Double-lap joints testing

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.

Single- or double-lap joint test Basic metal-to-metal single lap joint test... 

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

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... 

Lap joint tests conducted to yield a rate of shear strain of about 1.0/minute, except for Ref. 87 double lap specimen A adhesive failure C cohesive failure... 

In the absence of more rigorous methods or direct applicable test results, the following method of analysis may be used for simple concentrically and eccentrically loaded double lap joints between structural members, of which at least one must be of glass FRP (provided that the preceding design requirements are met). 

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... 

Tensile-Shear Strength n A measure of the shear strength of an adhesive bond in which two members are bonded in a Lap Joint, then pulled at both ends until the joint fails in shear. The strength is reported as the tensile force divided by the shear area (Pa). A double lap joint may be specified. Many tests of tensile-shear strength are listed among the ASTM Standards (www. astm.org). 

To facilitate the inclusion of bonded joints within an engineering structure, several stages of suitability must be demonstrated. First the mechanical properties of the adhesive itself must be satisfactory and these are quantified by bulk materials tests. Once knowledge of the material itself is gained, then some idea of how well it performs in a bonded joint is required. This is most often achieved via standard test specimens such as the single or double lap joints. 

Wood to wood bonds made with vinyl white glue are also tested with double lap shear test specimen (Fig. 32), in order to avoid cleavage effects at the ends of the joint. In this case the tensile shear resistance will be from 4 to 10 MPa and some wood fiber tear will occur. 

Nevertheless, there have been many attempts to improve the singlelap test. These include the laminated assembly (ASTMD 3165-73) shown in Fig. 80(a). Some of the load non-linearity is removed compared with the simple single-lap joint, but differential adherend straining and high transverse stresses still exist. The advantage of this type of specimen is that it may be taken from the scrap edge of a laminated sheet assembly so that production parameters may be more reliably checked. This type of joint is sometimes wrongly referred to as a double-lap joint. 

Another variation of the lap test is the true double lap illustrated in Fig. 80(b). Even in this case, as was shown in Chapter 2, we still have differential straining of the adherends and the loads are still non-colinear internally, even though the external forces balance. Such joints tend to fail at T where the adhesive is in transverse tension rather than at C where it is in compression. The double butt-strap joint shown in Fig. 80(c) is a form of double-lap joint and again it fails at T. Sometimes, if the gap between the two tension members is small, most of the load is carried directly across the stiff, butt faces. Premature failure ensues here but the joint is still able to carry a substantial load in excess of this. 

Geometry of a specimen of shear test for (a) the single-lap joint, and (b) the double-lap joint... 

D 3528 Test Method for Strength Properties of Double Lap Shear Adhesive Joints by... 

The paper is presented in three parts. First, the tests employed to determine the mixed mode fracture envelope of a glass fibre reinforced epoxy composite adhesively bonded with either a brittle or a ductile adhesive are briefly described. These include mode I (DCB), and mixed mode (MMB) with various mixed mode (I/II) ratios. In the second part of the paper different structural joints will be discussed. These include single and double lap shear and L-specimens. In a recent European thematic network lap shear and double lap shear composite joints were tested, and predictions of failure load were made by different academic and industrial partners [9,10]. It was apparent that considerable differences existed between different analytical predictions and FE analyses, and correlation with tests proved complex. In particular, the progressive damage development in assemblies bonded with a ductile adhesive was not treated adequately. A more detailed study of damage mechanisms was therefore undertaken, using image analysis combined with microscopy to examine the crack tip strain fields and measure adherend displacements. This is described below and correlation is made between predicted displacements and failure loads, based on the mixed mode envelope determined previously, and measured values. 

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