Butt joints stress distribution

Figure 3 Axial and shear stress distributions in butt joints, v = 0.49. Djh is diameter/thickness ratio.

Fig. 4.1. Elastic stress distributions in various kinds of bonded joints, (a) Lap shear, (b) Butt-tensile and scarf, (c) Cleavage, (d) Peel.

Fig. 4.11. Tensile (cylindrical) butt-joint test, (a) Unloaded, (b) Loaded, (c) Stress distribution.

Fig. 2. An example of the stress distribution in an axially loaded butt joint (after Adams et al.y (ao = applied average axial stress a = 2.5 GPa Eg = 69 GPa aspect ratio = 20)...

In a similar study, Brewis et al observed the same type of reductions in the mechanical properties of the adhesive. They demonstrated that the reduction in strength of single lap joints on exposure to moisture was linearly related to the fractional water content of the joints. On saturation with water the joints were weakened by 44%, whereas films of the adhesive were weakened by as much as 74%. It was pointed out that the lack of correlation between the bulk and adhesive joint strength was due to the different stress distributions in the two situations. They concluded that the principal mechanism of loss of adhesive joint strength was by diffusion-controlled water plasticization of the adhesive. There is an apparent discrepency between this work, in which water has a reversible effect on joint strength, and that of Butt and Cotter, in which the effect is irreversible. This will be discussed further in Section III.B. 

The axial-loaded butt joint is the simplest geometry for stress distributions arising from the application of tensile loads. Several investigators have addressed the stress analysis of this geometry using both analytical and finite element methods. ... 

Adams et al. [6] detailed axisymmetric analysis of a butt-tension joint and considered the influence of the detailed geometry at the edges of the joint on the stress distribution. The single lap joint was further analysed by Crocombe and Adams [7] using the finite element method and the effect of the spew fillet was included which was seen to significantly redistribute, and decrease, the stresses at the ends of the adhesive layer. In complementary work, Crocombe and Adams [8,9] analysed the mechanics of behaviour of the peel test and included the effects of non-linear deformations and also plasticity in their work. Harris and Adams [10] extended this work and accounted for the non-linear behaviour of the single lap joint. Crocombe et al. [11] quantified the influence of this non-linearity, both material and geometric. 

Fig. 5. Axial stress distribution in adhesive layer of butt tensile joint [12],...

Alwar and Nagaraja [22] and Adams et al. [10] have used an elastic finite-element method to analyse the stress distribution in butt joints loaded in tension and a typical stress distribution is shown in Fig. 6.7 for an epoxy adhesive bonding aluminium alloy substrate. The bonded area comprises two different regions. 

Figure 6.7 Stress distribution in a butt joint loaded in tension [10].

As stated in Section 6.4.2, an annular butt joint, or napkin-ring, specimen tested in shear minimizes the variation of shear stress in the adhesive and has been used by many workers to assess the shear strength and shear stress versus strain behaviour of adhesives [14,17,111-115] and is also listed for this purpose as ASTM E 229 (see Table 6.1). The independence of the measured strength upon specimen geometry has been substantiated by Bryant and Dukes [112] and Foulkes et al. [110] and the shear stress distribution calculated using linear elasticity theory is ... 

Typical Joint designs using brazing lap and scarf in thin Joints with large contact areas or a combination of lap and fillet. Fillets can help to distribute stresses at the Joint. Butt Joints are possible but can cause stress concentrators in bending. 

Figure 50 shows the adhesive shear stress distributions for a doublescarf joint and a double butt-strap joint and compares them with ordinary single- and double-lap joints. The double butt-strap joint gives the lowest stress concentration at its middle, but is then identical to the ordinary, parallel double-lap joint at the other end. For this reason, it is suggested that the straps should be bevelled or scarfed so that the stress concentration might approach that of the double-scarf joint. 

Fig. 64. Shear-stress distributions for solid butt joints in torsion (aspect ratio, 40) (from Adams et al, 1978b).

The shear stress distributions for an annular butt joint with and without a spew fillet are shown in Fig. 65. As with the solid butt joint,... 

Fig. 67. Stress distributions for solid butt joint in tension with and without spew fillet (aspect ratio, 20) and ere values at z = t/2 omitted for clarity -----with spew fillet ---------without spew fillet (from Adams et al, 1978b).

Thus, the complex strain distributions in axially loaded butt joints make it difficult quantitatively to predict their stress-strain behaviour and ultimate tensile strength from the bulk properties of the adhesive without the use of a non-linear stress analysis and a detailed understanding of the response of the adhesive to the local stress concentrations around the perimeter of the joint. Conversely, it would be very difiicult to use the stress-strain data obtained from axially loaded butt... 

Mechanical adhesion depends on surface topography, which can be considered a collection of many geometrical forms. Therefore, mechanical adhesion depends on the stress states of different adhesive joint geometries on the scale of the surface topography, which may include many lap, butt, and scarf joints in the interphase region. To address this issue, Ma et al. (2001) compared the stress distributions in adhesive joints as functions ofvarying geometrical interfaces described mathematically in polynomial or other functional forms, as well as the material properties of the adhesive and the adherend or two different substrates joined by an... 

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