Adhesive joints stress analysis

Daghyani. H.R.. Ye. L. and Mai. Y.W. (1995b). Mode I fracture behaviour of adhesive joints, 2, Stress analysis of constraint parameters. J. Adhesion 53, 163-172. 

Life prediction methodology embraces all aspects of the numerous processes that could affect the function of the element—in this case the bulk adhesive. The first step is to define the function of the adhesive clearly enough for a failure criterion to be derived. This failure criterion may be an unacceptable reduction in tensile strength, time to creep failure under a given stress, reduction in modulus due to moisture ingression, increase in modulus due to oxidation, unacceptable crack depth, or a variety of other possible criteria. It is also important that the criteria be related to practical adhesive joint performance. This is where it is difficult, and one must presume, at least for this limited analysis, that the adhesive will fail via a bulk (cohesive) property. 

The design analysis of a scarf may be considered similar to a single lap bonded joint, detailed analysis of which can be found in MIL-HDBK 17-3E 3. The analysis of a bonded joint is made complex, however, by the modulus difference of the adhesive compared to the adherends and the relative thicknesses of both which causes a non-linear distribution of the shear forces in a lap joint with peak stresses at the ends [1]. Scarf repairs provide a more uniform stress distribution however, to achieve this an adequate scarf angle is required [24] shown in Eigure 14.6. 

As shown earlier, adhesive joints are considered to be systems unequally resistant to the stresses of normal fracture and compression. Thus, it is expedient to limit our analysis to strength theories and the Mohr theory as the pure experimental one. 

Nearly a century after Fairbaim, in 1938, two ideas emerged from the new engineering of airframes which were to focus on the paradoxical notion embodied in Equations (15.1) and (15.2). Volkersen derived a stress analysis for the deformation of a lap joint, showing that infinite stresses could arise at the ends of a lap joint, and Chadwick measured the peel strength of soldered joints, raising the conundrum that a joint is much weaker in peeling than it is when overlapped. How can strength be different when the same adhesive is employed ... 

The science of adhesion is truly multi-disciplinary, demanding a consideration of concepts from such topics as surface chemistry, polymer chemistry, rheology, stress analysis and fracture mechanics. It is, nevertheless, important for the technologist to possess a qualitatively correct overall picture of the various factors influencing adhesion and controlling joint performance in order to make rational judgements concerning the selection and use of adhesives. 

Because there are so many geometries of adhesive joints encountered, and so many types of stress applied (tension, shear, torsion, thermal, etc.), the analysis of a given system must be tailored to meet the specific application. The processes of experimental design and data analysis, therefore, become quite complicated. It should also be kept in mind that flaws such as those often implicated in adhesive failure can also lead to apparent cohesive failure in the bulk material. 

Lunsford, L.R., Stress analysis of bonded joints. Applied Polymer Symposia No. 3, Structural Adhesive Bonding, Presented at a Symposium Sponsored by Picatinny Arsenal and held at Stevens Institute of Technology, September 14-16, 1965, pp. 57-73, Wiley-Interscience, New Yoik, 1966. 

The finite element (FE) technique is an approximate numerical method for solving differential equations. Within the field of adhesive technology, it is most commonly used to determine the state of stress and strain within a bonded joint. It can also be used to determine moisture diffusion, natural frequencies of vibration and other field problems. Although this article will concentrate on the stress analysis, the same concepts can be applied to these other applications of finite element analysis. The basis of any finite element method is the discretization of the (irregular) region of interest into a number of... 

The coefficient of expansion of a polymer is very much greater than that of a metal. Such a mismatch of expansion will introdnce stresses within an adhesive joint, especially if the joint is cured at an elevated temperatnre. The measnrement of the expansion coefficient can be carried out by a variety of techniques, and one of the simplest is thermomechanical analysis (TMA), where the dimensions of a small sample are measured over a range of temperatures. The change in the sample dimension gives the variation in expansion coefficient with temperature. 

The stress analysis of bonded joints remains in its infancy and, until it is better developed, simple shear strength values such as those given in Figure 2.16 (and typically provided by most manufacturers) should not be used alone to assess the ability of an adhesive to transmit power (see pp.l6 22 Supplementary role of adhesives in power transmission). 

Sawa, T., Yoneno, M., Motegi, Y. (2001). Stress analysis and strength evaluation of bonded shrink fitted joints subjected to torsional loads. Journal of Adhesion Science and Technology, 15(1), 23-42. 

This chapter will discuss the testing, analysis, and design of structural adhesive joints. Adhesive bond test techniques to be considered include tensile, shear, peel, impact, creep, and fatigue. Some considerations will also be given to the effect of environment and test rate. A continuum approach to the analysis of adhesive joints will discuss tensile, shear, and peel stresses which arise in various joint geometries. Classical theories by Volkersen, Goland and Reissner, and others will be included. References to finite element analysis will be made where appropriate throughout the chapter. 

Many researchers have performed subsequent analysis of lap shear joints.Plantema combined the results of Volkersen and Goland and Reissner and also included shear effects.Cornell analyzed the lap shear joint and characterized the adherends as simple beams while considering the adhesive to behave as a system of shear and tension springs.The stress analysis of an unusual lap joint consisting of circular tubes was studied by Lubkin and Reissner. Lubkin was able to define conditions for uniform stress distributions in both flat and tubular lap joints, specifically when the adhesive is perfectly elastic. ... 

So far all of the analyses discussed or mentioned have assumed linear elasticity of the joint. However, adhesives typically show either plastic or elastic-plastic behavior, depending on the nature of the joint materials. Several investigators have included the nonlinear or time-dependent behavior of the adhesive joint in the stress analysis of single-lap joints. 

As described in Section II of this chapter, there are many types of peel tests available to characterize adhesives. These tests are important because peel stresses arise in the loading of many joint geometries, such as lap joints. Peel tests are severe because they constitute a test of the adhesive in its weakest stress mode. However, the peel test is a comparative test for adhesives and is dependent on many parameters. These parameters, such as peel speed, peel angle, bond thickness, and temperature, must be held constant to obtain valid results. The stress analysis of peeling is highly complicated because of these variable dependencies. 

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