Single-lap joint

Under the best of conditions, single lap joint samples do not fail in pure shear due to the tensile and peel forces present at the ends of the overlap. These non-shear forces are exacerbated when using thin gauge adherends. Because of this, the lap joint dimensions as well as the testing rate were modified from the ASTM D-1002 standard as a result of earlier work on thin gauge steel adherends. 

Sawyer, J.W. (1985). Effect of stitching on the strength of bonded composite single lap joints. AIAA J. 23, 1744-1748. 

Fig. 4. The nominal breaking stress (Amsier testing machine) of aluminium alloy/epoxy single lap joints as a function of the hydroxyl content of the epoxy raised to the two-thirds power51 (Reprinted from Ref. 51, p.307, by courtesy of Society of Chemical Industry)...

D 4896 Guide for the Use of Adhesive Bonded Single Lap-Joint Specimen Test... 

Fig. 2. Photograph of the co-cured lap joint specimens, (a) Co-cured single lap joint (b) co-cured double lap joint.

In the case of the dynamic tensile test, co-cured single lap joint specimens selected in this paper... 

The bond length of the specimens under cyclic tensile test was 20mm. Cyclic tensile tests were performed under the condition of stress ratio 7 = 0.1 and a loading frequency / = 5 Hz. Cyclic tensile loads applied to the co-cured single lap joint specimens were 30%, 40%, 50%, 60%, and 70% of the tensile load bearing capacity obtained from the static tensile load test. 

Figure 4 shows typical failure surfaces obtained from tensile tests of the co-cured single and double lap Joint specimens. In the case of the co-cured single lap Joint, as the surface preparation on the steel adherend is better, a greater amount of carbon fibers and epoxy resin is attached to the steel adherend. Failure mechanism is a partial cohesive failure mode at the C ply of the composite adherend. In contrast with the co-cured single lap joint, failure mechanism of the co-cured double lap joint is the partial cohesive failure or interlaminar delamination failure at the 1 ply of the composite adherend because interfocial out-of-plane peel stress... 

Fig. 10. Interfacial out-of-plane transverse stress distribution, a , of the co-cured single lap joint along the interface between steel and composite adherends. (a) [0]32t stacking sequence and (b) [ 45]4s s stacking sequence.

Fig. 11. Interfacial out-of-plane shear stress distribution, a , of the co-cured single lap joint...

Fig. 14. Comparison of tensile load bearing capacities of co-cured single lap joint (fabricated under 0.7 MPa manufacturing pressure) calculated from the two failure criteria with experimental results.

D 1002 Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to Metal)... 

As previously mentioned, the single lap joint is the most common test used to evaluate adhesives because of its practical resemblance to many real-world joint designs. The adhesive lap joint has proven useful over the years and will likely continue to be widely used in the future. This paper discusses some of the complexities of the lap joint. The discussion will now concentrate on using finite element analysis to aid in a fracture mechanics approach to aid in understanding the mechanics of a lap joint. It will, to some extent, explore the validity of the design rules discussed above and look at the affect of some of the joint features that are not considered in these rules. 

FIG. 4—Experimental failure load versus bondline thickness for single-lap joint specimens withAraldite 2011 adhesive (from [8]). 

Creep tests have been carried out on single lap joints at different temperatures and loads as indicated in Fig. 33.4. The resulting shear strain y versus time relationships appear as straight lines in a double-logarithmic plot which show the same slope but are shifted by a factor. This indicates that creep data may be represented by a power law of the form of Eq. (2), where t is time and Aq, n and m denote material parameters [9]. 

S-N curves were measured at 23 °C, 5 Hz and a stress ratio of R=0.2 for single lap joints the results are given in Table 33.5. The S-N curves appear as a more or less straight line in a double-logarithmic plot (Fig. 33.6) and can therefore be represented by the power law of Eq. (6). 

Table 33.5 Fatigue test results for single lap joints.

Lap shear tests were carried out using single lap joint specimens of the aluminum alloy (5052H34). Sample dimensions are shown in Fig. 34.6. Surface pretreatment and curing conditions of the joint specimens were similar to the other specimens used for the dismantlement test The lap shear strength of the... 

Fig. 34.11 Fracture surfaces of single lap joints (a) with a grinding paper finish (b) with a grit-blasted finish.

Fig. 34.12 Variation of failure strength of single lap joints with respect to the weight fraction of thermally expansive particles.

In addition, a specific tests for composites ASTM 2733 [35] uses the double notched specimens (c.f single lap joint) to assess the shear strength at elevated temperatures. An extension of this test is in an internal aerospace test [36]. which uses thermal spiking to assess performance. In this test a sample is conditioned at the aerospace standard 70 C 85%RH for two weeks prior to a rapid excursion for a maximum of one minute to a higher service temperature. The specimen is then conditioned under the standard conditions for a further two weeks prior to the evaluation of the weight gain. 

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