Andrea Spaggiari

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.

Combined squeeze-shear properties of magnetorheological fluids: Effect of pressure

Several applications of magnetorheological fluids are nowadays present in the industrial world; however, sometimes material properties are not sufficient to meet the system requirements. Among the technical literature, there is experimental evidence of the squeeze-strengthen effect, which is a pressure dependency of the yield stress of the magnetorheological fluid. Since many magnetorheological systems are rotary devices, such as brakes and clutches, this article investigates the behaviour of magnetorheological fluids under pressure when a rotation is applied to shear the fluid. The system is designed to apply both the magnetic field and the pressure following a design of experiment method. The experimental apparatus comprises a cylinder in which a piston applies both pressure and a prescribed rotation. The magnetic circuit is designed to provide a tunable, nearly constant magnetic induction field inside the fluid. The experimental apparatus measures the torque produced by the magnetorheological fluid as a function of the variables considered, and consequently, the yield shear stress is evaluated. A statistical analysis of the results finds a positive interaction between the magnetic field and the pressure, which enhances the magnetorheological fluid performances, measured in terms of yield stress, up to more than two times the manufacturer’s datasheet values.

Effect of Mechanical Surface Treatment on the Static Strength of Adhesive Lap Joints

This work deals with an experimental investigation on the effect of mechanical surface treatments of adhesive bonded joints. The behaviour of an adhesively bonded joint can be considered good if cohesive failure is achieved, while when interfacial failure occurs the performances are normally much worse. A key parameter which drives the failure type is the surface treatment applied to the adherends. This work analyzes, by means of a structured experimental campaign, which surface mechanical treatment gives the best performance. The design of the experimental approach used involves different materials, joint geometries, and surface treatments. The results are investigated in terms of force, energy, and stresses in the joints and the performance of the several mechanical treatments tested is assessed, showing that a simple correlation with the surface roughness is not sufficient to predict the best joint performances. The reliable results obtained prove that sandpapering or sandblasting the adherends gives a strong improvement in terms of performance and leads to a higher probability of cohesive failure.

Effect of Pressure on the Flow Properties of Magnetorheological Fluids

Magnetorheological (MR) fluids are widely used in the industrial world; however, sometimes their properties fail to meet system requirements. In shear mode, MR fluids have been found to exhibit a pressure dependency called squeeze strengthen effect. Since a lot of MR fluid based devices work in flow mode (i.e., dampers), this paper investigates the behavior in flow mode under pressure. The system design consists of three steps: the hydraulic system, the magnetic circuit, and the design of experiment method. The experimental apparatus is a cylinder in which a piston displaces the fluid without the use of standard gear pumps, which are incompatible with MR fluids. The experimental apparatus measures the yield stress of the MR fluid as a function of the pressure and magnetic field, thus, enabling the yield shear stress to be calculated. A statistical analysis of the results shows that the squeeze strengthen effect is also present in flow mode, and that the internal pressure enhances the performance of MR fluids by nearly five times.

Design equations for binary shape memory actuators under arbitrary external forces

This article presents the design equations for an on–off shape memory alloy actuator working against an external system of arbitrary constant forces. A binary shape memory alloy actuator is considered where a cursor is moved against both conservative and dissipative forces, which may be different during the push or pull phase. Three cases are analysed and differentiated in the way the bias force is applied to the primary shape memory alloy spring: using a constant force, a conventional spring or a second shape memory alloy spring. Closed-form dimensionless design equations are developed, which form the basis of a step-by-step procedure for an optimal design of the whole actuator.

Efficient Post-elastic Analysis of Bonded Joints by Standard Finite Element Techniques

A simplified finite element approach based on reduced models with minimum degrees of freedom was applied to the post-elastic analysis of bonded joints. The reduced model describes the adherends by means of structural elements (beams or shells) and the adhesive by a single strip of solid elements (plane-stress or brick). Internal kinematic constraints were applied to link the adherends and adhesive meshes. The accuracy and the efficiency of the reduced models in providing the force–displacement curve of T-peel joints were evaluated through a numerical test campaign by comparison with full finite element analyses. The test campaign was designed as a 2-level factorial experiment involving four variables: the skew angle of the T-peel (45 and 90°), the thickness of the adherends (2 and 3 mm), the material of the adherends (aluminium and steel) and the stress–strain behaviour of the adhesive (brittle and perfectly plastic). The results show that the reduced model reproduces with fair accuracy the load–displacement curves of the joints at a fraction of the computational cost of the full model. The elastic stiffness, the yield load and the deformation energy were predicted within an error of 7%, 15% and 36%, respectively, with processing times that were typically 50 times shorter than the full model.

Robust Shape Optimization of Tubular Butt Joints for Characterizing Thin Adhesive Layers under Uniform Normal and Shear Stresses

Thin-walled tubular joints, bonded end to end, are commonly used specimens to measure the mechanical properties of thin adhesive layers subjected to uniform shear stress distributions. Unfortunately, the application of an axial loading to this geometry leads to strong stress concentrations at the edges of the adherend–adhesive interface. This drawback undermines the use of this test for characterizing adhesives under biaxial stress conditions. With the aim of removing these stress concentrations, this paper suggests the introduction of stress relieving grooves on the internal and external surfaces of the tubular adherends. The optimal shape of the groove is identified following the Taguchi robust optimization technique. Via finite element analyses, the stress concentrations at the edges of the adherend–adhesive interface are calculated. Many geometries are examined for different adherend and adhesive properties (noise factors) in order to identify the groove shape that minimizes the stress concentrations for all experimental conditions. The analysis shows that a shallow V-shaped groove close to the adherend–adhesive interface smoothes significantly the stress peaks due to axial loading. With this simple modification, a tubular butt joint becomes a universal specimen for applying any combinations of reasonably uniform shear and normal stresses to thin adhesive layers.

Experimental Tests on Tubular Bonded Butt Specimens: Effect of Relief Grooves on Tensile Strength of the Adhesive

This paper investigates experimentally a tubular bonded butt specimen with relief grooves carved close to the adherend-adhesive interface. The specimen is used to assess the strength of a thin adhesive layer, as usually occurs in structural bonded joints. Hence, this configuration overcomes the problems related to the differences in chemical and mechanical properties which could occur in bulk adhesive tests. The aim is to verify experimentally the reduction of the stress concentrations at the interface given by the presence of the grooves, observed in a previous numerical work of the authors. Finite element analyses show that the groove geometry adopted here, although slightly simplified with respect to the optimum shape previously proposed, produces a strong reduction of the edge effects. This work performs an indirect assessment by comparing tensile strength of bonded specimens with and without relief grooves. A two-level factorial experimental campaign is performed, according to Design of Experiment criteria. The variables are: presence of the grooves, adherends material, and adhesive thickness. The response of the tests is the maximum tensile load carried by the specimen which is found to depend strongly on the adherends’ materials. In the case of steel joints, the relief grooves near the adherend-adhesive interface lead to higher loads regardless of the adhesive thickness. In the case of aluminium joints the relief grooves play a minor role, while tensile strength decreases as the adhesive thickness increases.

Multiphysics Modeling and Design of Shape Memory Alloy Wave Springs as Linear Actuators

This paper explores the merits of shape memory wave springs as powering elements of solid-state actuators. Advantages and disadvantages of the wave construction in comparison to the traditional helical shape are presented and discussed by means of dimensionless functions. The main assets of the wave springs are the higher electrical resistance (leading to simpler electrical drives) and the lower cooling time (leading to enhanced working frequency). The wave geometry is also superior in purely mechanical terms to the helical counterpart when axial space is at a premium. A step-by-step design procedure is proposed, leading to the optimal wave spring meeting the multiphysics design specifications and constraints. A case study is finally reported, showing the application of shape memory wave springs to the design of a telescopic linear actuator.

Properties and applications of Magnetorheological fluids

This brief introduction describes the mechanical, rheological and magnetic properties of the magnetorheological (MR) fluids for feasible engineering applications. The typical modes of exploiting this technology are shown and discussed. An increasing number of industrial applications illustrate how the MR fluids peculiar properties may be used to provide optimal performance in semi active damping and dissipative devices.