Adhesive Stiffness Research Articles

Effect of post-cure on adhesively bonded functionally graded joints by induction heating

Functionally graded joints with an adhesive functionally modified by induction heating confer a more uniform stress distribution along the overlap and reduce the stress concentrations located at the ends of the overlap. The adhesive stiffness varies gradually along the overlap, being maximum in the middle and minimum at the ends of the overlap. This work studies the effect of post-curing on functionally graded joints obtained by induction heating. The performance of functionally graded joints, when submitted to different post-cure temperatures, was experimentally tested. Three different post-curing conditions were considered, with temperatures above and below the glass transition temperature of the fully cured network, Tg∞. The functionally graded joints (with and without post-cure) were compared with joints cured isothermally (with and without post-cure). The cure temperature values applied to the ends and to the middle of the graded joint are the same temperatures used to cure the isothermally cured joints. Analytical modelling was used to predict the failure load and to assess the effectiveness of the graded joint concept. The functionally graded joints subjected to post-cure at low temperatures (below Tg∞) show a slight decrease of the strength and the joints cured isothermally show a slight increase of the strength. With increase of the post-cure temperature (above Tg∞) the functionally graded joints exhibit strength similar to that of the joints cured isothermally. However, even for the highest post-cure temperatures, the functionally graded joints have a slightly higher strength.

Regularization of torsional stresses in tubular lap bonded joints by means of functionally graded adhesives

This paper describes the analytical stress analysis of a tubular single lap joint under torsion with a functionally graded modulus adhesive (FGA). The adhesive technology offers several advantages in structural applications, such as bonding dissimilar materials, but suffers from severe stress concentrations in the bondline which often act as a trigger for fracture phenomena. To overcome these problems, FGA with nanoparticles distributed inside the polymer can be used, as proposed recently in technical literature. The aim of the work is to retrieve which is the optimal stiffness of the adhesive layer to regularize the stresses. The peculiar geometry analyzed permits a straightforward analytical approach since pure torsion on tubes do not cause any bending and no Poisson's effect creates peel stress in the adhesive, therefore only shear stress are to be considered. The work first develops the equations which govern the shear stress distribution in the adhesive and then by forcing the shear stress to be constant is able to find out which is the stiffness profile along the bondline. The axial distribution of the stiffness of the FGA layer along the overlap is provided and the dependence on the elasto-geometrical parameters is discussed. The findings of the paper can be used to tailor the reinforcement distribution, under the hypothesis of a continuously changing adhesive stiffness.

Analysis of the behaviour of a bonded joint between laminated wood and ultra high performance fibre reinforced concrete using push-out test

Bonded composite constructions using timber and ultra-high performance fibre reinforced concrete (UHPFRC) are investigated as highly innovative structural elements technically and economically efficient, and having better environmental performances. Such technique could indeed offer new structures typologies for bridges for instance associating timber for main beams with UHPFRC slabs. In timber–concrete-composite structures, connection is traditionally achieved with mechanical means. The research presented herein describes the timber–concrete assembly by adhesive bonding and specially the behaviour of bonded joint between Laminated Wood (LW) and UHPFRC. This type of assembly has to be investigated concerning mainly the durability of the bonded assembly and its ability to transmit the forces concentrated on edges. The present study describes the development of a test called “Push-Out” used to characterize the behaviour of the timber–concrete bonded after or prior to environmental ageing. Following the development of the used test protocol, investigations are realized on four different commercial resins to study the influence of the adhesive stiffness on the force transfer focusing in particular on the failure mode, on the ultimate capacities and some local deformations. The mechanical behaviour of the bonded joint is investigated in particular from a theoretical point of view and from the determination of deformation field obtained by Digital Image Correlation (DIC).

Cell Traction Force Mapping in MG63 and HaCaTs

The ability of a cell to adhere and transmit traction forces to a surface reveals the cytoskeleton integrity of a cell. Shear sensitive liquid crystals were discovered with new function in sensing cell traction force recently. This liquid crystal has been previously shown to be non-toxic, linear viscoelastic and sensitive to localized exerted forces. This paper reports the possibility of extending the application of the proposed liquid crystal based cell force sensor in sensing traction forces of osteoblast-like (MG-63) and human keratinocyte (HaCaT) cell lines exerted to the liquid crystal sensor. Incorporated with cell force measurement software, force distributions of both cell types were represented in force maps. For these lowly contractile cells, chondrocytes expressed regular forces (10 – 90 nN, N = 200) around the circular cell body whereas HaCaT projected forces (0 – 200 nN, N = 200) around the perimeter of poly-hedral shaped body. These forces are associated with the organisation of the focal adhesion expressions and stiffness of the LC substrate. From the results, liquid crystal based cell force sensor system is shown to be feasible in detecting forces of both MG63 and HaCaT.

Adhesively bonded functionally graded joints by induction heating

The main objective of this work was to develop an adhesive functionally modified in order to have mechanical properties that vary gradually along the overlap, allowing a more uniform stress distribution along the overlap and to reduce the stress concentrations at the ends of the overlap. This allows for a stronger and more efficient adhesive joint. The adhesive stiffness varies along the overlap, being maximum in the middle and minimum at the ends of the overlap.In this study, grading was achieved by induction heating, giving a graded cure of the adhesive along the joint. The functionally graded joint was found to have a higher joint strength compared to the cases where the adhesive is cured uniformly at low temperature or at high temperature. Analytical analysis was performed to predict the failure load of the joints with graded cure and isothermal cure.

Quantifying the mechanical micro-environment during three-dimensional cell expansion on microbeads by means of individual cell-based modelling

Controlled in vitro three-dimensional cell expansion requires culture conditions that optimise the biophysical micro-environment of the cells during proliferation. In this study, we propose an individual cell-based modelling platform for simulating the mechanics of cell expansion on microcarriers. The lattice-free, particle-based method considers cells as individual interacting particles that deform and move over time. The model quantifies how the mechanical micro-environment of individual cells changes during the time of confluency. A sensitivity analysis is performed, which shows that changes in the cell-specific properties of cell–cell adhesion and cell stiffness cause the strongest change in the mechanical micro-environment of the cells. Furthermore, the influence of the mechanical properties of cells and microbead is characterised. The mechanical micro-environment is strongly influenced by the adhesive properties and the size of the microbead. Simulations show that even in the absence of strong biological heterogeneity, a large heterogeneity in mechanical stresses can be expected purely due to geometric properties of the culture system.Supplemental data for this article can be accessed online.

Finite element analysis of torsional free vibration of adhesively bonded single-lap joints

Adhesively bonding is a high-speed fastening technique which is suitable for joining advanced lightweight sheet materials that are dissimilar, coated and hard to weld. In this paper, the free torsional vibration characteristics of adhesively bonded single-lap joints are investigated in detail using finite element method. The effectiveness of finite element analysis technique used in the study is validated by experimental tests. The focus of the analysis is to reveal the influence on the torsional natural frequencies and mode shapes of these joints caused by variations in the material properties of adhesives. It is shown that the torsional natural frequencies and the torsional natural frequency ratios of the adhesively bonded single-lap joints increases significantly as the Young′s modulus of the adhesives increase, but only slight changes are encountered for variations of Poisson's ratio. The mode shapes analysis show that the adhesive stiffness has a significant effect on the torsional mode shapes. When the adhesive is relatively soft, the torsional mode shapes at the lap joint are slightly distorted. But when the adhesive is relatively very stiff, the torsional mode shapes at the lap joint are fairly smooth and there is a relatively higher local stiffening effect. The consequence of this is that higher stresses will be developed in the stiffer adhesive than in the softer adhesive.

Mechanical properties of adhesives for bonding wood—A review

In this review the current state of the art on mechanical properties of pure wood adhesives is summarised and discussed. Conventionally, mechanical adhesive properties were characterised by means of macroscopic tensile or bending tests of ex-situ cured adhesive films. More recently, nanoindentation was also used to characterise such ex-situ specimens, but more importantly, this method allows the mechanical characterisation of adhesive bond lines in-situ. Mechanical tests reveal high variability between, but notably also within specific groups of adhesives. For example, the modulus of elasticity covers a wide range of more than two magnitudes ranging from 0.1GPa up to 15GPa. Significant differences in adhesive stiffness were observed for adhesives intended to be used for solid wood products compared to wood based composite adhesives, the latter showing higher modulus values. In addition to mechanical adhesive properties as such, factors possibly influencing adhesive performance such as temperature, humidity or ageing of the bonds are taken into consideration.

A Computational Model for Collective Cellular Motion in Three Dimensions: General Framework and Case Study for Cell Pair Dynamics

Cell migration in healthy and diseased systems is a combination of single and collective cell motion. While single cell motion has received considerable attention, our understanding of collective cell motion remains elusive. A new computational framework for the migration of groups of cells in three dimensions is presented, which focuses on the forces acting at the microscopic scale and the interactions between cells and their extracellular matrix (ECM) environment. Cell-cell adhesion, resistance due to the ECM and the factors regulating the propulsion of each cell through the matrix are considered. In particular, our approach emphasizes the role of receptors that mediate cell-cell and cell-matrix interactions, and examines how variation in their properties induces changes in cellular motion. As an important case study, we analyze two interacting cells. Our results show that the dynamics of cell pairs depends on the magnitude and the stochastic nature of the forces. Stronger intercellular stability is generally promoted by surface receptors that move. We also demonstrate that matrix resistance, cellular stiffness and intensity of adhesion contribute to migration behaviors in different ways, with memory effects present that can alter pair motility. If adhesion weakens with time, our findings show that cell pair break-up depends strongly on the way cells interact with the matrix. Finally, the motility for cells in a larger cluster (size 50 cells) is examined to illustrate the full capabilities of the model and to stress the role of cellular pairs in complex cellular structures. Overall, our framework shows how properties of cells and their environment influence the stability and motility of cellular assemblies. This is an important step in the advancement of the understanding of collective motility, and can contribute to knowledge of complex biological processes involving migration, aggregation and detachment of cells in healthy and diseased systems.

Homogenization of a nonlinear elastic fibre-reinforced composite: A second gradient nonlinear elastic material

We study the homogenization of a nonlinear elastic material in contact with periodic parallel nonlinear elastic fibres of higher rigidity. The energy density of both materials is of Saint Venant–Kirchhoff type. The interactions between the matrix and the fibres are described by a local adhesion contact law with interfacial adhesive stiffness parameter depending on the period. Assuming that the Lamé constants in the fibres and the stiffness parameter have appropriate orders of magnitude, we derive a class of nonlinear elastic energies involving displacement gradient of second order.

Buckling induced delamination of graphene composites through hybrid molecular modeling

The efficiency of graphene-based composites relies on mechanical stability and cooperativity, whereby separation of layers (i.e., delamination) can severely hinder performance. Here we study buckling induced delamination of mono- and bilayer graphene-based composites, utilizing a hybrid full atomistic and coarse-grained molecular dynamics approach. The coarse-grain model allows exploration of an idealized model material to facilitate parametric variation beyond any particular molecular structure. Through theoretical and simulation analyses, we show a critical delamination condition, where ΔD∝kL4, where ΔD is the change in bending stiffness (eV), k the stiffness of adhesion (eV/Å4), and L the length of the adhered section (Å).

Percutaneous Multiple Kirschner Wire Fixation in the Treatment of Hand Fractures

The hand fracture is one of the most common traumas, and it accounts for up to 10% of all the human body fractures. Its etiologic factors include sports activities, traffic accidents and industrial work activities. There is a variability in the management of hand fracture. This poses challenging problems for surgeons. The treatment goal is to obtain good outcomes, for which surgeons should consider 1) restoration of the normal alignment, 2) achievement of the appropriate union, 3) recovery of the early range of movement and earlier return to full activities, and 4) absence of residual disabilities or deformities It is difficult to maintain reduction without causing undesirable side effects. The complications of hand fracture include infection, non-union, malunion, tendon adhesion and joint stiffness. Of these, the most serious potential problem is an inability to attain a full range of movement. There Percutaneous Multiple Kirschner Wire Fixation in the Treatment of Hand Fractures

The significance of lap-shear testing of wood adhesive bonds by means of Volkersen’s shear lag model

Lap-shear specimens of varying geometry were bonded with two different adhesives and tested. At the same time, the stress distribution along the adhesive bond lines of the examined specimens was modelled by means of Volkersen’s equation. A good agreement was found between apparent shear strength and a stress concentration factor derived from the model. Highly significant effects of adhesive stiffness were found at short overlap length and high specimen thickness.

Influence of material and geometrical parameters on stress field of composite plates with insert

Reduction of stress concentrations in the area of pin-loaded holes is one of the greatest challenges in the design of multilayered fibre-reinforced composite structures. Among many approaches to decrease concentration effects, putting metallic inserts in the holes is a convenient method. In this research, finite element analysis is applied to assess the effects of manufacturing parameters on stress concentration in the vicinity of the holes in composite plates with inserts. Edge and pitch distance, thickness and materials of insert, clearance between insert and composite part, and stiffness of the adhesive filling the gap are studied in this article. Investigations show that the optimum ratio of edge distance of insert to hole’s diameter is between 2 and 3. Results also show that the changes of the ratio of hole diameter to the width of plate do not have a considerable effect on the bearing concentration factor. Thicker insert reduces stress on the hole contour and higher clearance increases stress near the holes. Finally, increase of adhesive stiffness reduces the stress concentration caused by the clearance.

Experimental and numerical investigations on adhesively bonded timber joints

Adhesively bonding of timber structural elements provides new opportunities as it is well adapted for the anisotropic and fibrous nature of the material. Experimental and numerical investigations were carried out on adhesively bonded full-scale double-lap joints composed of timber adherends (spruce) and adhesive layers. The influence of different geometric parameters and adhesives on the joint strength was studied. The investigated geometric parameters were the thickness of the adhesive layer (0.5–2.0 mm), the overlap length (40–280 mm), and the ductility of the adhesive (using three different adhesives). It was found that the joint strength was independent of the adhesive layer thickness (for the thickness range investigated) that the joint strength increased with the overlap length up to an apparent maximum of approximately 200 mm and that strength was almost independent of the adhesive stiffness. The numerical investigation was in good agreement with the experimental results and allows for the model to be used for strength prediction of the investigated joints.

Strengthening of damaged structures with bonded prestressed FRP composites plates: an improved theoretical solution

In this article, a closed-form rigorous original solution for interfacial stresses in simply supported beams strengthened with bonded prestressed fiber-reinforced polymer (FRP) plates and subjected to a uniformly distributed load, is developed using an improved new model shear deformation coupled with the prestressed force effect. The main interest of this research puts in evidence a new originality approach theory, taking into account the mechanical load, the shear deformation of adherends, and the prestressed FRP plate model with a new shear lag model, which considers a strength rigidity and resistance of the damaged structures, which is one aspect that has not been taken into account by the previous studies. A parametric study has been conducted on factors such as laminate and adhesive stiffness and the thickness of the laminate and the adhesive, where all of them were found to have a marked effect on the magnitude of maximum shear and normal stress.

Dynamic Contact Stiffness of Adhesive Hertzian Contact

Based on the JKR and DMT models, dynamic contact stiffness of a rigid sphere against an adhesive semiinfinite solid is investigated by the consideration of dynamic contact deformation at the contact interface. The assumption of sufficiently small oscillating force yields a dynamic contact-pressure distribution of constant contact size, and then dynamic contact stiffness. It is found that except for the contact radius, two adhesive models predict the same expression of quasi-static contact stiffness and dynamic adhesive contact stiffness factor (DACSF). The influence of the oscillating frequency and specimen elasticity on the DACSF is discussed.

Application of plate theory to laminated wood composites with non-rigid adhesive

A closed-form solution for laminated wood and sandwich biocomposite panels assembled with non-rigid adhesives and subjected to edgewise loading is presented. The solution satisfies the equilibrium equations of the layers, the compatibility equations of stresses and strains at the interfaces and the boundary conditions. To investigate the effects of the finite values of bonding stiffness on the mechanical responses of the panels, numerical evaluations are conducted. The results obtained have shown that the adhesive stiffness has a strong effect on the panel mechanical response. Beyond a certain level of stiffness, the usual assumption of perfect bonding used in classical theories is acceptable. This could provide an answer to what constitutes perfect bonding in terms of the ratio of the inner layer stiffness to the bonding stiffness.

Contact atomic force microscopy of biological tissues

The forces of adhesion of the normal and tumor cells of the human stomach epithelium to the probe of an atomic force microscope have been measured for the first time. It is established that the adhesion and contact stiffness of malignant cells are significantly lower than those of the normal cells.

Finite difference three-dimensional solution of stresses in adhesively bonded composite tubular joint subjected to torsion

Traditionally, the adhesive layer in adhesively bonded joints is simplified as having constant shear and peel stresses across the thickness. In this study, it is modeled as a separate 3D elastic body without the uniform stress assumption. The stress state in an adhesively bonded fiber-reinforced composite tubular joint subjected to a torsional load is theoretically analyzed. A finite difference method is utilized to solve the system of equilibrium equations. The solution is validated by comparison with data found in the literature. The effects of fiber orientation, composite laminate stacking sequence, overlap length, adhesive layer thickness, and adhesive stiffness on the shear and peel stress distribution are evaluated and presented. The results show that all the six stress components are present and they vary considerably across the adhesive layer thickness; 3D analysis is desired, especially for general laminated composite adherends.