Stepped 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... 

Recent theoretical studies have become much more complex. New computer-assisted techniques permit the use of finite-element matrix-theory type approaches. The effects of important variables are being determined by parametric studies. More complex joints are also being studied. New adherend materials, including advanced filamentary composites, are also being evaluated. The elastic, low-deflection, constant temperature behavior of scarf and stepped-lap joints has been replaced by elastic-plastic, large-deflection behavior, combined with thermal expansion differences, or curing shrinkage-induced residual stresses. 

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. 

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. 

Hart-Smith (references 5.25, 5.26, 5.30 and 5.31) has conducted extensive studies of bonded joints using the elastic—plastic model for the adhesive. He has covered the analysis of lap, strap, scarf and step-lap joints. He has modified the load eccentricity induced peel stress approach by using a modified bending stiffness. He has studied the effects of non-uniform adhesive thickness, adhesive non-uniform moisture absorbtion and defects in the bondline. He has also included thermal stresses in his models. 

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). 

Stepped-lap joints with glass fibre fabrics and brittle resins are easily made as each layer can be identified and peeled back. With tougher resins and stiff fibres, such as carbon-fibre fabrics and aramids, peeling back the layers is difficult, so scarf joints are more often specified in recent Structural Repair Manuals (SRMs). 

A wide variety of adhesively bonded joint configurations are available to the designer based on the application and scope of use [7]. Commonly, joint configurations that have been analyzed in the literature are single-lap joints, double-lap joints, scarf joints, and step joints (Figure 1). 

Hart-Smith, L.J., Adhesive-Bonded Scarf and Stepped-Lap Joints, Tech. Rep. NASA CR-112237, NASA Langley Research Center, Hampton, VA, 1973. 

Hart-Smith, L. J. (1973b) Adhesive-bonded scarf and stepped-lap joints, CR-112237, NASA, Langley Res. Centre. 

Composite repair configurations (a) singie strap joint (b) double strap Joint (c) scarf joint (d) step joint... 

Hart-Smith LJ (1973d) Adhesive-bonded scarf and stepped-lap joints. NASA Langley contract report NASA CR-112237, January 1973 Hart-Smith LJ (1973e) Non-classical adhesive-bonded joints in practical aerospace construction. NASA Langley contract report NASA CR-112238, January 1973... 

Further improvements in the load bearing capacity and joint efficiency of bonded joints can be achieved by using stepped-lap or scarfed configurations where the stress concentrations are reduced significantly and the peel stresses minimised or eliminated. 

Composites may be joined by bonding, bolting or both. Scarf, stepped lap, supported single-lap joints are the most common, but single-lap or double-lap types are used (Shear tests). 

Modified-lap joints such as the scarf, stepped-lap, or strap have the ability to lessen the stress concentrations in the joint. Typically, this occurs because of reductions in differential strains and in eccentricity of loading. 

The second point to be established is the joint configuration — doublelap, single-lap, stepped-lap, or scarfed, as indicated in Fig. 37. The thicker the adherends, the more complicated the joint design needs to be and, conversely, the thinner the adherends, the simpler the joint configuration can be. 

It may be useful to the reader to consider as an example the work by Adams et al (1978c) and some of their hitherto unpublished results. They used finite-element methods to examine the stresses in high-performance composites in symmetrical lap joints with parallel, bevelled, scarfed and stepped adherends. The composite adherends were assumed to be linearly elastic type II carbon fibre reinforced epoxy composites with a 60% fibre volume fraction. The mechanical properties of this material are given in Table 3. 

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