Lap and butt joints

Figure 8.21a Comparison of lap and butt joints. (Reproduced by kind permission of SAE, Warrandale, PA, USA)...

Lap and butt Joints, Sandwich structures, Bushes/gears/ bearings/shafts/pipes/threaded fittings... 

This is a large and growing application for butyl rubbers. Butyl caulks are used in limited movement lap and butt joints, and in concealed or interlocking metal-to-metal joints such as are found in curtain wall... 

All brazed joint designs shall be suitable for the intended critical service of hydrogen applications. There are two basic types lap and butt. 

Sheet or panel (metal, plastic, etc.) wall siding. Seal tight all lap or butt joints. Make certain that caps, covers and flashings don t have hidden voids due to siding configurations (flutes, corrugations, etc.)... 

If the joint movement amounts to 15—35% of the total joint width, a shouldow sealant depth in a wide joint will minimize stress on the sealant and on its adhesive bond to the substrate (this apphes to expansion, butt, capping, and some floor, lap, and comer joints). 

Joints and fastenings should be arranged to give clean uninterrupted lines. Welds are generally preferable to bolted joints, and butt welds to lap welds. If lap joints have to be used, then appropriate welding or filling may be necessary to avoid the entrapment of moisture and dirt (Fig. 9.15). 

Countersunk rivets or screws give a surface profile which is compatible with painting protuding bolts and rivets are very much more difficult to coat adequately. Welded joints can aid optimum shaping for painting. Butt joints are preferable to lap joints. 

The life of equipment subjected to corrosive environments can be increased by proper attention to design details. Equipment should be designed to drain freely and completely. The internal surfaces should be smooth and free from crevasses where corrosion products and other solids can accumulate. Butt joints should be used in preference to lap joints. The use of dissimilar metals in contact should be avoided, or care taken to ensure that they are effectively insulated to avoid galvanic corrosion. Fluid velocities and turbulence should be high enough to avoid the deposition of solids, but not so high as to cause erosion-corrosion. 

Brazed joints are limited to tubular lap or butt-lap joints and shall meet the more stringent requirements of this Code or the engineering design. See Nonmandatory Appendix A, para. A-3.3 for AWS C3.3, Recommended Practices for the Design, Manufacture, and Examination of Critical Brazed Components. 

Imperfectionf When required examination is Girth and miter-joint butt welds Longitudinal butt welds 1 Fillet, socket, seal, and reinforcement attachment welds Welded branch connections and fabricated laps... 

Figure 6.2 Joint types (a) butt joint (b) expansion joint (c) (i) glass-to-glass lap joint, (ii) concrete-to-concrete lap joint (d) triangular joint (e.g. sanitary seals and glazing seals). H = sealant S = backing material [part (c)]...

The cured sealant gives a tough, elastic, rubber-like seal and gives excellent adhesion to concrete and masonry, glass, aluminium and stainless steel. The shore A hardness ranges from 15-35. The movement accommodation factor is 25% in butt joints and 50% in lap joints. These sealants have the capacity to accommodate continuous and pronounced cyclic movements. They are suitable for joints where the joint width may range between 5 mm and 50 mm. Joints which are expected to experience cyclic movements should have a width depth ratio of 2 1. Minimum sealant depths recommended for different environments are mentioned in Table 7.8. Primers... 

Different methods have been devised to test the bondability of the treated plastic. The specimen can be joined in different ways butt joint, disk joint, and lap shear. An adhesive layer or film is applied to the treated surface, which is then placed against the substrate and heated in a press or oven. The substrate could be a duplicate of the plastic itself. Table... 

FSW can be applied to most geometric structural shapes and to various types of joints, such as butt, lap, T-butt, and fillet shapes (Ref... 

While many have demonstrated that two plates in the butt joint conhguration can be readily friction stir welded, it is the lap joint that offers the most applications. Lap joints are frequently used in industry, and the replacement of fasteners (rivets or holts) with FSW would he faster if significant modihcations of current production parts were not necessary. The lap joint interface (and corresponding surface oxides) resides in a horizontal layer that is more difficult to break up than the vertical interface encountered in hutt joints. 

Modification of Butt Joint Pins. Derivatives of butt joint tools have shown some promise to produce quality lap joints. One such tool used a scrolled shoulder and a partially threaded pin. Threads were on the upper part of the pin... 

Joint Design Allowables and Service Life Assessments. Friction stir welding has been demonstrated in a variety of joint designs (Fig. 13.15). The most commonly used joints are the full-penetration butt joints and the parfial-penetration lap joints, followed by the edge joint, capture joint, and fillet joint, listed in order of ease of manufacture. Friction stir welding is not a dropin process, and existing riv-... 

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