Thick adherend shear strength

Thick adherend shear strength test piece... 

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First, we must (with one exception) disqualify as shear tests the single-lap joint and its allies described earlier. The exception is the thick adherend shear test. As discussed above, provided proper precautions are taken, this test can be used to determine the shear properties of an adhesive, although the ultimate strength cannot be attained (para. 4.2 of ASTM D 3983-81). 

The tensile and compressive properties were obtained in the bulk form and the shear properties were obtained with the thick adherend shear test (TAST). ( , Young s modulus Cy, yield strength strength tensile failure strain ly, shear yield strength shear strength jy, shear failure strain v, Poisson s ratio)... 

Pinto AMG, Magalhaes AG, Campilho RDSG, da Silva LFM, Chousal JAG, Baptista APM (2010) Shear modulus and strength of an acrylic adhesive with the notched plate method (Arcan) and the thick adherend shear test (TAST). Mater Sci Forum 636-637 787... 

Where S, and are out-of-plane peel and shear strengths between steel and composite adherends, respectively. is out-of-plane shear strength in the plane (i.e. zy-plane) perpendicular to the fiber direction. All the stress components are defined as the averaged stresses over a distance from the free edge, c. Table 2 shows various values used to calculate Ye-delamination failure indices. The distance c was defined as one-ply thickness and S =5 was assumed [23]. 

To enable the determination of almost pure strength values for the adhesive layer, the parameters eccentric application of load and adherend extension/ deformation must be eliminated. This is the case in the test piece geometry depicted in Figure 10.4 according to the standard ISO 11003-2 Shear testing method for thick adherends . 

The centrical force transmission is obtained by gluing sections of the same adherend thickness onto the adherends in the force transmission area. The adherend extension will be eliminated by a larger adherend thickness (5 mm instead of 1.6 mm) and a reduced overlap length. Due to the homogeneous stress distribution, the respective test results are thus based on defined shear stress conditions. Adhesive layer values, to a large extent independent of the adherend, such as shear modulus, shear strength and deformation behavior, are available as a basis for precise calculations. 

ASTMD3933-98 Standard practice for measuring strength and shear modulus of non-rigid adhesives by the thick adherend tensile lap specimen. 

Fig. 1. Comparison of destructive shear strength tests on laminates with the predicted strengths using a Fokker bond tester. A series of laminates is represented where the adherend thickness t varies as follows ( ), t = 0.6 mm (O), t = 0.8 mm (x), f = 1.0 mm (-h), f = 1.2 mm (A), t = 1.5 mm. To construct this standard plot, 120 specimens of Dural 2024-T3 were used 95% of all results are within the range 0.36 kg mm . (From R J Schliekelmann, Non-destructive testing of adhesively bonded joints, in Adhesion, Fundamentals and Practice, McClaren, London, 1966)...

Measuring Strength and Shear Modulus of Nonrigid Adhesives by the Thick Adherend Ihnsile Lap Specimen, Practice for (D 3983)... 

An adhesively bonded structure is considered in shear when the applied loads act in the plane of the adhesive layer. The loads tend to produce sliding of the adherends and this results in sliding or shearing of the adhesive. These shear stresses should not be confused with pure shear stresses, which are typical of uniform blocks of a homogeneous material. Interactions between adhesive thickness, adherend thickness and yield strength, and bond geometry produce nonuniform stresses. When this occurs, the shear bond may actually be dominated by tensile stresses rather than shear stresses. However, in service, adhesive structures rarely encounter these pure shear conditions. Therefore, standard shear tests provide adequate duplication of conditions which may exist in an actual structural adhesive application. 

Since the stress distribution across the bonded area is not uniform and depends on joint geometry, the failure load of one specimen cannot be used to predict the failure load of another specimen with different joint geometry. The results of a particular shear test pertain only to joints that are exact duplicates. To characterize overlap joints more closely, the ratio of overlap length to adherend thickness l/t can be plotted against shear strength. A set of l/t curves for aluminum bonded with a nitrile-rubber adhesive is shown in Fig. 7.11. 

It has been shown that a very simple approach can be used for predicting the lap shear strength of adhesively bonded Joints for ductile adherends and ductile adhesives. However, it cannot take into account such more complex situations as thermal stresses or changes in adherend thickness (stepped or tapered joints). 

It can be seen from the entries in Fig. 2 that the potential adhesive shear strength for the thinnest (0.040 inch) skin is roughly three times the adherend strength outside the joint. But, for the thickest skin shown (0.125 inch), this ratio has fallen to less than 2-to-l. The reason for this is that the adherend strength is directly proportional to its thickness, while the adhesive bond strength is proportional only to the square root of the thickness of the adherends (see [7]). At somewhere between 3/16 and 1/4 of an inch this ratio of strengths falls below unity, which is why the table is tenninated where it is. (Multi-step joints are needed for thicker skins, as is discussed later.)... 

Fig. 20. Effect of adherend thickness on (apparent) lap-shear strength of adhesive bonds.

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