Lap shear specimen

Fig. 2. Illustrations of forces to which adhesive bonds are subjected, (a) A standard lap shear specimen where the black area shows the adhesive. The adherends are usually 25 mm wide and the lap area is 312.5 mm. The arrows show the direction of the normal apphcation of load, (b) A peel test where the loading configuration, shown by the arrows, is for a 180° peel test, (c) A double cantilever beam test specimen used in the evaluation of the resistance to crack propagation of an adhesive. The normal application of load is shown by the arrows. This load is appHed by a tensile testing machine or other...

The principal type of shear test specimen used in the industry, the lap shear specimen, is 2.54 cm wide and has a 3.23-cm overlap bonded by the adhesive. Adherends are chosen according to the industry aluminum for aerospace, steel for automotive, and wood for constmction appHcations. Adhesive joints made in this fashion are tested to failure in a tensile testing machine. The temperature of test, as weU as the rate of extension, are specified. Results are presented in units of pressure, where the area of the adhesive bond is considered to be the area over which the force is appHed. Although the 3.23-cm ... 

The slow rate of hydration for buried surfaces is desirable from a service point of view, but makes the study and evaluation of the durability of surface treatments difficult unless wedge tests (ASTM D3762) or similar tests are used to accelerate the degradation. As for the wedge test, the stress at the crack tip, together with the presence of moisture at the tip, make this a more severe test than soaked lap shear specimens or similar types and therefore a better measure of relative durability. 

Fig. 27. Fracture toughness of cracked-lap shear specimens made with FM-x5 adhesive. Adapted from Ref. [109].

Forced sinusoidal uniaxial tension and shear imposed by mechanical drive to tensile bar or double-lap shear specimen... 

Actual joint preparation involved first acetone-degreasing the steel lap shear specimens for about 30 min (Independent evaluation via XPS found the metal surface to be 99% I C.O along with some adventitious carbon.) These joints coated with coupling agent underwent the following procedure ... 

ASTM D 1144 provides a recommended practice for determining the rate of bond strength development for either tensile or lap shear specimens. However, peel and can-teliever tests can also be used effectively. Measured bond strength values of partially cured test specimens are compared with those of a reference (i.e., fully cured adhesive joint) to assess the extent of cure. This method may suit some applications, but it is limited in accuracy because it does not directly measure the degree of cure in the adhesive, and the effect on the joint design and substrates may override the effect of cure development. 

FIGURE 20.5 Standard lap shear test specimen design (a) form and dimension of lap shear specimen (b) standard test panel of five lap shear specimens. (From ASTM D 1002.)... 

The width of the lap shear specimen is generally 1 in. The recommended length of overlap, for metal substrates of 0.064-in thickness, is 0.5 + 0.05 in however, it is recommended that the overlap length be chosen so that the yield point of the substrate is not exceeded. In lap shear specimens, an optimum adhesive thickness exists. For maximum bond strengths, the optimum thickness varies with adhesives of different moduli (from about 2 mils for high-modulus adhesives to about 6 mils for low-modulus adhesives).5... 

The lap shear specimen can be used for determining the shear strength of dissimilar materials in a manner similar to that described for the laminated button tension specimen. 

Two other variations are used to avoid the bending forces that occur with simple ASTM D 1002 specimens the laminated lap shear specimen (ASTM D 3165) shown in Fig. 20.6a and the double-lap specimen (ASTM D 3528) shown in Fig. 20.6b. These specimens minimize the joint eccentricity and provide higher strength values than does the singleoverlap specimen. For the specimen in Fig. 20.6a, the overlap joint can be made from saw cuts in the top and bottom substrates of a bonded laminate. This process negates the effects of extruded adhesive at the edges of the lap and the sheared edge of the standard type of lap shear specimen. As a result, the chances of deformation and uneven surface preparation are lessened. 

Compression shear tests are also commonly used. ASTM D 2182 describes a simple compression specimen geometry and the compression shear test apparatus. The compression shear design also reduces bending and, therefore, peeling at the edges of the laps. Higher and more realistic strength values are obtained with the compression shear specimen over the standard lap shear specimen. 

FIGURE 20.6 Modified lap shear specimens used to maintain axial loading (a) single-saw-cut specimen, (b) double-lap specimen.6... 

In order to examine the application of this approach to composite assemblie.-, tests were performed on single and double lap shear specimens as shown in Table 1. Test results were compared with predictions based on reference [5]. Figure 3 shows representative results for both adhesives. 

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

Fatigue Life. Lin et al. (Ref 13) investigated fracture and fatigue mechanisms of spot friction welds in aluminum 6111-T4 lap-shear specimens. A concave tool was used to make the spot welds. Optical and scanning electron micro-... 

Lap-shear specimens were first tested by using an Instron 4502 testing machine at a monotonic displacement rate of 1.0 mm/min. The tests were... 

Fig. 1 2.1 3 (3) Failed spot friction weld lap-shear specimen, (b) Top view of a spot friction weld on the upper sheet of the failed...

Fig. 12.20 A lap -shear specimen and the fixture are ° mounted in an Instron fatigue testing machine.

Static Strength Evaluation. Ami et al. (Ref 18) investigated the microstmctures and failure mechanisms of spot friction welds in aluminum 5754 lap-shear specimens. In this investigation, aluminum 5754 sheets with thickness of 1.0 mm were used. The lap-shear specimens are made by using two 25.4 by 101.6 mm coupons with a... 

Fig. 12.22 (a) Schematic plot of the cross-sectional symmetry of a lap-shear specimen made by a concave tool, with asheetthick-... 

Failure Mode. Figure 12.24 shows a cross-sectional view and close-up views of a spot friction weld made by a concave tool with a depth of 1.95 mm in a partially failed lap-shear specimen (Ref 18). The two arrows in Fig. 12.24(a) schematically show the loading direction. In Fig. 12.24(a), near the upper right portion of the... 

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