Environmental Failure of Structural Adhesive Joints

The deleterious effect of water on structural adhesive joints has been investigated using joints which consist of mild steel substrates bonded with a simple epoxy adhesive. The mechanisms and kinetics of environmental failure have been identified and the instability of the joints in hostile environments successfully predicted from thermodynamic considerations.     

Prediction of Failure Times for Some Adhesive Bonded Joints

Data are given for shear and tensile testing of adhesive bonds under conditions of a constant rate of loading. A rate equation is then used to predict lifetime from the mechanical data. The correlations appear to be satisfactory providing that the failure is cohesive within the adhesive.     

Prediction of Failure Times for Some Adhesive Bonded Joints

Data are given for shear and tensile testing of adhesive bonds under conditions of a constant rate of loading. A rate equation is then used to predict lifetime from the mechanical data. The correlations appear to be satisfactory providing that the failure is cohesive within the adhesive.     

Bonding and Failure Mechanisms in Aluminium Alloy Adhesive Joints

X-ray photoelectron spectroscopy and scanning electron microscopy have been used to investigate the surface chemistry of an aluminium-alloy which has been pretreated by various industrial methods commonly employed prior to adhesive bonding. The fracture surfaces of butt joints, consisting of the pretreated alloy bonded with an epoxy adhesive, have also been studied. The analyses have been conducted before and after exposure of the specimens to water, the main hostile environment that structural adhesive joints usually encounter. It is suggested that the concentration of magnesium, in the form of magnesium oxide, on the pretreated aluminium-alloy surface may be an important factor in influencing the durability of adhesive joints.     

Adhesive Joints for Low- and High-Temperature Use: An Overview

This work presents a review of several investigations on the topic of adhesive bonding at high and low temperatures. Durability and strength at extreme temperatures have always been a major limitation of adhesives that, given their polymeric nature, exhibit substantial degradation at temperatures where other structural materials (such as metals for example) have minute changes in mechanical properties. However, due to the inherent advantages of bonding, there is a large and continued effort aiming to improve the temperature resistance of adhesive joints, and this effort has been spread among the various topics that are discussed in this review. These topics include adhesive shrinkage and thermal expansion, adhesive properties, joint geometry optimization, and design techniques, among others. The findings of these research efforts have all found use in practical applications, helping to solve complex problems in a variety of high-tech industries where there is a constant need to produce light and strong components that can withstand large temperature gradients. Therefore, the final sections of this work include a discussion on two specific application areas that demonstrate the strict demands that extreme temperature use imposes on adhesive joints and the methods used to improve their performance.     

Some Aspects of Bond Formation and Failure in Adhesive Wood Joints

An investigation has been made of the effect of varying glue-spread on the bond strength of holly (Ilex aquifolium) using three adhesives and three different wood sections. The glue-spreads are lower than those normally used, and it has been found with edge-grain joints that 100% cohesive wood failure can occur with a glue-spread as low as 2.7mg/cm². Scanning electron microscopy shows that interlocking between adhesive and adherend does not occur. Factors leading to delamination and joint failure are discussed.

Lignin, without further addition, has been shown to be a useful wood adhesive. It has also been shown that it is possible to make end-grain joints without the use of an adhesive; the lignin present in the wood specimens is considered to be responsible for such joints.     

On the Character of Failure of Adhesive Joints Between Two Polymers

Up to the present there has been discussion of the locus of fracture of adhesive joints, i.e., if it proceeds precisely at the interphase border, or if cohesive fracture in a weak layer of one of the components takes place. There are only a few data available on this point.     

Some Aspects of Bond Formation and Failure in Adhesive Wood Joints

An investigation has been made of the effect of varying glue-spread on the bond strength of holly (Ilex aquifolium) using three adhesives and three different wood sections. The glue-spreads are lower than those normally used, and it has been found with edge-grain joints that 100% cohesive wood failure can occur with a glue-spread as low as 2.7mg/cm². Scanning electron microscopy shows that interlocking between adhesive and adherend does not occur. Factors leading to delamination and joint failure are discussed.

Lignin, without further addition, has been shown to be a useful wood adhesive. It has also been shown that it is possible to make end-grain joints without the use of an adhesive; the lignin present in the wood specimens is considered to be responsible for such joints.     

Ultimate Tensile Stress over a Zone: A New Failure Criterion for Adhesive Joints

The objective of this work was to develop a criterion for predicting the static strength of adhesively-bonded joints. Although there is a large body of literature on this subject, no satisfactory criterion has been proposed in any publication to date. To obtain a criterion, finite element models of widely differing joint geometries were developed, in which the stresses in the adhesive bonds were calculated in great detail. These were then compared with test measurements. After examining two toughened epoxy adhe-sives, the authors have developed a failure criterion that predicts failure loads to within approximately 15% of measured values. This is: that the maximum principal stress must exceed the ultimate tensile stress of the adhesive material over a finite zone normal to the direction of maximum principal stress. The size of this zone is a property of the adhesive that can be determined from a combination of analysis and test.     

Torsional Test Method for Adhesive Joints

A modified tubular butt assembly for testing adhesive joints was found to yield greater precision and sensitivity than previous methods. Effects due to modifying the surface pretreatment of aluminurn-epoxy joints could be identified and analyzed statistically by the Wilcoxon-Mann-Whitney test. The use of a very short tube integrally mounted on a solid rod minimizes axial stress components. The joints are easily handled and tested, and due to their greater reproducibility, fewer tests are necessary.     

Determination of Traction Separation Law for Interfacial Failure in Adhesive Joints at Different Loading Rates

Cohesive zone modeling (CZM) has been extensively used in recent years to simulate failure in adhesive joints. Accurate determination of the traction–separation law (TSL) (or parameters of the CZM) is very crucial to the success of this approach. Recent experimental investigations have indicated that loading rate influences the TSL/CZM parameters. In this work we have attempted to measure the TSL using two different approaches for an adherend/adhesive system which always fails by interfacial failure. In the first approach, the TSL is obtained by differentiating the experimentally measured J integral by the opening displacement. The second, an inverse approach, involves a finite element (FE) analysis in which the adhesive layer is also modeled and cohesive elements are used to model the interfacial failure. The TSL is then obtained iteratively by matching the numerical load–displacement data to that obtained in experiments. We show that the first approach yielded TSLs which are dependent on both adhesive layer thickness and the loading rate, whereas the second approach yielded a TSL which is independent of the adhesive layer thickness and the loading rate. Therefore, the TSL obtained from the second approach is intrinsic to the adhesive/adherend pair and in that sense is unique.     

Modelling Damage and Failure in Adhesive Joints Using A Combined XFEM-Cohesive Element Methodology

In recent years, cohesive elements based on the cohesive zone model (CZM) have been increasingly used within finite element analyses of adhesively bonded joints to predict failure. The cohesive element approach has advantages over fracture mechanics methods in that an initial crack does not have to be incorporated within the model. It is also capable of modelling crack propagation and representing material damage in a process zone ahead of the crack tip. However, the cohesive element approach requires the placement of special elements along the crack path and is, hence, less suited to situations where the exact crack path is not known a priori. The extended finite element method (XFEM) can be used to represent cracking within a finite element and hence removes the requirement to define crack paths or have an initial crack in the structure. In this article, a hybrid XFEM-cohesive element approach is used to model cracking in the fillet area using XFEM where the crack path is not known and then using cohesive elements to model crack and damage progression along the interface. The approach is applied to the case of an aluminium–epoxy single lap joint and is shown to be highly effective.     

Adhesive Failure of Model Acrylic Pressure Sensitive Adhesives

An axisymmetric adhesion apparatus was used to characterize the adhesive and viscoelastic properties of acrylic block copolymer layers that behave as model pressure sensitive adhesives. The mechanisms of deformation were summarized and related to the structure and linear viscoelastic response of each model adhesive. In cases where the area between the adhesive layer and adhering surface remained circular and shrunk uniformly during detachment, the adhesive failure criterion can be quantified and compared to predictions from linear elastic fracture mechanics. The nature of adhesive failure can not be reconciled with these traditional, low-strain approaches, but is consistent with models of large strain elasticity, provided that the finite thickness of the adhesive layer is taken into account. A dimensionless ratio involving the adhesive strength, elastic modulus and adhesive layer thickness can be used to define the regime in which the adhesive failure criterion can be quantified with linear elastic fracture mechanics.     

Adhesive Failure of Model Acrylic Pressure Sensitive Adhesives

An axisymmetric adhesion apparatus was used to characterize the adhesive and viscoelastic properties of acrylic block copolymer layers that behave as model pressure sensitive adhesives. The mechanisms of deformation were summarized and related to the structure and linear viscoelastic response of each model adhesive. In cases where the area between the adhesive layer and adhering surface remained circular and shrunk uniformly during detachment, the adhesive failure criterion can be quantified and compared to predictions from linear elastic fracture mechanics. The nature of adhesive failure can not be reconciled with these traditional, low-strain approaches, but is consistent with models of large strain elasticity, provided that the finite thickness of the adhesive layer is taken into account. A dimensionless ratio involving the adhesive strength, elastic modulus and adhesive layer thickness can be used to define the regime in which the adhesive failure criterion can be quantified with linear elastic fracture mechanics.     

金属胶接接头劈裂破坏的模型分析

对金属胶接接头劈裂强度测试方法和影响劈裂强度的主要因素的作用进行了分析与讨论。本文提出了金属胶接接头劈裂破坏模型，对完善劈裂强度测试方法提出了改进意见．     

Fracture Toughness of Adhesive Joints

To define the influence of the processing variables on the resistance of epoxy joints to brittle crack extension during short loading times, the fracture toughness, g_ic, of the joints was measured as a function of the following variables:

1. Hardener type (TEPA vs. HHPA)

2. Ratio of hardener to resin content

3. Post-cure temperature

and 4. Joint geometry (thickness and width)

It was found that the toughness of the TEPA hardened system varied by a factor of four-to-one as the ratio of hardener to resin content and post-cure temperature varied within what might be considered reasonable limits for manufacturing. The toughness of the HHPA hardened system varied only over the middle half of this same range.

For both systems, toughness increased with joint thickness over the range of 2 to 50 mils.     

The Strength of Adhesive Joints

The main rules pertaining to the strength of adhesive joints are: (1) This strength is a mechanical (or rheological) property. The local stress which causes the extension of a pre-existing crack can be determined only if the stress pattern in the whole adhint is known and the intensification of stress at flaws is taken into account. (2) The rupture occurs in a material, not between two materials. Consequently, the molecular forces across the adhesive-adherend interface are irrelevant, and the “adhesion tension” does not determine the adhint strength.     

Non Linear Fracture Mechanics for Adhesive Lap Joints

The Rice Cherepanov J is calculated for a lap joint in pure shear. By choosing as a condition for fracture a critical value J_c of this quantity the fracture load of the joint is calculated for linear elastic, perfectly plastic and linear hardening behavior of the adhesive. Comparisons are given with experiments with various adhesives and overlap lengths.     

A Model for the Diffusion of Moisture in Adhesive Joints. Part II: Experimental

The concentration dependence and temperature dependence of the diffusion coefficient, as discussed in Part I, are validated with literature data on poly(styrene) and on poly(vinylacetate). The effect on diffusivity, of a uniaxial tensile stress state and of a biaxial tensile stress state, is measured with permeation tests on stretched poly(ethyleneterephthalate) (PET) films. The influence of semi-crystallinity is briefly discussed. Further, diffusivity measurements under a tensile stress state, under a compressive stress state, and under a pure shear stress state are performed on Ultem^® polyimide films, using a modified sorption technique. Good agreement between theoretical predictions and experiment is found. Finally, predictions by the solubility model discussed in Part I are compared with data on low density polyethylene and on Ultem polyimide.     

Fatigue Behaviour of Adhesive Bonded Joints

This paper presents experimental results of the fatigue behaviour of adhesive bonded plastic-to-plastic joints and metal-to-plastic joints under both dynamic and static loading. The fatigue life of the joints was found to be independent of the test frequencies and humidity for the range of values tested, but dependent on the mean stress level and test temperature with greater reduction in fatigue life observed in metal-to-plastic joints at higher temperature. Empirical equations from which the fatigue life of joints could be predicted were obtained by regression analysis.