Lap joints are shaped to accommodate the stub end (Figure 4.23). The combination of flange and stub end presents similar geometry to the raised face flange and can be used where severe bending stresses will not occur. The lap joint flange is economical if costly pipe such as stainless steel is used because the flange can be carbon steel, and only the lap joint stub end needs to be furnished in the line material. A stub end is used in a lap joint, and the cost of the two items must be considered. If both stub end and flange are of the same material, they would be significantly more expensive than a weld neck flange.
6.13.3.2.2 Derivation of the shear-stress distribution in three related lap geometries
Three different lap-joint geometries can be defined (see Figure 4). The normal lap-joint is commonly known simply as a “lap-joint.” The lap-joint (Figure 4(a)) has a “reversed” partner (see Figure 4(b)). This joint geometry is often found in closed structures with limited structure height, such as the aileron of a glider. Figure 8 shows a section of the aileron of the GROB Astir-CS. This lap-joint can transfer shear load (e.g., due to torsion) in the upper to the lower adherent quite well, but it is not recommended for the transfer of normal loadings. In the latter case adherent bending can cause severe peel effect. The third lap-joint is a mirrored normal lap-joint (Figure 4(c)) This is a glued-on patch, or reinforced area.
Fig. 4. Three joint geometries that frequently occur in fastenerless composite structures.
Fig. 8. A section through the aileron of the Grob G 101 Astir glider. The design has limited the structural parts to a lower and upper skin. The single skin GRP structure is a closed box with a “reversed” lapjoint as trailing edge. Note the counterbalanclead weight (rod), embedded in resin. This is also a fastenerless joint, and strong enough to withstand 24 g's, as practice shows.
The ideal adhesive-bonded joint is one in which, under all practical loading conditions, the adhesive is stressed in the direction in which it most resists failure. Figure 7.3 shows several types of joints used in bonding flat adherends. These will be discussed briefly [6].
Butt joints. These joints are not able to withstand bending forces because under such forces the adhesive would undergo cleavage stress. If the adherends are too thick to design simple overlap joints, modified butt joints can be designed. Such joints reduce the cleavage effect caused by side loading. Tongue-and-groove joints are self-aligning and provide a reservoir for the adhesive. Scarf butt joints keep the axis of loading in line with the joint and require no extensive machining [6].
Lap joints. These are the most commonly used adhesive joints. They are simple to make, can be used with thin adherends, and stress the adhesive in its strongest direction. The simple lap joint, however, is offset and the shear forces are not in line, as seen in Figure 7.4[6]. It can be seen in this stress distribution curve that most of the stress (cleavage stress) is concentrated at the ends of the lap. The greater part of the overlap (adjacent to the center) carries a comparatively low stress. If the overlap length is increased by 100%, the load-carrying capability is increased by a much lower percentage. The most effective way to increase the bond strength is to increase the joint width [4]. Modifications of lap joint designs that improve efficiency include [6]:
•
Redesigning the joint to bring the load on the adherends in line.
•
Making the adherends more rigid (thicker) near the bond area (Figure 7.8).
Figure 7.8. Interrelation of failure loads, depth of lap, and adherend thickness for lap joints with a specific adhesive and adherend [6,10].
•
Making the edges of the bonded area more flexible for better conformance, thereby minimizing peel.
Modifications of lap joints are shown in Figure 7.3.
Joggle lap joints: This is the easiest design for aligning loads. This type of joint can be made by simply bending the adherends. It also provides a surface to which it is easy to apply pressure [6].
Double lap joints: These joints have a balanced construction that is subjected to bending only if loads in the double side are not balanced [6].
Beveled lap joints: These joints are also more efficient than plain lap joints. The beveled edges allow conformance of the adherends during loading, with a resultant reduction of cleavage stress at the ends of the joint [6].
Strap joints: These joints keep the operating loads aligned and are generally used where overlap joints are impractical because of adherend thickness. As in the case of the lap joint, the single strap is subject to cleavage stress under bending forces. The double strap joint is superior when bending stresses are involved. The beveled double strap and recessed double strap are the best joint designs to resist bending forces. However, both of these types of joints require expensive machining [6].
Lap joints are used quite frequently especially when attaching thin sheets together. They allow welding with less restrictive joint fit-up tolerances. Laser welding is a popular method of creating a lap connection in sheet metal owing to its simplicity, since a stake weld can be used where the laser energy penetrates the top sheet of a thin metal without the need for plug or spot welding. The net result is a weldment at the interface of the plates. The lap weld can be fabricated in various configurations as shown in Figs. 9.17 and 9.18. The fatigue performance is a function of weld configuration, among other effects. Since the actual weld dimensions are internal to the structure, the performance of lap-welds is often rated on a resistance per unit length basis in lieu of stress. In the case of a single lap, the connection is subjected, in general, to a combination of axial force, shear force and bending moments as shown in Fig. 9.19. The fatigue response of the lap connection has been analyzed, based upon the stress state at the weld root (Zhang, 2002a).
9.17. Various weld configurations for the lap joint: (a) simple single lap joint, (b) tapered lap, (c) scarf joint, (d) stepped lap, (e) strap joint.
9.18. Various weld configurations for the lap joint.
9.19. Stresses on an axially loaded single lap joint.
Determination of the stress at the weld root is done on an empirical basis or by using a structural stress approach. However, there are still difficulties in determining a proper value for structural stress in some cases. Fracture mechanics approaches using energy release rates, stress intensity factors and notch stress also have difficulty in predicting fatigue response. This difficulty can be attributed to variability in the weldment including post-weld material and geometric properties and to experimental uncertainties. During testing of a lap connection, the critical interface is not accessible until the material has broken apart. The critical stresses in this joint are the shear stress across the interface,τ, and the normal stress, σ, at the weld root. Zhang (2002b) presents a method using strain gages in combination with finite element analysis to estimate the stresses at the critical locations and shows that a more representative prediction of the structural stress results in an improved correlation of fatigue results.
Lap joints subjected to axial loads represent a critical condition in the structure where fatigue is possible or likely. An axially loaded single-lap joint will include bending stresses in addition to the axial and shear stresses, as demonstrated in Fig. 9.19. Any unintentional gap between the plates that exists owing to manufacturing will further exacerbate the bending stresses. Lap joints suffer from the notch effect, surface cracks and residual stresses (Cho et al., 2004). The relative strength of the lap joint weld to base plate material is also highly dependent on the thickness of the plates being welded. Thin plates have a tendency to fail in the base material, whereas thick plates normally fail in the weld.
Lap joints made from AISI 304 and AISI 316 stainless steel were tested by Dattoma (1994). Single lap specimens were welded with plate thicknesses of 1 mm and 2 mm using a CO2 laser. In the fatigue tests, the average slope was reported to be m = 4.73, which is more like the base metal than typical welded cruciform joints where the recommended value of m is 3.
Hsu and Albright (1989, 1991) demonstrated that multiple longitudinal welds parallel to the loading direction give a better performance than a transverse configuration. Increasing sheet thickness reduces the fatigue strength of the weld. The fatigue response of a laser welded lap joint was three-quarters that of an unwelded smooth material. The one-quarter reduction in strength was attributed to residual stresses, material changes and possible discontinuities caused by the welding.
Wang and Ewing (1991) reported the fatigue strength of laser welded sheets of SAE 1008 grade steel to be better than resistance spot welds. Laz-zarin et al. (1995) found similar strengths in laser welded bars and hot dipped galvanized sheets. The fatigue strength of most lap connections is generally lower than the base metal owing to the moment created by the offset of the sheets and any stress concentrations. Tests by Flavenot et al. (1993) on 0.7-mm thick mild steel sheets also demonstrated that laser welded lap joints have better fatigue resistance than spot welds. Microhard- ness values reported peaked at approximately 260 HV0.1 to a basis of about 110 HV0.1 with a HAZ of about 3 mm . A gap between the sheets can have a significant influence on the fatigue response. Intermittent welds also show less fatigue life than continuous beads owing to stress concentrations at the point where the weld starts and stops. A single transverse lap weld results in the poorest response to fatigue. Again, increasing the sheet thickness also tends to reduce the fatigue life.
Nordberg (2005) compared laser welded lap joints to spot welded, clinched and adhesively bonded joints in stainless steel. Compared to spot welding, the continuous laser weld dramatically reduces stress concentration in the connections. A line load resistance at 2 × 106 cycles of 118 N mm−1 for a 0.8-mm wide weld (nominal 147.5 MPa) was reported in 3-mm thick laser welded AISI 304 stainless steel. Doubling the welds or a wider bead resulted in a higher fatigue strength. The fact that the line load strength is related to the sheet thickness was also reported. A study on AISI 304CS steel reported line load resistance at 2 × 106 cycles of 60 N mm−1 for a 0.6-mm wide weld (nominal 100 MPa) in a 1-mm thick plate and 75 N mm−1 for a 1-mm wide weld (nominal 75 MPa) in a 2-mm thick plate.
Larsson et al. (1999) investigated the effect of laser bead on the fatigue properties of lap joints. Cold rolled sheet steel 0.9 mm thick was used at a laser power of 6 kW with speeds between 5 and 10 m min−1. The resulting bead widths ranged from 0.47 mm to 0.94 mm from higher to lower weld speeds. A single lap specimen with a single transverse weld bead was used. The test results show that fatigue life increased with weld bead, as expected.
Sonsino et al. (2006) tested tube connections fabricated by laser welding with an overlapped joint. The tube sections were subject to multi-axial loading. They modeled the weld root as a notch with fictitious radius of 0.05 mm . The effective equivalent stress hypothesis resulted in satisfactory prediction of fatigue life of these joints under complex multiaxial stress.
The lap joint is a type of flanged joint as well as a type of facing. If lap joint flanges are considered for an application then the type and nature of the seal must also be considered.
The lap joint uses a separate flange to carry the bolting and apply the axial load. Lap joint connections can be considered when the piping material is exotic, expensive or non-metallic. Exotic materials are very costly, so if the overall size of a component can be reduced, a considerable cost saving can be achieved. Figure 9.8 (g) shows a lap joint flange connected to a flat face flange. This type of connection would occur at vessels or other components where the lap joint principle could not be applied.
In theory, any of the common sealing methods could be applied to the lap joint flange face. If a metal-to-metal joint or plain gasket is proposed, then alignment may be a problem. In these cases alignment would be controlled by the bolting clearance and the clearance between the loose flange and the lap insert. The use of a ring-joint would ensure good alignment. Alternatively, an extension of the “spigot and recess” principle, using the od of the lap insert, could be used to maintain good alignment.
Lap joint configuration is widely used in conventional welding and friction stir lap welding (FSLW), shown schematically in Fig. 1, should be applied more and more widely. Lap welding of dissimilar alloys such as Al-to-steel or Al-to-Ti is also of considerable significance in many industries. In general, it is well known that fusion welding of Al-to-M, where M is an alloy with a melting point (or range) considerably higher than that of Al (or Al alloys), is very challenging [1]. Melting and solidification related problems can be avoided in FSW of Al-to-M. During FSLW of Al-to-M where the top plate is Al, the lower portion of the stir zone is normally the interface region. In this region, aided by frictional and deformation heat during FS, metallurgical bond is established joining the top and bottom plates, as indicated in Fig. 1b and 1c.
Figure 1. Schematics of (a) FSLW of Al-to-Fe/Ti, (b) interface region and penetration depth in a longitudinal view and (c) subsequent tensile-shear test with force normal to v direction.
For Al-to-steel FSLW, tool pin slightly penetrating axially to steel (Dp > 0) was recognised as a condition for a metallurgical joint to be established in the Al/steel interface region, resulting in a good joint strength [2]. The interface region of welds made with pin penetration is a highly irregular structure of mixed layers of α -Fe and Fe-Al intermetallics. This feature has been commonly observed in many Al-to-steel FSLW studies [2–6] and the region is named mixed stir zone (MSZ) by Coelho et al. [4] in their work on characterising the microstructures in the zone. It is clear that on one hand intermetallic formation is a condition for a joint to establish, on the other hand, intermetallics are commonly viewed to adversely affect the joint strength [2,3,5].
Recently [7], we have also confirmed that for Al-to-steel FSLW a small penetration depth (Dp as shown in Fig. 1b) can result in a good joint strength. Under tensile-shear loading, cracks propagate along the thin and irregular intermetallic layers inside the MSZ fracturing in a brittle manner, but cracks also need to propagate through the bridging α-Fe grains in the zone. The required load per unit width, Fm/ws (≈ 300-310 N/mm), for this fracture mode is not significantly affected by the size of the zone and by the coarseness of the microstructures in the zone. Thus, under the condition of small Dp, fracture load is not significantly affected by the variations of Dp, welding speed (v) and rotation speed (ω). We further illustrated that as Dp reduces to zero, an interface intermetallic layer remains without the MSZ. Evidences have suggested that this interface layer is relatively high in shear fracture resistance.
For Al-to-Ti FSLW, early work by Chen and Nakata [8] has shown the effect of Dp on joint strength although force control rather than Dp control was used in their experiments. They demonstrated, through the effect of v on Dp, that for Dp > 0 and thus for the weld containing a MSZ, a good strength value (≈ 7500 N / 20 mm = 375 N/mm) was obtained and joint strength increased significantly to 470 N/mm when Dp is less than but close to zero. They revealed voids in the MSZ and suggested that this void defect assisted cracking during subsequent mechanical testing. Thus, Fm/ws of the close-to-zero Dp weld is significantly higher than that of the weld made with Dp > 0. However, how the different modes of fracture due to the large difference in microstructures in the interface region was not explored further. In the more recent studies on FSLW of Al-to-Ti [9,10], values of joint strength are significantly lower.
In order to more clearly understand how interface microstructure formed during Al-to-M FSLW affects joint strength, in the present work, we have extended our FSLW experiments from Al-to-steel to Al-to-Ti. Experiments have been conducted with values of Dp both close to zero and higher than zero, ascertained by monitoring the down force (Fz). Al-to-Ti joint strength affected by the microstructure in the interface region is evaluated. This structure and property relationship for Al-to-Ti FSL welds is compared to that observed for Al-to-Fe FSL welds.
The lap joint is perhaps the easiest joint of all to assemble. It comprises two overlapping plates joined by a fillet weld (Fig. 5.4c). Variations in component sizes are easily accommodated and no edge preparation is required, although a bevel, as in Fig. 5.16, may be used to guarantee full root fusion.
5.16. Bevelling the plate edge in a lap joint to improve penetration.
The joint is uneconomical in terms of material as the overlapping material is waste. The overlap should be at least three times the thickness of the thinner plate. Care also needs to be taken to ensure that the weld does not melt away the corner of the upper plate as this results in a reduction in the effective throat thickness of the fillet.
The joint strength is set by the shear strength of the fillet weld. Weld sizes and lengths should be specified by the designer to guarantee adequate load- carrying capacity.
Lap joints of Alclad AA2024-T3aluminum alloy sheets were successfully welded by using friction stir welding process. Defect-free stir zone was achieved with tools of different probe geometries.
2.
While the pure aluminum layer was intractable to eccentric triangular probe and remained almost unaffected in the form of a continuous layer in the stir zone after FSW, the pure aluminum layer could be partially or completely scattered in the stir zone by using cylindrical threaded tool or sector-shape threaded tool. The friction stir lap weld that obtained by using sector-shape threaded tool exhibits the highest tensile resistance.
3.
Investigation on the macro- and microstructure of the stir zone and its adjacent area allows an interpretation of the tensile performance of the lap joint associated with the material flow patterns and tool geometries.
A lap joint is the most common type of joint in automotive assembly applications (36). Resistance spot welding is extensively used for lap joining of car body assemblies (36,37), which may contain around 4500 spot welds (38). Common weld defects in spot welding are stick weld, missing or open weld points, burned through weld points, and too small of a weld spot (39).
Observable defects in this process include metal expulsions during the welding cycle (40). Expulsion is the eruption of molten metal particles, visible as hot sparks thrown into the air, which are ejected from the welding area during the welding process. The metal expulsion decreases the cross section of the joint, weakens the weld (41), and may contain discontinuities which can spread with vibration and lead to weld failure (40). However, spot welding without expulsion seldom occurs in production, and some studies have indicated that although increased indentation may occur during expulsion, the welds are not necessarily of reduced strength (42). In addition, the expulsed molten particles may adhere and solidify on the BM as weld spatter. It is only considered a significant defect if it interferes with the part serviceability or subsequent operations, such as painting. Weld spatter can be carefully removed by blasting or mechanical grinding. Nevertheless, spatter is an important factor in most welding processes because of the cost of subsequent removal and the potential to cause in-service defects such as pit corrosion and microcracks (43).
Galvanized or zinc-coated steel sheets are used abundantly in the fabrication of automobile frames. Predominantly dip-coated steels are used, which may sometimes have an uneven coating thickness. This affects the resistance factor from weld to weld; thus, it is quite difficult to maintain the integrity of the galvanized coating when performing resistance spot welding (41). Welding a lap joint configuration would involve two layers of zinc coatings in between two steel sheets. Zinc vaporizes at 907 °C, whereas the melting point of steel is in the range of 1425–1540 °C. The different boiling points and melting points causes zinc vaporization, resulting in porosity in the weld and a general weakening of the expected shear strength. In a study by Marya and Gayden (44) on dual phase steel, it was found that the effect of zinc was most prominent in welds that were made abnormally quickly and resulted in solidification cracking. Furthermore, voids could be controlled by process parameters such as high welding forces and reduced sheet thickness.
Laser welding is used increasingly in the fabrication of various automotive body parts. The technique offers high scanning speed, high strength, and low distortion of joints, and flexible implementation of the system in the production line (36). The common defects of laser welding at high power include heavy spatter ejection (45), intrinsic pore formation (38), holes, drop outs, and LOP (46).
Laser welding is also used in the fabrication of tailor welded blanks (TWBs) (47), made up of two more sheets of metals welded together in a single plane prior to forming. The sheets can be identical, or have different thicknesses, mechanical properties, or surface coatings (48). TWBs are used in automobile manufacturing to produce body, frame, and closure panels (49). There are several challenges in the laser welding of zinc-coated steel sheets. Due to the high energy density of the laser, both zinc and steel in the weld pool would begin to vaporize and get trapped in between the sheets. A degassing process would occur if the vapor pressure exceeds the pressure of the weld pool (50). This would result in cavities in the weld seam when the liquid steel is spattered out of the welding zone. Improper degassing of the vapor would also cause porosities in the weld. Schmidt et al. (50) suggest that the zinc vapor is also generated beside the weld pool and, because it does not have a direct dissipation path, creates degassing channels as it expands toward the liquid weld pool.