Lap joints

This is the most standard adhesive joint and the one widely used as the standard for testing the performance of adhesives. 

In a standard lap shear joint, if the overlap length is increased, the strength of the joint does not necessarily increase linearly for the same standard conditions (i.e., same adhesive, same substrate, same surface preparation, and so on). 

Shear forces can canse mechanical deformations in the substrates and the appearance of peak stress points this is of particular concern when the components being assembled have a thin cross section and when the materials have a low modulus of elasticity. Failure is most likely to occur at the ends where maximum stress is present. 

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Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Manufacturers offer flanged-end pipe in only a few metals. Otherwise, flanges are attached to pipe by various types of joints (Fig. 10-133). The lap joint involves a modification of the pipe which ma ... 

Threaded slip-on socket welding Lap joint Welding neck ANSIB16.1, screwed (125-lh)... 

Lap-joint flange (with ANSI B16.9 lap-joint stub)... 

Cooling tower Threaded pipe joints Bolts, nuts, washers Partially exfoliated coatings Lap joints in sheet metal Between bushings and shafts on pumps Pump gaskets... 

The anode mountings are welded to lap joints in the yard, and the anodes are installed at a minimum distance of 30 cm from the structure to achieve the most uniform current distribution [1-3]. Nonuniform potential distribution occurs even with this distance. 

Nodal points of the platform require special attention for corrosion protection. Therefore the anodes have to be installed in the vicinity of these points, as indicated in Fig. 16-4. The spacing must be sufficiently large that the welded Joints of the nodes do not lie in the area of the lap Joints. The effort for calculating the optimal distribution with the lowest weight of anodes is considerable and has led to computer programs by which the anode distribution can be estimated [11]. 

Variations on the basic lap joint are preferred to give Large contact areas using uniform thin sections. If at all, the joint should be loaded in shear and... 

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. 

Fig.. 24. Schematic drawing of the visual appearance of the failure surfaces of lap joints prepared from hot-dipped galvanized steel substrates. Reproduced by permission of John Wiley and Sons from Ref. [41].

Concentration cell corrosion occurs in an environment in which an electrochemical cell is affected by a difference in concentrations in the aqueous medium. The most common form is crevice corrosion. If an oxygen concentration gradient exists (usually at gaskets and lap joints), crevice corrosion often occurs. Larger concentration gradients cause increased corrosion (due to the larger electrical potentials present). 

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