Fracture Behavior Of Adhesives Research Articles

Load-control vs. displacement-control strategy in fatigue threshold analysis of adhesives: Effects of temperature

For the design of adhesive joints for service in fatigue, parameters such as the rate of fatigue crack growth and the fatigue threshold energy release rate are of extreme importance. These parameters can be obtained through fatigue fracture tests using different strategies, namely Constant load control and constant displacement control. Despite the extensive studies conducted on the fatigue performance of bonded joints, the role of testing methodology on the fatigue fracture behaviour of adhesives is less considered, especially in conjunction with the effect of temperature in these tests. Accordingly, in this study, a structural polyurethane adhesive suitable for automotive applications is characterized in terms of its fatigue behaviour in pure mode I using DCB (double cantilever beam) specimens. Fatigue tests were performed at different temperatures under both constant load control and constant displacement control conditions. A compliance-based method was used to measure the crack size throughout tests. The threshold energy release rate was calculated for different test temperatures and the two testing strategies were compared. It was seen that a constant displacement control methodology is superior to a constant load control methodology when testing for the threshold energy release rate, due to its shorter duration and low sensitivity to initial testing conditions. Differences observed between fatigue testing methodologies were exacerbated by the effect of temperature on the adhesive.

Numerical investigation on the effect of fillers on the fracture behavior of adhesives

Adhesives are usually heterogeneous materials and contain fillers, e.g., glass beads, and voids, which are crucial for its fracture properties. Multiscale approaches using computational homogenization can link model descriptions of the microstructure and the mechanical properties of the constituents to the macroscopic properties, which allows for a virtual optimization of materials. The fracture behavior of full adhesive layers is often modeled by cohesive zone models. Recently, a Fast Fourier Transform (FFT) based homogenization scheme for cohesive zone modelling has been introduced, which is expected to be computationally more efficient than FE-based computational homogenization for many materials. Nevertheless, FFT-based homogenization still requires too many computational resources for a large scale industrial application of coupled multiscale simulations, which is why we propose a machine learning based surrogate cohesive law for mode I fracture which is trained offline. Our model can take into account the effect of different filler volume fractions and is implemented in the commercial FE software Abaqus. We furthermore demonstrate its capabilities with FE simulations of a Double Cantilever Beam (DCB) test exemplary.

A strain rate dependent cohesive zone element for mode I modeling of the fracture behavior of adhesives

Adhesives are widely used in the automotive industry as they can be used to manufacture lightweight multimaterial structures with improved strength-to-weight ratio, contributing to lower energy consumption and emissions. In this industry, it is also crucial to ensure passengers' safety and, thus, mechanical behavior of the complete automotive structure should be tested for impact conditions, including the adhesive joints within it. This work presents the development of a finite element, which models the mechanical behavior of adhesives and takes into account the strain rate dependent property variation, in mode I. The trends for the property variation were set based on an experimental study of two adhesives under three different loading conditions: quasi-static, intermediate speed, and impact. The strength was determined using bulk tensile tests and toughness using double cantilever beam, respectively. Both adhesives studied have shown an increase in ultimate stress and critical energy release rate with increasing strain rate. The property variation was then implemented on a finite element, which revealed a good agreement with the experimental results.

Effect of aligning graphene oxide nanoplatelets using direct current electric field on fracture behaviour of adhesives

AbstractIn this paper, the effect of aligning graphene oxide nanoplatelets (GONPs) using a direct current electric field on fracture behaviour of adhesives was studied experimentally and numerically. Aligning GONPs was modelled by using an analytical model. The fracture behaviour of the adhesive joints reinforced with randomly dispersed and aligned GONPs was studied experimentally by testing double cantilever beam specimens and modelled numerically by using a cohesive zone model that characterized by a compliance‐based method. The aligned GONPs resulted 56% and 42% further improvements in the maximum load and fracture energy of the adhesive joints, respectively, at one‐third nanofiller weight percentage (0.1 wt%) compared with the randomly dispersed GONPs that caused maximum improvements in the maximum load and fracture energy of the adhesive joints at 0.3 wt% nanofillers. The scanning electron microscopy fractography revealed that aligned GONPs promoted crack branching in the adhesive layer and caused the fracture resistance of material to further improve.

The intrinsic fracture property of a rubber-modified epoxy adhesive: Geometrical transferability

This study presents recent achievements in understanding the fracture behaviour of rubber-modified epoxy adhesives, notably the determination of the fully developed fracture process zone (FPZ) and its associated intrinsic fracture energy, G0. The shape and size of the FPZ are identified by inspecting the fracture surfaces using SEM, and exploring subsurface damage using optical microscopy and TEM. The thickness and failure strain of the FPZ are found to be essentially the same for different fracture tests, i.e. tapered double cantilever beam and single edge notched bending tests. As a consequence, the results from different fracture tests are linked by using the geometrically transferable, true fracture properties, i.e. FPZ thickness and G0. The variation of total fracture energy observed in different fracture tests is attributed to varying plastic deformation energy dissipated in plastic deformation zone (outside FPZ). The fracture behaviour of the adhesive is then successfully predicted by a cohesive zone model using parameters extracted from experiments.

Fracture Behaviour of Adhesives Modified with various Rubber Content under Combined Stress Conditions

Rubber-modified adhesives have attracted special interest recently due to their property of dramatically increase in fracture toughness of adhesively bonded joints, where the fracture toughness increases with the rubber content. However, an increase in rubber content decreases the joint strength without initial flaws. Besides, most adhesively-bonded joints have combined normal and shear stresses concentrated in the adhesive layer. Hence, the effect of rubber content on adhesively-bonded joints should be clarified under combined stress conditions. In this paper the aim is to characterize the rubber content dependent behaviour of the rubber-modified adhesive under combined stress conditions. Tensile tests were conducted for adhesively-bonded butt and scarf joints with various rubber contents, where these joints can cover the combined stress conditions in the adhesive layers of most joints in industrial applications. The influence of the rubber content on the joint strength is represented in the normal stress-shear stress diagram.

Adhesion-induced instabilities and pattern formation in thin films of elastomers and gels

A hydrostatically stressed soft elastic film circumvents the imposed constraint by undergoing a morphological instability, the wavelength of which is dictated by the minimization of the surface and the elastic strain energies of the film. While for a single film, the wavelength is entirely dependent on its thickness, a co-operative energy minimization dictates that the wavelength depends on both the elastic moduli and thicknesses of two contacting films. The wavelength can also depend on the material properties of a film if its surface tension has a pronounced effect in comparison to its elasticity. When such a confined film is subjected to a continually increasing normal displacement, the morphological patterns evolve into cracks, which, in turn, govern the adhesive fracture behavior of the interface. While, in general, the thickness provides the relevant length scale underlying the well-known Griffith-Kendall criterion of debonding of a rigid disc from a confined film, it is modified non-trivially by the elasto-capillary number for an ultra-soft film. Depending upon the degree of confinement and the spatial distribution of external stress, various analogs of the canonical instability patterns in liquid systems can also be reproduced with thin confined elastic films.

Studying Friction and Shear Fracture in Thin Confined Films Using a Rotational Shear Apparatus

This paper describes the effects of the elastic modulus and sliding velocity on the friction and shear fracture of smooth silanized rigid disks rotating against thin confined films of poly(dimethylsiloxane) (PDMS) elastomers. A rigid glass disk is rotated against thin PDMS films of different thicknesses and moduli bonded to a glass plate at various speeds. While the disk rotates on the PDMS coated glass plate, a load cell measures the resulting force with a cantilever beam. One end of the cantilever beam is glued to the glass plate, while its other end presses against a load cell. From the balance of forces and torques, the friction force at a given slip velocity is determined. The friction force increases with the slip velocity sublinearly, which is consistent with the results reported previously by Vorvolakos and Chaudhury (Langmuir 2003, 19, 6778). During rotation, however, the glass disk comes off the PDMS film when the shear stress reaches a critical value. This critical shear stress increases with the modulus of the film, but it decreases with its thickness, following a square root relationship, which is similar to the adhesive fracture behavior in thin films under pull-off conditions. A simple model is presented that captures the essential physics of the fracture behavior under shear mode.

Adhesive Fracture Energies of Some High Performance Polymers

Abstract A fracture mechanics approach was applied to determine the adhesive fracture energy of various high performance polymers. These polymers, including both thermosetting and thermoplastic materials, generally offer higher temperature capability than conventional epoxies. Double tapered cantilever beam specimens were used for fracture tests at both room temperatures and 225°C. The adhesive fracture energies of a tetrafunctional epoxy and a phthalocyanine resin were also determined at low temperatures. Adhesive fracture behavior of polymers at high temperatures was found to depend on polymer glass transition temperature, whereas at low temperatures it was related to secondary relaxation processes in the glassy state.

FRACTURE DESIGN CRITERIA FOR STRUCTURAL ADHESIVE BONDING‐PROMISE AND PROBLEMS

ABSTRACTThe joining of load bearing structures using organic resin adhesives is an established technology in aircraft, spacecraft, and automobile construction and is being considered in ship construction; especially for hydrofoil and other advanced craft. The failure analysis of structural adhesive joints must be based on fracture criteria since these joints fail by linear elastic brittle fracture. Accordingly, the Navy has supported work over the past decade on developing an adhesive fracture test methodology and data base.Recent work at the Naval Research Laboratory (NRL) had demonstrated that fracture testing of structural adhesive joints is fundamentally more complex than the fracture testing of monolithic structures. Specifically, there is a very strong bond thickness effect; and more importantly, testing must be done under combined stress conditions. Opening‐mode (cleavage) tests alone are not sufficient. A study of the adhesive fracture behavior of structural adhesives has established why this additional testing is necessary. Work remains to be done on developing analytical relationships for the effect of joint geometry on adhesive fracture behavior.

Effect of Temperature on the Adhesive Fracture Behavior of an Elastomer-Epoxy Resin

Abstract The bulk and adhesive fracture behavior of a diglycidyl ether bisphenol-A epoxy modified with 15% carboxy-terminated butadiene acrylonitrile was determined as a function of temperature. The bulk fracture toughness increased sharply near the resin Tg in a manner similar to unmodified epoxy resins. The adhesive fracture energy exhibited a maximum with respect to bond thickness and this maximum broadened and shifted to larger bond thicknesses with increasing temperature. These results are discussed in terms of the size and stress condition of the crack tip deformation zone.

The fracture of epoxy- and elastomer-modified epoxy polymers in bulk and as adhesives

The fracture behavior of a piperidine/bisphenol A diglycidyl ether (A) resin has been determined in bulk and as an adhesive using the linear elastic fracture methods developed by Mostovoy1. The effect of adding carboxy-terminated butadiene–acrylonitrile (CTBN) elastomer to resin A was investigated. The opening-mode fracture energy () of resin A was 120 to 150 J/m2, and largely attributable to plastic deformation. Fractographic evidence was obtained for plastic flow at the crack tip during crack initiation. Propagation was unstable due to the rate dependence of the plasticity. There were no significant differences in the bulk and adhesive fracture behavior. Addition of 5–15% CTBN to resin A produced minute elastomer particles which increased to ∼4000J/m2 (at 15%). Further CTBN addition resulted in an elastomer–epoxy blend and a decrease in fracture energy. Fractography again indicated that crack initiation involved plastic deformation but that the elastomer had greatly increased the volume in which the deformation occurred. The adhesive fracture of the elastomer–epoxy was found to be strongly dependent on the crack-tip deformation zone size (ryc) in that was a maximum when bond thickness was equal to 2 ryc. At bond thicknesses less than 2 ryc, there was a restraint on the development of the plastic zone resulting in lower values.