Bond line thickness bonded joints

Davies P, Sohier L, Cognard lY Bourmaud A, Choqueuse D, Rinnert E, CrracTicadec R (2009) Influence of adhesive bond line thickness on joint strength. Int I Adhes Adhes 29 724—736 Davies P, Choqueuse D, Bourbouze G (2010) Micro-tomography to study high performance sandwich structures. J Sandwich Struct Mater. doi 10.1177/ 1099636209344273... 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

Thickeners, fillers, and thixotropic agents are also commonly used to control flow and the bond line s thickness within a joint. Scrims, carriers, and woven reinforcements are other methods commonly used to control bond line thickness. 

The viscosity of epoxy resins and curing agents can be used to control the bond line thickness within the adhesive joint. But the bond line can also be regulated by the incorporation of fillers, by the use of scrim cloth or woven tapes as internal shims within the adhesive itself, or by the careful regulation of the cure cycle. 

It can provide for a practical and reproducible bond line thickness in the final joint. 

The type and amount of fillers are chosen so that a practical bond line thickness will result after application of the necessary pressure (usually only contact pressure, approximately 5 psi) during cure. Ordinarily, the objective is a bond line thickness of 2 to 10 mils. Consideration, of course, must be given to the curing temperature. Viscosity of the formulation could drastically be reduced at elevated temperatures, and unless there is a furrow designed into the joint to contain the adhesive, much of the adhesive could flow out of the joint area before the adhesive is completely cured. 

Application requires a relatively high degree of care to ensure nonwrinkling and removal of separator sheets. Films are often supported on scrim that distributes stress in the cured joint and ensures a uniform bond line thickness throughout the bonded area. 

The amount of the applied adhesive and the final bond line thickness must be monitored because they can have a significant effect on joint strength. Curing conditions should be monitored for pressure, heat-up rate, maximum and minimum temperatures during cure, time at the required temperature, and cool-down rate. The primary concerns are to ensure the following ... 

Results of experiments on S/I/S free films and on S/I/S bonded joints with thermoplastic glue line thickness of 0.3, 0.5 and 0.7 mm appear together in Fig. 4, previously published (3) All preparations were made with the 50% styrene content polymer. The preparation temperature was... 

It is highly desirable to have a uniformly thin (0.05-0.25 mm) adhesive bond line. Starved adhesive joints (where some areas have no adhesive), however, will result in poor bonds. Three basic methods are used to control adhesive (bond line) thickness ... 

Figure 2.10 Effect of bond line thickness on the peel strength of a toughened epoxy-based adhesive. Although not directly proportional, the effect is substantial and of considerable practical importance because bond line thickness is often difficult to control. In most cases the overall result of an increase in bond line thickness is beneficial, even though the shear strength may fall most joints fail because they cannot meet peel and cleavage overloads. Generally shear overloads are rare.

Keywords Adhesive modulus Adhesys expert system Co-axial joints Compression Concealed joints Creep Elastic limit Epoxy Epoxy composite Einite element analysis Glue line thickness Goland and Reissner Hart-Smith Heat exchanger Hooke s Law Joint designs Joint thickness Lap shear strength Peel Plastic behaviour Polyurethane Pipe bonding Shear stresses Shear modulus Stress distribution Thick adherend shear test Tubular joints Volkersen equation Young s modulus... 

Consider a lap joint with an overlap of 25 mm and a length for the purposes of stress calculation of only 25 mm. For the initial assessment two pieces of mild steel of thickness 1.5 mm are bonded together using an adhesive of modulus 0.4 GPa, an elastic limit of 19 MPa and a glue line thickness of 0.05 mm. When a high load of 3000 N is applied, the distribution of stress is calculated to be as follows (Fig. 11). 

The load-carrying capabilities of adhesive joints decrease as bond-line thickness increases. 

Increasing bond-line thickness affects the failure mode of bonded joints. 

The three-dimensional models predict that the stress level can be reduced if the bond line thickness is increased from 25 to 75 or even 175 xm. However, the curves of Figure 50 show, e.g. that the maximum shearing stress decreases by a factor of two, from 33 to 17 MPa, when the thickness of the adhesive layer increases from 25 to 100 xm. A bond line thickness of 50-75 xm is generally recommended for the die attachment because of the negligible thermal impedance penalty. The experimental results indicate that, between 20 and 80 xm, the thickness of the adhesive joint does not greatly affect the thermal transfer capability. This behaviour has been explained by the fact that the interfacial thermal resistances between the adhesive and both the die and the substrate are much higher than that contributed by the bulk thermal conductivity of the adhesive materials. 

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