Single-lap test

EN 302-6 (2004) [11] deals with determination of the conventional pressing time. Single lap test samples prepared with various pressing times at different temperature levels are tested according to EN 302-1 [2] until a failure strength exceeding 4 N/mm is found. 

ASTM D2339-82, used for wood substrates, is similar to the single-lap test for metals. Most wood substrates use some form of overlap construction, making performance in shear crucial. The samples usually consist of 0.0625... 

The advantages of the single-lap test are that it is simple, cheap, uses a standard tensile testing machine, and there are a lot of data available for comparison. Its main disadvantage is that the reported nominal shear stress bears little relationship to any intrinsic adhesive property. 

Curiously, this very disadvantage is also an advantage since no-one really believes that the average shear stress means anything fundamental while in other tests with apparently more precisely controlled conditions, it is common for misleadingly definite values to be quoted. Also, the complex stress situation which pertains in the single-lap test makes it quite representative of many structural applications and loading situations. This test is widely used, often abused, but remains one of the most trusted standards. 

There have been numerous attempts to improve the single-lap test. These include the laminated assembly (ASTM D 3165-73) shown in Fig. 3.2(a), but differential adherend straining and high transverse... 

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. 

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... 

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

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... 

What is the adhesive strength of a single-lap bonded joint with the above dimensions, if the testing machine measures a maximum force of 7500 N at the break of the bonded joint ... 

Only very rarely are bonded joints stressed exclusively under normal conditions. For this reasons, testing under the environmental effects of temperature, natural and artificial climates are required. Ageing tests are usually carried out on standardized test pieces, mainly on single-lap bonded joints exposed to respective environmental conditions and then tested according to the provisions of the respective standards. 

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. 

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 . ... 

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. 

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]. 

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... 

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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