The Simple Lap

When we wish to assemble two pieces of thin metal by overlapping them, it is clear that a relatively thick glue line especially at the ends of the joint will allow for a significant increase in joint capability. When metal sheets are guillotined or 

In the example of poor joint design, the distorted metal creates a favourable thick glue line over most of the joint, but minimises the thickness at the critical joint ends. If a separation dimple is placed in the middle of the joint (the low stress area), this will control the glue line without compromise of joint strength (see Fig. 39). 

A normal glue line thickness, without glue line control, is often less than 0.025 mm. In order to cope with high stresses, the glue line should be much thicker, in the region of 0.2 to 0.3 mm. Some adhesives are available today, which automatically control the glue line to a specific thickness. However, the use of dimples can have benefits in situations where electro-deposition of paint is important. Adhesives generally provide electrical insulation between components which would prevent the deposition of paint. 

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Lap joints are the most commonly used adhesive joint, because they are simple to make, are applicable to thin adherends, and stress the adhesive to be stressed in shear. However, the simple lap joint causes the adhesive to be stressed in shear. In this design, the adherends are offset, and the shear forces are not in-line, as was illustrated in Figure 7.15. This factor results in cleavage stress at the ends of the joint, which seriously impairs its efficiency. Modifications of lap-joint design (Figure 7.17) include ... 

The simple lap joint may not perform well in load-bearing structures when other considerations dictate the use of untoughened adhesives, because the distortion illustrated in Figure 2.29 may cause brittle failure at quite low load levels, making the joint collapse catastrophically. 

This excellent variant of the simple lap joint is effective for bonding metal to metal (particularly sheet metal) and metal to plastics. Unless gross distortion causes peel and cleavage, normal structural loads induce compression forces in the critical region indicated in Figure 2.31, ensuring very robust joints - particularly with toughened adhesives. 

It is now opportune to illustrate some of the points made above by considering examples of simple joints and how these can be treated. There is no point in trying to re-invent the wheel. We will therefore look critically at the existing literature and theories on adhesive joints rather than present an apparently all-pervading solution without discussing alternatives. First we will study the simple lap joint with parallel adherends. All parameters will be considered elastic, but not necessarily linear we will then allow for some unexpected effects at the ends of the bond-line and, finally, consider the case where the adhesive and/or the adherends may become plastic. Modifications of the simple lap joint, together with butt, tube, peel and other geometries, will also be considered. 

So far, we have only considered parallel-sided adherends in single- and double-lap joints. It has been shown that the mathematical treatment, whether it is by closed-form analytical methods or by finite-element techniques, is difficult if realistic results are to be obtained. For instance, it is essential to allow for adhesive and adherend plasticity if joint strength predictions are to be made. But, as shown in Fig. 5, there are several forms of the lap joint in which the adherends are not parallel-sided, constant-thickness sheets but can have a variety of forms. These deviants are an attempt to reduce the high stress and strain concentrations, which occur at the ends of the simple lap joint, by modifying the stiffness of the adherends. In these profiled joints, the load line direction must change and, in addition to the tensile stiffness, so the shear and bending stiffness of the adherends change. 

The lap joint and its variants (Figs 7.12a-g). The simple lap joint and its several variants are very often used, particularly in the assembly of large load bearing structural components. The joint s effectiveness depends upon the materials involved, the geometry of the assembly and the scale and disposition of the forces to be borne. 

Fig. 7.12. The lap joint and its variants (a) simple lap (b) tapered lap, (c) rebated lap, (d) strapped lap (e/f) double strapped and tapered lap (g) stepped, strapped lap. These various forms of the simple lap joint have been developed in order to reduce peel and cleavage forces and to maximise performance some lend themselves particularly well to fabrication in composite but some, as is obvious, could be very expensive to fabricate in metal.

Until now, all in the literature proposed LAPS devices are complete autarkic measurement systems. Further applications can be found by the integration of LAPS devices into existing analytic fields. This requires the development of inexpensive integrated electronic units to operate the LAPS and to provide a standardised communication with higher process levels. The LAPS devices need to be easy in use to allow the operation in commercial environments. Due to the simple structure of the LAPS, the integration into micro-electro-mechanical systems (MEMS), lab-on-chip and micro-total analysis systems (p-TAS) might be of special interest in the near future. 

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. 

Threadless Tools. A simple lap welding tool consisting of a tool with two shoulders (Fig. 2.21) was developed by TWI and designated the Multistage tool (Ref 100). The first shoulder rested on the top surface of the overlapping plates. The second shoulder was located at the interface between the two lapped plates and was designed to disrupt the oxides at the lap joint interface. A variation of the Multistage tool was later used to friction stir weld 2.4 mm (0.09 in.)... 

A simple nondestructive capacitance method is proposed (Adamyan et al, 2006) for the determination of basic PSi parameters such as layer thickness, porosity and dielectric permittivity. The method is based on two comparative measurements of the capacitance of the metal/PSi/single crystalline silicon/metal structure one measurement is taken when there are air-filled pores, while the other measurement involves pores filled by an organic compound with a high value of dielectric permittivity. Comparison of results obtained in Adamyan et al. (2006) by the ball lap and the gravimetric techniques before and after anodization, with the data of capacitance measurements carried out with the same samples prior to their destruction, shows sufficiently good agreement. 

The elastic adhesive layer 027, on the other hand, had no hardening problems. The simple explanation is that this adhesive was already a commonly used product hence the manufacturer had considerable experience in its handling. In fact, the small beech specimens for the shear lap tests had already been made with adhesives manufactured under normal factory conditions. 

The other coefficients have the simple form only for lAp 1 (Ip is correlation length forPf) ... 

Typically, the yardstick for qualitatively measuring the internal resistance of an adhesive bond to an external load has been the determination of the strain distribution in the adhesive and adherends. This is a difficult task. Even in simple lap joints, the actual stress-strain distributions under load are extremely complex combinations of shear and tensile stresses, and are very prone to disturbance by non-uniform material characteristics, stress concentrations or locaUzed partial failures, creep and plastic yielding, etc. It is extremely difficult to accurately measure the strains in adhesive joints with such small glue Une thicknesses and such relatively inaccessible adhesive. Extensometers, strain gauges, and photoelasticity are being used with limited success." ... 

Figure 2.31 The rebated (joggled) lap joint. When shear loaded, the correct geometric alignment of the adherends inhibits the generation of tensile forces, normal to the plane of the joint, (at X) which may cleave a simple lap joint prematurely (see Figure 2.29b). This type of joint is particularly useful for sheet metal assembly.

Figure 2.33 The strapped lap joint, a Simple and cheap joint but poor if there are any peel and cleavage forces generated by distortion, b Simple and cheap joint but it requires access to both sides of a structure. It can give a very robust performance, c An even more effective form but one which can involve considerable fabrication costs if metal machining is required and extrusions cannot be employed (see Figure 2.35).

Figure 2.36 The scarf and tapered lap joints. While effective in appropriate designs, these joints are always more expensive than simple lap joints - especially when formed from metal. Often the scarf will be quite impracticable.

Of all adhesives, only anaerobics have been formulated to give reproducible and different levels of strength. They are the only adhesives which can be reliably used for the assembly of fitted, co-axial mechanical components where disassembly may be required for maintenance or other reasons. However, other types can be considered to be temporary and may be used in simple lap joints, e.g. adhesive tapes. A particular problem is presented here by adhesives which, in some applications, may reasonably be considered permanent - elsewhere their role is less readily defined. A particularly good example being the hot melt adhesives. 

The thick adherend shear test also provides data on the modulus of the adhesive. If we use this data along with a knowledge of the modulus and thermal expansion of the two materials being bonded together, it is possible to compute the stress levels experienced by a simple lap joint. 

Diffusion of Be through polycrystalline unirradiated specimens of BeO was studied. The isotope was deposited on one end of cylindrical specimens by exchange and adsorption from aqueous solution. The depth of penetration perpendicular to the deposition surfaces was determined after annealing. A simple lapping method of high precision was used to determine the penetration depths. For poly crystalline cold-pressed and sintered material, the diffusion behaviour at 1150 to 1800C were described by ... 

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