Erol Sancaktar

Selective use of rubber toughening to optimize lap-joint strength

This paper introduces a novel approach to increasing the lap joint strength, different from the traditional methods of either increasing the lap joint area or altering the joint geometry. This is accomplished by the selective use of rubber toughening in epoxy to optimize lap joint strength. This was accomplished in three stages. In the first stage an adduct was prepared, this was used to make bulk tensile specimens to calculate the bulk properties for various concentrations of rubber, i.e. 0, 10, and 20 parts per hundred parts of resin (epoxy). In the second stage finite element models were developed using the bulk properties previously obtained. Interfacial stresses were used to access the trends obtained by the selective use of rubber toughening at different locations of the overlap in different configurations. The modeling of adhesive joints was done using ALGOR 2-D, linear and nonlinear finite element analyses (FEA). In the third stage, tensile shear tests conducted on the lap joints validated the trends from the finite element models. Finite element modeling and meshing of the lap joints having 25.4 and 50.8 mm adhesive overlap lengths were completed. Different configurations of rubber toughened and untoughened adhesive were tried in these two overlaps. The validation was done by lap joint tests conducted on an Instron mechanical tester coupled with an extensometer. Comparable strengths were obtained for completely toughened overlap and the configuration where only the edges of the adhesive overlap were toughened and the region in-between was untoughened. Also, the nonlinear FEA was shown to represent the experimental results more closely than the linear approach.

Pressure-dependent conduction behavior of various particles for conductive adhesive applications

Abstraet-The efficiency of electrical conduction in particle-filled conductive adhesives largely depends on the interparticle conduction. In order to gain insight into the pressure-dependent conduction behavior with particles of different sizes, shapes, and types, the effects of external pressure on the filler resistance were measured by the four-point probe method using different conductive fillers. The following types of particles were used: Ni powder, Ni flake, Ag powder, Ni filament, magnetite spindles, and Cu particles. Non-filament particle size was in the range 0.7-44 μm. The filaments were 20 μm in diameter and 160 or 260 μm in length. The particle treatment procedures investigated included silver coating using different methods, and the use of acid solutions including H3PO4, HF, and HCl to etch and remove the surface oxide layer. Resistivity levels were measured using a nonconductive hollow cylinder plunger device developed in our laboratories. The results of our work showed that when the pressur...

A study on the effects of surface roughness on the strength of single lap joints

In order to gain an insight into the aspects of surface roughness, surface profile, adhesive viscosity, and surface morphology, experiments were conducted using two different steels (1018 cold-rolled and hot-rolled weld steels). Four different surface modifications were performed on these samples by sand-blasting at 552 kPa from a distance of 25.4 mm, and etching for 2 and 10 min with a chemical recipe containing chromic acid. Two different adhesives were used, namely Epon 815 and Epon 830 with viscosities of 5-7 and 170-225 Poise, respectively, to assess the effect of viscosity in bonding to adherends with different surface topographies. Contact angles were measured on all surfaces using the two adhesives. To gain an insight into the effect of surface topography, scanning electron microscopy (SEM) was utilized to observe the modified surfaces at different magnifications. Both average and maximum peak-to-valley height values were measured using a profilometer and surface profiles were also obtained for all the samples. The single lap joint strength and displacement values were measured at two different crosshead speeds of 1 and 100 mm/min, to assess the interrelationship between the failure mechanisms and joint displacements, surface topography and adhesive viscosity. The results of our work showed a general decrease in the contact angle values with average surface roughness for the liquid, Epon/DETA, thermoset/hardener combination on both cold- and hot-rolled steel specimens. When tested at a 100 mm/min loading rate, and compared with the 1 mm/min loading rate condition the cold-rolled specimens, which had smaller surface roughness averages in comparison with the hot-rolled condition, exhibited a higher reduction in failure load values. It is believed that this indicates a higher proportion of interfacial failure in cold-rolled specimens at higher loading rates.

Anisotropic alignment of nickel particles in a magnetic field for electronically conductive adhesives applications

Anisotropically conductive adhesives were developed by magnetic alignment of conductive nickel particles in a non-conductive epoxy matrix. Conductivity occurs in the direction of the magnetic field applied to the adhesive film. The effects of magnetic and conducting nickel filler type (i.e. powders, filaments, flakes, and fibers), resin viscosity, and the magnitude of aligning magnetic field on the electrical properties of the resulting anisotropic adhesive were investigated. The electrical resistance of the anisotropic adhesive was measured using the four-point probe method. The resistance of the filled adhesive decreased with increasing viscosity and the intensity of the magnetic field.

The effect of pressure on the initial establishment of conductive paths in electronically conductive adhesives

In order to understand and to be able to affect the efficiency of electrical conduction in particle-filled conductive adhesives, the mechanism of interparticle conduction is modeled theoretically and assessed experimentally. We predict and show that the electrical resistance of powders is dependent on the external pressure applied on them. A pressure-dependent model is developed to predict interparticle electric conduction by representing interparticle contact resistance as the sum of constriction resistance and tunneling resistance. The theoretical model developed is confirmed experimentally using 9-μm diameter spherical nickel particles under pressure.

Increasing strength of single lap joints of metal adherends by taper minimization

The effect of tapering the ends of the adherend on the joint strength and joint deformation behavior of a single lap joint geometry was studied. The joints were geometrically modeled using finite element (FE) techniques involving linear, as well as nonlinear (bilinear) material behavior. The FEA results were then compared with the experimental results for different single lap configurations, which had aluminum and steel adherends with different surface etch conditions, bonded using two different adhesives. The FEA results were found to be consistent with the experimental results with the normal and shear stresses significantly decreasing in the modified (tapered) geometries over those in unmodified geometries. The joint strength increased with decreasing taper angle, reaching a maximum at the smallest value considered (~10°).

Free vibration analysis of symmetric cross-ply laminated composite beams with the help of the transfer matrix approach

The transfer matrix method is used for the numerical solution of both in-plane and out-of-plane free vibration problems of symmetric cross-ply laminated composite beams. The rotary inertia, and both shear and extensional deformation effects, are considered in the analysis. A distributed parameter model is used in the mathematical formulation of the free vibration problem. The overall transfer matrix of the beam is obtained making use of the series solution of a set of differential equations. All the effects of the boundary conditions, the slenderness ratio, the ratio of the height to the width of the rectangular cross-section, the number of plies, and material types on the free vibration frequencies are studied. Comparison of the exact natural frequencies obtained in this study with the existing results is quite good. Copyright

Fracture aspects of adhesive joints: material, fatigue, interphase, and stress concentration considerations

—The effects of secondary material phases added or created in the adhesive bulk during processing ; changes in joint material properties due to processing and adhesion (interphase properties) and/or loading, including fatigue loading; and effect of interphase material properties on overlap edge stress concentrations are considered in this paper in relation to the fracture behavior of adhesive joints. A theoretical discussion is provided on each of these issues along with specific examples and experimental and/or calculated data. The experimental data discussed were obtained using bulk specimens (material considerations), independently loaded mixed-mode specimens (ILMMS) (fatigue/fracture considerations), and single fiber fragmentation specimens (interphase considerations and its effect on the overlap edge stress concentration). Cause-effect relations between the above-mentioned fracture issues are highlighted as permitted by the data and the analytical models presented.

Dependence of electrical conduction on the film thickness of conductive adhesives: modeling, computer simulation, and experiment

The effect of film thickness on the conduction behavior of electrically conductive adhesives is presented. For comparison purposes, an analytical relation is developed to predict the three-dimensional resistivity of particle-filled conductive adhesives. This analysis reveals that the adhesives resistivity depends on parameters m, representing an average contact number, and Si, representing the average length of conductive paths between the conductive particles incorporated in the adhesive matrix. Both the parameters m and Si are functions of the conductive particle volume fraction Φ, and these functional relations are developed by computer simulation for the cases of spherical particles in three-dimensional (3-D) and two-dimensional (2-D) contacts. The latter (2-D) case represents thin film applications. The computer simulation utilized reveals that the average length of conductive paths in the planar direction of the 2-D case is much larger than that for the 3-D case. This leads to an electrical conduct...

Conduction Efficiency and Strength of Electronically Conductive Adhesive Joints

Abstract Electronically conductive adhesives are being considered as an alternative to solder for interconnection in microelectronics. In order to gain insight regarding electrical and mechanical performance properties of this class of adhesive interconnections, overlap joints were made. Joint resistance and mechanical bond strength were measured before and after environmental stressing.