Lap-shear joints

St. Clair et. al. investigated a series of maleimide and nadimide terminated polyimides and developed LARC-13 [8,9]. Changing the terminal group from maleimide to nadimide, the value of the lap shear strength of a titanium lap shear joint increased from 7 to 19 MPa [9]. They also added an elastomeric component to the adhesive formulation. The introduction of 15 wt% of a rubbery component, ATBN (amine terminated butadiene nitrile polymer) and ADMS (aniline terminated polydimethyl siloxane) enhanced the adhesive properties as follows 19 MPa to 25 MPa (ATBN) titanium T-peel strength 0.2 kN/m to 1.4... 

Surface cleaning/etches. As with aluminum and titanium, the most critical test for bonded steel joints is durability in hostile (i.e., humid) environments. The fact that the problem is a serious one for steel was illustrated in a study [117] that compared solvent cleaned (smooth) 1010 cold-rolled steel surfaces with FPL aluminum (microrough) substrates. Although the dry lap-shear strengths were not markedly different, stressed lap-shear joints of steel adherends that were exposed to a humid environment failed in less than 30 days, whereas the aluminum joints lasted for more than 3000 days. 

Figure 3. Graph of shear strength vs. exposure time for lap shear joints constructed from untreated, alkaline cleaned or lubricated CRS adherends.

FIGURE 14.7 Illustration of dielectric heating for the bonding of an electrogalvanized steel (EGS)/ SMC lap shear joint.30... 

A comparison of experimentally determined failure times for different stress levels and those predicted by the above equation for epoxy-aluminum lap shear joints aged at 60°C and 95 percent RH is presented in Table 15.3. These results indicate that the reaction rate method is satisfactory for predicting the effects of temperature and stress on the lifetime of adhesive bonds, provided that failure is cohesive within the adhesive. This, of course, should be validated by prototype testing. 

Bodnar, M. J., and Wegman, R. F., Effect of Outdoor Aging on Unstressed, Adhesive Bonded Aluminum to Aluminum Lap Shear Joints, Technical Report No. 3689, Picatinny Arsenal, Dover, NJ, May 1968. 

Fresh pig skin was harvested from the back of a pig within 5 hours postsacrifice and the fat attached to the inside skin surface trimmed away. The skin was cut into 2x1 coupons and covered by saline moist paper towels before use. The external surface of the skin was used as the bonding surface to prepare the lap shear joint samples. 

The coupons were dried prior to forming a lap shear joint. About 100 pi of the experimental adhesive was deposited to one coupon and smoothed to cover a 1/2 X 1 area. Another coupon was placed over the area of the initial coupon and a 1 lb weight placed on top. It was cured for 20 to 30 min and the strength of the joint... 

Another aspect to be considered is the difficulty in producing curved structures with the same fibre content as flat laboratory panels. This effect is shown in Figure 16, at the comer the laminate thickness is larger than at the flat section and fibre content is rather lower. This will affect the bending stiffness of the arm and the predicted failure load. This figure also shows the fillet, which is critical to initiation in the specimens without implanted defects. It is well known that fillets can significantly alter the load path in lap shear joints and increase the failure loads (see [1] and Figure 3 for example). If a fracture mechanics approach is to be applied this effect must be considered. Some recent studies on stress intensity factors for such cases may allow this to be addressed [22]. 

The shear adhesion strength of the PU formulations was tested by single lap shear joints, according to ASTM D 1002-94 (at a rate of 1.3mmmin ). Peel strength was evaluated by T-Peel (ASTM 1876-95) (at a rate of 254 mm min ). 

Figure 4 Standard lap shear geometries (a) simple lap joint test, ASTM D-1002 (b) laminated lap shear joint test, ASTM D-3165 (c) double lap joint test, ASTM D-3528.

To apply an external or internal patch (single lap joint) the surface is cleaned and lightly abraded prior to laying up the repair. Single lap shear joints are not as... 

The traditional view of lap joint failure is old and familiar and was, in essence, employed by Fairbairn(36) who considered the failure mode of riveted joints on iron bridges. However, the topic of bonded lap shear joints is probably best reviewed in detail by... 

Wake(37) and by Adams and Wake(5), and Kinloch(4) summarises the evolution of the approach of the many stress analysts. The most common shear test comprises the single lap shear joint embodied in BS 5350(10) and ASTM 01002-72(11) (Fig. 4.7(a)). With reference to Figs. 4.1(a) and 4.8 it can be seen that the resulting stress concentrations can be extremely large at the joint ends (points X and Y in Fig. 4.8(b)). The analysis of Volkersen(15) predicts that for identical adherends, the elastic shear stress concentration factor, for the adhesive due to adherend tensile strain is given by... 

Lap shear strength. Tests should be carried out over a range of temperatures, specifically including -25 °C, +20 °C and +45 °C. using bright mild steel adherends. The temperature should be measured by means of a thermocouple attached to the steel surface of the joint. Two alternative forms of lap shear joint may be employed. 

Subjected to stress, moisture and temperature uses peel joint As above, but uses single-lap shear joint loaded in tension As above but uses a wedge test Exposure to moisture and temperature... 

The fracture experiments were conducted on single lap shear joints with an overlap length of 1.27 cm, bond area of... 

We have determined that S/I/S triblock copolymers may be prepared to show anisotropic behavior with respect to fracture in adhesively bonded lap shear joints made with rigid substrates. Table 2 summarizes joint strength and fracture energy data for lap shear joints prepared with the orientation (machine) direction of the adhesive layer either parallel or perpendicular to the fracture axis of the joint. The important comparison is that of the fracture energies of the different orientations for the 50% styrene S/I/S copolymer (E2) sample. The high fracture energy for the parallel orientation corresponded exclusively to ductile drawing failure, the perpendicular orientation... 

An adhesive fillet, or the amount of excess adhesive squeezed outside of a lap shear joint on assembly, can also be used as a measure of air inhibition. The less the fillet cure, the greater the inhibition. A porous material, pine wood, was chosen as a substrate that would allow air to remain in intimate contact with the curing adhesive. The ability to lock the threads of an iron nut and bolt without use of an activator is a recognized test for determining whether or not a formulation is capable of anaerobic cure (1.4,5). ... 

ASTM D3163-01 Standard recommended practice for determining the strength of adhesively bonded rigid plastic lap-shear joints in shear by tension loading. 

For example, single-lap shear joints can have a known static load applied, then inserted into a specified environment, and the time-to-failnie of joints noted as a function of variables such as adhesive type, pre-treatment and stress (see Shear tests). Static loads can also be applied to other joint configurations. Many studies have shown that such tests exhibit good discrimination between different surface pre-treatments. The creep performance of adhesives can also be measured by means of static load testing (see Durability creep rupture and Durability sub-critical debonding). 

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