PSI - Issue 13

Mohammad Reza Khosravani et al. / Procedia Structural Integrity 13 (2018) 168–173 Author name / Structural Integrity Procedia 00 (2018) 000–000

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• core: honeycomb, plywood, aluminum, PVC foam, balsa, • skin: aluminum alloy, glass or carbon fiber reinforced polym er (GFRP,CFRP), plywood, • adhesive: epoxy, acrylic, film adhesive, polyester resin, p henolic, silicone, polyurethane.

In all cases of construction by above-mentioned materials, the bending moment is carried out by tension and compression by the skins, and the core supports the transverse loading. Although type of core, skin and adhesive play crucial roles in strength of sandwich joint, the bonding process also has e ff ects on failure process and strength. There are three main bonding processes in manufacturing of composite joints. These processes are (a) co-bonding, (b) co-curing, and (c) secondary bonding. In the first process on e adherend is cured with adhesive. In the second method, both parts are cured at the same time, and in the last process, adhesive layer should be cured between two pre-cured composites [13]. In several applications, particularly in aviation industry, adhesively bonded joints are preferred types of the joint. Although mechanical fasteners like bolts, pins and rivets have been used by aircraft manufacturers for many years, adhesively bonded composite joints show more e ffi ciency, specially in distributing loads and significant red uction of stress concentration. It should be noted that several facto rs influences the strength of the adhesively bonded joints, f or instance design of the joint, thickness of adhesive bond, length of the joint and mechanical properties of adhesive. In the present study, all of these factors are considered in implementation of the case-based system. Throughout the years, composite sandwich structural joints are constructed with various designs for aerospace, marine and other applications and have been examined by the di ff erent tests [14, 15, 16]. Various types of corner joints configuration made by sandwich-structured composites are s chematically shown in Fig. 1. For instance, reinforcement layers are used to support the corner joint as shown.

Fig. 1. Three types of corner joints in sandwich-structured composites.

Parallel to the developments in construction of sandwich joints, various experimental test methods are used to examine strength and sti ff ness of the joints [17, 18, 19]. For instance, in [19] experimental tests under static and dynamic loading conditions are conducted for evaluation of bonded sandwich pi-joints. By the obtained results it is claimed that the steady state crushing load is reduced under dynamic loading which attributes to the changing in failure mode due to the increased loading rate. However, determination of failure modes in sandwich joints is a complicated issue. The main failure modes are (a) cohesive failure: the crack initiates in the adhesive and propagates in it till final fracture, (b) adhesive failure: debonding oc curred between the adhesive and the adherent whereby all of the adhesive remains on one side of the separated materials, and (c) adjacent failure: adhesive and bonding area remain intact and the fracture occurs in the adjacent structure, e.g. shear failure consisting shear fracture of the core. Since sandwich joints experiences di ff erent environmental conditions during their service life, e ff ects of environment on the behavior of the joints should be noted in fracture analysis. It is worth mentioning that in this study we considered a wide range of experimental results which are achieved by tests on di ff erent sandwich joints under various loadings and environmental conditions. The reported results are saved in the case-base of the system which is described in the following section.

3. Implementation of CBR system

CBR is a methodology of solving problems and experiences are essential for this approach. Here experiences are recorded from previous experimental tests. In preparing the cases for the case base of the proposed system, the work of 26 researches is used. In collecting and recording data we considered various parameters (features) which are crucial