Single-lap shear joints

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 ). 

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... 

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... 

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... 

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). 

It has been shown that in comparison with spot welds, rivets and a variety of selfpiercing mechanical fastenings, toughened structural adhesives may yield single lap shear joints of superior energy absorption when used to bond aluminium alloy sheet the findings were confirmed in tests on full-size bonded vehicles (see Toughened adhesives, Structural adhesives). 

Fig. 2. Effect of pre-treatment and environmental exposure on the energy absorption of single lap shear joints. Environment deionized water 50 °C. Pre-treatments A degreased in acetone, then cleaned in alkali B as A then anodize in 10 vol. percent orthophosphoric acid at 60 V r.m.s. AC for 10 s C as B but anodized for 60 s...

Amanda et al., [15] established a technique which holds promise for detecting damage and evaluating different failure modes in adhesively-bonded composite-to-metal single-lap shear joints. They compared electrical resistance data using carbon nanotubes in adhesive with AE and found good agreement. 

For any load applied to the single-lap shear joint, dividing by the bond area produces an average shear stress. Yet these loads also produce stress concentrators at the ends of the bond area. The ratio of these large local stresses to the average bond stress is termed the stress concentration factor. Volkersen derived a formula for this stress concentration factor, n, given... 

Adhesive shear stresses originate because of different axial deformations of the adherends whereas peel stresses primarily develop due to eccentricity in the load path, as is the case for unstabilized single-lap shear joint configurations. Considerable local stress concentrations near discontinuities in the substrate, the adhesive bond or the FRP strengthening material are the major common attribute for both shear and peel stresses, as illustrated in Fig. 10.1. 

Fig. 23. A comparison of the numerically predicted and experimentally observed deformation of a 2.3-mm-thick single lap-shear joint [57].

Figure 26.18 The single lap shear joint has bending moment and stress concentrations at the ends of the overlap.

Figure 4.6 Single-lap shear joint strength versus critical surface tension, yc, for low-density polyethylene pretreated by various methods [47].

Only a few detailed studies have been reported on the static fatigue of adhesive joints and, even fewer, on modelling the data in order to predict the long-term behaviour of stressed joints from short-term data. Some workers [16-19] have concentrated on studying the single lap-shear joint whilst others [20,21] have used precracked specimens and adopted a fracture mechanics approach. 

Fig. 112. Effect of cyclic stressing on single-lap shear joints aluminium and...

Table 12.3 T values and single lap shear joint properties of ProAssure Wrap Extreme at RT and 55 °C...

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