Step lap joint

The basic types of joint can be used with either bonded or mechanical fastening, but there are special requirements when using mechanical fasteners with tapered thickness plates. The simplest design (unsupported single lap) is the weakest the most complex (stepped-lap joint) is the strongest (Figures 5.89 and 5.90). 

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 The elastic, low-deflection, constant temperature behavior of scarf and stepped-lap joints has been replaced by elastic-plastic, large-deflection behavior, combined with thermal expansion differences, or curing shrinkage-induced residual stresses. 

Figure 7.22 Joints used to reduce stress in adhesive bonding, (a) A tapered lap joint reduces stress at the joint ends, (b) A step lap joint avoids a large change in stress concentration from the middle to the ends of the joint when long overlap lengths are necessary. ...

Notation and a typical geometry for a stepped lap joint are given in... 

As step-lap joints have similar features to lap and donble-lap Joints, every design aspect that has been indicated in the design of lap Joints shonld also be considered in the design of step-lap joints. In step-lap Joints factors snch as fibre orientation on the bond surface still have a major effect on the Joint strength (see 5.3.1.4), while tapering the adherend ends becomes irrelevant. 

P(8) The steps of step-lap Joints may be formed during the laminate... 

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. 

Design of step-lap joints A typical laminated step-lap joint is shown in Figure 5.47. 

P(2) Laminated step-lap 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. 

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). 

In addition to the above mentioned, Hart-Smith (reference 5.31) has given three practical limitations for the optimisation of bonded step-lap joints ... 

When elastic-plastic adhesive behaviour is assumed, the analysis predicts that the adhesive will become plastic first between the butt faces at each end of the step-lap joint. At higher loads, the proportion of the load transferred by the butt faces is reduced as the joint efficiency is increased. 

Step-lap joints are more complicated and more expensive to manufacture than lap joints. Requirements for the geometrical tolerances of the joint are also much more demanding. 

There are no simple analytical methods for analysing bonded-bolted joints. The static strength of an undamaged bonded-bolted joint can be analysed identically as for the corresponding bonded joint. When the effect of defects needs to be evaluated, computer codes or finite element (FE) packages are required. Hart-Smith (reference 5.45) has developed a FORTRAN code called A4EK for analysing intact and flawed bonded-bolted step-lap joints with linearly elastic adherend deformations. 

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). 

Fig. 3. An ultrasonic C-scan image of two deliberate bonding defects shown by the dark areas between the dotted boundaries, in the bond area of a step lap joint. (From J L Rose, P A Meyer, Ultrasonic procedures for predicting adhesive bond strength.

Figure 2.32 The stepped lap joint. The two forms of this joint are commonly used, particularly for wooden and moulded plastic joints, where shaping and forming costs are low. However, their use on metal structures is limited by machining costs unless extrusions can be used (see Figure 2.35).

The single and double-stepped lap joint in Figure 2.32 is a further refinement of the rebated or joggled lap. The bonded vertical faces help to reduce the tendency to fail either by peel or cleavage. 

Erdogan and Ratwani( ) presented an analytical solution based on a one-dimensional model for calculating stresses in a stepped lap joint. One adherend was treated as isotropic and the second as orthotropic, and linear elastic behavior was assumed. The thickness variation of the stresses in both the adherends and the adhesive was neglected. 

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

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