Stepped adherends

THE EFFECT OF ADHEREND SHAPE—SCARFED, BEVELLED AND STEPPED ADHERENDS... 

It may be useful to the reader to consider as an example the work by Adams et al (1978c) and some of their hitherto unpublished results. They used finite-element methods to examine the stresses in high-performance composites in symmetrical lap joints with parallel, bevelled, scarfed and stepped adherends. The composite adherends were assumed to be linearly elastic type II carbon fibre reinforced epoxy composites with a 60% fibre volume fraction. The mechanical properties of this material are given in Table 3. 

The application of a primer is an additional step in the bonding process, and it comes with associated costs and quality control requirements. Therefore, primers should be used only when justified. The most likely occasions for a primer to be used are when (1) the adhesive or sealant cannot be applied immediately after surface preparation, (2) the substrate surface is weak or porous, or (3) the adhesive-adherend interface requires additional protection from service environments such as moisture. 

A proprietary alkaline cleaner, Prebond 700, appears to be satisfactory for a number of metal adherends including titanium and is recommended as a versatile one-step surface preparation process.48 A proprietary alkaline etch solution, Turco 5578, is available from... 

On the assumption of a clean and, respectively, pretreated surface, the next step will be the application of the adhesive. It has to be ensured, however, that in areas with adhesive forces emanating from the adherend surface, the adhesive molecules are really able to get closer. Only then can the adhesive distribute itself on the surface, that is, wet the surface despite a, more or less, existing roughness. Furthermore, sufficient flowability of the adhesive is important. A complete and equal wetting of the surface to be bonded is therefore an indispensible prerequisite for the production of a strong bonded joint. Figure 6.5 demonstrates the difference between a low-viscosity and high-viscosity adhesive. 

This work step is mainly required for the obtainment of equal adhesive layers. Here, particularly in the case of small adhesive surfaces as used for test purposes, it is necessary to remove the burr at the test specimen. In the case of larger adhesive surfaces, the rectification of the adherends is a prerequisite for parallel gluelines. 

Recent theoretical studies have become much more complex. New computer-assisted techniques permit the use of finite-element matrix-theory type approaches. The effects of important variables are being determined by parametric studies. More complex joints are also being studied. New adherend materials, including advanced filamentary composites, are also being evaluated. The elastic, low-deflection, constant temperature behavior of scarf and stepped-lap joints has been replaced by elastic-plastic, large-deflection behavior, combined with thermal expansion differences, or curing shrinkage-induced residual stresses. 

The second step, after determining the quality of incoming materials, is adherend surface preparation. Surface preparation must be carefully controlled for reliable production of adhesive-bonded parts. 

In the case of laminate thicknesses below 5 mm the use of scarf and step-lap configurations shall be considered with care, as producing the required shapes on the adherends may become complicated. Also the bonding may become impossible if adequate jigging cannot be provided, see 5.3,5,11. 

As step-lap joints have similar features to lap and donble-lap Joints, every design aspect that has been indicated in the design of lap Joints shonld also be considered in the design of step-lap joints. In step-lap Joints factors snch as fibre orientation on the bond surface still have a major effect on the Joint strength (see 5.3.1.4), while tapering the adherend ends becomes irrelevant. 

P (10) If scarf Joints or step lap Joints are used with adherend thicknesses of less than 5 mm, proper Jigging shall be used during bonding to guarantee an adequate bondline quality. 

The step surfaces should coincide as accurately as possible with the layer interfaces of the adherends (see Figure 5.48). 

The outer steps of any adherend should not have an excessive L/t ratio to ensure that it does not become overloaded by the higher-than-average load transfer near the ends of the overlap. 

There are no simple analytical methods for analysing bonded-bolted joints. The static strength of an undamaged bonded-bolted joint can be analysed identically as for the corresponding bonded joint. When the effect of defects needs to be evaluated, computer codes or finite element (FE) packages are required. Hart-Smith (reference 5.45) has developed a FORTRAN code called A4EK for analysing intact and flawed bonded-bolted step-lap joints with linearly elastic adherend deformations. 

In the case of concrete, the main reasons for preparing the surface before bonding are similar, namely (1) to produce a close fit between the adherends (2) to remove laitance and contaminants and to expose pieces of aggregate and (3) to produce a mechanically sound surface. Any typical sequence of steps in the process of concrete surface preparation should then include the removal of any damaged concrete and its replacement with new material, and the elimination of dust and other contaminants by brushing, air blast or vacuum. In some situations, additional steps like cleaning with a solvent to eradicate specific contaminants and application of a primer may be required. 

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