Scarf joints

An industry standard scarf repair first involves removal of topcoat and primer and then using a hand-held pneumatic router or grinder to machine the damaged portion of the laminate into a circular shape. The sides of the circular cutaway must then be machined to a taper at the required angle. Two approaches can be employed, either stepped or scarfed. The scarfed joint is theoretically superior (better stress distribution) but little difference is seen in practice. The joint face is sanded to a constant... 

Scarf repairs can be modelled to a first approximation as a simple scarf joint. If the patch matches the parent material in stifliiess and expansion coefficient, simple theory... 

Gunnion AJ, Herszberg I. Parametric study of scarf joints in composite structures. Compos Stmct 2006 75(1) 364—76. 

Fig. 4.14. Some adhesive joint fracture mechanics specimens, (a) Tapered double cantilever beam (TDCB). (b) Thick double cantilever beam (DCB). (c) Thin double cantilever beam or wedge cleavage specimen, (d) Independently loaded mixed-mode specimen (ILMMS). (e) Scarf joint.

Scarf joint. The importance of testing adhesive fracture under mixed-mode stress conditions has been noted. In the scarf joint (Fig. 4.14(e)), the applied load is resolved in the bondline to Mode I and Mode II components and their ratio changes with the scarf angle, < >. Bascom et al. 26, 70) investigated the effects of bondline thickness and test temperature, and G,i. n)c for < ) = 45° was calculated from the failure load and crack length using a finite-element analysis. A complex behaviour pattern emerged, as discussed... 

Figure 7.14 Recommended scarf joint configuration for flexible plastics and elastomers. ...

Figure 7.15 shows several types of joints for rubber under tension. The horizontal white lines are equidistant when the joints are unstressed. It is obvious that the scarf joint is least subject to stress concentration with materials of equal modulus, and the double scarf joint is the best for materials of unequal modulus. These designs offer the best resistance to peel and, all other factors being equal, represent the best choices. ... 

Feathering—The tapering of an adherend on one side to form a wedge section, as used in a scarf joint... 

P(7) Knife edges are not allowed in scarf joints. The adherend ends should have a minimum thickness of 1.0 mm. 

P(2) Instead of butt joints a strap, scarf or lap joint configuration shall be used (see Figure 5.41). The joint design is then undertaken according to the procedures for strap or scarf joints respectively. When using strap configurations the adherend ends shall also be bonded. 

P (10) If scarf Joints or step lap Joints are used with adherend thicknesses of less than 5 mm, proper Jigging shall be used during bonding to guarantee an adequate bondline quality. 

A typical laminated scarf Joint is shown in Figure 5.46. The scarf angle should be equal to or less than 10°. 

Laminated scarf joints may be designed according to the design... 

Although stress concentrations are a problem mainly with lap and strap joints, it should be noted that stress concentrations are present also in scarf and step-lap joints. However, in scarf joints the stress concentrations are irrelevant when the scarf angle is low, typically less than 20°. in step-lap joints the stress concentrations generally have to be taken into account only at the ends of the outermost steps. 

The procedure for the symmetrical double taper scarf joint is identical to that for the single taper scarf joint case except that the required bond length is half the bond length of the single-taper joint. 

Step-lap joints share features in common with both double lap joints and scarf joints. The scarf joint represents the mathematical limiting case of a step-lap joint with an infinite number of steps, (see reference 5.30). 

A scarf joint reaches its maximum strength when the scarf tip is sharp. However, in actual applications this kind of a sharp tip is difficult to manufacture. Also the bondline thickness becomes extremely difficult to control close to the tip. Therefore, it is required that the tip should have a butt end with a finite thickness of at least 1.0 mm. 

Only one scarf Joint geometry was tested. It had the maximum allowable scarf angle and therefore the Joint was not very effective. However, it provided a maximum loading density of 282 N/mm, compared to 218 N/mm in the single-lap joint, while the bond lengths of the scarf joint and the single-lap joint were 14 and 25 mm, respectively. 

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