Laminate joints

High stiffnesses and strengths can be attained for composite laminates. However, these characteristics are quite different from those of ordinary materials to which we often need to fasten composite laminates. Often, the full strength and stiffness characteristics of the laminate cannot be transferred through the joint without a significant weight penalty. Thus, the topic of joints or other fastening devices is critical to the successful use of composite materials. 

The purpose of this subsection is to familiarize the reader with some of the basic characteristics and problems of composite laminate joints. The specific design of a joint is much too complex for an introductory textbook such as this. The published state-of-the-art of laminate joint design is summarized in the Structural Design Guide for Advanced Composite Applications [7-5] and Military Handbook 17A, Plastics for Aerospace Vehicles, Part 1, Reinforced Plastics [7-6]. Further developments can be found in the technical literature and revisions of the two preceding references. 

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The two major classes of laminate joints are bonded joints as in Figure 7-39 and bolted joints as in Figure 7-40. Often, the two classes are combined, for example, as in the bonded-bolted joint of Figure 7-41. Joints involving composite materials are often bonded because of the natural presence of resin in the composite and are often also bolted for... 

D 5751 Standard Specification for Adhesives Used for Laminate Joints in Nonstructural Lumber Products... 

Laminated Joint—A joint where loads are primarily transferred from laminated on to cured members. 

Strap and tee Joint configurations are the most frequently used laminated Joints. 

P(l) The same requirements of qnaUty and snpervision shall be applied to the fabrication of laminated joints as to the manufacture of laminated structures and components. 

P(15) The laminated joint shall be cured so that the joint reaches the level of temperature resistance required from the whole structure or component. 

Laminated joints are often manufactured under on site conditions which may increase the number of defects or even be the primary reason for these defects. The on-site conditions may also introduce t3q>es of defects... 

When an adhesively bonded or laminated joint has failed due to adhesive/resin failure and none of the joint members is damaged, the joint may be reassembled using adhesive bonding with proper structural adhesive. Surfaces shall be treated according to 5.3.1.5 prior to bonding. 

This section also applies to bolted-bonded connections where structural adhesive is used, and laminated joint connections, as appropriate, unless otherwise specified by the designer. 

Laminated joints are formed by laminating fibre layers on cured composite parts, often by hand lay-up. The resin used is typically identical to that used in the components to be joined. However, different resins may also be used for fabricating laminated joints, for example epoxy on polyester parts. 

The mechanical behaviour of laminated joints can be characterized by using two... 

The first case includes the assumption that there is a distinguishable layer between the adherend and the on-laminated joint. However, there is no specific layer that can be said to be the adhesive layer thus the thickness of the adhesive layer is not accurately definable. This leads to the second assumption, that the joint strength is dependent on the internal strength of the laminate. In reality, the performance of laminated joints indicates that the first assumption is closer to the truth. 

A typical laminated joint is a butt joint on to which doublers are laminated, the result being a double-strap joint. The following features distinguish this laminated joint from the corresponding adhesively bonded double-strap joint ... 

A laminated joint is typically a weak link in the structure for the following reasons ... 

Basically similar manufacturing principles may be applied to laminated joints as to adhesively bonded joints, ffowever, some additional points should be considered, while some others become irrelevant. These additional points include the following ... 

The same manufacturing requirements should be applied to laminated joints as to fabricating laminated structures in general. 

Similar principles can be applied to the manufacture of moulded joints as to adhesively bonded and laminated joints, but owing to the special manufacturing techniques involved some additional points should be considered. Three major aspects are discussed below. 

Repairs may be made by building up the section thicknesses at the defect as if making a laminated joint. Thicknesses of repair material should be at least equal to the thickness of the parent defective tube, bar, plate, etc. and in no case less than 3 mm. The procedure should be as set down in section 5.3 for bonded or laminated joints. 

The test programme was necessary because of lack of a reliable joint data. For example, there are very limited data on the behaviour of laminated joints. Also, adhesive manufacturers are generally not able to provide those adhesive mechanical properties required in the design process. 

Laminates were manufactured mainly by Neste subsidiary Plastilon Oy. The laminates A300 WR, A300 UDl and A300 UD2 were manufactured by the Helsinki University of Technology, Laboratory of Lightweight Structures (HUT/LLS). Adhesively bonded joints and specimens for the mechanical characterisation of the adhesives were made by HUT/LLS. Laminated joints were made by Neste and HUT/LLS. All laminates, laminated joints, and adhesives were cured at room temperature for 24 hours minimum and then postcured according to the procedures given in Table 3. 

In laminated joints, the other adherend or the strap or straps were laminated onto the treated laminate surface or surfaces. Samples 25 mm wide were cut from the cured laminates. Typically live specimens from each batch were tested. 

Table 8 Compilation of laminated joint tests in which a mat layer was used on laminate surfaces, hand lay-up laminates.

It may be noticed that adhesively bonded joints and laminated joints appear to have similar maximum loading densities, i.e. load per unit width, as indicated in Figure 4, while the joint efficiencies of laminated joints are typically higher than those of adhesively bonded joints (Figures 5 to 7). One possible explanation of this behaviour is that in laminated strap joints the bond surface of the strap had a woven roving surface layer, while in adhesively bonded strap joints a laminate having a mat surface layer was used as straps. 

According to these results it seems justifiable to use the design procedures of adhesively bonded joints also for laminated joints. 

Figure 6 Joint efficiencies of laminated joints with A300 HL adherents.

The effect of the adhesives and resins on the joint efficiency is shown in Figure 9. A possible explanation for the better performance of laminated Joints compared to adhesively bonded joints is the one given above concerning the strap lay-up sequence. However, that does not explain the behaviour of the laminated single-lap joint. 

Adhesively bonded and laminated joints can be treated identically. 

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