Stress Analysis of Tubular Adhesive Joints with Delaminated Adherend

The use of adhesive joints is becoming increasingly important in aerospace, automotive and other industries where the use of traditional fasteners is discouraged. When using composite adherends, the use of adhesively bonded joints is preferable rather than the traditional bolts and other types of fasteners, because they do not require holes, thereby removing the problems of stress concentrations around the holes. However, when using an adhesively bonded joint, there will be concentrations of the distributions of shear and peel stresses within the adhesive layer which should be controlled effectively. Therefore, the investigation of such stress variation has attracted many researchers. The aforementioned stress distributions become more complicated if the composite adherend contains a pre-existing delamination. Delamination is one of the most common failure modes in laminated composite materials; it can occur due to sudden impact by an external object, during the manufacturing process (e.g., during the filament winding process), or as a result of excessive stresses due to an applied load. It is clear that the existence of a delamination in any composite structure causes a reduction in its stiffness and in some critical situations, it may cause complete failure. This paper investigates the effect of delamination on the structural response of an adhesively bonded tubular joint with composite and aluminum adherends. The finite element method, using the commercial package ABAQUS, is used to conduct a parametric investigation. The effects of the delamination's spatial location, length, width, and the applied loading are studied. Results provide interesting insight (not necessarily intuitive) into the effect of an interlayer delamination on the stress distribution within the adhesive.     

Improvement of the Adhesive Fracture Toughness of Bonded Aluminum Joints Using E-Glass Fibers at Cryogenic Temperature

Fracture toughness and crack resistance of aluminum adhesive joints were measured at the cryogenic temperature of ?150°C, with respect to the orientation and volume fraction of the E-glass fibers in the epoxy adhesive. Cleavage tests on the DCB (Double Cantilever Beam) adhesive joints were performed using two different test rates of 1.67 × 10^?2 and 8.33 × 10^?4 mm/s to observe the crack propagation trends. From the experiments, it was found that the DCB joints bonded with the epoxy adhesive reinforced with E-glass fibers not only showed a stable crack propagation with a low crack propagation speed, but also higher fracture toughness and crack resistance than those of the DCB joints bonded with the unreinforced epoxy adhesive at a cryogenic temperature of ?150°C.     

Yield in Adhesive Joints and Design of Zones with Constant Elastic Shear Stresses

Yield in adhesive joints has been investigated by several scientists among whom L. J. Hart-Smith¹ especially is to be mentioned.

In the following, a method is demonstrated which is based on a simple elastic-plastic model. It shows the distribution of stresses in the adhesive and gives a total picture of the development of the length of the yield zones and their strain as a function of load.

Methods are given for the design of adhesive joints with constant elastic shear stresses at their ends or throughout their whole length. These stresses are obtained by varying the thickness of the adherends, the adhesive, or a combination of both. The constant elastic shear stress zones can be designed to take into consideration all known factors as temperature and hardening stresses, moments, etc. The characteristic yield properties as well as internal stresses after yield and unloading are determined together with the modified stress distribution for a new load.     

Comparative Study of the Results of Various Experimental Tests Used for the Analysis of the Mechanical Behaviour of an Adhesive in a Bonded Joint

The use of adhesively bonded joints is often limited by a lack of reliable models able to accurately predict their behaviour in industrial applications, in which the stress distribution is often complex. The mechanical behaviour of an adhesive in a bonded joint is often heavily dependent on its stress state (i.e., the tensile–shear combinations). Thus, a large experimental database is required to accurately represent the complex behaviour of an adhesive in a bonded joint. On the one hand, the initial yield surface (initial elastic limit) often has to be described taking into account the two stress invariants, hydrostatic stress and von Mises equivalent stress, and on the other hand the non-linear behaviour of the adhesive is also quite complex to model. However, the mechanical response of adhesively bonded joints often presents quite large stress concentrations; thus, the analysis of experimental tests is made particularly difficult. Obtaining reliable experimental results makes it possible to contribute to optimization of an adhesive in a bonded joint. This paper presents comparisons between results of different experimental tests (with bulk and bonded joints), some of them are designed to greatly limit the edge effects. Results are presented for two adhesives under proportional monotonic loadings. The two adhesives have very different behaviours (a ductile adhesive and a brittle adhesive) and two different surface preparations of aluminium substrates (a mechanical preparation and a chemical preparation recommended by the adhesive manufacturer) were studied.