Double lap

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

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

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

The measurement method described in this article is an embodiment of the non-resonance, direct-force-excitation approach that subjects a double-lap shear sample of damping polymer to force from a vibration shaker. In concept this approach can be applied irrespective of whether the material is in a rubbery, glassy, or intermediate state. Each material specimen is small in size and behaves as a damped spring over the entire frequency range. The small specimen size is in contrast with some alternate approaches in which the specimens have sufficiently large dimensions to be wave-bearing. 

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. 

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. 

BOND PARAMETERS AFFECTING FAILURE OF CO-CURED SINGLE AND DOUBLE LAP JOINTS SUBJECTED TO STATIC AND DYNAMIC TENSILE LOADS... 

Fig. 2. Photograph of the co-cured lap joint specimens, (a) Co-cured single lap joint (b) co-cured double lap joint.

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... 

Bond Parameters Affecting Failure of Co-Cured Single and Double lap Joints... 

In this study, residual thermal stresses were also eonsidered because co-cured lap joints generally undergo temperature drop (from 120D to 20D) during the curing process. The stress distributions in the co-cured single and double lap joints were analyzed using ABAQUS i.8 to be commercial finite element analysis software [21],... 

Fig. 8. Coordinate system of the co-cured single and double lap joints for calculating stress distributions in the ply-axis.

The co-cured single and double lap joints were modeled as a three-dimensional solid structure. The resin layer was ignored in this analysis because the average thickness of the resin layer... 

Fig. 12. Interfacial out-of-plane transverse stress distribution, a , of the co-cured double lap joint along the interface between steel and composite adherends. (a) [0]i6t stacking sequence and (b) [+45]4S stacking sequence.

Figures 12 and 13 show interfacial out-of-plane transverse and shear stress distributions of the co-cured double lap joints with [0]i6t and [ 45]4s stacking sequences along the interfaces between steel and composite adherends, respectively. It is important to consider interfacial out-of-plane shear stress rather than interfacial out-of-plane transverse stress because of the compressive stress distribution due to the symmetric configuration of the co-cured double lap...

Based on the failure mechanisms and stress distributions at the interface between steel and composite adherends of the co-cured single and double lap joints, tensile load bearing capacities of the two joints were evaluated. Since failure started at the edge of the interface between steel and composite adherends, it is important to consider the failure criterion using interfacial out-of-plane stress distributions at the interface. Three-dimensional Tsai-Wu and Ye-delamination failure criteria were used to predict partial cohesive failure or interlaminar delamination failure in the co-cured single and double lap joints. 

Equation (1) must be less than 1 failure is predicted when F/ is > 1. Based on the three-dimensional Tsai-Wu failure criterion, tensile load bearing capacities of the co-cured single and double lap joints were calculated. 

Fig. 15. Comparison of tensile load bearing capacities of the co-cured double lap joint (fabricated under 0.7 MPa manufacturing pressure) calculated from the two failure criteria with experimental results.

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