actuator geometry and are validated numerically against finite element results. For a particular actuator configuration, the analytical forecasts are also checked experimentally on a proof-of-concept prototype. The analytical model shows that large strokes (up to one-half of the drum’s diameter) are achieved if the frictional coefficient is kept below 0.01. Rolling-contact architectures or sonic-pulse excitations of the drum are discussed as technical solutions to obtain such low friction values.