Bulk adhesive tests

Thin-film versus bulk adhesive testing... 

A stress-strain diagram identifying upper and lower stress-whitening stress limits, tru and respectively, as well as the visually observed stress-whitening stress. The modulus of elasticity, E, and the elastic limit stress, 0 for the bulk adhesive tested are 3.3 x 10 and 3.5 x 10 psi (2,276 and 24 MPa), respectively... 

Because of the high scattering of experimental results and the great difficulty in reaching the fully cohesive failure of wooden adhesive Joints, a numerical analysis has been made to give a better knowledge of their mechanical behaviour for various parameters (adhesive used. Joint thickness, loading mode, etc...). For the PU resin tested previously in shear, such an analysis has been made on two steps first simulations have been made on bulk adhesive specimen to determine the mechanical behaviour of the resin and the numerical results obtained have been implanted in the FE code CASTEM 2000 [21] for the mTENF bonded specimen loaded by shear. 

Fracture tests on notched bulk adhesive specimen have been made in three point bending to observe the real mechanical behaviour of the resin [16]. The Young s modulus is estimated to be equal to 96 10 MPa and fracture energy measured reaches 3900 600 J/m and the PU resin displayed softening behaviour. 

Fig. 7 Experimental testing device of bulk adhesive specimen.

Macroscopic adhesion tests between two dissimilar materials are essentially fracture tests and, like the cohesive or bulk fracture of a material, they require an understanding of the mechanics in the separation zone and of the... 

It is evident that a large number of parameters are involved in the fabrication and testing of bulk adhesive specimens and adhesive joints these must be controlled if meaningful experimental data are to be obtained. Joint tests evaluate not only the mechanical properties of the adhesive, but also the degree of adhesion and the effectiveness of surface treatments. The standard test procedures listed by ASTM, BSI, DIN and other official bodies are essentially for testing adhesives and surface treatments rather than joints (e.g. Table 4.3). Unfortunately, most of these tests consist of joints in which the adhesive stresses are far from uniform. The designer and the researcher therefore have to select appropriate tests, and to know what the results mean in terms of their own particular investigations and applications. 

In summary, the study of bond durability and, in particular, the development of predictive models is now a mature topic. Most bond durability tests provide only comparative data. Given the large number of adherend-pre-treatment-adhesive/primer combinations available and the range of exposure conditions to which a structure might be exposed, it is difficult to predict durability performance in the general case. There are limitations in the usefulness of current models once failure is observed away from the bulk adhesive. 

Bulk modification of paint and varnish coatings is achieved by the incorporation of various additives in the paint or varnish. In particle adhesion tests on certain coatings modified by this method, simultaneous measurements of wetting angle were made. The results obtained on chlorovinyl KhV-124 and penta-phthalic PF-115 coatings are listed in Table VIII.3, and those on chlorinated PVC coatings in Table VIII.4. 

The evaluation of adhesives by mechanical test methods is complex due to the state in which the adhesives are tested. Common adhesive tests require that the sample actually act as a bonded system formed by the adhesive in question and the appropriate substrates. Contrary to the usual testing of materials in their bulk form, adhesives are usually tested as one component of a system of many parts. An adhesive test actually tests the total bonded assembly, i.e., the substrates, the joint geometry, the interface, the primer coat, the surface preparation, the curing cycle, and the adhesive itself. 

The advantage of including the adhesive s non-linear constitutive stress-strain model, in terms of the accuracy of the FE stress predictions, can be clearly demonstrated with reference to Fig. 10.16. This figure represents the stress-strain curve for a specific resin (i.e. Araldite 420 epoxy) bulk coupon tested experimentally in direct tension inside an environmental chamber where the temperature of the coupon reached 0°C. Path OAB represents the actual measured nominal stress-strain curve, which is characterized by two parts the first part (i.e. OA) is linear, whereas the second part (AB)... 

To facilitate the inclusion of bonded joints within an engineering structure, several stages of suitability must be demonstrated. First the mechanical properties of the adhesive itself must be satisfactory and these are quantified by bulk materials tests. Once knowledge of the material itself is gained, then some idea of how well it performs in a bonded joint is required. This is most often achieved via standard test specimens such as the single or double lap joints. 

In a recent case study (see Svendsen et al, 2007 and also Problem 6.1), in collaboration with a paint company, the adhesion of six different epoxies-silicon systems has been studied. These paints are used in marine coating systems. Some epoxies showed adhesion problems in practice while others did not. The purpose of the study was to understand the origin of these problems and whether adhesion could be described/ correlated to surface characteristics, e.g. surface tensions. An extensive experimental study has been carried out including both surface analysis (contact angle measurements on the six epoxies, surface tension of silicon at various temperatures, atomic force microscopy (AFM) studies of the epoxies), as well as measurements of bulk properties (pull-off adhesion tests and modulus of elasticity). Theoretical analysis included both estimation of Zisman s critical surface tensions and surface characterization using the van Oss-Good theory. 

However, there are still many areas where further work is required. There is a need to confirm that the basic engineering properties that are currently being measured by different methods are yielding similar values and that when tests on the bulk adhesive are undertaken that they give representative properties of the adhesive as a thin layer in a joint. There is also a need to model the complete stress distributions in complex joint designs more readily and accurately, and to be able to undertake conveniently parametric design and fracture prediction studies. In connection with this last requirement, the failure criteria necessary for predicting joint fracture have yet to be established firmly. 

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