Single Lap Shear Tests

In order to understand the aim of surface treatments for composite bonding, we wiU concentrate on a real composite bonding problem for aeronautical purposes. Since classical surface treatments like peel ply can be limited by a cohesive failure occurring in the material, we wiU focus on a new kind of surface treatment (excimer laser) which can completely change surface parameters. The different aspects are presented in two steps the first consists in the surface characterization of the composite material and the second is related to results of destructive single lap shear tests of composite assemblies. Finally, both steps are Hnked in order to derive general mles on phenomena governing adhesion properties of polymer composites. 

Generally, multiple experimental test data of adhesive material are necessary for an adequate representation of the joint behavior under loading. Uniaxial tension, compression and single lap shear tests were therefore performed. 

Plastic substrates constitute another variation of the single-lap shear test. ASTM D3163-73 addresses the lap shear testing of rigid plastics. This test is important because plastics are becoming more useful in structural applications involving adhesives instead of mechanical fasteners. 

A variant on the above single-lap shear tests is where one, or more, holes are drilled through the centre of the adhesively bonded overlap in order to try to accelerate further the ingress of water, and hence the kinetics of attack [54,55]. However, such holes are not located near the end regions of the lap joint, where the maximum stress concentrations will occur and failure will initiate when the joint is loaded. Whether the drilling of such holes has any significant effect on the kinetics of attack is a much debated topic, but it certainly appears that any resulting increase in the kinetics of attack may in some cases be relatively small [56,57]. 

EN 2243 1 Structural adhesives Single-lap shear test method. 

Fig. 7 Anisotropic deformation of Sn-Ag3.8-Cu0.7 due to shear strain. Straight lines have been etched across the solder joint of a single lap shear test specimen with FIB and have been deformed during the test. Courtesy of P. Jud, EMPA...

In applications where possible degrading elements exist, candidate adhesives must be tested under simulated service conditions. Standard lap shear tests, such as ASTM D1002, which use a single rate of loading and a standard laboratory environment, do not yield optimal information on the service life of the joint. Important information such as the maximum load that the adhesive joint will withstand for extended periods and the degrading effects of various chemical environments are addressed by several test methods. Table 15.2 lists common ASTM environmental tests that are often reported in the literature. 

Figure 3. Test-prediction comparisons for 3mm adherend single lap shear specimens.

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 ). 

In some cases a silane (A-187 Union Carbide) was applied to the A1 adherends as a coupling agent for better chemical bonding. Single lap shear (SLS) joints were primed with all four PAMAMs, bonded with epoxy or PU adhesives, and tested in shear according to ASTM D-1002 at a loading speed of 2 mmmin . ... 

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... 

Single Lap-Joint Specimen Test Results, is intended to give insight into the interpretation of the results from all the lap shear tests. At other points in ASTM Vol. 15.06 and in this chapter, reference is made to other standard tests that use shear-type specimens to explore moisture, and other environmental, fatigue, and creep effects in adhesive joints. 

Lap shear tests are close to peel tests however, this method is more quantitative in the base. Two sheets are bonded to each other by toughened adhesives (acrylic) in the test configuration and tensile tester applies load at a rate of usually 1 mm/min [96]. The lap shear specimen represents the most utilized geometry for studies of adhesive bonding because specimens as single- or double-lap shears (Fig. 8.13a, b) are easy for the measmement [97]. 

A total of 45 single lap shear specimens made of beech wood were produced by the adhesive manufacturer as described in Section 3.2. The samples were glued with the five adhesive types used in the delamination test 3 plastic adhesives no. 009, 013 and 014, and 2 elastic adhesives no. 062 and 071. The adhesive layers of the lap shear specimens had the following nominal thicknesses 0.5, 1.0 and 3.0 mm. For each adhesive type and thickness batch, 3 specimens per batch were tested. 

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... 

EFFECT OF ELASTOMERS ON ADHESIVE PROPERTIES Single Lap Shear Bond Testing... 

Pure shear stresses are those which are imposed parallel to the bond and in its plane (Figure 11.1). Single-lap shear specimens do not represent pure shear, but are practical and relatively simple to prepare. They also provide reproducible, usable results. The preparation of this specimen and method of testing are described fully in ASTM D1002-01. Two types of panels for preparing multiple specimens are described. ... 

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). 

There are three conunonly used test configurations tensile, shear and peel. Probably the conunonest test piece used is the lap shear test piece. This comes in two principal forms, the single- and the double-lap joints. The advantage of this test piece is that it can be easily... 

Basic metal-to-metal single-lap joint test giving the single-lap-shear strength ... 

Figure 17. Single-lap shear fatigue test setup.

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