Test procedures, bonded joints

MIL-STD-304 is a commonly used accelerated-exposure technique to determine the effect of weathering and high humidity on adhesive specimens.66 In this procedure, bonded panels are exposed to alternating cold (-54°C) and heat and humidity (71°C, 95 percent RH) for 30 days. The effect of MIL-STD-304 conditioning on the joint strength of common structural adhesives is presented in Table 15.20. However, only relative comparisons can be made with this type of test it is not possible to extrapolate the results to actual service life. 

Proof of durability and safe performance are, rightly, onerous requirements for any innovations in the construction industry. The parameters affecting environmental durability have been summarised, and water has been identified as the most hostile environment for bonded joints that is commonly encountered. Identification of the general failure mechanisms is useful because it highlights the procedures necessary for the satisfactory fabrication of reliable and durable bonded joints. It also enables the development and adoption of appropriate test methods, since real joint configurations are of limited use in assessing experimentally environmental effects (e.g. bonded areas must be minimised in order to allow environmental access within a reasonable time-scale). Fracture mechanics methods. 

Appropriate test methods for the control of fabrication procedures and non-destructive testing (NDT) are basic requirements for the formation of structural adhesive joints. Such methods should be based upon relatively easily measurable parameters that have a close identity with the properties of the bonded assembly that need to be controlled. However the quality of bonded joints depends upon many factors, requiring a range of very different procedures. 

Secondly, the layered structure causes interlaminar shear stresses at each layer interface together with other in-plane shear stresses. Therefore, it should also be verified that the allowable in-plane shear stress of the adherend is not exceeded. This is not included, however, in the conditions above, as the interlaminar shear failure is typically preceded by a throughthickness tensile failure. The required value of the interlaminar shear strength is also seldom available, and there are no standardised test procedures to determine such a value. As a result, bonded joint induced interlaminar shear stresses are not calculated in the design procedures presented. 

Table 13 shows that the design procedures for bonded joints in the EUROCOMP Design Code seem to provide a reasonable conservatism in the joint design when short term static loadings are considered. However, owing to the relatively thick adhesive layers in most of the test joints, the applicability limits of the current theory may have been exceeded. This might have caused some inaccuracy in the calculated joint strengths. 

Based on the test programme the EUROCOMP Design Code provides acceptable procedures for the design of bonded joints. However, the test programme provided only a limited amount of information on the behaviour of bonded joints. Excessive generalisations on the basis of this test programme should be avoided. 

No account has been taken so far of the thickness of the adhesive within the joint line, but conventional wisdom, based on standard test procedures, dictates a bond line that is as thin as possible. While, to a degree, this is equally true for both the older types of conventional high-strength - though brittle - adhesives and the new toughened , non-brittle adhesives, the peel strength of the latter increases with bond thickness, as shown in Figure 2.10. And, it should be borne in mind that real joints are not usually stressed like standard test joints. 

The higher inherent corrosion resistance of aluminum alloys in saltwater, as compared to steel, has led to the use of a wet/dry saltwater immersion-type test being used at the Alcoa Laboratories instead of the ASTM B 117 procedure. In fact, the continuous immersion of aluminum bonded joints in 3.5% sodium chloride solution or even natural seawater may not deteriorate such joints as much as distilled or deionized water exposure. The amount of... 

In this test, small spots of the adhesive are placed onto a substrate. Surface preparation, application procedures, and curing conditions are to be as similar as possible to those used in the quantitative test and/or the actual adhesively bonded Joint. The adhesive spots are allowed to cure according to the manufacturers specifications. To test adhesion, this ASTM standard practice recommends the use of a thin stainless steel spatula or similar probe as a prying lever to assess the relative difficulty of removing the adhesive from the substrate. If the results are acceptable, standard quantitative adhesive test procedures can be used to obtain quantitative measurement of the adhesive s performance. 

When one or more adhesives have been chosen as candidates for a given bonding application, samples should be obtained and joints, representative of intended production, made and tested. Testing will usually be done to measure strength and/or durability. However, testing for physical and working properties may also be desirable. Some test procedures call for equipment not routinely available in all user laboratories. In those cases, the services of an independent testing laboratory should be considered. 

D-4299 and D-4300. Procedures for measuring the deteriorating effects of bacteria and molds, respectively, on joint strength (see details under durability testing for wood joints). Also useful for metal bonding adhesives which might be susceptible to molds or bacteria. 

Strength Under Exposure to a Hostile Environment. Ideally, a durability test should measure an adhesive bond s retention of strength and bond while exposed to the elements of its service environment. However, the execution of such tests can be physically and procedurally quite difficult. The following two are D-14 s only durability standards which are designed to test strength while joints are under exposure. 

There have, for decades, now, been reliable analytical tools available for the design and analysis of adhesively bonded joints. Even so, there is still far too much reKance on the oversimplified model whereby the bond strength is assumed to be the product of some fictitious uniform adhesive allowable shear stress and the bond area. If more joint strength were needed, all one had to do, according to this procedure, was to increase the bonded area. Bonded joints do not obey such rules. Such a formula is valid today only in the context of short-overlap test coupons in which the goal is to create as closely as possible a uniform state of... 

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