Shear specimen, double lap

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

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. 

Fig. 4a and b must distort, so that the forces applied to the sample fall on the same line of action. This induces cleavage stresses in the adhesive near the bond termini. Double lap shear specimens described in ASTM D-3528 are proposed as a means of alleviating this problem (see Fig. 4c). However, based on our computer analysis and experimental studies, we feel that failure of double lap joints is still dominated by cleavage stresses [5].

The commercially available MTS Laboratory Automation System was the major tool used in mechanical characterization.The MTS system, as shown in Figure 7, is equipped with a computer-controlled servohydraulic capability. It is a very versatile mechanical tester which can be used to test specimens of a variety of geometries under a broad range of conditions of temperature, frequency, and strain. The double lap shear specimen used to obtain dynamic mechanical properties data is shown in Figure 8. 

Experimental load-displacement history and creep data were used to estimate the partitioned viscoplastic constitutive properties. Load isplacement loops from double lap-shear tests were used in conjunction with finite element (FE) models to refine the estimates of the effective viscoplastic constitutive properties of the three Pb-free solders and eutectic Sn Pb. The constitutive equations for the four solders were used as initial estimates when iterating to match experimental and predicted hysteresis loops. That is, experimentally determined double lap-shear specimen load-displacement hysteresis loops were compared with the FE simulations. The constitutive properties were then adjusted iteratively to improve agreement the equations and properties are presented in Table 16. 

Temperature, load, strain data from double-lap shear specimens simultaneously subjected to in-phase mechanical and thermal cycles —55 to + 125°C (—55 to +90°C for Sn-58Bi alloy), 6-min ramp time, 3-min dwell time 2.9x10 deformation rate Solder alloy creep data... 

Measurement of the adhesion between two surfaces is often performed by a tensile test of a lap shear or double-lap shear specimen. The configurations of the tests are shown in Figure 11.3. Historically, due to its ease of sample construction and... 

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...

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... 

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. 

Lap shear tests are close to peel tests however, this method is more quantitative in the base. Two sheets are bonded to each other by toughened adhesives (acrylic) in the test configuration and tensile tester applies load at a rate of usually 1 mm/min [96]. The lap shear specimen represents the most utilized geometry for studies of adhesive bonding because specimens as single- or double-lap shears (Fig. 8.13a, b) are easy for the measmement [97]. 

Fig. 10. Double-lap shear adhesive specimen configurations (ASTM D3528).

Figure 14.6 Schematic of the double lap shear test specimen.

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. 

FIGURE 11.3 Lap shear specimens, (a) Single lap shear and (b) double lap shear. 

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... 

Several testing techniques are possible which use different specimen configurations, different loading modes, and various loading frequencies. Marceau et al. have described results for thick adherend lap shear and double cantilever beam (DCB) specimens exposed to 140 F/100%... 

The lap-shear stress distribution, the failure pattern and ultimately the bond strength of FRP joints are also functions of the mechanical properties of the FRP reinforcing fibres. This behavioural dependency is depicted in Fig. 10.3 where lap-shear stress distributions along the bondlength for two identical double-strap CFRP/steel specimens, with different elastic moduli of their reinforcing CF (carbon fibres), are presented. 

This fact is demonstrated in the lap-shear stress distribution for the CFRP/steel double-strap specimen of Fig. 10.3(a) where the outer adherend... 

Lap-shear stress variation with exposure temperature for identical double-strap CFRP/steel plate specimens (Al-Shawaf, 2010). 

Figure 10.18(a) and (b) displays and validates the FE lap-shear stress predictions, with the same experimental double-strap CFRP/steel specimens discussed in Section 10.6.1, along the CFRP/steel joint bondline. The lap-shear stress divergence between the FE predictions and experimental results disclosed in Fig. 10.18(a) close to the x = 0 end is due to erroneous reading of the ERSG in this location, and is discussed in Al-Shawaf (2010). 

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

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