Comportamiento del VBLAST

The unsupported single-lap joint (Fig. 5-2B) is a nonstructural joint that has low effi ciency because of the eccentricity in the load path. This con-fi guration is suitable for quality control testing because it tests simultane-ously for a large number of effects (Hart-Smith 1974). However, adhesive shear stresses play only a minor role in the failure of unsupported single-lap bonded joints. It is diffi cult to predict the reliability of the joint strength experimental results due to the complex infl uence of joint geometry on the measured strength values.

Typical structural effi ciencies for unsupported single-lap joints are 25%

to 35% (Hart-Smith 1974). As shown in Fig. 5-3, the unsupported single-lap bonded joint is the weakest joint as compared to four joint confi gura-tions illustrated in this fi gure. Beyond small thickness, the critical failure for unsupported single-lap bonded joints is governed by the peel stresses developed at the adhesive layer at the end of the overlap. In the case of thick adherends, the joint effi ciency drops rapidly. Figure 5-4 illustrates the effect of eccentricity on the structural behavior and failure of unsup-ported single-lap joints.

Joint Comments 1.

Single-Lap (Unsupported) Joint

Nonstructural joint having low effi ciency because of bending of the adherend due to the eccentricity in the load path. Thick adherends are associated with failures by the peel rather than by shear.

2.

Supported Single-Lap Joint

Practical joint for the thin adherends. Needs to be mounted on moment-resistant support to avoid the above limitations.

Joint load capacity does not increase indefi nitely with overlap. Load capacity is limited by the single bond surface.

3.

Supported Single-Strap Joint

Same as for No. 2. Suitable for fl ush exterior applications but limited to thin adherends, and needs either good moment-resistant supports or very large l/t ratios.

4.

Balanced Double-Lap Joint

Balanced Double-Strap Joint

Effi cient practical joint for thin and

moderately thick adherends. Simple joint with tolerant fi t requirements. Joint strength is limited by adherend thickness and independent of overlap beyond very short (uniformly stressed) lengths of bond.

Maximum strength limit is set by peel stresses for moderately thick adherends rather than by adhesive shear stresses. For thin adherends, peel stresses are negligible and shear strength usually exceeds

adherend strength.

5.

Unbalanced Double-Lap Joint

Weaker than No. 4 due to the unbalanced nature of the design. In this construction the thin adherend is used or extends to its capacity, while the thicker adherend has an unusable reserve.

6.

Tapered-Lap Joint

Effi cient practical joints for moderately thick adherends, but shares similar overpeel-stress limitations of No. 2. Strength limited by adhesive shear strength for thick adherends. Best strengths are obtained with optimum stiffness imbalance between adherends to compensate for shear

strength loss due to taper. Only moderate precision requirements

Source: Hart-Smith (1974), courtesy of Dr. John Hart-Smith and the Boeing Company.

Table 5-2. Bonded Joint Concepts (Part II)

Joint Comments

7.

Flush Joints

Nonstructural joints suffering from net-section loss just outside the joint regions.

8.

Stepped-Lap Joint

Used extensively in advanced composite-to-titanium bonded joints. Detail design can be critical. Need to avoid composite net-section reduction at the end of the

titanium. End titanium step needs to have lower l/t ratio than other steps.

9.

Stiffness-Balanced Stepped-Lap Joint

Improvement on No. 8 because both ends of the joint are then loaded equally instead of concentrating the load transfer through the thin end of the titanium (or stiffer)

adherend in No. 8.

10.

Double-Stepped-Lap Joint

Needed for thick sections beyond the practical capabilities of No. 9.

11.

Scarf Joints

Most effi cient of all bonded joints. Necessary for thick adherends, unnecessary for thin adherends. Strength is maximized by balancing adherend stiffness at each end of the joint. Precise fi t requirements for effi cient joints can be reduced in some situations by co-curing and bonding of composite laminates.

12.

Joggled Lap Joint

Nonstructural joint used (because of

aerodynamic smoothness requirements) on exterior skins subjected to normal rather than in-plane loads. See also comments on No. 1.

Source: Hart-Smith (1974), courtesy of Dr. John Hart-Smith and the Boeing Company.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i) Figure 5-2. Bonded joint confi gurations.

Figure 5-3. Infl uence of member size on selection of optimum joint confi guration.

Source: Hart-Smith (1974), courtesy of Dr. John Hart-Smith and the Boeing Company.

In designing unsupported single-lap bonded joints, adherend bending must be taken into account due to the eccentricity of the load path, as shown in Fig. 5-4. The analysis should involve both adhesive shear stresses and peel stresses that are coupled, rather than independent, for all except joints between identical adherends (Hart-Smith 1981). The in-plane shear

loading of a single-lap bonded joint does not involve out-of-plane defor-mations of the adherends, and the appropriate analysis is to treat the unsupported single-lap bonded joint as one-half of a double-lap joint. This also applies to supported single-lap bonded joints, which are restrained against out-of-plane defl ection. One way to rectify the problems associ-ated with unsupported single-lap bonded joints is to fi rmly support these joints to nullify the effects of the load path eccentricity. This can be done by providing a kick-load at each end of the overlap to balance the applied bending moment (refer to row 2 of Table 5-1). For single-lap bonded joints, the peel stresses can be reduced by tapering the ends of the adherends and by increasing the overlap length. The latter approach also improves the effi ciency of the joint by alleviating the induced bending moment.